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I IntroductionTwo adjacent conductors are sandwiched by a layer of a non-conductive insulating medium to form a capacitor. Capacitors are one of the most commonly used electronic components. They play an important role in circuits like tuning, bypassing, coupling, and filtering. For example, they are often used in the tuning circuit of the transistor radio, coupling circuit and bypass circuit of the color TV. This article mainly introduces how to properly use multimeters to test capacitors and aluminum electrolytic capacitors (solid state capacitor), including detailed operating steps, working principles, notice, and explaining some fundamental knowledge about capacitors. We also have a related post about how to check start capacitors you may be interested in. Don't miss it! How to Test Capacitors with a Digital MultimeterCatalogI IntroductionII Definition of CapacitorIII The Reasons and Effects of Testing Capacitors and Withstand Voltage Performance  3.1 Why Should We Measure the Capacitance of A Capacitor?  3.2 Why Should Capacitors Undergo A Withstand Voltage Test?IV The Difference of Capacitors with Different Capacity in Test  4.1 Small-capacity Capacitor Test  4.2 Large-capacity Capacitor Test  4.3 Supercapacitor TestV How to Test Capacitors with A Multimeter?  5.1 Direct Test with A Capacitor  5.2 Test with Resistance File  5.3 Test with Voltage File  5.4 Test with Buzzer  5.5 Use a Digital Multimeter to Measure Capacitance Greater Than 20μFVI How to Detect Capacitors in Aluminum Capacitors  6.1 Appearance Physical Inspection  6.2 Capacity and Loss Test  6.3 Ripple Voltage Test  6.4 Leakage Current Test  6.5 Explosion Test  6.6 Temperature TestVII Considerations for Capacitor TestingVIII One Question Related to Testing Capacitor  8.1 Question  8.2 AnswerⅨ Frequently Asked Questions about How to Test a CapacitorII Definition of CapacitorCapacitors comprise components that store electricity and electrical energy (potential energy). A conductor is surrounded by another conductor, or the electric field lines emitted by one conductor all terminate in the conduction system of the other conductor, called a capacitor. This is a short introduction of capacitor. Under what circumstances do you need to test the capacitors, that's when you have capacitor uncertainty in use. So let's analyze it here. III The Reasons and Effects of Testing Capacitors and Withstand Voltage Performance3.1 Why Should We Measure the Capacitance of A Capacitor?The purpose of measuring the capacitance value of a capacitor in a general sense of electricity is to check the change of its capacitance value. By comparing the measured value with the value on the nameplate, you can judge whether the internal wiring is correct and whether the insulation has deteriorated because of moisture, whether the component has broken down, and whether oil leakage has caused the capacitance to decrease. So be careful during the substantial operation. 3.2 Why Should Capacitors Undergo A Withstand Voltage Test?The withstand voltage test refers to the test of the capability of withstanding voltage of various electrical devices and structures. The process of applying a high voltage to an insulating material or an insulating structure without damaging the performance of the insulating material is considered a withstand voltage test. Broadly speaking, the primary purpose of the capability of withstanding voltage test is to check the ability of the insulation to withstand working voltage or overvoltage, and then to check whether the insulation performance of the product equipment meets safety standards capability of withstanding voltage test is to check the ability of the insulation to withstand working voltage or overvoltage, and then to check whether the insulation performance of the product equipment meets safety standards.Figure1. Capacitor TestingIV The Difference of Capacitors with Different Capacity in Test4.1 Small-capacity Capacitor TestThe capacitance of a small-capacity capacitor is generally below 1 UF because the capacity is too minor, the charging phenomenon is unobvious, and the angle of the hand to the right is not large when measuring. Therefore, it is generally impossible to estimate its capacitance with a multimeter, but only to detect whether it has leakage or breakdown. Under normal conditions, the resistance value of both ends of the multimeter R × 10 k should be infinite. If the certain resistance value is measured or the resistance value is close to 0, it means that the capacitor has leaked electricity or has been damaged by a breakdown.Related recommendation: How to Test Ceramic Disc Capacitor 4.2 Large-capacity Capacitor TestLarge capacity can generally be tested by 1K-10K, see the sweep of the meter during charging, and the resistance value indicated by the last meter. The closer to the left, the better. If the resistance is too small, it cannot be used. 4.3 Supercapacitor TestThe method of measuring supercapacitors is completely different from other types of capacitors. Supercapacitors have exceptionally large capacitance values that cannot be measured directly by standard equipment. Ordinary methods for testing the capacitance of these capacitors are by charging the supercapacitors at the rated voltage and discharging the supercapacitors by a constant current load.Figure2. Different CapacitorsV How to Test Capacitors with A Multimeter?5.1 Direct Test with A CapacitorSome digital multimeters have the function of measuring capacitance, and their ranges are divided into five ranges of 2,000p, 20n, 200n, 2μ and 20μ. When measuring, you can directly insert the two pins of the discharged capacitor into the Cx jack on the meter board and select the appropriate range to read the display data. 2,000p file, suitable for measuring capacitance less than 2000pF; 20n file, suitable for measuring the capacitance between 2000pF and 20nF; 200n file, suitable for measuring the capacitance between 20nF and 200nF; 2μ file, suitable for measuring between 200nF and 2μF Capacitance; 20μ range, suitable for measuring the capacitance between 2μF and 20μF. Experience has shown that some types of digital multimeters (like DT890B +) allow a considerable error when measuring small-capacity capacitors below 50pF, and there is almost no reference value for measuring capacitance below 20pF. At this time, the small value capacitance can be measured by the series method. Method: First find a capacitor of about 220pF, use a digital multimeter to measure its actual capacity C1, and then connect the small capacitor to be tested in parallel to measure its total capacity C2. The difference between the two (C1-C2) is subsequently the capacity of small capacitors under test.It is extremely accurate to measure the small capacitance of 1 ~ 20pF with this method.Figure3. How to Test a Capacitor with a Multimeter5.2 Test with Resistance FileThe practice has proved the charging process of capacitors can also be observed by using a digital multimeter, which actually reflects the change of charging voltage in discrete digital quantities. Assuming that the digital multimeter's measurement rate is n times/second, in the process of observing the charging of the capacitor, you can see n readings that are independent of each other and increase sequentially. According to this display characteristic of the digital multimeter, it is possible to detect the quality of the capacitor and estimate the size of the capacitance. The following describes the method of detecting the capacitor using the resistance meter of a digital multimeter, which is of practical value for instruments without a capacitor. This method is suitable for measuring large-capacitance capacitors from 0.1 μF to several thousand microfarads. 5.2.1 Operation Method of MeasurementAs shown in Figure 4, set the digital multimeter to the appropriate resistance level. The red and black test leads respectively to touch the two poles of the capacitor Cx under test. At this time, the displayed value will gradually increase from "000" until the display Overflow symbol "1."If"000" is consistently displayed, it means the capacitor is short-circuited internally; if it is constantly displayed, the internal poles of the capacitor may be open-circuited, or the selected resistance level may be inappropriate. When checking electrolytic capacitors, pay attention to the red test lead (positive charge) is connected to the positive electrode of the capacitor, and the black test lead is connected to the negative electrode of the capacitor.Figure4. Digital Multimeter 5.2.2 Measurement PrincipleFigure5 shows the measurement principle of measuring capacitors with resistance files. During the measurement, the positive power source charges, the capacitor Cx to be measured through the standard resistor R0. At the moment when charging starts, Vc = 0, so “000” is displayed. As Vc gradually increases, the displayed value increases. When Vc = 2VR, the meter starts to display the overflow symbol "1." The charging time t is the time required for the displayed value to alter from "000" to overflow. This time interval can be measured with a quartz meter.Figure5. Principle of Measurement 5.2.3 Measured Data Using DT830 Digital Multimeter to Estimate CapacitanceThe principle of selecting the resistance range is: when the capacitance is small, a high resistance should be selected, and when the capacitance is large, a low resistance should be selected. If you use a high-resistance range to estimate a large-capacity capacitor, the measurement time will last a long time because the charging process is very slow. If you use a low-resistance range to check a small-capacity capacitor, the meter will always show an overflow because the charging time is extremely short, and you cannot see the change. 5.3 Test with Voltage FileDetecting capacitors with the DC multimeter of a digital multimeter is actually an indirect measurement method. This method can measure small-capacitance capacitors from 220pF to 1μF, and can accurately measure the capacitor leakage current.5.3.1 Measurement Methods and PrinciplesThe measurement circuit is shown in Figure6. E is an external 1.5V dry battery. Set the digital multimeter to the DC 2V range, connect the red test lead to one electrode of the capacitor Cx under test, and the black test lead to the battery negative. The input resistance of the 2V range is RIN = 10MΩ. After the power is turned on, battery E charges Cx via RIN and starts to establish voltage Vc. The relationship between Vc and charging time t isFigure6. Wiring Diagram of Measuring Capacitor with Voltage Block Here, because the voltage across RIN is the instrument input voltage VIN, so RIN actually has the function of a sampling resistor. obviously,VIN (t) = E-Vc (t) = Eexp (-t / RINCx) (5-2)Figure7 is the change curve of the input voltage VIN (t) and the charging voltage Vc (t) on the capacitor under test. It can be seen from the figure that the change process of VIN (t) and Vc (t) is just the opposite. The curve of VIN (t) decreases with time, while Vc (t) increases with time. Although the meter shows the change process of VIN- (t), it indirectly reflects the charging process of the capacitor Cx under test. During the test, if Cx is open (no capacity), the displayed value will always be “000”. If Cx is internally short-circuited, the displayed value will always be the battery voltage E and will not change with time.Figure7. Change Curve of VIN (t) and Vc (t) Equation (5-2) shows that when the circuit is turned on, t = 0, VIN = E, the initial display value of the digital multimeter is the battery voltage, and then as Vc (t) increases, VIN (t) gradually decreases. Until VIN = 0V, the Cx charging process ends, at this timeVcx (t) = EUsing digital multimeter voltage level detection capacitor, not only can check small-capacitance capacitors from 220pF to 1μF, but also measure the capacitor leakage current. Let the leakage current of the capacitor being measured be ID, and the stable value displayed by the meter at the end is VD (the unit is V), thenFigure8. Equation (5-3) 5.3.2 ExamplesExample 1:The measured capacitance is a 1μF / 160V fixed capacitor, using the 2VDC range of the DT830 digital multimeter (RIN = 10MΩ). Connect the circuit according to Figure6. Initially, the meter displayed 1.543V, and then the displayed value gradually decreased. After about 2 minutes, the displayed value stabilized at 0.003V. Find the leakage current of the capacitor under test.Figure9. Equation The leakage current of the capacitor under test is only 0.3nA, indicating good quality.Example 2:The capacitor under test is a 0.022μF / 63V polyester capacitor. The measurement method is the same as in Example 1. Due to the small capacity of this capacitor, VIN (t) decreases rapidly during measurement, and after about 3 seconds, the displayed value decreases to 0.002V. Substituting this value into equation (5-3), the leakage current was calculated to be 0.2nA. 5.3.3 Notes(1) Before measurement, the two pins of the capacitor should be short-circuited and discharged, otherwise, the change process of the reading may not be observed.(2) Do not touch the capacitor electrode with both hands during the measurement to avoid meter jumping.(3) During the measurement, the value of VIN (t) changes exponentially, and decreases rapidly at the beginning. With the increase of time, the decline rate will become slower and slower. When the capacitance of the capacitor Cx under test is less than a few thousand picofarads, because VIN (t) initially drops too quickly, and the meter's measurement rate is too low to reflect the original voltage value, the initial display value of the meter is lower than the battery Voltage E.(4) When the measured capacitor Cx is greater than 1 μF, in order to shorten the measurement time, a resistance file can be used for measurement. However, when the capacitance of the capacitor under test is less than 200pF, it is difficult to observe the charging process because the change in the reading is very short. 5.4 Test with BuzzerUsing the buzzer file of the digital multimeter, you can quickly check the quality of the electrolytic capacitor. The measurement method is shown in Figure10. Set the digital multimeter to the buzzer position, and use two test leads to contact the two pins of the capacitor Cx under test. A short beep sound should be heard, the sound will stop, and the overflow symbol "1" will be displayed. Then, measure the two test leads again, and the buzzer should sound again, and the overflow symbol “1” will be displayed at last, which indicates that the electrolytic capacitor under test is basically normal. At this time, you can dial to 20MΩ or 200MΩ high resistance to measure the leakage resistance of the capacitor to determine its quality.Figure10. Wiring Diagram For Testing Electrolytic Capacitor with Buzzer The principle of the above measurement process is: At the beginning of the test, the charging current of the instrument to Cx is large, which is equivalent to the path, so the buzzer sounds. As the voltage across the capacitor continues to increase, the charging current rapidly decreases, and finally, the buzzer stops sounding. During the test, if the buzzer keeps sounding, it means that the internal of the electrolytic capacitor has been short-circuited. If the buzzer keeps sounding and the meter always shows "1" when the meter pen is repeatedly measured, it means that the capacitor under test is open or the capacity disappears. 5.5 Use a Digital Multimeter to Measure Capacitance Greater Than 20μFFor common digital multimeters, the maximum measurement value of the capacitance file is 20 μF, which sometimes cannot meet the measurement requirements. For this reason, the following simple method can be used to measure the capacitance of more than 20μF with the capacitance file of the digital multimeter, and the maximum capacitance of several thousand microfarads can be measured. When using this method to measure large-capacitance capacitors, there is no need to make any changes to the original digital multimeter circuit. The measurement principle of this method is based on the formula C string = C1C2 / (C1 + C2) of two capacitors in series. Since two capacitors with different capacities are connected in series, the total capacity after the series connection is smaller than that of the capacitor with the smaller capacity. Therefore, if the capacity of the capacitor to be measured exceeds 20 μF, only one capacitor with a capacity of less than 20 μF is used. In series with it, you can measure directly on the digital multimeter. According to the formula of two capacitors in series, it is easy to derive C1 = C2C string / (C2-C string). Using this formula, the capacitance value of the measured capacitor can be calculated. Here is a test example to illustrate the specific method of using this formula. The component under test is an electrolytic capacitor with a nominal capacity of 220 μF, and is set to C1. Select an electrolytic capacitor with a nominal value of 10μF as C2, use a digital multimeter 20μF capacitor to measure the actual value of this capacitor as 9.5μF, and connect the two capacitors in series to measure the C string as 9.09μF. Substituting C2 = 9.5 μF and C string = 9.09 μF into the formula, thenC1 = C2C string / (C2-C string) = 9.5 9.09 / (9.5-9.09) ≈211 (μF)Figure11. Digital MultimeterNote: No matter how much the capacity of C2 is selected, a capacitor with a larger capacity must be selected under the premise of less than 20μF, and C2 in the formula should be substituted into the actual measured value instead of the nominal value, which can reduce errors. The two capacitors are connected in series and measured with a digital multimeter. Due to the capacitance error and measurement error of the capacitor itself, as long as the actual measured value is close to the calculated value, the capacitor C1 to be measured is considered good. capacity. In theory, this method can measure the capacitance of any capacity, but if the capacity of the capacitor under test is too large, the error will increase. The error is proportional to the size of the capacitor to be measured.Do you want to know about other tools to test capacitors? You can Three Measuring Tools to Test Capacitors. VI How to Test Aluminum Electrolytic Capacitors6.1 Appearance Physical Inspection(1) First check whether the capacitor under test has a formal "Product Specification", which includes the product name, specifications, installation dimensions, process requirements, technical parameters, and supplier name, address and contact information to ensure this. Batch products are provided by regular manufacturers. The logo on the capacitor should include the trademark, working voltage, standard capacitance, polarity, and operating temperature range. (2) Refer to the process parameters in the “Product Specification” and observe whether the appearance, color, and material of the capacitor are consistent with the process indicators marked on it. (3) Use a vernier caliper to confirm the installation size of the capacitor to ensure that the diameter, height, and diameter and spacing of the lead-out terminal are within the tolerance of the product process, and the external dimensions must meet the company's selection requirements. (4) Check the appearance of the capacitor to ensure its appearance is neat, without obvious deformation, breakage, cracks, spots, dirt, rust, etc., and its marking is clear, firm, correct and complete. (5) Check the lead-out terminals to ensure that their terminals are straight, free from oxidation, rust, and have no effect on their conductive properties and that the lead-out terminals are free of distortion, deformation, and mechanical damage that affects insertion and removal. (6) Check that the production date marked on the electrolytic capacitor should not exceed six months, and make a record.Figure12. Aluminum Electrolytic Capacitor6.2 Capacity and Loss Test(1) Use the electric bridge to test whether the actual capacity is consistent with the nominal capacity (the electrolytic capacitor generally has an error range of ± 20%). The loss tangent value tanθ (that is, the D value) is in compliance with the standard. (2) How to use the Zen tech bridge tester: After connecting the power supply correctly, press the "POWER" key to turn on the tester's working voltage; press the "LCR" key to select the test type (L: Inductance, C: Capacitance, R: resistance). (3) Press the "UP" and "DOWN" keys to select the test range (μF, nF, pF) and press the "FREQ" key to select the test frequency (100HZ,(120HZ, 1KHZ) can choose the required test frequency according to the technical parameters provided by the manufacturer, the test in this article selects "100HZ". (4) Press "SERIES" (parallel) and "PARALLEL" (parallel) to select the connection mode for the test, small capacitance (less than 10μF)To use parallel mode, use large mode (10μF and above) in series mode. (5) After the setting is completed, connect the bridge test ports ("LOW" and "HIGH") to the two ends of the capacitor, and use the label paper to record the capacity value and loss value on the display respectively. And attach the label paper to the corresponding capacitor for subsequent analysis. 6.3 Ripple Voltage Test(1) Connect the circuit as shown below, and connect the capacitor to be tested to the adjustable DC power supply (note that the positive and negative poles are not connected reversely). Connect the positive electrode of the oscilloscope probe with a non-inductive capacitor (1μF 1200V.DC) in series to the positive electrode of the capacitor to be tested.Figure13. Circuit of Ripple Voltage Test (2) For the setting of the oscilloscope, it must be set to the DC test position first, and the fine adjustment knob of the oscilloscope voltage must be locked. (3) During the test, the DC voltage should be slowly increased to the rated voltage with a voltage regulator, and the changes displayed by the oscilloscope should be closely monitored. The correct range should be selected to ensure that the voltage can be accurately read from the oscilloscope waveform. (4) Take the ripple waveform with the camera, and record the range and division of the oscilloscope with label paper (that is, calculate the ripple voltage and paste it on the corresponding capacitor for subsequent analysis and comparison. (5) After the recording is completed, disconnect the DC power supply, discharge the capacitor under test and the non-inductive capacitor with the bulb load, and then remove the capacitor under test from the test bench. 6.4 Leakage Current Test6.4.1 Indirect Measurement Method OneConnect as shown below. Connect a 1K resistor in series with the capacitor under test and connect it to a DC adjustable power supply. Use an oscilloscope probe to connect to both ends of the resistor. Indirectly calculate the leakage current of the capacitor to be measured by sampling the voltage signal across the resistor. Operating essentials and precautions: After the circuit is connected, adjust the DC adjustable power supply to the rated voltage of the capacitor. After the circuit is equilibrated for two minutes, read the voltage value across the resistor. When reading the oscilloscope, the voltage trimming knob should be locked. Record the maximum value of the voltage waveform as the voltage value and divide it by the resistance value to obtain the value of the leakage current. The current is too large and the resistor is burned out. After the test, the capacitor should be discharged and then removed to avoid accidents.Figure14. Circuit 6.4.2 Indirect Measurement Method TwoConnect the wiring as shown in the figure, and add an air switch in series between the capacitor and the DC power supply. First close S1 and S2 respectively, and adjust the voltage regulator to the rated voltage to charge the capacitor for two minutes.Figure15. Circuit After that, both S1 and S2 are disconnected. At this time, the adjustable power supply is at the rated value. Do not move. Add a milliamp meter between S1 and S2, as shown in the figure below: S1 and S2 are both closed, and the leakage current can be directly read through the milliamp meter after one minute of stabilization.Figure16. Circuit 6.4.3 PrecautionsRemember not to connect the milliamp meter to the line directly when the capacitor is not charged, because the initial charging current is large, the milliamp meter will be burned out by accident. In the disassembly process, first discharge the capacitor with the bulb load. When discharging, remove the milliamp meter first, and ensure that the discharge current does not pass the test resistor to prevent damage to the test resistor and the millimeter meter.6.4.4 Leakage Current at 1.2UnAdjust the DC voltage to 1.2 times the rated voltage of the electrolytic capacitor, measure its leakage current again and compare different samples. 6.5 Explosion Test6.5.1 DC TestApply reverse DC voltage to the capacitor under test, slowly adjust the adjustable DC voltage, and observe the current closely with a clamp meter.The DC power setting is generally not more than 30V. The current value is set according to the size of the capacitor as follows:When the capacitor diameter is 6mm ≤ 22.4mm, the current cannot exceed 1A; when the capacitor diameter is> 22.4mm, the current cannot exceed 10A. 6.5.2 Observe The Surface Temperature of The CapacitorDuring the experiment, use a thermometer to closely observe the surface temperature of the capacitor (the sensing contact of the thermometer can be wrapped around the capacitor with tape). Note that the initial current is very small and almost zero. When the temperature of the capacitor rises (about 35-40 ° C) The current is significantly increased. At this time, close observation should be made. When the current reaches or approaches 10A, the voltage should be lowered to ensure that the current is controlled within 10A. 6.5.3 Capacitor Safety ValveWithin 30 minutes after the start of the test, the capacitor safety valve should be opened. If the capacitor fuse is open, the power should be cut off immediately (the electrolytic capacitor of 350V 6800F will automatically open under the following conditions, the current is about 8A, the surface temperature is about 45-60 ° C.), If the current is close to 10A and the fuse is still 30 minutes later, If it is not turned on, this function is missing.Figure17. DC Digital Voltmeter6.6 Temperature TestThe capacity of a capacitor will change due to different ambient temperatures. In general, the capacity will increase as the temperature rises. The temperature test is to test the change of capacitance after equilibration under the set temperature. 6.6.1 High-Temperature Test(1) Connect two small wires to the lead-out terminal of the capacitor to be tested respectively, and test the capacity of the two lead terminals at normal temperature, and label them for record.(2) Put the capacitor into the high and low temperature alternating humidity and heat test box, and leave the leads outside the test box to test the capacitance.(3) Turn on the test box switch button, click "Temperature Setting" on the screen, set the temperature to 100 ° C, and click "Run" to start the test box.(4) Test the capacity again about 2 hours after the temperature reaches 100 ° C, and calculate the percentage change in capacity (the initial measurement of the difference). 6.6.2 Low-Temperature Test(1) Put the capacitor to be tested into the test box (be careful not to use capacitors that have been tested at high temperatures, except for special needs).(2) Turn on the test box switch button, click "temperature setting" on the screen, set the temperature to -25 ° C, and click "run".(3) Test the capacity again about 2 hours after the temperature reaches -25 ° C, and calculate the percentage change in capacity (the initial measurement of the difference). 6.6.3 PrecautionsThe test should pay close attention to whether there is any obvious change in the capacitor. If serious conditions such as cracking of the capacitor surface and opening of the safety valve occur, the test box should be stopped immediately. During the test, the operating procedures of the test box should be strictly followed, and the door of the test box should not be opened at will. At the end of the high temperature test, the capacitor can only be taken out after the temperature inside the test box has dropped to prevent accidents such as burns.Figure18. CapacitorsVII Considerations for Capacitor Testing(1) When measuring with a multimeter, select the gear according to the rated voltage of the capacitor. For example, the capacitor voltage commonly used in electronic equipment is low, only a few volts to dozens of volts. If the multimeter RX10k is used for measurement, the battery voltage in the meter is 12 ～ 22.5V, which is likely to cause capacitor breakdown. Therefore, the RXlk file should be used. measuring.(2) For the capacitor just removed from the line, be sure to discharge the capacitor before measurement to prevent the residual charge in the capacitor from being discharged to the meter and damage the meter.(3) For capacitors with high working voltage and large capacity, the capacitors should be sufficiently discharged, and the operator should have protective measures to prevent electric shocks during discharge. VIII One Question Related to Testing Capacitor8.1 QuestionWhat should we do when checking a capacitor with an ohm meter？8.2 AnswerTo remove the capacitor from the circuit. It's usually easy to remove a start or run capacitor – you simply unhook it from its harness and disconnect the wires. However, be careful to avoid touching the capacitor terminals. If the capacitor isn't dead, it might have a full charge, and if so, you could get a serious shock.  Ⅸ Frequently Asked Questions about How to Test a Capacitor1. How do you check if a capacitor is bad with a multimeter?Use the multimeter and read the voltage on the capacitor leads. The voltage should read near 9 volts. The voltage will discharge rapidly to 0V because the capacitor is discharging through the multimeter. If the capacitor will not retain that voltage, it is defective and should be replaced. 2. How do you test a capacitor at home?Set your voltmeter to read DC voltage (if it's capable of reading both AC and DC). Connect the voltmeter leads to the capacitor. Connect the positive(red) lead to the positive (longer) terminal and the negative (black) lead to the negative (shorter) terminal. Note the initial voltage reading. 3. How to test capacitor using multimeter? 4. Can you test capacitor on board?You just cannot test a bad capacitor inside or outside a circuit board by measuring its capacitance value with a capacitor meter or a multimeter. ... When the capacitor is outside the board, sometimes a bad capacitor may give you a proper capacitance value on the multimeter or capacitor meter. 5. What is the best capacitor tester?Best Capacitance Meter Review:Signstek MESR-100 V2 Auto Ranging in Circuit ESR LCR Meter CapacitorELIKE Digital Capacitor Tester 0.1pF to 20mFHoneytek A6013l Capacitor TesterMESR-100 circuit tester, KKMOON mesr-100 capacitor testerMultimeter Digital Capacitance Meter Capacitor Tester 0.1Pf to 2000uFExcelvan M6013 Digital Auto Ranging Capacitance Meter Capacitor TesterDigital Capacitance Meter Professional Capacitor 0.1Pf – 20000Uf 6. How do you test a capacitor with a cheap multimeter? 7. How many ohms should a capacitor have?1,000 ohmsSet it to its highest ohm (Ω) setting, at least 1 kΩ (1,000 ohms). At this setting, the meter generates a small current when you connect the meter leads to the capacitor terminals. 8. What is the capacitor symbol on a multimeter?Most digital multimeters use a symbol similar to –|(– to signify capacitance. Move the dial to that symbol. If several symbols share that spot on the dial, you may need to press a button to cycle between them until the capacitance symbol appears on the screen. 9. What if a capacitor reads high?It is reading as if there is a short circuit across it. If we read a very high resistance across the capacitor (several MΩ), this is a sign that the capacitor likely is defective as well. It is reading as if there is an open circuit across the capacitor. ... But not 0Ω or several MΩ. 10. What is the first step in testing a capacitor?The first and most simple is to inspect the capacitor. If it appears “blotted” or swelled, it is a safe bet that it is bad. It is good practice to go ahead and perform the following test even though it is swelled. Make a sketch of the wires connected to the capacitor and note the colors or numbers that identify them.

I IntroductionThe light sensor is developed based on the photoelectric effect principle of semiconductors. It can be used to detect the intensity of ambient light, and it can also be used to detect the difference in light between different colored surfaces. Users can make projects that interact with light with it, such as smart dimming lights, a laser communication system or something more awesome.Light Sensor Using Arduino and LDR | Arduino Light SensorCatalogI IntroductionII Definition  2.1 What is a Sensor?  2.2 Definition of the Light SensorIII Spectrum and Photometric Physical Quantity  3.1 Spectrum  3.2 Photometric Physical Quantities  3.3 MID Display's Perception of Backlight Brightness Under Different IlluminationIV How the Light Sensor WorksV Types and Characteristics of Light Sensors  5.1 Photodiode Type  5.2 Photoresistor TypeVI Applications of Light Sensors  6.1 Types of Light Sensors in Application  6.2 Typical Applications  6.3 Practical Application CasesVII The Circuit Diagram of a Light Sensor  7.1 Model Introduction  7.2 Appearance and Size  7.3 Application  7.4 Functional Framework Diagram  7.5 Application CircuitVIII Programming Guide  8.1 mBlock Programming  8.2 Arduino Programming  8.3 SchematicIX A Related Question about Light Sensor  9.1 Question  9.2 AnswerⅩ FAQII Definition2.1 What is a Sensor?In a broad sense, a sensor is a sensor that converts a measurement into a signal that can be perceived or quantified. In a narrow sense, a device that senses the measurement and converts it into an output signal of the same or another nature according to a certain law. The sensor is generally composed of a sensor element, a conversion element, a measurement circuit, and an auxiliary power source. The sensor element and the conversion element may be combined into one, and some sensors do not require an auxiliary power source.2.2 Definition of the Light SensorThe light sensor usually refers to a device that can sensitively sense the light energy of ultraviolet light to infrared light and convert the light energy into an electrical signal. The light sensor is a kind of sensing device, which is mainly composed of light-sensitive elements. It is mainly divided into four categories: ambient light sensor, infrared light sensor, sunlight sensor, and ultraviolet light sensor. It is mainly used in the field of changing body electronics applications and intelligent lighting systems. Modern electrical measurement technology is becoming more and more mature. Due to its advantages such as high accuracy and easy microcomputer connection for automatic real-time processing, it has been widely used in the measurement of electrical and non-electrical quantities.  However, the electrical measurement method is susceptible to interference. In the AC measurement, the frequency response is not wide enough and there are certain requirements on the withstand voltage and insulation. Today, the rapid development of laser technology has been able to solve the above problems.Figure1. Light SensorIII Spectrum and Photometric Physical Quantity3.1 SpectrumThe spectrum is a pattern in which monochromatic light, which is dispersed by the dispersive system (such as a prism and a grating), is sequentially arranged according to the size of the wavelength (or frequency). The largest part of the visible spectrum is the visible part of the electromagnetic spectrum of the human eye. Electromagnetic radiation in this wavelength range is called visible light. The spectrum does not include all the colors that the human brain can distinguish, such as brown and pink.Figure2. Spectrum3.2 Photometric Physical Quantities3.2.1 Light Intensity(I/Intensity)(1) Definition: the intensity of light emitted by a monochromatic light source (frequency 540 × 1012 Hz, wavelength 555nm) in a unit solid angle in a given direction (radiation intensity in this direction is 1/683 watts per spherical degree) .(2) Unit: cd (Candela)(3) Luminous intensity of common light sources:●  Sun, 2.8E27 cd●  Highlight flashlight, 10000 cd●  5mm super bright LED, 15 cd 3.2.2 Luminous Flux(F/Flux)(1) Definition: The energy emitted by a point light source or a non-point light source in a unit time. Among them, the visual person (radiation flux that humans can feel) is called luminous flux.(2) Unit: Lm (lumens)(3) Efficiency of common light sources (lumens / watt, Lm / W)● Incandescent, 15● White LED, 20● fluorescent lamp, 50● The sun, 94● Sodium lamp, 120 3.2.3 E/Illuminance(1) Definition: Luminous flux irradiated onto a unit area.(2) Unit: Lx / Lux (1), 1 (Lx) = 1 Lm / m2.(3) Common Illumination (Lx):● Direct sunlight (noon), 110,000● Overcast day, 1000● Inside the mall, 500● Cloudy room with window, 100● Under normal room lighting, 100● Full moon, 0.2 3.2.4 L / Luminance(1) Definition: The intensity of light emitted by the unit light source area in the normal direction and within the unit solid angle.(2) Unit: nt (nits), 1 (nt) = 1 cd / m2.(3) Brightness of common luminous body (nt):● Solar surface, 2,000,000,000● Incandescent filament, 10,000,000● White paper under the sun, 30,000● Brightness that human eyes can get used to, 3,000● The human eye can better distinguish the brightness of the color, 1● No moon night sky, 0.00013.3 MID Display's Perception of Backlight Brightness Under Different IlluminationFigure3. Ambient Illumination-LUXIV How the Light Sensor WorksThe light sensor actually works according to the principle of the photoelectric effect. The so-called photoelectric effect refers to the phenomenon that certain special substances can convert light energy into electrical energy after absorbing light. The photoelectric effect can be divided into two types: an external photoelectric effect and an internal photoelectric effect. The external photoelectric effect refers to the fact that under light irradiation, electrons can be emitted from the inside of the material to generate electricity. The photocell and photomultiplier are originals based on the external photoelectric effect.  Correspondingly, the internal photoelectric effect occurs inside the substance. When light is irradiated onto the substance, the resistivity inside the substance is changed, thereby generating electromotive force. Photoelectric elements such as photoresistors and photovoltaic cells are made based on the internal photoelectric effect. Take the light sensor on the mobile phone as an example:The light sensor in a mobile phone should actually be an ambient light sensor, which is mainly composed of two parts, a light projector, and a light receiver. The white dot next to the front camera acts as a lens that focuses the light in the environment and transmits it to the receiver via the projector. According to the photoelectric effect, the light receiver can convert various light signals into corresponding electrical signals, and then further process them into various switching and control actions to realize the sensitivity adjustment of the mobile phone. An infrared cut-off film is often attached to the chip of the ambient light sensor to eliminate the interference of infrared light so that our electronic devices such as mobile phones and laptops can accurately detect the visible light intensity in the environment. When the display consumes too much power, the light sensor can also automatically reduce the screen brightness to extend the operating time of the battery. Figure4. Light Sensor in PhoneV Types and Characteristics of Light Sensors5.1 Photodiode TypePhotodiodes and semiconductor diodes are similar in structure, and their die is a PN junction with photosensitive characteristics, which has unidirectional conductivity, so a reverse voltage needs to be added when working. When there is no light, there is a small saturation reverse leakage current, that is, a dark current, at which time the photodiode is turned off. When exposed to light, the saturation reverse leakage current greatly increases, forming a photocurrent, which changes with the intensity of the incident light. When light irradiates the PN junction, an electron-hole pair can be generated in the PN junction, which increases the density of minority carriers. These carriers drift under the reverse voltage, causing the reverse current to increase. So you can use the light intensity to change the current in the circuit. It is turned off when there is no light and turned on when there is light. Features:(1) High sensitivity can reduce the influence of stray light(2) Photodiode (photodiode) is a photoelectric conversion device, which can convert the received light into a current change(3) The working mode of the photodiode (photodiode) is to increase the reverse voltage or not increase the voltage. When a reverse bias is applied to it, the reverse current in the tube will change with the intensity of the light. The greater the light intensity, the greater the reverse current.Figure5. Photodiode5.2 Photoresistor Type(1) PrincipleIt works based on the semiconductor photoelectric effect. The photoresistor is non-polar and is purely a resistive element. It can be applied with DC voltage or AC voltage.(2) Working characteristics of the photoresistor: When the light is on, the resistance is small; when the light is off, the resistance is large. The stronger the light, the smaller the resistance; when the light stops, the resistance returns to its original value.(3) Spectral range: from ultraviolet to infrared.(4) Features:● The internal photoelectric effect has nothing to do with the electrode (only related to the photodiode), that is, a DC power supply can be used.● Sensitivity is related to the semiconductor material and the wavelength of the incident light● Epoxy resin package, high reliability, small size, high sensitivity, fast response speed, and good spectral characteristics.Figure6. PhotoresistorVI Applications of Light Sensors6.1 Types of Light Sensors in Application(1) Ambient light sensorThe ambient light sensor can sense the surrounding light conditions and tell the processing chip to automatically adjust the backlight brightness of the display to reduce the power consumption of the product. On the other hand, the ambient light sensor helps the display provide a soft picture. When the ambient brightness is high, the LCD monitor using the ambient light sensor will automatically adjust to high brightness. When the external environment is dark, the display will be adjusted to low brightness to achieve automatic brightness adjustment. (2) Infrared light sensorThe infrared light sensor uses a charged thermopile and a scandium bromide iodide (KRS-5) window to sense wavelengths from 580 to 40,000 nm. The sensor can be used to measure a range of phenomena, including infrared radiation from the palm of your hand. (3) Sunlight sensorSolar sensor. It can recognize horizontal and vertical 360 degrees. The location of the sun, identification, cloudy, cloudy, semi-cloudy, sunny and evening during the day. Tracking bearing identification. Identification circuit processing and server drive. A digital chip is used to complete the processing of the above information. It can serve a variety of ordinary motors, stepper motors. The power consumption of the whole machine is 3mA, and the chip working voltage is 5V.  International advanced solar tracking equipment uses computer data theory, which requires data and settings for the latitude and longitude of the earth. The circuit principle and equipment technology are complicated. Intelligent sun tracker uses recognition theory technology, simple circuit and few components, no theory of latitude, longitude and data information. There is no need to consider the route that the sun runs through the year. From which direction the sun rises and from which direction it falls, it can accurately identify the position where the sun rises and falls. If he is placed on a walking car or boat, the tracker can face the sun no matter where he goes. (4) UV light sensorThe UV light sensor uses a filter to measure the UV light band (315nm-400nm). Remove the filter, the sensor can sense visible light at the same time. The sensor includes a UV filter, a sight, and a sensor handle. Figure7. Types of Light Sensors6.2 Typical ApplicationsBacklight adjustment: TV, computer monitor, LCD backlight, mobile phone, digital camera, MP4, PDA, GPS;Energy-saving control: outdoor advertising machines, induction lighting appliances, toys; instruments and meters: instruments and industrial controls for measuring light intensity;Environmentally friendly replacement: Replace traditional photoresistors, photodiodes, phototransistors6.3 Practical Application Cases6.3.1 Changing Body Electronics Applications(1) Ambient light detectionIn body electronics applications, ambient light sensors are used to adjust the backlight intensity of the dashboard, as well as the LCD backlight intensity in navigation systems (GPS), temperature control, and DVD screens. This is especially important for displays like BMW's iDrive and Prius' Multi-Info. For example, when daylight becomes dim and dark, the dashboard backlight will be adjusted to varying degrees to achieve the best visibility and reduce the glare that may be caused to the driver. Using these sensors eliminates the problem of turning on the headlights during the day, and the display automatically adjusts brightness. The key function of the ambient light sensor is to use the sensitivity visible wavelength of 380nm ~ 780nm to replicate the sensitivity of the human eye. (2) Tunnel detectionTunnel detection requires the input of two sensors. The first sensor has a wider field of view "looking up" and a relatively long average moving period, which prevents the lights from turning on and off. The second sensor has a narrower field of view "looking forward" and a relatively short average moving time. This allows the tunnel sensor to respond quickly to sudden changes in daylight, turn on the car's headlights, and adjust the display's backlight brightness when entering the tunnel. Forward-facing sensors eliminate the need to turn lights on and off when entering under a bridge or a tree covering the sun. In these cases, the sensor will still "see" the light ahead. When entering the tunnel, the signal from the tunnel sensor will drop, while the signal from the wide-field sensor will remain high; the headlights of the car will be turned on. When exiting the tunnel, the signal from the tunnel sensor will increase and the signal from the wide field of view sensor will decrease; the headlights of the vehicle will be turned off. With different average moving periods, the controller makes a clear distinction. 6.3.2 Intelligent Lighting SystemTo improve the comfort of the working environment, the lighting control system adopts a light sensor to automatically control the lighting equipment according to the illuminance of the current environment, so that the illuminance is controlled within a comfortable range. In traditional lighting control systems, ordinary light sensors are often combined with A / D converters (ADCs). Because the light signal detected by the light sensor contains both visible light components and infrared light components, the infrared light is filtered to detect the light sensor detection results.VII The Circuit Diagram of a Light Sensor7.1 Model IntroductionThe light sensor shown below is a low-cost I2C digital light sensor (ALS), which can convert light intensity into a digital output signal that can directly interface with I2C, providing a wide dynamic range from 0.01lux to 64K lux The linear response is very suitable for applications under high ambient brightness.Figure8. Model7.2 Appearance and SizeFigure9. Appearance and Size of the model7.3 Application(1) Back-lighting Control in mobile / portable devices(2) Touch Panel Control in mobile / portable devices7.4 Functional Framework DiagramFigure10. Functional Framework Diagram7.5 Application CircuitFigure11. Application CircuitVIII Programming GuideThe programming described below is based on the Me light sensor developed based on the photoelectric effect principle in semiconductors.8.1 mBlock ProgrammingThe light sensor module supports the mBlock programming environment. The following is a brief description of the module instructions:Figure12. Programming GuideHere is an example of how to use mBlock to control a light sensor moduleWhen the LED receives the light, M-Panda will move left and right and say I love sunshine; Cover the LED light, M-Panda will stop moving and say I love night. The results are as follows:Figure13. Result8.2 Arduino ProgrammingIf you write a program using Arduino, you should call the library Makeblock-Library-master to control the Me Light Sensor. This program instructs Me Light Sensor to read the current light intensity through Arduino programming.Figure14. Arduino ProgrammingFunction list of light sensor:Figure15. Function List of Me Light Sensor8.3 SchematicFigure16. SchematicIX A Related Question about Light Sensor9.1 QuestionHow to combine these 2 circuits together so that during complete darkness on the LDR, the LED would turn on instantly and when light falls on the LDR there would be around a 1 or 2-second delay before completely shutting off?The circuit would be running on a 5V DC power supply and powering an LED array.How to combine them together? Figure17.Circuit1Figure18. Circuit29.2 AnswerIn the 555 circuit the capacitor controls the wait time, if the capacitor is short-circuited the circuit will wat forever.In the LDR circuit the transistor acts like a switch but unfortunately it's switching to ground but the capacitor in the 555 circuit is connected to +9VTo resolve this I swapped the parts in the 555 circuit upside down to have the capacitor to ground. Then it was simple to I merge the two circuits.Figure19. AnswerIn the dark R1 turns Q1 on the keesp C1 duscharged so 555 output will be high.when there is light the LDR turns Q1 off and C1 charges , once it gets enough charge the 555 output goes low.We could have instead built the upside-down version of the LDR circuit using a BC557 transitor (or other similar PNP type) instead of the BC547 NPN transistor and merged that with the original 555 circuit.Ⅹ FAQ1. How is a relay added to a light sensor circuit?Presumably, your light sensor will be generating a variable voltage signal in response to how much light is hitting it, and you want to trip a relay when this light is above (or possibly below) a threshold. One way to do this is with a comparator circuit, which will compare two voltages and output a high or low depending on which one is higher. You then compare the signal from the light sensor to a reference voltage that you can set with a potentiometer and generate a high or low output signal from that. You can also use a microcontroller and read the signal from the light sensor with an analog input pin. This is more complex but useful if you want to implement features like hysteresis in the comparison. Now, the logic level signal can’t drive a relay coil directly, so you will need to use a transistor to switch the relay coil current. Which transistor to use will depend on the voltages involved and the amount of current you need to switch, but it’ll be a small signal transistor of some kind. You also need a current limiting resistor on the gate, possibly a pull-down on the gate as well, and a flyback diode across the relay coil. 2. What is a light sensor?Light sensors respond to changes in infrared light to detect motion or proximity to another object. Proximity sensors help robotic machines navigate obstacles and avoid bumping into objects. They are also used for devices in vehicles that sound an alarm when the vehicle is close to bumping into an object. 3. What are the disadvantages of a light sensor?Following are the disadvantages of Light sensor :• LDRs are highly inaccurate with high response time (about 10s or 100s of milliseconds).• Resistance varies continuously (analog) in photoresistors and is rugged in nature.• Photodiodes are temperature sensitive and are uni-directional, unlike photoresistors. 4. What does a light sensor do?Light sensors are electronic devices that indicate the intensity of daylight or artificial light. They convert light energy to electrical signal output. Light sensors have several uses in industrial and everyday consumer applications. 5. Where are light sensors used?Light sensors have a lot of uses. The most common use in our daily lives is in cell phones and tablets. Most portable personal electronics now have ambient light sensors used to adjust brightness. 6. How many types of light sensors are there?By using LDR as a circuit, we can calibrate the changes in its resistance to measure the intensity of Light. There are two other Light Sensors (or Photo Sensors) that are often used in complex electronic system design. They are Photo Diode and Photo Transistor. All these are Analog Sensors. 7. How long does a light sensor last?Long Duration Settings – In most cases, your motion detector light should only stay on for 20 to 30 seconds after it's triggered. However, you can manipulate the settings so it will stay on longer. For example, many lights come with settings ranging from a few seconds to an hour or more. 8. Is a light sensor analog or digital?Analog sensors that are used for detecting the amount of light striking the sensors are called light sensors. These analog light sensors are again classified into various types such as photo-resistor, Cadmium Sulfide (CdS), and, photocell. 9. What is a light sensor in a phone?Ambient-light sensors (ALS) are widely used in smartphones to provide information about ambient-light levels, in support of the backlight LED power circuit. 10. How do you wire a light sensor to an outside light?Connect one black wire on the photocell to the black wire that comes from the building. Be sure to twist the exposed copper wire so that it forms a tight connection. Connect the second black wire on the photocell to the black wire on your light fixture, making sure that the copper wire is twisted together completely.  

CategoryⅠ IntroductionⅡ Development Background  2.1 Limitations of Microwave Oscillators  2.2 Origin of OEOⅢ Working Principle of OEO  3.1 The basic structure of OEO  3.2 Principle-based improvement directionⅣ Operating Characteristics of OEO  4.1 Advantage Performance  4.2 Disadvantage PerformanceⅤ Application of Optoelectronic Oscillator  5.1 Light Pulse Output  5.2 Clock ExtractionⅥ SummaryⅦ FAQ Ⅰ IntroductionThe optoelectronic oscillator (OEO) represents the first practical microwave oscillator that uses optical energy storage elements to generate signals with high spectral purity in the frequency range of several hundred MHz to more than 100 GHz. Many light wave energy storage components, such as fiber Fabry-Perot resonators, fiber ring resonators, optical micro disc resonators, etc. can be used to form OEO. It is a long fiber loop. The use of optical resonators can greatly reduce the size of OEO. Especially the optical microdisk resonator, which is a key component of integrating OEO in a single chip.  Figure1. Opto-Isolator OscillatorⅡ Development Background2.1 Limitations of Microwave OscillatorsGenerally speaking, the quality of the microwave signal generated by the microwave oscillator depends on the energy storage performance of the oscillation cavity. To produce high-quality microwave signals, a high-Q and low-loss energy storage unit is required. Current microwave oscillators are mostly based on electronics (such as dielectric oscillators) and acoustic (such as crystal oscillators) energy storage elements. When these components operate at frequencies above GHz, the energy storage characteristics will drop sharply, and the phase noise and spectral purity of the high-frequency microwaves produced will be poor. 2.2 Origin of OEOIn 1996, XSYao and L. Maleki of the California Institute of Technology Jet Power Laboratory developed a microwave oscillator based on a photonic energy storage unit during the use of photonics technology to improve the performance of a microwave system. This oscillator was named optoelectronic oscillator (OEO). Compared with microwave oscillators based on electronics and acoustic energy storage units, optoelectronic oscillators can generate high-purity microwave or millimeter-wave signals from several MHz to hundreds of GHz, and the Q value of their energy storage elements is as high as 1010, which generates high-frequency signals. The phase noise is as low as -163dBc / Hz at a frequency offset of 10kHz, and has both optical and electrical outputs. It is a very ideal high-performance microwave oscillator and is expected to be widely used in the future. Ⅲ Working Principle of OEO3.1 The basic structure of OEOThe basic structure of the optoelectronic oscillator is shown in Figure 2. It is a positive feedback loop composed of laser, electro-optic modulator, high Q optical energy storage unit (such as a certain length of optical fiber), photodetector, bandpass filter, microwave amplifier, phase shifter and microwave coupler. The energy of the oscillation comes from the injected light in front of the electro-optic modulator. After the injected light is modulated by the electro-optic modulator, it becomes an optical signal carrying a specific frequency. This optical signal is converted into an electrical signal by a photodetector, amplified, and then band-pass filtered. The filter filters out a specific frequency, part of which is used for output, and part of which is fed back into the microwave input port of electro-optic modulation to complete a cycle. After continuous cycling, a stable oscillation is finally formed. Since the optical oscillator uses a high-Q optical energy storage unit such as a low-loss long fiber, the output signal has extremely low phase noise. Figure2. Basic Structure of OEO3.2 Principle-based improvement directionIn addition, the loss in the optical energy storage unit such as optical fiber does not change with the change of microwave frequency, so theoretically the performance of the output signal of the optoelectronic oscillator will not deteriorate with increasing frequency. After nearly two decades of continuous exploration, the research on opto-electronic oscillators has made rapid progress. In the United States, opto-electronic oscillators have been successfully applied in cutting-edge technologies such as drones as high-quality local oscillators.  Nevertheless, in order to obtain a wider range of applications, optoelectronic oscillators need to be continuously improved in terms of performance and stability. Current research on optoelectronic oscillators is mainly focused on reducing phase noise, improving side mode suppression ratio, improving frequency stability, expanding output frequency, improving frequency tuning performance, miniaturization and multi-frequency oscillation, etc.Details are as follows:  (1) Phase NoiseThe phase noise of the output signal of the optoelectronic oscillator mainly comes from the thermal noise, scattered noise, and relative intensity noise of active devices such as lasers, photodetectors, and amplifiers. Phase noise can be reduced by optimizing the structure of microwave photonic links and the way the devices work. In experiments by D. Eliyahu and some others that produced extremely low phase noise (-163 dBc / Hz @ 6kHz) signals, a high power Nd: YAG laser with low relative intensity noise and an array amplifier with low phase noise were used. P.S.Devgan et al. Used low-biased Mach-Zehnder modulators and optical amplifiers to achieve an all-optical gain optoelectronic oscillator.  Compared with optoelectronic oscillators using electric amplifiers, the phase noise of this solution has been improved by 10dB. In addition, the use of high-power photodetectors can effectively reduce white noise, while the use of photodetector arrays to receive signals can effectively reduce the effects of flicker noise.Figure3. Phase Noise Modulation(2) Side Mode SuppressionIn order to obtain microwave output with low phase noise, the resonator of the photo-electric oscillator must have a very high Q value (Q = 2πfτ, f is the center frequency, and τ is the energy decay time), that is, a very large energy decay time is required. A larger τ can be obtained by increasing the fiber length, but as the fiber length increases, the longitudinal mode spacing (Δf = 1 / τ) in the cavity of the photo-electric oscillator decreases（As low as several tens of kHz）, in order to effectively suppress the non-oscillation mode and select a single oscillation frequency, a relatively narrow microwave band-pass filter is required. ①Dual-loop optoelectric oscillatorOne way to suppress side modes is to use a dual-loop optoelectronic oscillator. Two optical fiber loops of different lengths are formed in the cavity of the photo-electric oscillator. Only modes that satisfy the conditions for selecting the two loops at the same time can start oscillation. By selecting appropriate loop lengths, single-mode vibration can be achieved. The dual-loop optoelectronic oscillator scheme can be divided into an optical-domain coupled dual-loop structure and an optical-domain coupled dual-loop structure.  This research group proposed a dual-loop optoelectronic oscillator based on polarization modulation and polarization division multiplexing. The polarization beam splitter not only realizes the conversion of polarization modulation to intensity modulation, but also realizes that the incident light wave is divided into two orthogonal polarization states to form a double loop. The side-mode rejection ratio of the 10GHz signal generated by this solution reached 78dB. Compared with the electric-domain coupled dual-loop scheme, the optical-domain coupled scheme requires only one photodetector. Optical domain coupling dual loop schemes can also be implemented using wavelength division multiplexing technology.Figure4. A Dual-loop Optoelectronic Oscillator②Coupled optoelectronic oscillatorAnother method to suppress side modes is to use a coupled optoelectronic oscillator (COEO). The coupled optoelectronic oscillator includes two parts: an actively mode-locked laser loop and an optical feedback loop. The active mode-locked fiber laser loop can effectively increase the Q value of the oscillator. Therefore, a shorter fiber length can be used to obtain low phase noise. This research group used a non-pumped erbium-doped fiber to achieve a 10.7GHz stable coupled photo-electric oscillator with a phase noise below -120dBc / Hz @ 10kHz. (3) Frequency StabilityThe factors that affect the frequency stability of the optoelectronic oscillator are mainly two aspects:  ①The high-Q components in the system (including long optical fibers and narrow-band electrical filters) are susceptible to changes in the environment, and the output frequency is changed to cause the output frequency. Instability, especially the change of equivalent cavity length caused by environmental factors such as temperature. ②Because the filters used in optoelectronic oscillators usually have a relatively large passband range, they are within the gain bandwidth of the loop. There will be many side molds. One of these side modes may obtain sufficient gain during the change of cavity length to replace the original starting frequency, resulting in unstable starting frequency. In addition, the bias point drift problem of common electro-optic modulators will also affect the stability of the output frequency, but isolating the optoelectronic oscillator from the environment or using a temperature control device can reduce the impact of environmental changes on the system. For example, in experiments of XSYao, the optoelectronic oscillator was placed in a foam-filled box to isolate the influence brought by vibration. The active phase-locked loop circuit control is used to lock the oscillation signal of the optoelectronic oscillator to an external reference source, which can also effectively improve the frequency stability of the optoelectronic oscillator.Figure5. Frequency Stability(4) Working FrequencyTheoretically, the optoelectronic oscillator can generate signals from several MHz to hundreds of GHz, and the phase noise has nothing to do with frequency, but the high-frequency millimeter wave optoelectronic oscillator is difficult to realize. This is mainly due to the use of microwave devices such as photoelectric modulators, microwave couplers, microwave phase shifters, microwave amplifiers, and microwave transmission lines in optoelectronic oscillators, whose operating frequency is limited by electronic bottlenecks. Although there have been recent reports of high-frequency microwave or millimeter-wave devices, these devices are generally expensive, consume large power, and have poor performance. In response to the above problems, M. Shin et al. Used the LiNbO3 Mach-Zehnder modulator's half-wave voltage to the proportional relationship between the wavelength to achieve the simultaneous generation of 10GHz fundamental frequency and 20GHz octave signal. (5) TunabilityIn order to generate a broadband adjustable microwave signal, the optoelectronic oscillator needs to use a broadband adjustable high Q filter, which can be a tunable electrical filter, an optical filter, or a microwave photon filter. Limited by the electronic bottleneck, the tuning range of the output signal of the optoelectronic oscillator using a tunable electrical filter is limited. Optoelectronic oscillators based on microwave photonic filters usually have a large tuning range.Figure6. Schematic of The Tunable Opto-electronic Oscillator(6) Miniaturization ResearchOptoelectronic oscillators usually include laser sources, intensity modulators, long fiber delay lines, photodetectors, electrical amplifiers, electrical phase shifters, electrical bandpass filters, and other electrical or optical devices. These discrete electrical and optical components make the optoelectronic oscillator bulky and cause large power losses. By using high-Q optical resonators (such as whispering wall mode resonators) to replace fiber lengths of several kilometers, the size of the energy storage unit of a photo-electric oscillator can be significantly reduced. (7) Multi-frequency OscillationOptoelectronic oscillators usually only produce a pure single frequency signal. In applications such as wideband channelized receivers and multi-band radars, signals of multiple frequencies are required. In 2012, F. Kong et al. Used a birefringence characteristic of a phase-shifted Bragg grating to implement a dual-frequency optoelectronic oscillator. The disadvantage of this solution is that it can only generate signals of two frequencies, and the system is very sensitive to the environment. If a multi-frequency optoelectronic oscillator based on a single-phase modulator and a multi-wavelength light source are used, a single-passband tunable microwave photon filter can be formed on each optical carrier. By increasing the number of optical carriers, it will be easy to obtain more channels of different frequency signal output. Ⅳ Operating Characteristics of OEO4.1 Advantage PerformanceOptoelectronic oscillator is generally a positive feedback loop composed of light source, intensity modulator, filter and photodetector (PD). It takes advantage of the low loss characteristics of modulators and optical fibers to turn continuous light into stable, clean spectrum RF/microwave signals. The continuous light emitted by the laser is transmitted to the photodetector through the optical fiber after passing through the electro-optic modulator. The photodetector converts the light into an electrical signal and enters the frequency selection, amplification, and feedback modulation device.  During this process, the active device will generate noise disturbances of different frequencies. These disturbances are filtered by the filter at the output to the desired frequency and used to feedback and control the electro-optic modulator. The amplifier in the loop provides gain, and after several cycles of the signal, a stable oscillation can be established, and its oscillation frequency is mainly determined by the passband characteristics of the filter. 4.2 Disadvantage PerformanceAlthough the performance of the optoelectronic oscillator is outstanding, its system composition also determines some of its shortcomings. First of all, in order to obtain a high Q signal output, a long fiber is generally used in the cavity. At this time, the length of the cavity also determines the interval between the oscillation modes. The longer the cavity, the smaller the mode interval. In theory, a sufficiently narrow filter can be used to filter out unwanted modes, but it is quite difficult to obtain the device.  Secondly, in terms of the phase noise of the signal, the relative intensity noise of the light source, the photodetector and the electric amplifier will all affects the phase noise of the resulting microwave signal. Excessive bandwidth of filters and amplifiers will also reduce the signal-to-noise ratio in the passband range and affect the quality of the oscillation frequency.  Finally, because the loop is mainly composed of optical fibers, its cavity length is easily affected by environmental conditions and stress. The change causes the change of the fundamental frequency of the oscillation to cause the output frequency to drift or hop. In addition, the long optical fiber occupies a relatively large volume, which causes obstacles to the miniaturization and integration of the entire optoelectronic oscillator system. Solving the above problems is some of the key work for the final practical use of optoelectronic oscillators. Figure7. Cristal Oscillator Ⅴ Application of Optoelectronic OscillatorThe basic function of the optoelectronic oscillator is to generate high-quality optical and electrical microwave signals, but after being updated, it has also derived some new applications. In these applications, the electrical output of the photo-oscillator basically keeps the microwave signal output, but some changes occur in the light output part.5.1 Light Pulse OutputIn 1997 and 2000, X. Steve Yao and others successively analyzed and demonstrated the hybrid structure (COEO) of the optical resonator and optical oscillator loop provided by SOA to generate electric microwave signals and light pulses. This solution is similar to a regenerative mode-locked laser.  The main difference is that the photoelectric loop of COEO needs to be oscillated, and the final output mode is constrained by the selection of the two loops. In 2007, Ertan Salik demonstrated a COEO structure based on erbium-doped fiber amplifier (EDFA) to provide optical path gain, and obtained a 9.4 GHz microwave signal with ultra-low phase noise of -150 dBc / Hz (at a frequency offset of 10 to 100 kHz). Output and light pulse output with only 2 fs jitter. The optical pulse output mechanism of this structure is based on a fiber mode-locked laser. Therefore, in order to obtain high-performance output, there are high requirements on the design of the cavity length stabilization, dispersion control, and polarization maintenance of the optical cavity. Another feasible solution is to generate light pulses by changing the photoelectric modulation characteristics in the optoelectronic oscillator loop. In 2003, Jacob Lasri et al. Used electro-absorption modulator (EAM) to replace Mach-Zehnder intensity modulation (MZM) in the traditional scheme. By controlling the bias of EAM, a narrow modulation transmission window was obtained. Electric microwave signal and light pulse output. If a multi-wavelength light source is used in the light source part, this structure can also conveniently generate multi-wavelength light pulses. The structure of this scheme is relatively simple, but EAM generally has a large insertion loss, and the resulting pulse width is also wide.Figure8. Electro Absorption ModulatorIn addition, using a semiconductor laser operating under gain switching conditions or using a large-signal direct-modulation as the light source of the photo-electric oscillator, it is possible to obtain an electric microwave signal and an optical pulse output without requiring an additional modulator.5.2 Clock ExtractionBecause the structure of the optoelectronic oscillator has the function of frequency selection and amplification feedback, no matter whether the optical or electrical signal is injected into the optoelectronic oscillator, its clock signal (or frequency-divided clock) can be changed as long as it falls within the passband of the filter. The output can be recovered after locking and regeneration.  The maximum recoverable clock frequency is determined by the center frequency of the filter in the loop and the bandwidth of the modulator and the photodetector. X. Steve Yao et al.  Later, Caiyu Loun and others analyzed the extraction scheme of the frequency-divided clock based on the optoelectronic oscillator in 2002. By using the output electrical signal of the optoelectronic oscillator as a trigger signal to observe the injected optical signal on an oscilloscope, the electrical signal at this time can be determined. Whether the output is a divided clock of the injected signal, and experimentally verified the divided clock extraction under the condition of 10 Gb / s injected signal. In 2005, Hidemi Tsuchida and his partners demonstrated a frequency-divided clock extraction experiment with an injected signal rate of 40 Gb / s and 160 Gb / s. Figure9. Clock RecoveryIt should be noted that this method also provides a new idea for clock extraction of non-return-to-zero (NRZ) signals. In theory, there is no obvious clock component for NRZ signals to be extracted, but as long as the frequency selection of the optical oscillator filter is carefully adjusted. The clock signal of the injected NRZ code signal can be found and generated by the window. Li Huo et al. proposed the clock of the injected 10 Gb / s NRZ code signal, and obtained the converted zero (RZ) at the same time in the optical output part of the optoelectronic oscillator. The EAM-based optoelectronic oscillator can also complete the clock recovery of the RZ code signal. In the experiments demonstrated by Jaoob Lasri et al., In order to obtain the optical clock pulse signal at the same time, a DC light with a wavelength different from the wavelength of the injected signal light was added. Since the power change of the injected signal light will form a periodic switching window on the EAM and transfer the clock information to the simultaneously injected DC light, the wavelength of this DC light is selected by the optical filter to complete the Oscillation can generate an electrical clock signal and simultaneously obtain an optical clock pulse at that wavelength. It should be said that in addition to optical and electrical microwave sources, pulse sources and clock extraction systems, there are other applications, such as generating dual-frequency signals, inserting encoders to form multi-function signal generators, and so on. However, various applications are based on the feature that the photo-electric oscillator structure can automatically generate stable low-phase noise microwave signals. Therefore, as long as it focuses on various fields that require high-quality microwave signals, many new applications can be developed. Ⅵ SummaryIt can be seen that as a high-quality optical and electrical microwave signal generator, the optoelectronic oscillator has great advantages and wide application prospects. Various unique application methods also lay the foundation for the multifunctionalization of the optoelectronic oscillator.  However, it is undeniable that the current optoelectronic oscillator is still mainly in the laboratory research stage. There is still a period of time before it can be practically applied in the national economic construction and the development of national defense science and technology. Its main constraints focus on how to make the optoelectronic oscillator system into a compact, integrated, and compact frequency control system. The realization of these requirements depends on the development and manufacturing process of new photonic microwave devices and corresponding active devices.  Although there are no direct targets for optoelectronic oscillators, recent literature reports show some opportunities. For example, utc-pd (uni-traveling -Carrier Photodiode) in optoelectronic detection can receive high optical power and have high power electrical signal output, which can reduce or avoid the use of electric amplifiers in optoelectronic oscillators. The development of integrated semiconductor laser and modulator technology makes it possible to miniaturize the light source and feedback modulation of the photoelectric oscillator. The high Q value photonic filter with semiconductor structure is helpful to realize the system integration and tunability of optoelectronic oscillator. It is believed that with the gradual maturity of these technologies, the optotoelectric oscillator will be applied in practice and play its due contribution.  Ⅶ FAQ1. What do you mean by optoelectronic devices?Optoelectronic devices are electrical-to-optical or optical-to-electrical transducers or instruments that use such devices in their operation. ... Optoelectronics is based on the quantum mechanical effects of light on electronic materials, especially semiconductors, sometimes in the presence of electric fields. 2. What are optoelectronic devices give example?Examples of optoelectronic devices are: laser diodes, superluminescent diodes and light-emitting diodes (LEDs), converting electrical energy to light. photodetectors (e.g. photodiodes and phototransistors), converting optical signals into electrical currents. 3. What is the working principle of optoelectronic devices?Optoelectronic devices are primarily transducers i.e. they can convert one energy form to another. These devices produce light by expending electrical energy. They can also detect light and transform light signals into electrical signals for processing by a computer. 4. What are Optoelectronics used for?Optoelectronic devices refer to components used to detect or emit electromagnetic radiation, typically in the visible and near-infrared (NIR) regions of the electromagnetic spectrum. Each of these functions exploits the photoelectric effect of materials, also known as light-matter interaction. 5. Is LDR an optoelectronic device?There are two types of optoelectronic devices. These are Photoconductive devices and Photovoltaic devices. Photoconductive devices detect variations in light intensity to activate or inhibit electronic circuits. LDR, Photodiodes and Phototransistors fall in this category. 6. What are optoelectronic junction devices?Optoelectronic junction devices are p-n junction devices in which, carriers are generated by photons. Photodiodes, light-emitting diodes (LEDs) and solar cells are examples of optoelectronic devices. A photodiode is a device that is used to detect optical signals. 7. Which substance has optoelectronic property?Unlike the majority of electronic devices, which are silicon-based, optoelectronic devices are predominantly made using III–V semiconductor compounds such as GaAs, InP, GaN, and GaSb, and their alloys due to their direct bandgap. 8. Who discovered optoelectronics?Three Bell Laboratories scientists, William Shockley, John Bardeen, and Walter Brattain, demonstrated the first transistor-based on point-contact germanium (Ge) device. On the other hand, the semiconductor laser was discovered 15 years later in 1962. 9. Is solar cell an optoelectronic device?Solar Cell is another example of an Optoelectronic device based on the p-n junction, and the operating mechanism of a solar cell is essentially the same as that of Photodiode in that, a p-n junction is illuminated by light and the photogenerated carriers are separated by the built-in electric field across the p-n junction. 10. What are optoelectronic devices Name any two optoelectronic devices?Examples of optoelectronic devices include telecommunication laser, blue laser, optical fiber, LED traffic lights, photo diodes and solar cells. The majority of the optoelectronic devices (direct conversion between electrons and photons) are LEDs, laser diodes, photo diodes and solar cells. 

CategoryⅠ IntroductionⅡ Electronic Ballast Circuit Diagram Research Application 2.1 Overview 2.2 Circuit Structure of High-Performance Electronic Ballast       2.2.1 Power Factor Correction Circuit       2.2.2 Inverter Circuit       2.2.3 Lamp Circuit Network       2.2.4 Control Circuit2.3 High-Performance Electronic Ballast Dedicated Integrated Controller of ML4830 Series       2.3.1 Introduction to ML4831/32 Function       2.3.2 The Improvement of the Internal Function of ML48332.4 High-performance Electronic Ballast Built by ML4833Ⅲ FAQ Ⅰ IntroductionIn the 1970s, a worldwide energy crisis emerged. The urgency of energy conservation has led many companies to focus on energy-saving light sources and electronic ballasts for fluorescent lamps. With the rapid development of semiconductor technology, various high-return power switching devices are emerging, which provide conditions for the development of electronic ballasts. In the late 1970s, foreign manufacturers took the lead in launching the first generation of electronic ballasts, which was a major innovation in the history of lighting development. Because it has many advantages such as energy-saving, it has aroused great concern and interest around the world. It is considered to be an ideal product to replace the inductance ballast. Later, some well-known enterprises have invested considerable manpower and material resources to carry out higher-level research and development. Due to the rapid advancement of microelectronics technology, the development of electronic ballasts to high performance and high reliability has been promoted. Many semiconductor companies have introduced a series of products for dedicated power switching devices and control ICs. In 1984, Siemens developed an active power factor correction IC such as the TPA4812 with a power factor of 0.99. Subsequently, some companies have successively launched integrated electronic ballasts. In 1989, Finland's Hell Valley Company successfully launched electronically adjustable ballast monolithic integrated circuit ballasts. Electronic ballasts have been promoted and applied throughout the world, especially in developed countries. Figure 1. BallastChina's research and development of electronic ballasts started late, the technology is not advanced, early understanding of the difficulty and complexity of this product is insufficient, the development of special semiconductor devices has not kept up, the quality of products has not passed, and the market is extremely irregular. A large number of low-priced inferior goods were thrown to the market, causing losses to consumers and seriously damaging the image of electronic ballasts.  In the late 1990s, due to the rapid development and improvement of production levels, from circuit design to electronic components, the products entered a relatively mature stage, and high-quality products entered the construction project. The implementation of China's green lighting project paved the way for the promotion and application of electronic ballasts. Knowledge of Electronic Ballast for Fluorescent Lamps and Germicidal Lamps The electronic ballast is an electronic control device that uses a semiconductor electronic component to convert a direct current or low frequency alternating current voltage into a high frequency alternating current voltage, and drives a light source such as a low pressure gas discharge lamp (sterilization lamp) or a tungsten halogen lamp. The most widely used is the electronic ballast for fluorescent lamps. Due to the adoption of modern soft-switching inverter technology and advanced active power factor correction technology and electronic filtering measures, the electronic ballast has good electromagnetic compatibility and reduces the self-loss of the ballast. Ⅱ Electronic Ballast Circuit Diagram Research Application2.1 OverviewOn October 1, 1997, China's "Green Lighting Project" was officially launched. This is a major decision and measure in the field of lighting technology, which has a huge impact on China's energy, electric light source and lighting technology, and even environmental protection. As an important target of the "green lighting project", China will replace the incandescent lamp with an integrated energy-saving lamp composed of electronic ballasts and compact fluorescent lamps and promote more than 300 million energy-saving lamp, forming the terminal's ability to save 22 billion kWh, which is equivalent to saving about 49-63 billion yuan electricity construction funds. In addition to saving electricity, it can actually reduce social expenditures by 30-40 billion yuan. According to relevant experts from the Ministry of Information Industry, under the same luminous flux conditions, energy-saving lamps can save 80% of energy compared with incandescent lamps, and the cost of purchasing energy-saving lamps can be recovered in the 8-10 months of electricity savings. The use of electronic energy-saving lamps in ordinary households, enterprises and institutions, hotels, restaurants, and commercial systems is more cost-effective than incandescent lamps. However, the old-fashioned inductance ballasts currently working at the industrial frequency generally have the disadvantages of high energy consumption, low efficiency, large volume, and large amount of copper needed. Therefore, the state has set a policy which is to replace traditional inductance ballasts with high frequency electronic ballasts. Currently, some electronic ballasts have appeared on the market, and Table 1 lists the performance comparison of these electronic ballasts. According to the International Electrotechnical Commission standard IEC929 and China's professional standard ZBK74012-90, the electronic ballast should be used in "normal conditions, the lamp should be activated, but it does not cause damage to the lamp performance"; "The shortest time to apply the cathode preheating voltage should not be less than 0.4s" and "the crest factor of the open circuit voltage shall not exceed 1.8; during the minimum warm-up period, no extremely narrow voltage peaks that do not affect the rms value shall be generated", etc.  As listed in table 1, except for high grade electronic ballasts, they are unqualified products. In particular, as early as 1982, the International Electrotechnical Commission (IEC) developed a standard called “interference of household equipment and similar electrical equipment to the power supply system”, namely the IEC555-2 standard. In 1987, Europe also developed a similar EN60555-2 standard. Both standards strictly limit the power factor of the equipment to be close to 1, and it also clearly stated that, all products that do not meet the standards are not allowed to be sold. In view of the great harm caused by the low power factor, it is very important and necessary to impose regulations on the power factor of electronic equipment and products that must be close to 1. Figure 2. Brief Comparison of Low, Medium and High Grade Electronic Ballasts We believe that the high-performance electronic ballast should be a product that has both power factor correction and lamp filament preheating, lighting adjustment and lamp circuit protection, and is fully compliant with IEC555-2 and similar standards. The basic principles of the circuit structure and power factor correction circuit that must be provided for high-performance electronic ballasts are briefly discussed in this article. The integrated controllers for electronic ballasts ML4831, ML4832, ML4833 and high-performance electronic ballast circuits composed of them are highlighted. 2.2 Circuit Structure of High Performance Electronic BallastThe RFI and EMI filters in the figure filter out conducted RF interference and electromagnetic interference from the grid, while obstructing the conducted RF and electromagnetic interference generated by the ballast circuit from entering the grid. The bridge rectifier circuit converts the input AC to DC. The power factor correction circuit acts to improve the input AC current waveform, ensuring that the input current is sinusoidal and in phase with the input voltage, achieving a power factor close to or equal to one.  The inverter circuit completes the conversion of the DC high voltage to the high-frequency AC, and finally transmits the input power to the fluorescent tube through the lamp circuit network. In addition to transmitting electrical power, the lamp network will also perform preheating of the fluorescent filament, sampling and feedback of the lamp operating state signal. The feedback signal of the working state of the lamp is taken from the power factor correction circuit and the dimming signal, and processed by the control circuit to obtain the driving pulse of the switching device in the correct inverter circuit. 2.2.1 Power Factor Correction CircuitThe power factor of the system is defined as PF=γcosφ1 In the formula, γ=I1/IRMS, which is the ratio of the fundamental rms value of the input current to the rms value of the input total current and is also called the distortion factor of the current. φ1 is the phase shift angle of the fundamental current and voltage. If the input voltage of the system has no phase shift (ie, the system is purely resistive) and there is no harmonic component (ie DF=1), the PF of the system must be one. Unfortunately, the input rectification filter units that most of the current devices connect with the power frequency grid are composed of uncontrolled diodes and large-capacity electrolytic capacitors. The instantaneous value of the current on the grid side is quite high (generally about 2 to 3 times that of IRMS), the duration is very short (usually no more than 4ms), and it is severely non-sinusoidal, so the PF of the system is much lower than 1.  The power factor correction is aimed at the drawbacks of the traditional uncontrolled rectifier circuit, and adopts corresponding circuit measures. While increasing the DF value of the system, the phase shift of the input fundamental current and voltage is minimized, and finally the target with the PF value equal to 1 is achieved. As a boost-type active power factor correction circuit commonly used in electronic ballasts, the control circuit uses the input voltage signal as a reference, and the product of the input current and the output voltage signal is used as a modulation source to obtain a sinusoidal pulse width modulation (SPWM) signal to the step-up DC/DC power conversion circuit to adjust the on/off time ratio of the power switch. In the end, a stable DC high voltage is obtained.  The power switching device in the step-up power conversion circuit is driven by the SPWM signal outputted by the control circuit to turn on and off at a high speed, thereby ensuring that the current waveform flowing through the inductor connected in series with the rectifier bridge is a sine wave, and is in phase with the input voltage. Thus, the distortion factors γ=1 and φ1=0 of the system input current are obtained, that is, cosφ1=1, and the system power factor is 1. 2.2.2 Inverter CircuitThe most important function of the inverter circuit is to convert the high-voltage direct current outputted by the power factor correction circuit into a high-frequency alternating current for the fluorescent lamp. The power MOSFET push-pull tubes (V1 and V2) are alternately turned on and off under the driving pulse with a duty cycle of 50%, and is commutated when the current crosses zero in the parallel resonant loop of the power transformer primary inductance and capacitance thus to realize zero voltage switching(ZVS) and perform chopping on high voltage DC. The zero-voltage switching eliminates switching losses associated with output capacitance and parasitic capacitance charging of MOSFET tube, and the gate drive charge is minimal, which helps reduce gate losses.  Since the high frequency AC obtained by the secondary coupling of the power transformer is directly fed to the lamp network, there is no phase shift between the lamp current (ie, secondary current of the power transformer) and the output current of the inverter circuit (ie, primary current of the power transformer). Considering that the total impedance of the lamp network is reduced at high frequencies, and the negative resistance characteristic of the fluorescent lamp itself, it can be found that as the lamp current decreases (corresponding to the weakening of the light intensity of the lamp), the output current of the inverter circuit will increase. 2.2.3 Lamp Circuit NetworkThe lamp circuit network not only needs to deliver the high-frequency AC power to the lamp tube to complete the efficient conversion of electricity and light, but it also needs to implement functions such as filament warm-up, lamp current detection feedback, and auxiliary power supply for the entire electronic ballast system.  The power transformer primary T is connected to the inverter circuit, and the lamp current is directly transmitted to the lamp through the capacitor, and the secondary winding supplies the lamp with filament current for preheating and maintaining the operation. The current transformer TA performs detection and sensing of the lamp current, and sends a signal about the operation of the lamp to the control circuit at any time by the change of the lamp current. The control circuit can judge the light intensity of the lamp according to the magnitude of the lamp current (even including the disconnection and short circuit of the lamp), and then send corresponding control signals to the inverter circuit. 2.2.4 Control CircuitThe control circuit for high-performance electronic ballasts should have a series of functions including power factor correction, lighting adjustment, light-on preheating, lamp disconnection alarm, and lamp restart program control. At present, some integrated circuit controllers for electronic ballasts appearing in the domestic and international device market are mostly based on PFC control, with appropriate addition of lamp control functions, or implementation of lamp control by external circuits. It is worth mentioning that the ML4830/31/32/33 series products can be said to be integrated controllers for high-performance electronic ballasts. 2.3 High-Performance Electronic Ballast Dedicated Integrated Controller of ML4830 SeriesML4830/31/32/33 are integrated circuit controllers developed by American Micro Linear Corporation for high-performance electronic ballasts. The first generation ML4830 has been eliminated; the second generation ML4831 is manufactured by bipolar integrated circuit technology; the third generation ML4832 uses Bicmos process to replace the original bipolar process, the circuit bias current is greatly reduced, and the consumption is greatly reduced. The fourth-generation ML4833 not only adopts the Bicmos process but also has a major improvement in the internal structure, so the function is enhanced and the performance is better. Although these devices can use the functional block diagram of figure 3, the internal structure of ML4831 and ML4832 and the internal structure of ML4833 are respectively shown in figure 4 and figure 5. Figure 3. Functional Block Diagram of ML4831, 32, 33  Figure 4. Internal Block Diagram of ML4831, 32  Figure 5. Internal Structure Block Diagram of ML4833 2.3.1 Introduction to ML4831/32 FunctionThe ML4831/32 is composed of a continuous current type boosting power factor correction stage controlled by an average current. It has a dedicated control circuit for electronic ballasts with various ballast control links. Lamp start-up and restart timing can be achieved by using external circuit components to provide a wide range of control over different types of lamps. The ballast link uses an additional programmable method of frequency modulation and adjustment of the frequency range of the voltage-controlled oscillator to control the lamp power, so it is suitable for various types of output networks. The gain modulator in the ML4831/32 is highly immune to interference caused by switching high-power switching devices. The output of the gain modulator appears as a reference to the current error amplifier at the inverting input of the amplifier. Isine is the current drawn from the AC input; UEA is the output of the error amplifier (pin 1). The output of the gain modulator is limited to 1V. The PWM regulator in the PFC control section compensates for the positive voltage generated by the multiplier output through the negative voltage developed across the pin 4 sense resistor. At the same time, the power MOSFET is protected against high-speed current transients by weekly current limiting. Once the voltage at pin 4 is below 1V, the PWM cycle is terminated immediately. The overvoltage protection (OVP) terminal (pin 18) of the ML4831/32 is used to protect the power circuit from high voltage damage when the lamp is suddenly disconnected. The OVP take-off point can be set by directly tapping the voltage divider resistor to the high-voltage DC bus. As long as the voltage at pin 18 exceeds 2.75V, the power factor correction (PFC) transistor will be turned off and the ballast operation can continue.  The threshold of the OVP should be set to a value that the power device can operate safely, but is not too low to affect the operation of the boost power conversion link. The internal operational transconductance amplifier performs PFC voltage feedback, current sensing and loop amplification. The transconductance amplifier is designed with a low signal forward transconductance so that a large value resistor can be used as a load and a small (<1μF) ceramic capacitor for AC coupling in the compensation network. The compensation network can take the form of figure 6, introducing a zero point and a pole at frequencies fz and fP, respectively: fZ=1/2πR1C1fP=1/2πR1C2 It is noted that the DC-to-ground path and the output of the transconductance amplifier may be out of tune, and the offset error voltage value reflected at the input is determined by uos=iO/gm. Capacitor C1 in figure 6 is used to block DC and minimize the adverse effects of offset. All of the operational transconductance amplifiers in the ML4831/32 incorporate a Slew Rate enhancement to improve recovery under circuit power-up and transient response conditions because the transconductance amplifier changes from a small transconductance state to a large transconductance state. The response to large signals is essentially non-linear. Figure 6. Compensation Network for Transconductance Amplifier The ML4831/32 controls the output power of the lamp by frequency modulation of the non-overlapping conduction of the power switch tube in the inverter part of the ballast circuit. That is to say, during the discharge of oscillation timing capacitor CT, the output of both ballast power tubes is low. The frequency range of the voltage controlled oscillator (VCO) in the device is controlled by the output of the LFB amplifier (pin 6). As the lamp current decreases, the voltage at pin 6 rises, causing the CT charging current to drop, thus causing the oscillation frequency of the oscillator to become lower. Because the ballast output network attenuates high frequencies, the power fed to the lamp increases accordingly. In general, the frequency of the oscillator can be calculated as follows: fosc=1/（tchg＋tdis） Attention: A zero charge current occurs when LFBOUT (pin 6) is high level. Typically, the charge current varies with the two inputs to the oscillator: One is the output of the warm-up timer, and the other is the output of the lamp feedback amplifier (pin 6). During the warm-up phase, the charging current is fixed at a value of Ichg (preheat) = 2.5 / Rset (3). During normal operation, the charging current varies with the voltage of pin 6 from 0 to UOH. When the voltage at pin 6 is zero, the oscillator frequency is lowest and the lamp power is maximum. The discharge current is much larger than the current flowing through the timing resistor RT. For example, when the discharge current is 5 mA, the discharge time is：  tdis ≈ 490 × CT. The ML4831/32 also includes a parallel regulator that limits the UCC voltage to 13.5V. When the UCC is 0.7V lower than 13.5V, the quiescent current of the device will be less than 1.7mA, and the output will be turned off, allowing the device to be started directly using the resistor attached to the rectified AC bus. In addition, because the ML4831/32 contains a temperature sensing function, the ballast operation is stopped as soon as the junction temperature of the device exceeds 120 °C. In order to better utilize the internal sensing function without using an external sensor, the position of the ML4831/32 must be carefully determined on the ballast's circuit board to ensure that the device can properly transfer the operating temperature of the ballast. The chip temperature of ML4831/32 can usually be estimated by the following formula:  Tj=65TA/PD(°C/W) It is worth noting that fully and reasonably using the sensing function inside the device is useful for reducing the total cost of the ballast. The starting scheme of the device is specifically designed for the ML4831/32 in accordance with the principle of ensuring the longest lamp life and minimizing the ballast heating. Figure 7(a) contains a starting scheme including preheating of the filament and sudden breaking of the lamp. When the ballast is energized, the time that the voltage on the CX rises from 0.7V to 3.4V is called the warm-up time of the filament.  During this time, the oscillator's charging current Ichg = 2.5/Rset, the oscillator produces a very high frequency, but does not produce a voltage sufficient to start the lamp. After the filament is preheated, the frequency of the inverter circuit drops to a minimum, and a high voltage is generated to start the lamp. If the voltage of the inverter circuit does not jump when the lamp should start to work, the lamp feedback voltage entering pin 9 will rise above Uref, the CX charging current will be bypassed, and the inverter circuit will stop working until CX drops to a 1.2V threshold by RX discharge. Stopping the inverter circuit in this way can avoid the failure of the lamp to start or the inverter circuit to overheat when it is disconnected from the socket.  In general, it is better to choose a large resistance RX to make this period longer. When CX reaches the 6.8V threshold, the oscillator will turn off LFBOUT, so the lamp will be driven to full power, then dimmed, and the potential of the CX pin is clamped at approximately 7.5V. The whole process is shown in the waveform of figure 7(b). Figure 7. Lamp Start Preheat and Interrupt Timing Scheme and Its Waveform 2.3.2 The Improvement of the Internal Function of ML4833The ML4833 is a modified version of the ML4831/32. In addition to the full functionality of the ML4831/32 described above, the most prominent improvement is in the power factor correction section. The power factor correction part of the ML4833 is a step-up type PFC control circuit for peak current sensing. This form of circuit only requires voltage loop compensation, which is simpler than the ML4831/32 with average current control mode circuit. It consists of a voltage error amplifier, a current sense amplifier without compensation, an integrator, a comparator, and a logic control circuit.  In the boost type power conversion part, the correction of the power factor is performed by the current sensing resistor to output the sensing voltage and the current flowing through, and the duty ratio is adjusted by comparing the integrated voltage signal of the error amplifier with the voltage across the Rsense. The control timing of the duty ratio is as shown in figure 8. Considering that all of the high-performance electronic ballast integrated control chips of Micro-Linearity are packaged in 18-pin DIP or SOIC packages, the improvement of the device structure will inevitably bring about changes in the internal functional frame and external pin functions. Figure 8. PEC Link and Duty Cycle Control of ML4833 2.4 High-performance Electronic Ballast Built by ML4833Figure 9 shows the complete circuit diagram of a high-performance electronic ballast built by ML4833. The circuit is a typical AC/DC/AC structure: the RFI suppression filter circuit is added to the input terminal, the booster active power factor correction circuit is composed of AC/DC in the front stage, and the high-frequency inverter circuit is composed of DC/AC in the rear stage. A closed-loop is formed through T5, VD11, R23 and pin 8 of the control to make the system works stably. Figure 9. Complete Circuit Diagram of High-performance Eectronic Ballast Built with ML4833  Ⅲ FAQ1. What is the use of electronic ballast?An electronic ballast will convert power frequency to a very high frequency to initialize the gas discharge process in Fluorescent Lamps – by controlling the voltage across the lamp and current through the lamp. 2. What is the output voltage of an electronic ballast?This unit operates off the AC mains with a voltage of 230 Volts and voltages generated within the unit can reach 600 to 800 Volts. 3. What is inside an electronic ballast?Lighting ballasts generate an initial high voltage to start the arc that excites the gases in fluorescent and HID lamps and makes them shine. ... Lighting ballasts for fluorescent light bulbs and HID lamps made before 1980 may contain polychlorinated biphenyls (PCBs). 4. How do you make an electronic ballast for tube light?An electrical ballast is nothing but a simple high current, mains voltage inductor made by winding number of turns of copper wire over the laminated iron core. Basically, as we all know a fluorescent tube requires a high initial current thrust to ignite and make the electrons flow connect in between its end filaments. 5. How do you wire an electronic ballast?Connect the ballast to the power from the breaker panel by wiring the black wire from the breaker panel to the black wire on the ballast, using a wire nut. Connect the white wire from the breaker to the white wire from the ballast. 6. What's the difference between electronic and magnetic ballast?A magnetic ballast uses coiled wire and creates magnetic fields to transform voltage. ... An electronic ballast uses solid-state components to transform voltage. It also changes the frequency of the power from 60 HZ to 20,000 HZ or higher depending on the ballast. 7. How do you test an electronic ballast with a multimeter?Insert one probe of the multimeter into the wire connector holding the white wires together. Touch the remaining probe to the ends of the blue, red and yellow wires leading from the ballast. Depending on the ballast, you may have only red and blue wires. 8. Are electronic ballasts non-linear loads?Rectified input, switching power supplies and electronic lighting ballasts are the most common single-phase non-linear loads. 9. Which is not the advantage of electronic ballast?Electronic ballasts are more efficient and more compact in size and weight. They also provide the ability for continuous power adjustment in different settings. A disadvantage is that power fluctuations may cause a failure but this can be offset by adding a buffer capacitor. The operation of the ballasts generates heat. 10. Can you repair an electronic ballast?I eventually replaced the 2 switching transistors in this ballast as well and it worked. So the next time you have a problem with an electronic ballast from a fluorescent fitting open it and check before buying a new one. They can be expensive and more often than not they can be repaired. 

IntroductionThyristor, commonly known as silicon controlled rectifier(SCR), its normative term is reverse blocking three-terminal thyristor. Thyristors are high-power semiconductor devices that have both switching and rectifying functions, and are used in various circuits such as controllable rectification and frequency conversion, inverters, and non-contact switches. As long as it is provided with a weak point trigger signal, it can control the strong electric output. So it is a bridge for semiconductor devices to enter the field of strong electricity from the field of weak electricity. So far, thyristors are the most widely used semiconductor devices in the electronics industry. Despite the continuous emergence of various new semiconductor materials, 98% of semiconductor materials are still silicon materials, which are still the basis of the integrated circuit industry. It is widely used due to its small size, light weight, high power and long life.Intro to Thyristors: the SCRCatalogIntroductionⅠ Thyristor Basics1.1 Brief Introduction of Thyristor1.2 Working Principle of  ThyristorⅡ The Main Characteristics of Thyristors2.1 Basic Structure of Thyristor2.2 Volt-ampere Characteristics of Thyristors2.3 Static Characteristics of Thyristors2.4 Characteristic Equation of ThyristorⅢ The Main Parameters of Thyristor3.1 Main Parameters of Unidirectional Thyristors3.2 Main Parameters of TRIACⅣ Main Function of ThyristorⅠ Thyristor Basics1.1 Brief Introduction of ThyristorThyristor, also called silicon controlled rectifier, is an abbreviation of semiconductor thyristor. It is a high-current switching semiconductor device that uses small currents to control. There are two commonly used types: ordinary thyristors (also called unidirectional thyristors) and TRIAC(triode for alternating current). Because of its small size, light weight, high efficiency, long life, vibration resistance and because it is noiseless, easy to use, it has attracted great attention from domestic, foreign, industrial and agricultural production departments in a short period of time and has been widely used in various production equipment and household appliances. According to its working principle, it can be roughly divided into four categories: f— Rectification: change AC power into adjustable DC power. — Inverter: converts DC power to AC power with a certain frequency. — DC switch: used for DC loop switch or DC voltage regulation. — AC switch: used for AC loop switch or AC voltage regulation. According to its service objects, it can be used in industries, agriculture, national defense, transportation, mining, metallurgy, light industry, chemical industry and other departments.In performance, thyristors not only have unidirectional conductivity, but also have more valuable controllability than silicon rectifier elements (commonly known as "dead silicon"). It has only two states: on and off.Thyristors can control high-power electromechanical equipment with milliamp currents. If the frequency exceeds this value, the average switching current allowed to pass will decrease due to the significant increase in the switching losses of the components. At this time, the nominal current should be degraded.Thyristors have many advantages, such as: controlling high power with low power, power amplification multiples up to several hundred thousand times; extremely fast response, turn on and off in microseconds; non-contact operation, no spark, no noise; high efficiency, low cost and so on.Disadvantages of thyristors: poor static and dynamic overload capacity; easy to be misguided due to interference.The two types of thyristors, unidirectional thyristors and three-terminal TRIAC, are briefly introduced below.1.2 Working Principle of  Thyristora. Unidirectional ThyristorThe internal structure of the unidirectional thyristor is shown in figure 1 (a). It can be seen from figure 1 (a) that the unidirectional thyristor is composed of four layers semiconductors P1N1P2N2. There are three PN junctions in the middle: the junction J1, J2, and J3. The anode A is drawn from P1, the cathode K is drawn from N2, and the control electrode (or gate) G is drawn from the middle P2. The circuit symbol of the unidirectional thyristor is shown in figure 1 (b). Figure 1. Schematic Diagram and Circuit Symbol of Unidirectional ThyristorIn order to understand the working principle of the unidirectional thyristor, the unidirectional thyristor can be equivalently regarded as a combination of a PNP transistor T1 and an NPN transistor T2. The middle layer P2 and layer N1 are shared by two transistors. The anode A is equivalent to the emitter of T1, and the cathode K is equivalent to the emitter of T2, as shown in figure 2. Figure 2. Working Principle of Unidirectional ThyristorThe key to understanding how unidirectional thyristors work is to understand the role of the control electrode.(1) No voltage or reverse voltage is applied to the control electrodeWhen the control electrode is left floating or a reverse voltage is applied between the control electrode and the cathode, that is, UGK<0, there must be IG=0. If a reverse voltage is applied between the anode and the cathode, that is, UAK＜0. Due to J, and J2, the transmitting junctions of T1, T2, are both reverse biased and T1 and T2 are in the off state, at this time, the current flowing through the unidirectional thyristor is only the reverse saturation current of the J1 and J3, IA≈0, and the unidirectional thyristor is in the blocking state; if a forward voltage is applied between the anode and the cathode, that is, UAK>0, J2 is in a reverse biased state, because IG=0, T2 must be in the off state. and the current in the unidirectional thyristor is only the reverse of J2. At this time, the current in the unidirectional thyristor is just the reverse saturation current of J2, IA≈0, and the unidirectional thyristor is still in the blocking state. Therefore, when no voltage is applied to the control pole or reverse voltage is applied, IG = 0, the unidirectional thyristor is in a blocking state, and has positive and negative blocking capabilities.(2) Apply forward voltage to the control electrodeWhen a forward voltage is applied between the control electrode and the cathode, that is, UGK> 0, the emitter junction J3 of T2 is in a forward bias, and IG≠0. If a reverse voltage is applied between the anode and the cathode, that is, UAK <0, because the emission junction J1 of T1 is reverse biased and T1 is in the off state, the unidirectional thyristor is in the blocking state, IA≈0; If a forward voltage is applied between the anode and the cathode, that is, UAK> 0, because the emission junctions J1, J3 of T1, T2 are forward biased, and the collector junction J2 is reverse biased, T1, T2 will be in an amplified state. After IG is amplified by T2, the collector current of T2 is IC2 = β2IG. The collector current of T2 is the base current of T1, after being amplified by T1, the collector current of T1 is IC1 = β1β2IG. This current flows into the base of T2 for amplification, and in this cycle, a strong positive feedback is formed, which makes T1, T2 quickly enter the saturation state, and the unidirectional thyristor is in the on state. After the unidirectional thyristor is turned on, UAK, the value of the voltage between the anode and the cathode is very small, and the external power supply voltage is almost completely dropped on the load.(3) Turn-off of the unidirectional thyristorFrom the above analysis, it can be seen that after the unidirectional thyristor is turned on, the base of T2 always has the collector current IC1 of T1 flowing, and the value of IC1 is much larger than the IG applied at the beginning. So even if the control electrode voltage disappears and IG = 0, it can still rely on the positive feedback of the tube itself to maintain conduction. Therefore, once the unidirectional thyristor is turned on, the control electrode will lose the function of controlling. After the unidirectional thyristor is turned on, if you want it to turn off again, the anode current IA must be reduced so that it cannot maintain positive feedback. To this end, the anode can be disconnected or a reverse voltage can be applied between the anode and the cathode.To sum up, under the condition that a forward voltage is applied between the anode and the cathode of the unidirectional thyristor, if a forward voltage is added between the control electrode and the cathode at a certain time, the unidirectional thyristor will change from the blocking state to the conducting state. This is triggered into conduction. After the unidirectional thyristor is turned on, the control electrode will lose the function of controlling. If you want to turn off the unidirectional thyristor again, you must make its anode current less than a certain value IH (called the holding current) or reduce the voltage UAK between anode and cathode to zero. b. TRIACA TRIAC is a three-terminal element with a five-layer structure of N1P1N2P2N3. It has three electrodes: a main electrode A1, a main electrode A2, and a control electrode (or gate) G. It is also a gate control switch. Regardless of its structure or characteristics, it can be regarded as a pair of anti-parallel ordinary thyristors. Its structure, equivalent circuit and symbols are shown in figure 3. Figure 3. Symbol, Structure and Equivalent Circuit of the TRIACThe main electrodes A2 and A1 of the triac are connected in series with the control object (load) RL, which is equivalent to a non-contact switch. The "on" or "off" of this switch is controlled by a signal uG (called a trigger signal) on the control electrode G. When there is a voltage (u ≠ 0) between the main electrodes A2 and A1, the moment the trigger signal uG appears, it will be conductive between A2 and A1 of the TRIAC, which is equivalent to the closed state of the switch. And once it is turned on, even if uG disappears, it can be kept on until u = 0 or the current in the series circuit of the main electrode and the load is reduced to a certain value, then it is turned off. After the cutoff, it is equivalent to the off state of the switch. In this way, the small current signal on the control electrode can be used to control the large current in the main electrode circuit. Figure 4. Volt-ampere Characteristic Curve of TRIACGenerally speaking, regardless of the voltage polarity between the two main electrodes A2 and A1 of TRIAC, as long as a certain amplitude of positive and negative pulses is applied to the control electrode, it can be turned on. So i represents the current in the main electrode and u represents the voltage between A2 and A1. The functional relationship between the two (called the volt-ampere characteristic curve) is shown in figure 4. It can be seen from the curve that the TRIAC has basically the same symmetrical performance in the first quadrant and the third quadrant.According to the voltage u on the main electrode and the polarity of the trigger pulse voltage uG on the control electrode, combined with the volt-ampere characteristic curve, the TRIAC can be divided into four trigger modes, which are defined as follows:(1) I+trigger: In the first quadrant of the characteristic curve (A2 is positive), the control electrode is a positive trigger relative to A1.(2) I-trigger: In the first quadrant of the characteristic curve (A2 is positive), the control electrode is a negative trigger relative to A1.(3) Ⅲ+trigger: In the third quadrant of the characteristic curve (A2 is negative), the control electrode is a positive trigger relative to A1.(4) Ⅲ-trigger: In the third quadrant of the characteristic curve (A2 is negative), the control electrode is a negative trigger relative to A1.Among these four trigger modes, I+ and III- have higher sensitivity, and are two commonly used trigger modes.In the control circuit of the new type electric heating electric appliance, the trigger signal applied to the control electrode of TRIAC is output by a single chip microcomputer or an integrated circuit. Some output a continuous positive (or negative) voltage signal, and some output a series of zero-crossing trigger pulses synchronized with a 50Hz sinusoidal AC power supply. The former is called a potential trigger, while the latter is called a pulse trigger. Their waveforms are shown in figure 5 and figure 6, respectively. Figure 5. Figure 6. Ⅱ The Main Characteristics of Thyristors2.1 Basic Structure of ThyristorA thyristor (also known as semiconductor controlled rectifier) is a high-power semiconductor device with a four-layer structure (PNPN). It has three lead-out electrodes, namely anode (A), cathode (K) and gate (G). Its symbolic representation and device cross-section are shown in figure 7. Figure 7. Symbol Representation and Device Cross-sectionOrdinary thyristors bidirectionally diffuse P-type impurities (aluminum or boron) in an N-type silicon wafer to form a P1N1P2 structure, and then diffuse N-type impurities (phosphorus or antimony) to form a cathode in most regions of P2, and at the same time lead out a gate electrode on P2 and form an ohmic contact is formed in the P1 as the anode.2.2 Volt-ampere Characteristics of ThyristorsThe on and off states of the thyristor are determined by the anode voltage, anode current and gate current. Volt-ampere characteristic curves are usually used to describe the relationship between them, as shown in figure 8. Figure 8. Volt-ampere Characteristic Curve of ThyristorWhen the thyristor VAK applies a forward voltage, J1 and J3 are forward biased, and J2 is reverse biased. The applied voltage almost falls on J2, and J2 plays a role of blocking the current. With the increase of VAK, as long as VAK <VBO, the passing anode current IA is small, so this region is called a forward blocking state. When VAK increases beyond VBO, the anode current suddenly increases, and it will be in a low voltage and high current state at the moment the characteristic curve passes the negative resistance. The on-state current IT determined by the load flows through the thyristor, the device voltage drop is about 1V, and the state corresponding to the CD section of the characteristic curve is called the on-state. VBO and its corresponding IBO are usually referred to as forward breakover voltage and breakover current. After the thyristor is turned on, it can maintain the on-state by itself. The transition from the on-state to the off-state is usually controlled by an external circuit without using a gate signal, that is, the device can be turned off only when the current is below a certain threshold value called the holding current IH.When the thyristor is in the off-state (VAK <VBO), if the gate electrode is made positive with respect to the cathode and the gate electrode is supplied with current IG, the thyristor will breakover at a lower voltage. The breakover voltage VBO and the breakover current IBO are both functions of IG. The larger the IG, the smaller the VBO. As shown in figure 3, once the thyristor is turned on, the device is turned on even if the gate signal is removed.When the anode of the thyristor is negative with respect to the cathode, as long as VAK <VBO, IA is small and has nothing to do with IG. However, when the reverse voltage is large (VAK≈VBO), the reverse leakage current through the thyristor increases sharply, showing thyristor breakdown. Therefore, VBO is called the reverse breakover voltage and breakover current.2.3 Static Characteristics of ThyristorsThe thyristor has 3 PN junctions, and the characteristic curve can be divided into (0 ~ 1) blocking area, (1 ~ 2) breakover area, (2 ~ 3) negative resistance area and (3 ~ 4) conducting area. a. Forward Working Area— Forward blocking (0 ~ 1) areaWhen a forward voltage is applied between AK, J1 and J3 bear the forward voltage, while J2 bears the reverse voltage, and the applied voltage falls almost entirely on J2. The reverse-biased J2 acts to block the current, and the thyristor is not conducting at this time.— Avalanche area (1 ~ 2 is also called breakover area)When the applied voltage rises close to the avalanche breakdown voltage VBJ2 of J2, the width of the space charge region of the reverse-biased J2 expands, and the internal electric field is greatly enhanced, which causes the multiplication effect to be strengthened. As a result, the current through J2 suddenly increases, and the current flowing through the device also increases. At this time, the current passing through J2 is transformed from the original reverse current to the current which is mainly attenuated by J1 and J3 through the base region and multiplied in the space charge region of J2. This is the avalanche area where the voltage increases and the current increases sharply. Therefore, the characteristic curve turns in the area, so it is called the breakover area.— Load area (2 ~ 3)When the applied voltage is greater than the breakover voltage, a large number of electron-hole pairs generated by the avalanche doubling of the space charge region of J2 are extracted by the reverse electric field. The electrons enter the region N1 and the holes enter the region P2. Due to the inability to recombine quickly, carrier accumulation occurs near both sides of J2: holes in region P2 and electrons in region N1, compensating for the charge of the ionized impurities and narrowing the space charge region. As a result, the potential in region P2 increases and the potential in the region N1 decreases, which acts to offset the external electric field. As the applied voltage at J2 decreases, the avalanche multiplication effect also weakens. On the other hand, the forward voltage of J1 and J3 has been enhanced, and the injection has increased, causing the current through J2 to increase, so a negative resistance phenomenon has occurred in which the current increases and the voltage decreases.— Low resistance on-state region (3 ~ 4)As mentioned above, the multiplication effect causes the accumulation of electrons and holes on both sides of J2, causing the reverse bias voltage of J2 to decrease; at the same time, the injection of J1 and J3 is enhanced, and the circuit is increased, so that charges continue to accumulate on both sides of J2, and the junction voltage continues to decrease. When the voltage drops to the point where the avalanche multiplication stops and all the junction voltages are cancelled, holes and electrons still accumulate on both sides of J2, and J2 becomes forward biased. At this time, J1, J2, and J3 are all forward biased, and large currents can pass through the device  because it is in a low-resistance on-state region. When fully conducting, its volt-ampere characteristic is similar to that of a rectifier element.b. Reverse Working Area (0 ~ 5)When the device is operating in reverse, J1 and J3 are reverse biased. Due to the very low breakdown voltage of the heavily doped J3, J1 withstands almost all of the applied voltage. The volt-ampere characteristic of the device is the volt-ampere characteristic curve of the reverse bias diode. Therefore, the PNPN thyristor has a reverse blocking region, and when the voltage increases above the J1 breakdown voltage, the current increases sharply due to the avalanche multiplication effect, at which time the thyristor is broken down. 2.4 Characteristic Equation of ThyristorA two-terminal device of a PNPN four-layer structure can be regarded as P1N1P2 and N1P2N2 transistors with current amplification coefficients of α1 and α2, respectively, where J2 is a common collector junction. When a forward voltage is applied to the device, the forward-biased J1 injects holes and passes through region N1 to reach the collector junction (J2). The hole current is α1IA; while the forward-biased J3 injects electrons and passes through region P2. The current carried to J2 is α2IK. Because J2 is in the reverse direction, the current through J2 also includes its own reverse saturation current, ICO.The current through J2 is the sum of the above three, that is,（1）Assuming the emission efficiency γ1 = γ2 = 1, according to the principle of current continuity IJ2 = IA = IK, so formula (1) becomes:（2）The formula shows that when the forward voltage is less than the avalanche breakdown voltage VB of J2, the multiplication effect is small and the injection current is also small. So α1 and α2 are also very small, thus（3）The ICO at this time was also small. Therefore, J1 and J3 are forward biased, so increasing VAK can only increase the reverse bias of J2. It cannot increase the ICO and IA a lot, so the device is always in the blocking state, and the current flowing through the device is the same order of magnitude as the ICO. Therefore, formula (3) is called a blocking condition.When the increase in VAK causes the reverse bias of J2 to increase and avalanche multiplication occurs, assuming multiplication factor Mn = Mp = M, then ICO, α1, and α2 will all increase by M times, so (2) becomes（4）At this time, the denominator becomes smaller, and IA will increase rapidly with the growth of VAK, so when（5）The avalanche steady-state limit is reached (VAK = VBO), and the current will tend to infinity, so equation (5) is called the forward breakover condition., , Using this feature, the breakover point conditions are derived from the characteristic curve equation (4). Because α1 and α2 are functions of current, M is a function of VJ2, which can be approximated with M(VJ2)=M(VAK), ICO is a constant and derive  with respect to (4). The outcome is（6）Since the breakover voltage is lower than the breakdown voltage,  must be a constant value. Because , the numerator must also be zero and obtain （7）According to the definition of transistor DC voltage amplification factor,                      （8）We can get the small signal current amplification factor                      （9）Using formula (9), formula (7) can be changed to                        （10）That is, at the breakover point, the product of the multiplication factor and the sum of the small signal is exactly 1. As long as the PNPN structure satisfies the above formula, it has switching characteristics, that is, it can be switched from an off-state to an on-state.Because α changes with the current IE, when IA increases, both α1 and α2 increase. It can be seen that, when the current is large, the value of M satisfying (6) can be reduced instead. This shows that IA increases and VAK decreases accordingly.α is both a function name of the current and a function of the collector junction voltage. When the current increases as α is constant, the corresponding reverse bias of the collector junction decreases. When the current is large,                             （11）According to equation (2), J2 provides an on-state current (ICO <0). Therefore, J2 must be forward biased, so J1, J2, and J3 are all forward biased, and the device is conducting. The off-state of the device changes to the on-state. The key is that J2 junction must be changed from reverse-biased to forward-biased. The condition for J2 to reverse to the forward direction is that holes and electrons should accumulate in regions P2 and N1, respectively. The condition for the accumulation of holes in region P2 is that the amount of holes α1IA injected by the J1 and collected by J2 into region P2 is greater than the amount of holes that disappear by recombination with (1-α2) IK, that is                           （12）Since IA=IK, α1+α2＞1 is obtained. As long as the conditions are true, the hole accumulation in region P2 is the same, and the region electron accumulation condition is（13）Thus （14）It can be seen that when the condition of α1+α2＞1 is satisfied, the potential of region P2 is positive, and the potential of region N1 is negative. J2 becomes forward-biased and the device is in a conducting state, so α1+α2＞1 is called a conducting condition.Figure 9. SCR (Silicon Controlled Rectifier) Symbol Ⅲ The Main Parameters of Thyristor3.1 Main Parameters of Unidirectional ThyristorsIn order to correctly use a unidirectional thyristor, it is necessary not only to understand its working principle, but also to master its main parameters.(1) Forward repetitive peak voltage UFRMUnder the condition that the control electrode is disconnected and the unidirectional thyristor is in the forward blocking state, when the junction temperature of the unidirectional thyristor is the rated value, it is allowed 50 times per second, and the duration should not exceed 10 ms. The forward peak voltage that can be repeatedly applied to the unidirectional thyristor is called the forward repetitive peak voltage, which is expressed by UFRM. Generally, the secondary voltage is specified as 80% of the forward breakover voltage.(2) Reverse repetitive peak voltage URRMUnder the same conditions as the forward repetitive peak voltage, the reverse peak voltage that can be repeatedly applied to the unidirectional thyristor is called the reverse repetitive peak voltage, which is expressed by URRM and is generally 80% of the reverse breakover voltage.(3) Rated voltage UNUsually, the smaller one of UFRM and URRM is used as the rated voltage of the unidirectional thyristor. This is because the voltage added to the tube in practice is generally a positive and negative symmetrical voltage, so the voltage with a smaller value shall prevail. But because the transient over-voltage will also damage the tube, when selecting the tube, for safety reasons, the rated voltage of the tube is required to be greater than 2 to 3 times the actual peak voltage.(4) Rated forward average current IFThe average value of the power frequency sinusoidal half-wave current allowed to pass through the unidirectional thyristor under the ambient temperature of 40°C and specified heat dissipation conditions is called the rated forward average current IF. How many amp of the unidirectional thyristors we generally say refers to this current value. The amount of IF is related to factors such as the ambient temperature, heat dissipation conditions, and the conduction angle of the component. The rated current of the unidirectional thyristor is calibrated by the power frequency sinusoidal half-wave average current under certain conditions. This is because the load connected to the rectifier output often requires the average current to measure its performance. However, from the perspective of the unidirectional thyristor heating, regardless of the current waveform flowing through the unidirectional thyristor and the conduction angle of the unidirectional thyristor, as long as the effective value of the designed current is equal to the effective value of the rated current IF, then the heating of the unidirectional thyristor is equivalent and allowed.(5) Holding current IHAt room temperature, under the condition of the control electrode short circuit, the minimum anode current required to maintain the unidirectional thyristor to continue conducting is called the holding current IH. If the anode current of the unidirectional thyristor is less than this value, the unidirectional thyristor will change from the conducting state to the blocking state.(6) Control electrode trigger voltage UGK and trigger current IGAt room temperature, under the condition that the voltage between the anode and cathode of the unidirectional thyristor is 6V, the minimum DC current value of the control electrode required to change the unidirectional thyristor from the blocking state to the conducting state is called the trigger current IG. The DC voltage UGK between the control electrode and the cathode corresponding to the trigger current IG is called a trigger voltage. Generally, UGK is about 1 to 5V, and IG is tens to hundreds of mA.3.2 Main Parameters of TRIACIn various control circuits, the TRIAC is a relatively easy-to-damage component. Once the TRIAC is found to be damaged, you just need to replace the TRIAC with the same parameters. There are many characteristic parameters of TRIAC, and the following are the main parameters that should be considered during maintenance.— Off-state repetitive peak voltage-rated voltage VDRMWhen the control electrode is disconnected and the component is at the rated junction temperature, the voltage corresponding to the sharp bending point of the forward and reverse volt-ampere characteristics is called the off-state non-repeating peak voltage. 80% of it is called the off-state repetitive peak voltage. It is also called rated voltage, which is expressed by VDRM.When the TRIAC works, the peak value of the applied voltage momentarily exceeds the reverse non-repetitive peak voltage, which can cause permanent damage to the TRIAC. Moreover, due to the increase in ambient temperature or poor heat dissipation, the reverse non-repetitive peak voltage value may decrease. Therefore, when a TRIAC is selected, its rated voltage value should be 2 to 3 times the possible maximum voltage in actual operation. If the power supply voltage is 220V, a TRIAC with a rated voltage above 500V should be selected so that the selected components can withstand the surge voltage.— Rated on-state average current—rated current IT(AV)Under the specified conditions, the maximum average on-state current allowed when the TRIAC is on is called the rated on-state average current. According to the standard series of TRIAC, this current is taken to the corresponding current level, which is often referred to as the rated current for short and represented by IT(AV).Because the current overload capacity of the TRIAC is much smaller than that of ordinary motors and electrical appliances, the rated current of the TRIAC should be 1.5 to 2 times the maximum current in actual operation when selected.— Gate trigger current IGT (voltage UGT)This refers to the minimum trigger signal current (voltage) value that can make the TRIAC conduct reliably and add to the control electrode. If the trigger current (voltage) obtained by the TRIAC control electrode is less than the number of times, the TRIAC may not be turned on.— On-state average voltage UT(AV)Once the TRIAC is turned on, it is equivalent to the closed switch. Because the TRIAC is connected in series with the load, the smaller the voltage between the two main electrodes, the better. After the TRIAC is turned on, the average value of the voltage between the two main electrodes is called the on-state average voltage, which is usually referred to as the tube voltage drop. If the tube pressure drop of the TRIAC is too large, the motors and solenoid valves it controls may not work properly because they cannot get the full voltage.— Holding currentWhen the control electrode is disconnected at room temperature, the TRIAC is reduced from a large on-state current to a minimum main electrode current that is just necessary to maintain conduction, which is called a holding current. The TRIAC is turned off only when the main electrode current decreases below the holding current. Ⅳ Main Function of ThyristorThe functions of thyristors are as follows: first, converter rectification; second, voltage regulation; third, frequency conversion; fourth, switch (contactless switch). The most basic use of ordinary thyristors is controlled rectification. The diode rectifier circuit we are familiar with is an uncontrollable rectifier circuit. If the diode is replaced by a thyristor, it can constitute a controllable rectifier circuit, inverter, non-contact switch, achieve motor speed control, motor excitation, automatic control and so on. In electrical technology, the half cycle of alternating current is often defined as 180°, which is called the electrical angle. In this way, in each positive half cycle of U2, the electrical angle experienced from the beginning of the zero value to the moment when the trigger pulse arrives is called the control angle α; the electrical angle at which the thyristor conducts in each positive half cycle is called the conduction angle θ. Obviously, both α and θ are used to indicate the on or off range of the thyristor during the half cycle of the forward voltage. Controllable rectification is achieved by changing the control angle α or the conduction angle θ, and changing the average value UL of the pulsed DC voltage on the load. The function of a thyristor is not only rectification, it can also be used as a non-contact switch to quickly turn on or off the circuit, to achieve the inverter that converts DC power to AC power, to change AC power of one frequency to AC power of another frequency, etc. This article mainly introduces the basic principle, characteristics and main parameters of thyristors. Frequently Asked Questions about Thyristors (SCR)1. What are the characteristics of SCR?Characteristics of Thyristor or Characteristics of SCRReverse Blocking Mode of Thyristor. Initially for the reverse blocking mode of the thyristor, the cathode is made positive with respect to anode by supplying voltage E and the gate to cathode supply voltage Es is detached initially by keeping switch S open.Forward Blocking ModeForward Conduction Mode 2. Why SCR is called as thyristor?Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor device made of silicon. This device is the solid state equivalent of thyratron and hence it is also referred to as thyristor or thyroid transistor. 3. Is SCR and thyristor are same?Thyristor is a 4 layer device formed by alternate combination of p and n type semiconductor materials. It is a device used for rectification and switching purpose. SCR is the mostly used member of thyristor family and it is the name commonly used when we talk about thyristors. 4. What is a thyristor used for?Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to switch off automatically, referred to as "zero cross" operation. 5. How does a SCR thyristor work?So how does it work? With no current flowing into the gate, the thyristor is switched off and no current flows between the anode and the cathode. When a current flows into the gate, it effectively flows into the base (input) of the lower (n-p-n) transistor, turning it on.

IntroductionThyristor is a four semiconductor layers or three PN junctions devicea solid-state semiconductor device with four layers of alternating P- and N-type materials. It is also known as “SCR” (Silicon Control Rectifier). The term “Thyristor” is dervid from the words of thyratron (a gas fluid tube which work as SCR) and Transistor. And It acts exclusively as a bistable switch in electronic circuit.What is a Thyristor?CatalogIntroductionⅠ Types of ThyristorsⅡ Thyristor Selection2.1 Specific Requirements of Applying Circuit2.2 Main Parameters of the ThyristorⅢ Replacement of ThyristorⅣ Detection of Thyristor4.1 Detection of Unidirectional Thyristors4.2 Detection of TRIACⅠ Types of ThyristorsCommonly used thyristors include unidirectional thyristors, TRIAC, and turn-off thyristors, etc., which should be selected reasonably according to the needs of the circuit.— Unidirectional ThyristorThe unidirectional thyristor is characterized in that the current can only flow from the anode A to the cathode k, and is mainly used in the control of DC power supply or pulsating direct current, AC power rectification, and DC power inverter.Unidirectional thyristors can be divided into ordinary thyristors and high-frequency thyristors (the working frequency is above 110kHz). Commonly used unidirectional thyristors are 3CT series, 3DT series, KP series and KK series (high frequency thyristors), and imported MCR series, SF series, BST series etc.— TRIACTRIAC was developed on the basis of the unidirectional thyristor and is an AC power control device. TRIAC can not only replace two unidirectional thyristors in anti-parallel, but also requires only one trigger circuit, which is more convenient to use.The characteristic of TRIAC is that alternating current can pass through it, which is mainly used in the control of AC power supply and the adjustment of AC voltage. Commonly used TRIAC include 3CTS series and KS series, as well as imported MAC series, SM series, BCR series, etc.— Gate Turn-off ThyristorThe characteristic of the gate turn-off thyristor is that it can be switched off by the control electrode. It is mainly used in gate turn-off contactless switches, DC inverters, dimmer, speed regulation and other occasions.Gate turn-off thyristors are power-type control devices developed on the basis of ordinary thyristors. After the ordinary thyristor is triggered to be turned on, its control electrode does not work. To turn off the thyristor, the power must be cut off, or the forward current flowing through the thyristor must be less than the holding current. Gate turn-off thyristor overcomes the above drawbacks. When the control electrode G is added with a positive pulse voltage, the thyristor is turned on, and when the control electrode G is added with a negative pulse voltage, the thyristor is turned off.Gate turn-off thyristors are ideal high-voltage, high-current switching devices. For example, the DG series high-power gate turn-off thyristors can reach a maximum voltage of 4500V and a maximum current of 3000A. Ⅱ Thyristor Selection2.1 Specific Requirements of Applying CircuitThere are many types of thyristors, which should be selected reasonably according to the specific requirements of the application circuit.For AC/DC voltage control, controllable rectification, AC voltage regulation, power inverter, switching power supply protection circuit, etc., ordinary thyristors can be selected.For AC switch, AC voltage regulation, AC motor linear speed regulation, lamp linear dimming, solid state relay, solid state contactor, etc., a TRIAC should be selected.For AC motor variable frequency speed regulation, chopper, power inverter and various electronic switch circuits, you can choose gate turn-off thyristor.For sawtooth wave generator, long time delay, over voltage protector and trigger circuit with power transistor, etc., BTG thyristor can be selected.In electromagnetic cookers, electronic ballasts, ultrasonic circuits, superconducting magnetic energy storage systems, switching power supplies and other circuits, reverse conducting thyristors can be selected.In the photocoupler, light detector, light alarm, light counter, photoelectric logic circuit and operation monitoring circuit of automatic production line, the light-control thyristor can be selected.2.2 Main Parameters of the ThyristorThe main parameters of the thyristor should be determined according to the specific requirements of the application circuit.The selected thyristor should have a certain power margin, and its rated peak voltage and rated current (on-state average current) should be higher than the maximum operating voltage and maximum working current of the controlled circuit by 1.5 to 2 times.The parameters of the thyristor's forward voltage drop, gate trigger current, and trigger voltage should meet the requirements of the application circuit (this refers to the control circuit of the gate), and should not be high or low, otherwise it will affect the normal operation of the thyristor. Ⅲ Replacement of ThyristorAfter the thyristor is damaged, if no thyristor of the same type is replaced, another type of thyristor with similar performance parameters can be used instead.When designing an application circuit, a large margin is generally left. When replacing the thyristor, just pay attention to its rated peak voltage (repeated peak voltage), rated current (on-state average current), gate trigger voltage and gate trigger current, especially the two indicators of rated peak voltage and rated current.The switching speed of the thyristor used for replacement should be consistent with the switching speed of the damaged thyristor. For example: After the high-speed thyristor used in the pulse circuit and high-speed inverter circuit is damaged, only the same type of fast thyristor can be used instead of the ordinary thyristor.When selecting a thyristor to be used for replacement, it is not necessary to leave too much margin for any parameter, and the parameter of it should be as close as possible to the parameter of the replaced thyristor, because an excessively large margin is not only a waste, but also sometimes has side effects, such as non-triggering or insensitive triggering.In addition, the appearance of the two thyristors should be the same, otherwise it will cause inconvenience to the installation. Ⅳ Detection of ThyristorThyristors are usually represented by the letters "SCR" in circuit schematic diagrams. For example, SCR2 refers to the thyristor numbered 2. The symbol of the thyristor in the schematic diagram is shown in figure 1. Figure 1. Symbols of Thyristor4.1 Detection of Unidirectional Thyristors(1) Discrimination of each electrode: According to the structure of an ordinary thyristor, it can be seen that there is a PN junction between the gate G and the cathode K, which has unidirectional conductive characteristics, while there are two PN junctions of opposite polarities connected in series between the anode A and the gate. Therefore, by measuring the resistance between the pins of an ordinary thyristor with the R × 100 or R × 1 k Q level of the multimeter, three electrodes can be determined.The specific method is: use the black probe of the multimeter to connect one electrode of the thyristor, and use the red probe to touch the other two electrodes in turn. If the measurement result has a resistance value of several thousand ohms (kΩ) and another resistance value of several hundred ohms(Ω), it can be determined that the black probe is connected to gate G. In the measurement with a resistance value of several hundred ohms, the red probe was connected to the cathode K, and in the measurement with a resistance value of several thousand ohms, the red probe was connected to the anode A. If the measured resistance values are both very large, it means that the black probe is not connected to gate G. Apply the same method to test other electrodes until three electrodes are found.You can also measure the forward and reverse resistance between any two pins. If the forward and reverse resistance are close to infinity, the two electrodes are anode A and cathode K, and the other pin is gate G.Each electrode of the ordinary thyristors can also be judged according to its packaging form.For example, the bolt end of the bolt-type ordinary thyristor is anode A, the thinner lead end is gate G, and the thicker lead end is cathode K.The lead end of the flat thyristor is gate G, the flat end is anode A, and the other end is cathode K.A thyristor of metal package (T0-3) is a common thyristor and its shell is anode A.The middle pin of the plastic thyristor (T0-220) is anode A, and it is mostly connected with its own heat sink. Figure 2. Pin Arrangement of Several Common Thyristors(2) Judging whether it is good or bad: Use the R×1 kΩ level of a multimeter to measure the forward and reverse resistance values between anode A and cathode K of ordinary thyristor, which should normally be infinite (∞) ; If the forward and reverse resistance values are zero or the resistance values are both small, it indicates that a breakdown short circuit or leakage occurs inside the thyristor.Measure the forward and reverse resistance values between gate G and cathode K. Normally, there should be forward and reverse resistance values similar to diodes (the actual measurement results are smaller than those of ordinary diodes), that is, the forward resistance value is small (less than 2 kΩ) and the reverse resistance value is large (greater than 80 kΩ). If the resistance values of the two measurements are both large or small, it means that the thyristor is open or short-circuited between electrode G and K. If the forward and reverse resistance values are equal or close, it indicates that the thyristor has failed, and the PN junction between its electrodes G and K has lost its unidirectional conduction effect.Measure the forward and reverse resistance value between anode A and gate G. In normal conditions, both resistances should be several hundred kiloohms (kΩ) or infinite. If the forward and reverse resistance values are not the same (there is unidirectional conduction like a diode). One of the two PN junctions connected in reverse series between gate G and electrode A has been short-circuited.(3) Detection of triggering capability: For ordinary thyristors with low power (working current is below 5A), it can be measured with R×1 level of the multimeter . During the measurement, the black probe is connected to anode A and the red probe is connected to cathode K. At this time, the watch hand does not move, and the resistance value is displayed as infinite (∞). Use tweezers or wires to make anode A and gate G of the thyristor be short-circuited(see figure 3), which is equivalent to applying a forward trigger voltage to gate G. At this time, if the resistance value is several ohms to tens of ohms (the specific resistance value will vary according to the part number of the thyristor), it indicates that the thyristor is conducting due to the forward trigger. Then disconnect electrode A and gate G(the probes on electrode A and K do not move, only the trigger voltage of gate G is cut off). If the value indicated by the watch hand is still in the position of several ohms to tens of ohms, it indicates that the triggering performance of the thyristor is good. Figure 3. Detection of Triggering CapabilityFor medium and high power ordinary thyristors with a working current above 5 A, the on-state voltage drop VT, holding current IH and the gate trigger voltage Vo are relatively large. The current provided by the R × 1 kΩ level of the multimeter is low, and the thyristor cannot be completely turned on, so a 200Ω adjustable resistor and one to three 1.5 V dry batteries can be connected in series at the end of the black probe (depending on the capacity of the thyristor being tested, if its working current is greater than 100 A, three 1.5 V dry batteries are applied), as shown in figure 4. Figure 4. Detection of Trigger VoltageYou can also use the test circuit in figure 5 to test the triggering capability of an ordinary thyristor. In the circuit, vT is the thyristor under test, HL is a 6.3 V indicator (small electric beads in a flashlight), GB is a 6 V power supply (four 1.5 V dry batteries or 6 V regulated power supply can be used), and S is the button, R is the current limiting resistor. Figure 5. Test Circuit to Test the Triggering CapabilityWhen the button S is not connected, the thyristor VT is in a blocking state, and the indicator light HL is not on (if HL is on at this time, there may be breakdown of vT or leakage damage). After pressing the button S once (turn S on for a moment to provide the trigger voltage for gate G of the thyristor VT), if the indicator HL is always on, it means that the thyristor has a good triggering capability. If the brightness of the indicator is low, it indicates that the thyristor has poor performance and a large conduction voltage drop (the conduction voltage drop should be about 1 V under normal conditions). If button S is on, the indicator light is on, and when button S is off, the indicator light is off, indicating that the thyristor is damaged and the triggering performance is poor.4.2 Detection of TRIAC(1) Discrimination of each electrode: Use the R×1 or R×10 level of the multimeter  to measure the forward and reverse resistance values between three pins of the TRIAC. If it is measured that one pin is not connected with the other two pins, then this pin is the main electrode T2.After finding the electrode T2, the remaining two pins are the main electrode T1 and the gate G3. Measuring the forward and reverse resistance values between these two pins will gain two smaller resistance values. In a measurement with a small resistance value (about tens of ohms), the black probe is connected to the main electrode T1, and the red probe is connected to gate G.One end of the bolt of the bolt-shaped TRIAC is the main electrode T2, the thinner lead end is gate G, and the thicker lead end is the main electrode T1.    The shell of the metal-encapsulated (TO-3) TRIAC is the main electrode T2.The middle pin of the plastic-encapsulated (TO-220) TRIAC is the main electrode T2, which is usually connected to its own small heat sink. Figure 6. Pin Arrangement of Several TRIAC(2) Judging whether it is good or bad: Use the R×1 or R×10 level of a multimeter to measure the forward and reverse resistance values between the main electrode T1 and the main electrode T2 and between the main electrode T2 and gate G of the TRIAC. Normally it should be close to infinity. If the measured resistance values are all very small, it means that the electrodes of the TRIAC have been broken down or are short-circuited.Measure the forward and reverse resistance of the main electrode T1 and gate G. Normally, it should be between tens of ohms (Ω) and one hundred ohms (Ω) (when the black probe is connected to electrode T1 and the red probe is connected to gate G, the measured forward resistance value is slightly smaller than the reverse resistance value). If the forward and reverse resistance values between electrode T1 and gate G are measured to be infinite, it indicates that the thyristor has been damaged by an open circuit.(3) Detection of triggering capability: For small power TRIAC with working current below 8A, it can be measured directly with R×1 level of the multimeter. When measuring, first connect the black probe to the main electrode T2 and the red probe to the main electrode T1, then use tweezers to make electrode T2 and gate G be short-circuited, and add a positive polarity trigger signal to gate G. If the resistance value measured at this time changes from infinity to more than ten ohms (Ω), it means that the thyristor has been triggered to conduct, and the conduction direction is T2 → T1.Then connect the black probe to the main electrode T1, and the red probe to the main electrode T2. Use tweezers to make electrode T2 and gate G be short-circuited, and add a negative polarity trigger signal to gate G. If the resistance value measured at this time changes from infinity to more than ten ohms (Ω), it means that the thyristor has been triggered to conduct, and the conduction direction is T1 → T2.If gate G is disconnected after the thyristor is triggered to be turned on, the low-resistance conduction state cannot be maintained between electrode T2 and T1 and the resistance value becomes infinite, it indicates that the TRIAC has poor performance or is damaged. If a positive (or negative) polarity trigger signal is added to gate G, the thyristor still does not conduct (the forward and reverse resistance values between T1 and T2 are still infinite), then the thyristor is damaged and has no trigger continuity.For medium and high power TRIAC with a working current of 8A or more, when measuring their triggering capability, one to three 1.5V dry batteries can be connected in series to a probe of a multimeter, and then measure by using R×1 level as described above.For a TRIAC with a withstand voltage of 400V or more, its trigger capability and performance can also be tested by using 220V AC voltage.Figure 7 is a test circuit of a TRIAC. In the circuit, FL is a 60W /220V incandescent bulb, VT is the TRIAC under test, R is a 100Ω current limiting resistor, and S is a button. Figure 7. TRIAC CircuitAfter the power plug is connected to the working frequency AC, the TRIAC is in the off-state and the light bulb is off. (If the bulb is glowing normally at this time, it means that electrode T1 and T2 of the thyristor under test have been broken down and short-circuited; if the light bulb is slightly light, it means that the thyristor under test is damaged by leakage). Press the button S once to provide the trigger voltage signal for gate G of the thyristor. In normal conditions, the thyristor should be immediately triggered to turn on, and the light bulb will glow normally. If the bulb fails to emit light, the internal circuit of the tested thyristor is damaged. If the light bulb is turned on when the button S is pressed, and the light bulb is turned off when the button is released, it indicates that the triggering performance of the tested thyristor is poor.When using a multimeter to detect low-power light-controlled thyristors, put the multimeter in R × 1 level, connect one to three 1.5V dry batteries in series to a black probe, and measure the forward and reverse resistance values between the two pins. Normally it should be infinite. Then use a small flashlight or laser pen to illuminate the light receiving window of the light controlled thyristor. At this time, a small forward resistance value can be measured, but the reverse resistance value is still infinite. In a measurement with a small resistance value, the black probe is connected to the anode A, and the red probe is connected to the cathode K.The following method can also be used to measure light-controlled thyristors. Turn on the power switch S and illuminate the light receiving window of the thyristor VT with a flashlight. After adding a trigger light source (high-power light-controlled thyristor has its own light source, as long as the light-emitting diode or semiconductor laser in its optical cable is added with the working voltage, no external light source is required), the indicator EL should be on. After the light source is evacuated, the indicator light EL should remain illuminated. There is only one PN junction. Therefore, you just need to measure electrode A and G with a multimeter.Put the multimeter in the R × 1 kΩ level, and the two probes can be connected to one of the two pins of the thyristor under test (measure their forward and reverse resistance values). If a pair of pins is measured with a low resistance value, the black probe is connected to the anode A, while the red probe is connected to gate G, and the other pin is the cathode K. (2) Judging whether it is good or bad: Use the R×1 level of a multimeter to measure the forward and reverse resistance values between the electrodes of the BTG thyristor. Under normal conditions, the forward and reverse resistances between the anode A and the cathode K are infinite; the forward resistance between the anode A and gate G (when the black probe is connected to electrode A) is several hundred ohms to several thousand ohms and the reverse resistance value is infinite. If the forward and reverse resistance values between two electrodes are measured to be very small, it indicates that the thyristor has been short-circuited and damaged.(3) Detection of triggering capability: Put the multimeter in the R × 1 Ω level, connect the black probe to anode A, and the red probe to cathode K. The measured resistance should be infinite. Then touch gate G with your finger and add a human body induction signal to it. If the resistance between electrodes A and K changes from infinity to low resistance (a few ohms) at this time, it indicates that the thyristor has a good triggering ability. Otherwise, the performance of the thyristor is poor. Frequently Asked Questions about Thyristors1. What is thyristor and its types?A thyristor is a four-layer device with alternating P-type and N-type semiconductors (P-N-P-N). In its most basic form, a thyristor has three terminals: anode (positive terminal), cathode (negative terminal), and gate (control terminal). The gate controls the flow of current between the anode and cathode. 2. What is thyristor diagram?In general, Thyristors are also switching devices similar to the transistors. ... SCR or Thyristor is a four-layered, three-junction semiconductor switching device. It has three terminals anode, cathode, and gate. Thyristor is also a unidirectional device like a diode, which means it flows current only in one direction. 3. Where is thyristor used?Thyristors may be used in power-switching circuits, relay-replacement circuits, inverter circuits, oscillator circuits, level-detector circuits, chopper circuits, light-dimming circuits, low-cost timer circuits, logic circuits, speed-control circuits, phase-control circuits, etc. 4. Why SCR is called Thyristor?Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor device made of silicon. This device is the solid state equivalent of thyratron and hence it is also referred to as thyristor or thyroid transistor. 5. What is thyristor diagram?In general, Thyristors are also switching devices similar to the transistors. ... SCR or Thyristor is a four-layered, three-junction semiconductor switching device. It has three terminals anode, cathode, and gate. Thyristor is also a unidirectional device like a diode, which means it flows current only in one direction.