The Kynix Blog

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.  

IntroductionLED is a new type of light source that has entered the application field in the past ten years, and it has been used in the medical field for about 6 years, showing excellent therapeutic effects in such a short period of time. LED is a cold light source, which can well moisturize the skin while treating diseases. Besides, since high-power LEDs have strong light intensity and a good effect on the deep layer of the human skin, they are widely used in various departments in hospitals. The LED light sources currently used in the medical field include red, blue and purple. The efficacy of LED in disinfection and sterilization, wound healing, wound treatment, inflammation elimination, edema reduction, and photodynamic tumor treatment has been fully affirmed in the medical field. Can LED Light Improve Your Skin?CatalogIntroductionCatalogI What is LED Light Therapy? Is it Safe?  1.1 LED  1.2 LED Light Therapy  1.3 Safety&Side EffectsII LED Light Therapy Colors: Which do you Need?  2.1 Does LED Light Therapy Actually Work?  2.2 How to Choose the LED Light Therapy Colors?III Specific Uses and Benefits of LED Light TherapyIV Procedure of Performing a LED Light Therapy  4.1 Perform a LED Light Therapy at a Professional’s Office  4.2 How to use At-home Devices?V LED Light Face Mask  5.1 Does LED Light Face Mask Work?  5.2 Are LED Face Masks Safe?VI Frequently Asked QuestionsI What is LED Light Therapy? Is it Safe?1.1 LEDLED is the abbreviation of Lighting emitting diode, which is a light-emitting diode, which is made of compounds containing gallium (Ga), arsenic (As), phosphorus (P), nitrogen (N), etc. Semiconductor. When electrons and holes are recombined, they can radiate visible light, so they can be used to make light-emitting diodes. The gallium arsenide diode emits red light, the gallium phosphide diode emits green light, the silicon carbide diode emits yellow light, and the gallium nitride diode emits blue light. Light-emitting diodes appeared as early as 1962. In the early days, it could only emit low-light red light, and later developed other monochromatic light versions. The light that can be emitted today has reached visible light, infrared light and ultraviolet light, and the light level has increased to a considerable level Degree.Figure1. LED 1.2 LED Light TherapyFor many years, scientists have studied how the sun's rays, including so-called burning rays of the sun, or ultraviolet B radiation, ultraviolet A rays, or UVA, affect the skin. Only recently have we started to talk about the effects of visible light on the skin — not necessarily LED light, but visible light in general," says Dr. Buzney, assistant professor of dermatology at Harvard Medical School. LED lights have been around since the 1960s, but have only recently been used as a skin treatment that uses varying wavelengths of light, including red, purple, green and blue. Different wavelengths of the visible light spectrum correspond to different colors of LED light and penetrate the skin to different depths. Depending on how deeply they penetrate, LED lights are thought to have different biological effects.1.3 Safety&Side EffectsAccording to Harvard Health Publishing, for the most part, these LED light therapies appear to be relatively safe, at least in the short term. The FDA has approved some products for home use. LED skin devices don't have a lot of power, so they're unlikely to burn our skin. However, it is important to shield your eyes from the light while using them. In addition, a recent study named Phototherapy with Light Emitting Diodes indicates that a device using LEDs with frequencies of 415nm (blue), 633nm (red), and 830nm (infrared) has demonstrated significant results for the treatment of medical conditions, including mild-to-moderate acne vulgaris, wound healing, psoriasis, squamous cell carcinoma in situ (Bowen’s disease), basal cell carcinoma, actinic keratosis, and cosmetic applications.  Although photodynamic therapy with the photosensitizer 5-aminolevulinic acid might cause stinging and burning, phototherapy is free of adverse events. We determined that phototherapy using LEDs is beneficial for a range of medical and aesthetic conditions encountered in the dermatology practice. This treatment displays an excellent safety profile. The above content is extracted from the journal directly, Let’s put it simple:● In the short term, LED light therapies are relatively safe, but you have to care about the device's potential to damage the eyes, especially for people with underlying eye conditions or those who are taking medication that makes the eyes more sensitive to light.● Light Therapy doesn't use UV light so there's no risk of tanning whatsoever. Therefore, you do not have to worry for regular use.● LED light therapy doesn’t cause burns compared to other anti-aging treatments such as chemical peels, dermabrasion, and laser therapy. In general, side effects are rare, but there may be(1) increased inflammation(2) Redness(3) Rashes(4) TendernessYou should not use LED therapy if having taken certain medications, such as isotretinoin (Accutane), for acne or use topical treatments that cause sensitivity to sunlight. People with skin conditions should speak to a dermatologist before using LED light therapy.Figure2. LED Light TherapyII LED Light Therapy Colors: Which do you Need?2.1 Does LED Light Therapy Actually Work?The light therapy effect of LEDs is based on the wavelengths of different spectra. Red light LEDs of 633nm to 660nm can promote collagen proliferation of the skin and even lighten fine lines and dark spots. Near-infrared LEDs with a wavelength of 780nm to 1200nm can be used Anti-inflammatory and analgesic. The blue LED and the green LED can also be used in the medical beauty market, which are respectively suitable for preventing the growth of acne and brightening, helping to absorb nutrients in skin care products, and improving the effect of allergic skin. Laura Ferguson, founder of The Light Salon, a company focused on LED beauty, said that LED light therapy has many benefits, different effects at different wavelengths, such as anti-wrinkle or blemish, and the light will not have a band that will damage the skin Intrusive, painless light therapy, it even has a mood-relieving effect. Ferguson said that LED can also solve the problem of skin sensitivity. After light therapy, the newly stimulated cells will have a protective layer to help prevent sunburn from being exposed to strong sunlight. LED light therapy is not like a solarium, so you don't have to worry about melanin precipitation. However, the current price of LED medical beauty products is still high. In the future, the price is expected to decline as the technology of LED medical products matures. At that time, consumers' choices and needs will be more and more diversified. Figure3. Four Types of LED Light2.2 How to Choose the LED Light Therapy Colors?LED light stimulates fibroblasts, which can produce collagen, thereby improving skin elasticity, scarring and enhancing skin metabolism. It can also make use of the organism's absorption of different visible light wavelengths to stimulate the mitochondria and glands to produce more energy and extend the life cycle of cells. LED light can be used in all parts of the human body, whether it is psychological or physical, scalp or toes, internal medicine or surgery, as long as the light reaches the place, it can be used for effective treatment. After laser and pulsed light, LED light therapy has become a new light therapy. (1) Red lightThe red light with a wavelength of 635nm has the characteristics of high purity, strong light source, and uniform energy density. Red light has bactericidal, repairing and pain-removing effects, which can increase cell activity and promote cell metabolism. (2) Blue LightBlue light with a wavelength of 405, 415nm has a strong bactericidal effect, which can quickly inhibit inflammation. During the formation of acne, it is mainly caused by Propionibacterium, and blue light can cause no damage to skin tissue, effectively destroy this bacteria. (3) Purple LightThe violet light with a wavelength of 313, 410nm has the advantages of sterilizing, purifying the skin, activating cells, and promoting protein and collagen synthesis. It has a good effect on specific dermatitis, vitiligo, scleroderma and so on. (4) Green Light (560nm)Natural and soft light color, which has the effect of neutralizing and stabilizing nerves, can improve anxiety or depression; regulate skin gland function, effectively dredge lymph and improve edema, improve oily skin, acne, etc. Figure4. Function of Diffrent Light III Specific Uses and Benefits of LED Light Therapy(1) PhotorejuvenationPhoton skin rejuvenation technology is defined as non-exfoliation skin rejuvenation treatment using low-energy density under continuous high pulse light. At present, it has become one of the main methods to improve skin photoaging. This technology can significantly improve skin wrinkles and texture.  Rough, irregular pigmentation and enlarged pores.Such phenomena have been recognized by many technical professionals and beauty applicants. The characteristic histological changes of skin photoaging are elastic fibrosis and collagen fiber maturation disorder in the dermal matrix, which leads to skin relaxation and wrinkles. The study found that visible to near-infrared LED light penetrated the epidermis of the skin and reached the dermal layer of the skin, and promoted the regeneration and rearrangement of elastic fibers and collagen fibers through photothermal and photochemical effects, thereby reducing skin wrinkles and increasing elasticity. (2) Prevent or treat pigmentation after inflammationPigmentation of skin after inflammation is a common and difficult to avoid phenomenon in skin physical and chemical cosmetology, and it is especially easy to appear in Asian people. For example, in order to reduce the degree of skin pigmentation after laser treatment in the clinic, generally avoid the season of excessive ultraviolet rays, ask patients to avoid sun, apply sunscreen, etc., but it is still difficult to completely avoid pigmentation after inflammation. Recent studies have found that 660nm LED light can prevent or even treat skin pigmentation that occurs after this inflammation, which will be a new research hotspot in the skin and beauty industry. (3) Promote wound healingIt can be seen that LED light of various wavelengths in the near infrared can promote the growth of epithelial cells after trauma, and promote wound healing. At the same time, it also has a good therapeutic effect on the healing of chronic ulcers in the lower extremities of diabetic patients. (4) Reduce inflammationA series of studies have shown that LED has anti-inflammatory effects. Research has found that 635nm LED light can inhibit the release of the inflammatory mediator prostaglandin E2 (PGE2) by the fibroblasts of the gum, thereby reducing the inflammatory response of the gum. Before using pulsed dye laser to treat skin photoaging, if LED light source is used to irradiate in advance, it can reduce the discomfort of skin erythema, swelling and pain caused by dye laser. The use of LED light sources in advance of radiation therapy for breast cancer patients can reduce the side effects of radiotherapy. (5) Scar preventionKeloid keloids is a skin disorder that affects beauty and is difficult to treat clinically. It is caused by excessive proliferation of connective tissue after skin damage. Patients often have a scarring constitution. It started clinically as a small, hard red pimples, which slowly increased, producing round, oval, or irregular scars, higher than the skin surface, extending outward in the shape of a crab foot, with smooth and shiny skin, which may be accompanied by Pain, itching, etc. The clinical treatment is difficult and the effect is not ideal. Studies have found that LED can significantly improve the patient's pain, itching and other discomfort, make the scar flat, and have the advantages of non-invasive. (6) Other functionsIn addition, LED can also be used as a non-ultraviolet light therapy instrument, used in photodynamic therapy, hair loss treatment, skin damage reduction after ultraviolet irradiation, and so on. In short, LED as a new type of light source has been gradually applied to dermatology. With the continuous innovation of LED lamps and the study of the biological effect of LED on medicine, the application of LED in dermatology will have unlimited prospects. . At the same time, LEDs have higher security and can be used more widely as home medical equipment.Figure5. LED Light Therapy DevicesIV Procedure of Performing a LED Light Therapy4.1 Perform a LED Light Therapy at a Professional’s OfficeWhen getting an LED treatment, you really don’t have to do much but lie in a comfy bed with your face positioned directly under panels that emanated different colored lights. Generally, LED panels will be placed a few inches away from your face after microneedling or microdermabrasion. Each session lasts approximately 15-20 minutes. At first, it feels warm, and many people report it is really like the feeling of relaxation.4.2 How to use At-home Devices?Using at-home light therapy devices is like working out by yourself and things done in the office are like working out with a trainer. Both are good. But you’re not going to get as intense of a treatment at home. This means at-home LED devices may be more convenient, but they may be less effective than professional treatments. When using an at-home device, it is important to follow the manufacturer’s instructions. These devices typically come in the form of a mask that a person applies to the face for several minutes or a wand that they use on the skin. LED light therapy is suitable for use on any body part, including the face, hands, neck, and chest.Following treatment, no recovery time is necessary.Does LED Light Therapy Really Work? Can You Do it At HOME?V LED Light Face Mask5.1 Does LED Light Face Mask Work?The LED mask was invented by John Tsagaris, a Chinese medicine doctor. It is understood that John has a degree in human biological sciences from Chinese Medicine, and also holds a graduate diploma in skin treatment and beauty care. LED blue light can play a bactericidal and anti-inflammatory effect to improve the surface skin, and has a good effect on the treatment of acne and rosacea. LED red light can promote the growth of skin collagen, for deep skin beauty care. The working principle of LED mask is similar to photosynthesis. It treats skin folds from the depth by changing the energy of LED red light. It can not only calm the skin, but also prevent the growth of bacteria inside the skin. The use of LED mask masks not only does not have a claustrophobic feeling, but the LED red light delivered to the skin in large quantities can make people feel comfortable. Just wear the LED mask for 25 minutes every day, and the LED red light can gradually improve your skin. According to a survey report from the United States, LED masks have a significant effect on eliminating wrinkles. Another report showed that 90% of people believe that LED mask can reduce skin aging, make the skin more delicate and smooth, and greatly improve the crow's feet, red blood, and melanin deposition in the corners of the eyes.Figure6. Depth of Light Energy Penetration5.2 Are LED Face Masks Safe?Actually, it is the same concern as whether LED light therapy is safe. There’s no doubt that one of the most important aspects of LED phototherapy devices is their safety. Phototherapy with Light Emitting Diodes also points out that LEDs are nonablative and nonthermal, and when used alone (i.e., without topical photosensitizers in PDT applications) do not cause damage to the epidermis or dermal tissue.  There are no adverse events associated with the use of these devices and little to no downtime for the patient. When LED phototherapy is used alone, patients do not experience redness, peeling, blistering, swelling, or pain. In fact, patients can have a treatment during their lunch hour and return to work immediately afterwards. While home use devices have been available for several years, there are many differences between those devices and those specifically designed for use by physicians. The home use devices necessarily deliver significantly less power and typically do not have light panel arrays large enough to treat the entire face at once, for example. As they often are hand-held, it might be cumbersome, time-consuming, and impractical to treat the entire face in a single session.  In contrast with the medical LED units and their protocols, home use devices have not been validated by controlled clinical studies published in peer-reviewed journals. In some cases, home units may be used adjunctively with dermatologist-provided treatment to address specific areas of concern, but they are dissimilar enough from the medical-grade units to not be considered an alternative to these tested technologies.Figure7. LED Face MasksVI Frequently Asked Questions1. Does LED light therapy actually work?LED light therapy appears to be a safe treatment for several skin conditions, including acne, skin aging, skin wounds, and other problems. Research indicates that this therapy offers promising results, although people should not expect a 100% improvement. 2. What does LED light therapy do?LED light therapy is now used by some aestheticians to help regenerate the skin from aging. It's also used for acne. Your healthcare provider uses red or blue light frequencies based on the skincare concern. Red is primarily used for anti-aging, while blue is used for acne treatment. 3. Does LED light reduce wrinkles?LED light therapy can stimulate collagen production, which reduces fine lines and wrinkles, as well as eliminate acne-causing bacteria, which improves skin clarity. 4. Can you overdo LED light therapy?Light therapy cannot be overdone for most people. If you notice any extraordinary results, stop treatment, and contact your physician. For the best results, choose the right device style and LED color, and use it as directed. 5. Does red light therapy tighten loose skin?Amber light stimulates collagen and elastin. Red light is most commonly used to promote circulation. White light penetrates the deepest and works to tighten and reduce inflammation. Blue light kills bacteria. 6. Are LED lights bad for your eyes?New findings confirm earlier concerns that "exposure to an intense and powerful [LED] light is 'photo-toxic' and can lead to irreversible loss of retinal cells and diminished sharpness of vision," the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) warned in a statement. 7. Are led masks effective?The research behind LED masks is centered on the lights used, and if you're going on those findings, LED masks can be beneficial to your skin. 8. What are the side effects of red light therapy?Red light therapy is considered safe and painless. However, there have been reports of burns and blistering from using RLT units. A few people developed burns after falling asleep with the unit in place, while others experienced burns due to broken wires or device corrosion. 9. Is red light therapy the same as laser therapy?In clinical practice, low-level laser (LLT) therapy involves exposing tissues to red and near infrared (NI) light, which are lower in energy than the lasers used in surgery. 10. How long does it take for light therapy to work?Light therapy can start to improve symptoms within just a few days. In some cases, though, it can take two or more weeks.

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. 

Ⅰ IntroductionFlyback Diodes, which are also known as freewheeling diodes, generally refer to diodes that are inversely paralleled across the ends of energy storage elements such as inductors, relays, and thyristors. When a voltage or current changes suddenly in a circuit, it protects other components in the circuit. When using a flyback diode, the circuit current can be changed more gently to avoid the occurrence of voltage spike. This article will introduce in detail what is flyback diode, how freewheeling diode works, flyback diode selection and the flyback diode function.How Freewheeling Diode WorksCatalogⅠ IntroductionⅡ DesignⅢ How It Works?Ⅳ SelectionⅤ Applications5.1 Summary5.2 In Forward Switching Power Supply5.3 In Converter Technology5.4 In Unidirectional Half Wave Silicon Control Rectifier Circuit5.5 In BUCK CircuitⅥ Something Has to CareIn electronics, a flyback voltage or an inductive flyback is a voltage spike created by an Inductor when its power supply is removed abruptly. The reason for this voltage spike is the fact that there cannot be an instant change to the current flowing through an Inductor.In addition, time constant of the inductor determines the rate at which the current can change through an inductor. This is similar to the time constant of a capacitor, which determines the rate at which its voltage can change.The freewheeling diode is named because it plays the role of freewheeling in the circuit. It is generally used in the circuit to protect components from being damaged or burned out by voltage breakdown, connected in parallel to both ends of the elements that generate the induced electromotive force(EMF), and form a loop with them, so that the high electromotive force generated in the loop is consumed by the continuous current method, thereby protecting the components in the circuits.Flyback diodes are connected in parallel at both ends of the coil. When the current passes through the coil, it will generate induced electromotive force at both ends. When the current disappears, its induced electromotive force generates a reverse voltage to the components in the circuit. When the reverse voltage is higher than the reverse breakdown voltage of the elements, it will cause damage to the elements such as triode and thyristor. When the current flowing through the coil disappears, the induced electromotive force generated by the coil is consumed by the work formed by the diode and the coil, thereby protecting the other elements in the circuit.Ⅱ DesignIn the following figure, it is showed that a flyback diode is placed across the inductor. An ideal flyback diode will have a very large peak forward current; capacity which helps in handling the voltage transients from damaging the diode, and inductor’s power supply is suited for reverse breakdown voltage and low forward voltage drop. Voltage spike can be 10times to the voltage of power supply which depends on the equipment involved and the application. So it is understood that not to underestimate the energy which contain within an energized inductor. Figure 1. Flyback DiodeFor an ideal flyback diode selection, a diode which has very large peak forward current capacity (to handle voltage transients without burning out the diode) should be selected, moreover, low forward voltage drop, and a reverse breakdown voltage fitted the inductor's power supply. Depending on the application and equipment in real requirement, some voltage surges can be upwards of 10 times the voltage of the power source, so it is critical not to underestimate the energy contained within an energized inductor.Flyback Diode Selection Note You Should KnowWhen used with a DC coil relay, a flyback diode can cause delayed drop-out of the contacts when power is turned off, due to the continued circulation of current in the relay coil and diode. When rapid opening of the contacts is important, a small value resistor can be placed in series with the diode to help dissipate the coil energy faster, at the expense of higher voltage at the switch.Schottky diodes are preferred in flyback diode applications as switching power converters, because they have the lowest forward drop (~0.2V rather than >0.7V for low currents) and are able to quickly respond to reverse bias (when the inductor is being re-energized). They therefore dissipate less energy while transferring energy from the inductor to a capacitor.When the flyback diode is used to simply dissipate the inductive energy, as with a solenoid or electric motor, cheap 1N540x and 1N400x general-purpose diodes are used instead. Ⅲ How It Works?Flyback diodes are often used with energy storage elements to prevent sudden changes in voltage and current to provide a pathway. The inductor can provide continuous current to the load through it to avoid sudden changes in load current and smooth the current. In the switching power supply, you can see a freewheeling circuit composed of a diode and a resistor connected in series, which is connected in parallel with the primary side of the transformer. When the switch is turned off, the freewheeling circuit can release the energy stored in the transformer coil to prevent the induced voltage from being too large and breakdown the switch. Generally, it is often to choose the fast recovery diode or the Schottky diode as flyback diode.Circuit Expressions Figure 2. Flyback Diode in Switching Power Supply CircuitIn Figure 2(c), when KR is turned on, the upper is positive voltage and the lower is negative voltage,  and the current direction is from top to bottom. When the VT is turned off, the current in the KR is suddenly interrupted and an induced potential is generated. The current direction is kept constant, that is, keeping the KR current direction from the top to bottom, which based on the Lenz's law. The induced potential and the power supply voltage are superimposed and applied across the VT, making it easy for the VT to breakdown. To avoid it, VD is used to short-circuit the induced potential generated by KR, that is, The current flows clockwise in the small circuits of the diodes and relays to protect the VT. R and C in Figure 2(b) also use the principle that the voltage on C cannot be abruptly changed to absorb the induced potential.In short, the flyback diode is connected in parallel to the relay or the inductor at both ends of the circuit. When the inductor is powered off, the electromotive force at both ends does not disappear immediately. At this time, the residual electromotive force is released through a freewheeling diode to reverse the reverse generated by the coil (the EMF is consumed in the form of current). It can be seen that the freewheeling diode is not a substantial component, but plays a "freewheeling" role in the circuit.For example, reversely connect a flyback diode at both ends of a relay coil or at both ends of a unidirectional thyristor. In practice, electromagnetic relays are usually controlled by triodes or MOS tubes to achieve automatic control of electrical loads (such as through a single-chip microcomputer), and the coil of the relay is a large inductance, which can store electrical energy in the form of a magnetic field. So when it pulls in, it stores a lot of magnetic field. When the triode controlling the relay changes from on to off, the coil is powered off, but there is a magnetic field in the coil. At this time, the back electromotive voltage can be as high as 1000v to destroy other circuit components. This is because the access of the diode is exactly the same as the direction of the reverse electromotive force. So that the reverse potential is neutralized by the freewheeling diode in the form of current to protect other circuit components. In addition, it is generally a diode with a fast switching speed.  Figure 3. Freewheeling Diode CircuitBecause the relay coil exists inductive load, which will absorb the self-inductive voltage of the relay coil when the triode is turned off. According to Lenz's law, when the current on the inductor decreases, a self-inductive voltage is generated. The direction of this voltage is that the forward terminal is negative and the collector of the driving tube is positive. This voltage will break through the triode, so an freewheeling diode is connected in parallel with the relay to absorb this self-inductive voltage.1) The influence of the time parameter of the circuit below the ms level on the mechanical contact is ignored.2) Even the 1N4000 reverse recovery time is far below the ms level, and the forward conduction time is shorter.3) Capacitance between the driving tubes and parasitic capacitance of the relay is enough to disable the high-speed diode.4) The consumption of inductive energy storage mainly depends on the winding resistance, which is generally in an overdamped state.It is general to use transistors as switches. As shown in Figure, a transistor TR1 is used to control the conduction of the relay coil, and the relay contact is used to control the load circuit.In a thyristor circuit, the thyristor is generally used as a contact switch, if a large inductive load is controlled, a high-voltage back electromotive force will be generated, and the principle is the same as that of a relay.Flyback diode also used on displays coils commonly used in relays. It is often used with energy storage elements to prevent sudden changes in voltage and current and provide a path. The inductor can provide continuous current to the load to avoid sudden changes in load current and smooth the current. In the switching power supply, it is common to see a freewheeling circuit composed of a diode and a resistor connected in series. The following circuit is connected in parallel with the primary side of the transformer. Figure 4. Flyback Diode in Relay CircuitThe freewheeling diode is added to both ends of the inductive load, and the inductive here is to have an inductive characteristic. The characteristic of the inductive load is that the current cannot be abruptly changed, in other words, it can't be all of a sudden. Common inductive loads include relay coils and solenoid valves.Figure 5.  Typical Freewheeling CircuitThe Figure 5 shows the typical application circuit of the flyback diode, where the resistor R determined whether it is needed or not. When the energy storage element VT is turned on, the upper voltage is positive, and the lower voltage is negative, and the current direction is from top to bottom. When the VT is turned off, the current in the energy storage element is suddenly interrupted, and an induced potential is generated at this time. This induced potential and the power supply voltage are superimposed and applied to both ends of the VT, which can easily cause VT to break down. VD can be added for this purpose, so that the induced potential generated by the energy storage element can be short-circuited to achieve the purpose of protecting the VT. Ⅳ Selection1) Based on working voltage2) Based on working current1N4007 is a not bad choice but not the best, because the PLC may be damaged before the diodes have time to play the freewheeling effect. Therefore, it is best to use FR107 to protect the freewheeling circuit, which can better protect the PLC output interface, and the cost will not rise too much. It is also possible to choose IN5819 or IN5817, which has better performance than FR107, but the cost is a little higher. Ⅴ Applications5.1 SummaryFlyback diodes are usually used with energy storage elements, and their role is to prevent sudden changes in voltage and current in the circuit and provide a power-consuming path for reverse electromotive force. The inductive coil can provide continuous current to the load through EMF, so as not to change the load current and smooth the current. In the switching power supply, a freewheeling circuit always composed of a diode and a resistor connected in series. This circuit is connected in parallel with the primary side of the transformer. When the switch is turned off, the freewheeling circuit can release the energy stored in the transformer coil to prevent the induced voltage from being too large and breakdown the switch.5.2 In Forward Switching Power SupplyIn the forward switching power supply, when the MOS is turned off, the secondary side of the transformer provides current to the outside by the energy stored in the inductor. In order to make the inductor play this role under load, a freewheeling diode is added on the secondary side of the transformer. The inductor, load, and freewheeling diodes create paths to transfer the energy in the inductor to the outside.5.3 In Converter TechnologyIn the electronic converter circuit, the single-phase bridge rectifier in the rectification section is the single-phase rectifier circuit with the most practical applications. And three-phase bridge rectification is the most widely used method for power systems, especially generator excitation systems. Both of these circuits must be connected to a flyback diode. Its function is almost the same. Take a single-phase bridge circuit as an example: When the rectifier bridge is connected to an inductive load, because the inductor current cannot be abruptly changed, during the thyristor off time, it must connect freewheeling diode at both ends of the load to provide a smoothing path  to prevent dangerous overvoltages across the inductive load, and also the thyristor can be commutated to conduct.The three-phase bridge rectifier circuits used in generator excitation systems are divided into three-phase half-control bridges and three-phase full-control bridge circuits. Therefore, in order to ensure reliable commutation of the rectifier components, the half-control bridge needs to connect flyback diodes in parallel at both ends of the inductive load, while the full-control bridge does not need to do so. In addition, when the conduction angle is changed, the average voltage and line current of the half-controlled bridge change more slowly than the full-controlled bridge.At present, current converters such as rectifiers and inverters are now used in a large number of devices, in which flyback diodes are typically added to the internal DC bus of the converter. Because if the load is an inductive element, when a large-capacity inverter on the bus fails, the DC bus will generate huge reverse surge energy. At this time, it is necessary to provide a discharge channel for this energy, otherwise it will break down or burn the converter. This channel needs a diode to form, that is a flyback diode.5.4 In Unidirectional Half Wave Silicon Control Rectifier CircuitFor unidirectional half-wave silicon control rectifier circuit with large inductive load, when the  silicon control is turned off in the negative half cycle, the inductive load will generate a high reverse induced electromotive force. This reverse electromotive force is sufficient to cause the silicon control to break down and burn. After that, the reverse electromotive force can be discharged into the forward voltage drop of the diode (about 0.7V), thereby effectively protecting the circuit components. 5.5 In BUCK Circuit Figure 6. BUCK CircuitIn the BUCK circuit, fast recovery diodes or Schottky diodes are generally selected as freewheeling diodes. It is generally used in the circuit to protect components from being broken down or burned by induced voltage. The two ends of the element form a loop with it, so that the high electromotive force generated in the loop is consumed in a continuous current manner, thereby protecting the elements in the circuit.In theory, the diode is selected at least 2 times the maximum current. In actual use, due to the strong transient overload resistance of the diode, an ultra-fast diode with a maximum current of 50A can also be used. In addition, a reasonable heat sink generally has little damage in actual use. The total impedance when conducting is the internal resistance of the motor plus the equivalent internal resistance of the drive tube. And the total impedance during freewheeling is the internal resistance of the motor plus the equivalent internal resistance of the freewheeling diode. In general, the AC equivalent internal resistance of the freewheeling diode is smaller than the AC equivalent internal resistance of the driving transistor. Therefore, in conventional design, the maximum current of the freewheeling diode is generally doubled to the maximum current of the motor.The transient current is only a moment, and the anti-overload capability of the surface-contact diode is enough, as long as it is not used in overvoltage, if necessary, a small resistor can be connected in series to limit the current. The flyback diode is to protect the switching device. The transient current during freewheeling is related to the working voltage of the motor and the internal resistance of the winding, and has nothing to do with the power of the motor. If necessary, the peak value of the transient current is the reverse self-inductance voltage minus diode junction voltage drop and then divided by the loop resistance. The reason why a diode with a certain current used is because the internal resistance of the winding of the low-voltage high-power motor is low, so the transient current will be relatively large. A series of small resistors can suppress the peak current, the transient voltage of the switch tube rises slightly because the operating voltage is not high, and now the current withstand voltage of transistors is at least 50V or more. Ⅵ Something Has to CareFreewheeling diodes are commonly used in switching power supplies, relay circuits, thyristor circuits, IGBTs, and other circuits. They are widely used, so it is necessary to pay attention to the following points when using them: 1) Fylback diode is an effective method to prevent the high voltage generated by self-inductive potential from causing damage to related components when the DC coil is powered off.2) The polarity of the flyback diode must not be connected wrongly, otherwise a short circuit situation will be caused.3) The flyback diode is always reversed to the DC voltage, that is, the negative pole of the diode is connected to the positive pole of the DC power supply.4) The flyback diode works in the forward conduction state, not in the breakdown state or the high-speed switching state, that is, the flyback diode does not used in electrical breakdown, recoverable situation, but its unidirectional conduction effect is the key point.5) Zener diodes can't be regarded as flyback diode. Because the zener diodes use reverse characteristics, and the flyback diodes use forward characteristics. Frequently Asked Questions about Flyback Diode or Freewheeling Diode1. What is a flyback diode?A flyback diode is a diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. 2. What is the role of freewheeling diode?A Flyback diode is also called as freewheeling diode. ... Here catch diode is used to eliminate flyback, when the abrupt voltage spike is witnessed across the inductive load when the supply current abruptly reduced. It helps the circuit from damaging. 3. What is a flyback diode used for?A flyback diode is a diode connected across an inductor used to eliminate flyback, which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted. 4. How does a flyback diode work?The Flyback diode makes inductor to draw current from itself in a loop until the energy is dissipated in diode and wires. When the current flow to an AC induction motor is suddenly interrupted, then the inductor tries to maintain increasing the voltage and the current by reversing polarity. 5. How do you choose a freewheeling diode?The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating. A diode in your application having 50 volts would be more than adequate. Again 1N4001 will do the job. 6. How do I choose a flyback diode for a relay?Specify a diode for at least 79.4 mA current. In your case, a 1N4001 current rating far exceeds the requirement. The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating. 7. What are the advantages of freewheeling diode?What are the advantage of free wheeling diode in a Full Wave rectifier? It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load. 8. Why freewheeling diode is used in controlled rectifier?When the inductive circuit is switched off, this diode gives a short circuit path for the flow of inductor decay current and hence dissipation of stored energy in the inductor. This diode is also called Flywheel or Fly-back diode. circuits, inverter circuits, and chopper circuits by making it continuous. 9. What is the effect of adding free wheeling diode?It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load. 10. What is the use of freewheeling diode in converter circuit?A free wheeling diode is used in converter circuits . It is connected across the load. During positive cycle of input it is reverse biased. During negative cycle of input the diode conducts and the energy stored in the circuit inductor during the previous half cycle is delivered to the load itself.

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.