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IntroductionA relay is an electronic control device, which has a control system (also called an input loop) and a controlled system (also called an output loop). It is often used in automatic control circuits. In fact, it is an automated switch using a smaller current to control a larger current. Therefore, it plays the role of automatic adjustment, safety protection, and converter in the circuit. Relay has the features of fast response speed, stable work, long service life and small size. In order to ensure that these performances can be better played, the test and maintenance of the relay (solid state relay) are particularly important. This paper will introduce main relay test parameters, how to test a relay, an example of an automotive relay test.Testing a RelayCatalogIntroductionⅠ Understanding Relays  1.1 Relay ParametersⅡ How to Test A Relay?  2.1 General Test Ideas  2.2 Types of Relay TestⅢ Relay for Life: Automotive Relay Test  3.1 Automotive Relay  3.2 Common Faults of Automotive Relays  3.3 Detection Method  3.4 Specific OperationⅣ One Question Related to Relay Test and Going Further  4.1 Question  4.2 AnswerⅤ Frequently Asked Questions about Relay TestⅠ Understanding Relays1.1 Relay ParametersMain relay parameters include rated working voltage, rated working current, coil resistance, contact load, etc.1) Rated working voltage refers to the voltage required by the coil when the relay is working normally. For DC relays it refers to DC voltage (Figure a), and for AC relays it refers to AC voltage (Figure b). Relays of the same type often have multiple assessed operating voltages for circuit requirements, and the specification number is added to the end of the component to distinguish.Figure 1. Relay Symbol2) The rated working current refers to the current required by the coil when the relay is working normally.Coil resistance refers to the DC resistance of the relay coil. When selecting a relay, you must ensure that it is rated working voltage and rated working current meet the requirements.Figure 2. Rated Working Current3) Contact load refers to the load capacity of the relay contact, also known as the contact capacity. For example, the contact load of the jzx-10m relay is:  DC 28v×2a or AC 115v×1a. When used, the voltage and current passing through the relay contact should not exceed the rated value, otherwise, the contact will be burned out and the relay will be damaged. A load of multiple sets of contacts of a relay is generally the same.Figure 3. Contact LoadRecommended Reading: Basic Knowledge of Relay Electronics Tutorial with Video    The Role of the Relay and Its Working Principle Ⅱ How to Test A Relay?Relays are widely used in power protection, automation, sport, remote control, measurement and communication devices, so it is very important to check and maintain the normal operation of relays. There are many types of relays. Therefore, the inspection of relays cannot only be judged by measuring the resistance value of the coil. It is necessary to adopt multiple detection methods according to different relay types.2.1 General Test Ideas1) measuring contact resistanceApply the specified working voltage to the relay coil, and use a multimeter to detect the on-off condition of the contact at the “R×1k” gear. When the power is not applied, the normally open contact does not work, and the normally closed contact conducts. When the power is turned on, you should be able to hear the pick-up sound of the relay. At this time, the normally open contact is conducting and the normally closed contact is opposite, and the switching contact should be switched accordingly. Otherwise, the relay is damaged. For multi-group contact relays, if some of the contacts are damaged, the remaining contacts can still be used.Figure 4. Relay Test 2) measuring coil resistanceThe resistance value of the relay coil can be measured with the multimeter at R×10Ω gear, so as to determine whether the coil is open. 3) measuring of pull-in voltage and currentUse an adjustable regulated power supply to input a set of voltage to the relay, and connect an ammeter in the power supply circuit to monitor. Increase the power supply voltage slowly, and when you hear the pull-in sound of the relay, write down the voltage and current. In order to be accurate, you can try several times to get the average value. 4) measuring the release voltage and currentSame test connection like the above. When the relay pulls in, then gradually reduce the supply voltage. When you hear the relay release sound again, write down the voltage and current at this time. Try several times to get the average release voltage and release current. Under normal circumstances, the release voltage of the relay is about 10-50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), it can't be used normally, which will affect the circuit stability resulting in abnormal operation. 2.2 Types of Relay TestElectromagnetic Relay TestFigure 5. Electromagnetic RelayThe multimeter is placed in the “R×100” or “R×1k” gear, and the two test leads (regardless of positive and negative) are connected to the two pins of the relay coil (shown in Figure 5). The indication of the multimeter should basically match the coil resistance of the relay. If the resistance value is obviously too small, it means that the coil is short-circuited locally; if the resistance value is 0, it means that there is a short circuit between the two coil pins; if the resistance value is infinite, it means that the coil is open or the pins are disconnected. Reed Relay TestReed relay is also one of the most commonly used relays. It consists of a reed switch and a coil, as shown in Figure 6. The reed switch is made by sealing two non-interconnected ferromagnetic metal strips in a glass tube, and the reed switch is placed in the coil. When the current passes through the coil, the magnetic field generated by the coil magnetizes the metal strips in the reed pipe, and the two metal strips attract due to opposite polarities to connect the controlled circuit. Several reed pipes can be placed in the coil, and they will act simultaneously under the action of the coil's magnetic field.Figure 6. Reed Relay Reed relay has a pair of coil pins and several pairs of reed switch pins, and there are corresponding marks on the shell for identification.Figure 7. Reed Relay Reed relays can also use a multimeter to detect their coils and contacts, and the detection method is the same as that of electromagnetic relays.Figure 8. Reed Relay Solid State Relay (SSR) TestThe input end can be tested with a multimeter. The multimeter is placed in the "R×10k" gear, the black test lead (the positive electrode of the battery in the meter) is connected to the positive electrode of the SSR input terminal, and the red test lead (that is, the negative electrode of the battery in the meter) is connected to the negative electrode of the input terminal of SSR. The hands should deflect more than halfway (Figure 9). Re-testing after swapping the two test leads, the hands should not move. If the needle deflects to the top or does not move regardless of the forward or reverse voltage access, the solid-state relay has been damaged.Figure 9. Solid State Relay  SSR You can also make a test circuit according to Figure 10. When the control voltage of the SSR input terminal is turned on, the light-emitting VD is on; when the control voltage of the SSR input terminal is cut off, the light-emitting diode VD is off.Figure 10. SSR Thermal Relay Test1) heating elements detectionThe heating element is composed of an electric heating wire or electric heating sheet, and its resistance is very small (close to 0Ω). The detection is shown in Figure 11. The normal resistance of the three groups of heating elements should be close to 0Ω. If the resistance is infinite (the digital multimeter displays the symbol "1" or "OL" for exceeding the range), the heating element is open.Figure 11. ① 200Ω gear is selected.② The red and black probes are respectively connected to the two ends of a heating element.③ The resistance is close to 0Ω, indicating that the resistor as a heating element is normal. 2) contact detectionThermal relays generally have a normally closed contact and a normally open contact. This detection includes working and non-working conditions. The first picture is the detection of the normally closed contact resistance when it is not in operation. Normally it should be close to 0Ω. Then the detection is taken in the opposite condition. Move the test rod, as shown in the second picture, simulates the over-current heating and bending of the heating element to make the contact action. The normally closed contact becomes an open circuit, and the resistance is infinite.Figure 12. ① 200Ω gear is selected.② The red and black probes are connected to both ends of the normally closed contact.③ The resistance is close to 0 Ω, indicating that the normally closed contact is closed.④ Move the test rod by hand.⑤ The out-of-range symbol "1" is displayed to indicate that the normally closed contact is open. Intermediate Relay TestThe electrical part of the intermediate relay is composed of coils and contacts, both of which use the resistance gear of a multimeter.1) The contact is detected when the control coil is not powered. Contacts include normally open contacts and normally closed contacts. When the control coil is power off, the normally open contacts are open and the resistance is infinite, at this time, the normally closed contacts are closed and the resistance is close to 0Ω. The above-mentioned detection of the normally open contact is shown in the figure below.Figure 13.① 200Ω gear is selected.② The red and black probes are connected to both ends of normally open contact.③ The out-of-range symbol "1" is displayed to indicate that the normally open contact is open. 2) Control coil detection of the intermediate relay is shown in Figure 14. Generally, the greater the rated current of the contact, the smaller the resistance of the control coil. This is because the greater the rated current of the contact, the larger the volume of the contact. Only a small control coil resistance (thicker line diameter) can flow through a larger current to produce a stronger magnetic field suction contact.Figure 14. ① 200Ω gear is selected for the gear switch.② Connect the red and black lead to the two pins of the control coil.③ The display of "6.60" indicates that the resistance of the control coil is 6.6kΩ.3) Power on the control coil to detect the contacts. Apply a rated voltage to the control coil, then use a multimeter to detect the resistance of the normally open and normally closed contacts. The normally open contact should be closed and the resistance should be close to 0Ω; the normally closed contact should be open and the resistance is infinite. Time Relay TestThe detection of time relay mainly includes contact normal state detection, coil detection and coil energization detection.1) Normal-state detection of contacts. It refers to the detection of the resistance of the contact when the control coil is not energized. The normally open contact is open and the resistance is infinite, while the normally closed contact is closed, and the resistance is close to 0Ω. Normal detection processes are shown in the figure below.Figure 15. ① 200Ω gear is selected for the gear switch.② The red and black lead is connected with two pins of a normally closed contact.③ The resistance is close to 0Ω, indicating that the normally closed contact is closed. 2) Detection of control coil. It is shown in Figure 16.Figure 16. ① 20kΩ gear is selected for the gear switch.② Connect the red and black lead to the two pins of the control coil.③ The display of "4.93" indicates that the resistance of the control coil is 4.93kΩ.3) Power on the control coil to detect the contacts. Apply a rated voltage to the control coil, then check whether the contact status has changed according to the characteristics of types of the time relay. For example, for a delay time relay, after a period of time delay, check whether the delay contact is closed (resistance is close to 0Ω) and whether the delay contact is disconnected (resistance is infinite).  Ⅲ Relay for Life: Automotive Relay Test3.1 Automotive RelayRelays are widely used in automotive circuits, such as starting system circuits, wiper circuits, and rear window heating circuits. When the vehicle starts, a larger starting current is required. If the ignition switch is used for direct control, the starting contacts will ignite and burn, which will affect the service life of the ignition switch and even cause serious consequences such as line ablation and fire. Using a relay to control a large current with a small current will not cause the above problems. When a certain voltage or current is applied to both ends of the electromagnetic relay coil, the magnetic flux generated by the coil passes through the magnetic circuit composed of the core, yoke, armature, and the working air gap of the magnetic circuit. Under the action of the magnetic field, the armature attracts the pole face of the iron core, making the normally closed contact opens and the normally open contact close. When the voltage or current at both ends of the coil is less than a certain value, the mechanical reaction force is greater than the electromagnetic attraction force, and the armature returns to the initial state: the normally closed contact is on and the normally open contact is off. One of the automobile relays functions is a switch; the other is load overload protection; the third is fault protection. 3.2 Common Faults of Automotive RelaysIncluding coil burnt, short circuit, insulation part aging, contact ablation, etc.1) Relay MalfunctionWhen the controlled circuit is required to be closed, the relay will not act, on the contrary, when the controlled circuit is not required to be closed, the relay will act. This kind of problem occurs mainly because the interference voltage in the circuit exceeds the allowable range of the drive circuit of the relay. When designing the circuit, pay attention to the factors that can cause interference (such as chip command errors, short circuits, grid fluctuations, etc.). 2) Relay BurnedThere are many reasons for burnout. For example, the actual switching current exceeds the rated switching current of the relay, and the actual inrush current exceeds the rated switching current of the relay. According to design experience, in order to avoid these problems, the rated current should be selected to be 2-3 times the actual switching current, and the impact current of the relay is 2-3 times the actual current. 3) Contact WeldingGenerally speaking, the temperature rise of the AC conversion relay coil is higher than that of the DC conversion relay. This is because of the eddy current loss and hysteresis loss in the magnetic circuit. In addition, when the AC conversion relay is operating at a voltage lower than the rated voltage, a bounce phenomenon may occur. This will cause burnout, welding of contacts and damage to the relay, or disconnection of the self-protection circuit. Therefore, measures must be taken to prevent fluctuations in the power supply voltage.In addition, regardless of the length of the fluctuation time, it will cause the failure of the relay. So ensure that there is a power supply with sufficient capacity. 4) Coil Temperature Rise is Too HighThe loss of magnetic materials such as copper wires and iron cores or the heat transfer of the contacts will cause the temperature rise. Therefore, the heat resistance of the insulating material and the distance between the relay and the heat-generating device should be paid special attention in the circuit design. 3.3 Detection MethodStatic detection: check the resistance of the coil and the resistance of the normally closed contact.Dynamic detection: energize the coil and detect the resistance of the normally open contact. 3.4 Specific OperationTurn on the ignition switch and hear whether there is a pull-in sound in the control relay or feel the relay with your hands for vibration. If so, it means that the relay is basically in routine. The failure of the circuit may be caused by other reasons. On the contrary, it means that the relay is faulty.Replace the relay to be tested with an identical working relay. Turn on the switch, and if the electrical equipment is working normally, it can be determined there is a problem with the relay to be tested.Use the multimeter Rx100Ω gear and combine the resistance of each pin of the circuit to analyze. If the conduction and disconnection are normal, it means that there is no problem with the relay, otherwise, it means the relay is faulty.Open the relay shell to check whether the contacts are ablated or oxidized. If there are bumps and rust on the contact, it means that the contact is ablated or oxidized and does not work properly.Check whether the coil is ablated or discolored. If the coil is ablated with jelly, the coil is black or has a gluey smell, which means the coil is short-circuited by ablation.Ⅳ One Question Related to Relay Test and Going Further4.1 QuestionWhat are the symptoms of a bad car relay?4.2 AnswerThe car suddenly stalls while operating. One of the most common symptoms of a failed ignition relay is a car that suddenly stalls while operating. Car not starting. Another symptom of a faulty ignition relay is a no-power condition.Dead battery. A dead battery is another symptom of a faulty ignition relay.Burned relay. Ⅴ Frequently Asked Questions about Relay Test1. How do you check if a relay is bad?The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 positions in the fuse box where the relay plugs in (or wherever the relay is located). 2. How do you test a 12-volt relay? 3. How do you check an overload relay with a multimeter?CEP7 Overload Relay test proceduresMeasure the normal motor running current (i motor).Turn off the motor and let it cool for about 10 minutes.Calculate the following ratio: i (motor) / i (overload min FLA).Set the overload to its minimum FLA and turn on the motor.Wait for the overload to trip. 4. How do I test a solid-state relay?The SSR can be tested as described below if a load is connected. Connect a load and power supply, and check the voltage of the load terminals with the input ON and OFF. The output voltage will be close to the load power supply voltage with the SSR turned OFF. 5. Can a bad relay drain your battery?Battery drain or dead batteryA failed ECM power relay can also cause a battery drain or a dead battery. If the relay shorts, it can leave power on to the computer, even when the vehicle is turned off. This will place a parasitic drain on the battery, which will eventually cause it to go dead. 6. What happens when the main relay goes bad?The engine will not startIf the main relay is not supplying the engine computer with the power it needs, then the engine will not be able to crank and run the right way. Failing to get the main relay replaced will usually lead to the car being unusable. 7. How do you test a battery relay? 8. How do you test a protection relay?Protection relay self-test procedureThis will normally involve checking the relay watchdog circuit, exercising all digital inputs and outputs and checking that the relay analog inputs are within calibration by applying a test current or voltage. 9. How do you check if a relay is working?The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 positions in the fuse box where the relay plugs in (or wherever the relay is located). 10. How do you test an electromagnetic relay?Grab a multimeter and set it to Ohms. Touch the leads across the electromagnet coil pins and measure resistance. Anywhere from 50-120 ohms is OK. Out of range or open means a bad electromagnet coil winding and time for a new relay.

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.