How to use a digital multimeter to judge the quality of general capacitors
1. Detection of small capacitors below 10pF: Because the capacity of fixed capacitors below 10pF is too small, measuring with a multimeter can only qualitatively check whether there is leakage, internal short circuit or breakdown.
When measuring, you can choose the R×10k block of the multimeter, and use the two test leads to connect the two pins of the capacitor arbitrarily, and the resistance should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it means that the capacitor is damaged by leakage or internal breakdown.
2. Detect 10PF～001μF fixed capacitor: by judging whether there is charging phenomenon, and then judging whether it is good or bad. The multimeter selects the R×1k block. The β values of the two triodes are both above 100, and the penetration current should be small. 3DG6 and other types of silicon transistors can be used to form composite tubes.
The red and black test leads of the multimeter are respectively connected to the emitter e and collector c of the composite tube. Due to the amplification effect of the composite triode, the charging and discharging process of the measured capacitor is amplified, and the pendulum of the pointer of the multimeter is increased, which is convenient for observation.
It should be noted that during the test operation, especially when measuring a capacitor with a small capacity, it is necessary to repeatedly switch the pins of the capacitor under test to touch points A and B, so that the swing of the pointer of the multimeter can be clearly seen.
For fixed capacitance above 001μF, the R×10k block of the multimeter can be used to directly test whether the capacitor has a charging process and whether there is an internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swinging to the right.
Troubleshooting:
1. Detection of small capacitors below 10pF: Because the capacity of fixed capacitors below 10pF is too small, measuring with a multimeter can only qualitatively check whether there is leakage, internal short circuit or breakdown.
When measuring, you can choose the R×10k block of the multimeter, and use the two test leads to connect the two pins of the capacitor arbitrarily, and the resistance should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it means that the capacitor is damaged by leakage or internal breakdown.
2. Detect 10PF～001μF fixed capacitor: by judging whether there is charging phenomenon, and then judging whether it is good or bad. The multimeter selects the R×1k block. The β values of the two triodes are both above 100, and the penetration current should be small. 3DG6 and other types of silicon transistors can be used to form composite tubes.
The red and black test leads of the multimeter are respectively connected to the emitter e and collector c of the composite tube. Due to the amplification effect of the composite triode, the charging and discharging process of the measured capacitor is amplified, and the pendulum of the pointer of the multimeter is increased, which is convenient for observation.
It should be noted that during the test operation, especially when measuring a capacitor with a small capacity, it is necessary to repeatedly switch the pins of the capacitor under test to touch points A and B, so that the swing of the pointer of the multimeter can be clearly seen.
For fixed capacitance above 001μF, the R×10k gear of the multimeter can be used to directly test whether the capacitor has a charging process and whether there is an internal short circuit or leakage 1. Common faults of the capacitor. When one of the following conditions of the capacitor is found, the power supply should be cut off immediately.
(1) Capacitor shell expansion or oil leakage.
(2) The casing is broken, and flashover occurs with sparks.
(3) Abnormal sound inside the capacitor.
(4) The temperature of the shell is higher than 55°C and the temperature indicator chip falls off.
2. Troubleshooting of capacitors
(1) When the capacitor explodes and catches fire, immediately disconnect the power supply, and use sand and a dry fire extinguisher to extinguish the fire.
(2) When the fuse of the capacitor is blown, it should be reported to the dispatcher, and the circuit breaker of the capacitor should be opened after obtaining consent.
Cut off the power supply and discharge it, first conduct an external inspection, such as whether there is any flashover trace on the outside of the bushing, whether the shell is deformed, whether there is oil leakage and whether there is a short circuit in the grounding device, etc., and shake the insulation resistance between the poles and the pole to the ground Value, check whether the wiring of the capacitor bank is complete and firm, and whether there is a phase loss phenomenon. If no fault phenomenon is found, replace the insurance and put it into operation.
If the fuse is still blown after power transmission, the faulty capacitor should be withdrawn, and power transmission to the rest should be resumed. If the circuit breaker trips at the same time as the fuse blows, do not force it at this time. It must be put in after the above-mentioned inspection is completed and the insurance is replaced.
(3) If the circuit breaker of the capacitor trips, but the shunt insurance is not broken, the capacitor should be discharged for three minutes before checking the circuit breaker, current transformer, power cable and capacitor exterior.
If no abnormality is found, it may be caused by the voltage fluctuation of the external fault bus. After inspection, you can try to cast; otherwise, you should further conduct a comprehensive power-on test for protection. Through the above inspections and tests, if the reason is still not found, it is necessary to follow the system and gradually test the capacitor. Do not try to vote until the reason is found out.
