May 27, 2019

What method should I use to test whether the capacitor is leaking?

Leakage occurs when the electrolytic capacitor is used for a long time. The circuit shown in the figure allows you to test the leakage of the capacitor and determine if they are worth using. You can suppress the leakage by the ratio of CREF/RREF.

The ratios in the figure apply to general testing of all capacitors from 1 nanofarad ceramic capacitors to 1000 microfarad electrolytic capacitors. The value of CREF in the circuit is similar to the value of capacitor CX to be tested. You can also select RREF through a rotary switch to make it larger or smaller than 22MΩ.

working principle

When the push button switch is closed, capacitors CREF and CX are charged by their respective PNP transistors. When the switch is turned off, capacitors CREF and CX begin to discharge. Assuming CREF is in good condition, it has an additional discharge external resistor RREF through which the capacitor CX to be tested is discharged.

If the discharge of CX is faster than the discharge of CREF through RREF, then the voltage will drop faster, so that the input voltage of the op amp will be lower than the input voltage of its opposite terminal, forcing the output of the op amp to go low. Light up the red LED. The LED indicates the leakage of the capacitor CX to be tested. The test circuit indicates that even a 1 nanofarad ceramic capacitor is adapted for reference. Before testing, check that the nominal voltage of the capacitor CX to be tested should be higher than the voltage to be charged.

The operational amplifier LF357 has a minimum supply voltage of 10V. The test circuit only selects 6V to allow a low upper limit voltage of the capacitor CX to be tested.

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