A circuit for qualitatively testing capacitor leakage

Electrolytic capacitors will leak over time. The circuit shown in the attached image allows you to test the leakage of capacitors and decide whether they are worth using. You can suppress leakage by the ratio of CREF/RREF.

The ratios in the attached figure are suitable for universal testing of all capacitors from 1 nanofarad ceramic capacitors to 1000 microfarad electrolytic capacitors. The value of CREF in the circuit is close to the value of the capacitor CX to be tested, and you can also select RREF to be greater or less than 22MΩ through a rotary switch.

How it works

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

If CX discharges faster than CREF through RREF, its voltage will drop faster, so that the op amp's non-inverting input voltage will be lower than its inverting input voltage, forcing the op amp's output to go low, thus lighting up the red LED. The LED indicates that the capacitor CX under test is leaking. The test circuit shows that even a 1 nanofarad ceramic capacitor is suitable for comparison as a reference. Before testing, please check that the nominal voltage of the capacitor CX under test should be higher than the voltage to be charged.

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