Circuit for testing capacitors

Electrolytic capacitors leak over time. The circuit in Figure 1 can be used to test capacitors to determine if they are worth using. The CREF/RREF ratio sets the limit on leakage. The values ​​shown are suitable for general testing of all capacitors, from 1nF ceramic capacitors to 1000μF electrolytic capacitors. In the circuit, the value of CREF is close to the value of the capacitor to be tested, CX. Alternatively, the rotary switch can be used to select RREF to be greater or less than 22 MΩ.

When the pushbutton switch is closed, capacitors CREF and CX are charged through their respective PNP transistors. When the switch is opened, these capacitors begin to discharge. CREF (assuming it is in good condition) has additional external discharge resistors. The capacitor under test, CX, discharges through its own internal resistance. If the leakage of CX is greater than the leakage of CREF through RREF, the voltage on CX drops faster. As a result, the voltage at the non-inverting input of the op amp will be lower than the inverting input, making the op amp output low and the red LED illuminated. This LED is used to indicate that the capacitor under test has leakage. Testing of the circuit has shown that even 1nF ceramic capacitors can be detected. Check the rating of the test capacitor to make sure it is higher than the voltage it will be charged to, in this case VSUPPLY is -1.8V.

The minimum supply voltage of the LF357 is 10V, but the test only requires 6V, so a low upper voltage limit is allowed for the test capacitor. Make sure the capacitor has a FET or MOSFET input stage.