Summary of Circuit lets you test capacitors
This article describes a circuit for testing capacitor leakage using an LF357 op-amp, PNP transistors, and LEDs. By comparing the discharge rate of a test capacitor against a reference capacitor and resistor, the circuit indicates failure via a red LED if leakage is excessive. It supports testing various capacitor types from 1-nF ceramics to 1000-μF electrolytics by adjusting reference values and ensuring the test voltage remains within safe limits.
Parts used in the Capacitor Leakage Tester:
- LF357 Op Amp
- PNP Transistors
- Red LED
- Pushbutton Switch
- CREF (Reference Capacitor)
- CX (Test Capacitor)
- RREF (Reference Resistor)
- Rotary Switch
When the pushbutton switch closes, capacitors CREF and CX charge through their respective PNP transistors. When the switch opens, the capacitors begin to discharge. CREF, assuming that it is in good condition, has an additional discharge external resistance, RREF. The capacitor under test, CX, discharges through its internal resistance. If the leakage in CX is greater than that of CREF through RREF, then its voltage will fall faster. Thus, the voltage at the op amp’s noninverting input will be lower than at its inverting input, forcing the op amp’s output low and lighting the red LED. This LED indicates that the test capacitor leaks. Testing of the circuit reveals that even a 1-nF ceramic capacitor holds against the reference. Check the voltage rating on the test capacitor to make sure that it is higher than the voltage to which it will be charged—in this case, VSUPPLY is −1.8V.
The LF357 has a minimum supply voltage of 10V, but the testing took place at only 6V to allow a low upper-limit voltage for the test capacitor. Make sure the capacitor has a FET or a MOSFET input stage.
For more detail: Circuit lets you test capacitors
- How does the circuit determine if a capacitor leaks?
The circuit compares discharge rates; if the test capacitor voltage falls faster than the reference, the op amp output goes low and lights the red LED. - What components are used to charge the capacitors when the switch closes?
Capacitors CREF and CX charge through their respective PNP transistors. - Can this circuit test both ceramic and electrolytic capacitors?
Yes, it can test general capacitors ranging from 1-nF ceramic versions to 1000-μF electrolytic types. - Does the value of RREF affect the leakiness constraint?
Yes, you can choose RREF to be greater than or less than 22 MΩ to set the constraint on leakiness. - What input stage must the LF357 have for this application?
The capacitor under test must have a FET or MOSFET input stage. - Why was the testing performed at 6V instead of the minimum supply voltage?
Testing occurred at 6V to allow a low upper-limit voltage for the test capacitor despite the LF357's 10V minimum requirement. - What happens if the leakage in CX is greater than that of CREF?
The voltage at the noninverting input becomes lower than the inverting input, forcing the op amp output low and lighting the red LED. - Is there a specific voltage rating requirement for the test capacitor?
Yes, the voltage rating on the test capacitor must be higher than VSUPPLY, which is −1.8V in this case.
