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Use this Capacitor Tester to Find Leaky Capacitors Quickly

This simple capacitor tester is capable of testing electrolytic capacitors in the range of 1uf to 450uf. It can test large start and run capacitors as well as 1uf miniature capacitors rated at 10v. Once you understand the timing cycle, you can test down to 0.5uf and up to 650uf.

By Henry Bowman

How to Make this Capacitance Tester


The tester was made out of some junk parts I had on hand as well as a couple of op-amps and a 555 timer. The test is based on a timed cycle of charge, where two voltage compartors indicate 37% and 63% of charge.

Referring to the schematic, the capacitor is connected to the terminals labeled C. One side is ground and the other side is connected to a rotary selector switch and also to the inputs of two op-amps. The “G” position on the rotary switch is a low resistance ground to discharge capacitors when connected. Large value capacitors should always be discharged before connecting.

Circuit Diagram

Capacitor Tester Circuit to Find Leaky Capacitors



The 12 volt zener is also for voltage protection. If the capacitor is polarity marked, the red dot, or + should be connected to the positive test lead. The selector switch should also be in position “G” when connecting. S2 should be in “discharge” position.

The rotary switch resistor sizes were determine by inverting the formula T=RC, so that R=T/C. Each value of resistor on the rotary switch is selected to provides an approximate time of 5.5 seconds to charge. The actual average charge time takes 4.5 to 6.5 seconds.

Resistor tolerances and slight differences in capacitor values create the difference in the 5.5 second design. The supply voltage needs to be very close to 9 volts. Any lower, or higher voltage will effect the voltage at the resistance dividers at IC 2 and IC 3 input pins 3.

How to Test


The voltage from the ac/dc adapter plug was higher than it's stated 9 volts. I used a 110 ohm dropping resister in series to bring it down to 9v. When the capacitor is connected to the test terminals, the selector switch should be moved from “G' to same value, or nearest value, of capacitor to test.

When S2 is operated to charge, 9 volts is placed on the selector switch resistor through the common wiper to the capacitor to start the capacitor charge. The 9 volts is also placed on the emitter of Q1, a high current gain transistor. Q1 will immediately conduct and power the 555 as Q1's base is at resistive ground potential from IC 3's output pin 6.

The 555 timer lights led 2, once each second, until 63% of charge is reached. The two op-amps are configured as voltage comparators. When 37% (3.3v) of charge is reached, IC2's output goes high, lighting led 3.

When 63% of charge (5.7 volts) is reached, IC 3 goes high, lighting led 4 and also stops Q1 from supplying power to the timer. Operating S2 to discharge provides ground through the same resistor that charged the capacitor.

The 555 does not operate during discharge. Led 4 will go out first indicating that the voltage has fell below 63%, then led 3 will also go out after the voltage has dropped below 37%. Below are the trouble indicators for capacitor tests after verifying that you have selected the proper range and the polarity is correctly connected::

Open capacitor: Will light led 3 and 4 immediately after charge switch is operated. No current flowed through the capacitor, so both comparators will provide high outputs immediately.

Shorted capacitor: led 3 and 4 will never light. Timer light led 2 will flash continually.

High resistance short or change in value: 1. led 3 may light and led 4 stay unlit. 2. both led 3 and 4 may light, but with an charge time greater, or smaller, than designed charge time. Try a known good capacitor and retest.

I had a capacitor labeled 50uf that was taking 12-13 seconds to charge to 63%. I tested it with a digital capacitor tester and it showed an actual value of 123 uf !

If you have a capacitor that falls in the mid range between two capicator values, test on both values. The average between high and low charge intervals should fall within the 4.5-6.5 second range.

A 0.5 uf will have a charge time of 2.5-3 seconds on the 1uf position. Also, testing a 650 uf capacitor on the 450 uf position will provide a charge time of 8-10 seconds. An alternative to the rotary switch would be spst switches for each resistor. Use a digital ohmmeter to verify the resistance of each resistor before installing. The 6K and 3.4K resistors used at the op amp voltage divider networks should be chosen for low tolerances. A voltage of 3 volts and 6 volts on the dividers would be close enough for the charge cycle.

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