Capacitance Checker

The circuit, in Fig. 1, came about at a time when I couldn’t locate my old Eico capacitor checker to test the leakage of a tattered electrolytic capacitor that was needed for a project under construction. Granted that this simple capacitor tester won’t give you a digital readout, but with a little practice you can tell a good deal about what’s tied to its input, and in the process learn a little more about capacitors in general.

The capacitor tester, using the component values shown, can indicate a leakage resistance of 100 megohms, and more. That’s a bunch of ohms. But a number of good-quality Tantalum capacitors, most all non-electrolytic, and a number of good-quality electrolytic capacitors test well above that leakage resistance value.

When checking electrolytic capacitors be sure to get the polarity right or the unit is sure to leak like a sieve. Most electrolytic capacitors take a long time to charge unless S1 is pressed for a few seconds, speeding up the charging process.

As soon as a good capacitor is charged, the leakage current should be very low or almost non-existent, and the LED should remain dark. After a long period of time, depending on the size and quality of the capacitor, the LED may begin to glow dimly as a small charging current flows to replace the capacitor’s voltage loss due to the slight internal leakage current and Q1’s minute base current.

A good 1-microfarad non-electrolytic capacitor connected to the tester causes the LED to light and remain on for about one minute as the capacitor charges. A 0.1-microfarad unit takes about 6 seconds, and a .01 microfarad unit takes about I second to charge. If the LED does not go out after a sufficient charging time, it’s a good indication that an internal leakage resistance is lurking somewhere between the capacitor’s leads. So, not only does our simple checker test for internal leakage resistance, but also gives a clue to the capacitance value. If you are only interested in checking for leakage resistance press S1 to speed up the testing procedure.