[Rob]

Carmine, All,

The idea of using an ohm meter to check capacitors for leakage is simple. The ohm meter has a battery inside that sends voltage out to the test leads through the meter circuit. When you place a dead short (touch the leads together) this is zero ohms and the maximum current can flow in the ohm meter which is callibrated to read zero ohms under this condition. The higher the external resistance placed across the leads the lower the current flow. The meter is calibrated to read higher ohms as this circuit current goes down. There is a low DC voltage across the leads of the ohm meter which comes from the battery inside the meter case. When you place a capacitor across the ohm meter, at first the cap will read low ohms because it looks like a short circuit because it needs current to charge itself. As the internal voltage of the capacitor under test grows (cap is getting charged up by the meter) the ohms reading will increase because the cap is looking like an increasingly larger value resistor while the charging current drops. Eventually the cap, if not leaky, will, charge to the full potential supplied by the ohm meter and the meter will go to infinity ohms or OL if a digital ohm meter. For the most accurate testing of small coupling capacitors in circuits they should be tested with one lead disconnected from the circuit and the highest ohm range available on the meter, 2 meg ohm, 10 meg ohm or 20 meg ohm should be used. For electrolytics it is best to also disconnect one side, usually the positive connection in the case of a chassis mounted can type cap where the negative terminal is almost always the negative terminal. When testing electrolytics the lower ohms range may be needed because the lower ohms range can supply more charging current to the capacitor and large electrolytics will take more current to charge, and even a good electrolytic cap has some leakage meaning it will never read infinity on the ohm meter. With an electrolytic you should have the ohm meter red test lead connected to the positive terminal of the capacitor. You can shorten the test time on the big electrolytics by starting at low ohms range to help charge the cap quickly and then keep switching to higher and higher resistance ranges as the ohms reading continues to climb. If you can reach over range or infinity reading on the meg ohm scale of your meter with an electrrolytic that shows either a very good low leakage electrolytic (if it took time to get to thet reading) or if it reads high right away it is dried out and 'open' inside and should be thrown away.

When working on testing capacitors in a chassis it is important to unplug the chassis from the mains and also to discharge any of the capacitors in the circuit with a shorting wire placed across their terminals for several seconds before unsoldering them or attaching the ohm meter. Touching an ohm meter to a charged capacitor can blow out the ohm meter.

Rob