T&L Publications

How can I bench test a capacitor without any specialty capacitor test equipment? The tools available are a Fluke Meter(with capacitor test function) and some jumper leads. The type of capacitors are used in central air conditiong units and are about the size of a small soda can. The one I tested today was 370vac 15 uf . The meter said 15.4uf (good) and I hooked it into a wall outlet (120vac) with the jumper leads. After it sat for about a minute it read 159 volts on the dc scale of the meter. so I figured it could hold a charge. There was no continuity between the terminals on top (2 unmarked) and the case. So I concluded this capacitor was good. When I drained it (jumper wires between terminals) I got a good spark. Is there a better more accurate way to test and how about putting a load on it to test for a breakdown?

Accurately you cant very well.

But using a DC source.....[and AC]

You can charge it up measuring the voltage, and using ohms law you can determine the drain time using a preset load.

Then like you did, you check it for general storing of the voltage.

You also charge it up using DC and check to see how fast it leaks on its own.
[no shorting resistor]

You also run both the AC and then the DC into it and check for warmth.

New Guy
You cannot determine the caps internal leakage current very well with a voltmeter as the voltmeters impedance(10 meg?) will discharge a cap faster than normal internal leakage would. Also trying to charge a cap with AC isn't reliable as the capacitor voltage read will be the result of the applied voltage at the instance of removal and could well read zero volts. With your cap directly across the power line and of the value stated, it would draw about 1 peak ampere. This would have been enough to warm the capacitor after a period of time but would not indicate it was bad. Be careful of doing this as large values might produce disastrous results. If you must you a voltmeter for leakage testing, charge the cap with dc and leave it sit for a while before measuring voltage and then note the voltage the second you touch the probes to it. this will be more accurrate than clipping the voltmeter on right after charging and then taking a reading. I am assuming this is an oil filled capacitor and should hold its charge for a long time.

Warmth is OK.

HOT is very questionable.

I don't know much about these types of caps so my question is is it safe to connect them to ac . I know most large caps have polarity markings.

AC and DC type caps are marked different, and they are completely different.

You might try using your nose to smell it. If you plugged it into ac for too long it might smell burnt or you might have diminished the quality of it. If you have a good one to use for an example then see if you can smell the difference between the two.

Hi there,

Another aspect of some of the capacitors is the voltage rating.
If you have a voltage supply that goes from say 0 to 500vdc you
can check for leakage current as you turn the voltage up.
I dont remember what the leakage spec is for caps but perhaps
this info could be found on the manufacturers sites.

I used to have an old capacitor/inductor tester/bridge. It had a built
in 0 to 500v dc supply and one of those "green eye" vac tubes. As
you crank up the voltage, the eye closes more and more which tells
you the leakage is increasing. It was a very old model but wow it
worked so well for checking the values and the rating.
You could use it for inductors too if you had a few reference inductors
laying around to use as a reference L for the bridge.