Capacitor Testing with a Multimeter

by admin on October 18, 2011

Let me begin this rant by letting you know that you CANNOT, unequivocally, test a capacitor and know that it is good with just a multimeter. If anyone tells you that you can, they’re full of baloney. I’d say something else, but I’m trying to keep this post “G” rated. ;)

You can tell if it is bad, and you can get an idea if it’s good, but you won’t know for certain. Why? A multimeter is designed to measure Voltage, Current and Resistance. A capacitor stores charge. The charge that it stores is represented by current and time. Furthermore, a capacitor has limits as to how high of a voltage it will work with.
To tell if a capacitor is bad, here are some tests you can do with a multimeter: 1st before you even break out the multimeter, do a visual inspection. Does it bulge or is it symmetrical? Is it discolored or does it look “nice”, is it leaking anything? Any yes’s means don’t waste your time with a meter, just get a replacement capacitor with the same specifications. If you can’t get exactly the same the first thing you can change is the working voltage, get one with a higher WV. Difference in capacitance is dependent on the circuit that it is in and beyond the scope of this article.

For small value capacitors, say 0.01uF and less, about all you can do with a multimeter is check for a short or open. Either condition indicates it is bad, but lack of both does not guarantee “good”. Connect a voltmeter on the ohms range (x 10K) range to the capacitor. Make sure your meter is connected properly, the leads may or may not be positive or negative as you expect (check them with either another meter or a diode). If the capacitor is bad, the needle will either

  • Stay at infinity (indicating that it is open) OR
  • Drop to zero (indicating that it is shorted)

If you have an electrolytic capacitor of several uF or more, then you should be able to observe the following: It might be good if you see the needle jump towards zero and then slowly return to infinity. Note that if you have a DMM and it does not have a bar-graph display like a Fluke 7x/8x series you won’t be able to observe this. What you are actually seeing is the current from the meter charge the capacitor. Initially there is no charge and the current is high which makes the needle move towards 0. As the capacitor builds charge, the current becomes less and the needle returns to infinity.

To more completely test a capacitor, you need to test capacitance, leakage, equivalent series resistance (ESR), and maybe even the breakdown voltage. Each of these can be tested with specialized test equipment. Any testing you do should be done with the capacitor out of the circuit (at least one lead) as other components can (and if Murphy is has anything to say about it, will) cause an erroneous reading.

To more thoroughly test a capacitor, I highly recommend getting Jestine Yong’s book Testing Electronic Components. Not only will he tell you how to test the capacitor, but he’ll tell you which test equipment you should have. He devotes several chapters to capacitors and also goes through every electronic device you’re likely to ever want to know how to test.  Many of them don’t require any more than a multi-meter, unfortunately, capacitors do.  :-(
If you got lucky and were able to determine that your capacitor was bad for sure, great, get a replacement and restore the circuit. If not, you’ll need to do more in depth testing.

The book Testing Electronic Components will tell you what additional test equipment you’ll need and then how to use that equipment to see if it’s good or bad. You’ll be well on your way to being a professional electronics repairer or at least know how to test those tricky capacitors.

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