How to Test Capacitors


I know I am dating myself and I really hate to say this, but “when I started in this industry” I learned to test capacitors with an analog volt-ohm meter (Simpson 260 VOM to be exact and still a great meter). At that time it seemed to be an effective test.  I would pull the wires off the terminals of the capacitor, set my analog meter on the highest ohm scale, place the leads on the terminals of the capacitor and then switch the leads and watch the needle rise across the scale and then slowly drop back to zero.  If it did not discharge all the way to zero, the capacitor was bad. If the needle went all the way up and stayed there, the capacitor was bad.  If the needle didn’t move and I switched the leads again and it still didn’t move, the capacitor was bad. I would also check each terminal to ground. Any reading to ground was a shorted capacitor.  I used this method for years as a way to check capacitors. This method checks for opens, shorts, grounds, and if the capacitor will accept a charge and discharge.  The only thing this method did not do was tell me if the capacitance is correct.  It is still a good test for capacitors if you happen to have an analog meter.

Of course, one of the other checks of a capacitor didn’t even require a meter and that was to see if a run capacitor showed any signs of “bulging” or a start capacitor was “blowing it’s top”.  These were sure signs that a capacitor was bad.

Finally I broke down and bought a digital meter with a capacitance scale on the meter.  The problem with digital meters is you cannot do the charge/discharge test of the capacitor like I still can do with my analog meter but it did have a capacitor analyzing scale built into the meter.  I now could see if the actual capacitance of the capacitor was within tolerance of the rating printed on the capacitor.  All I had to do now was remove the wires from the capacitor, put the leads from my new digital meter across the terminals and the meter would actually read the capacitance.   I was now hi-tech!

The point I want to make here is too often, motors, compressors, etc  get condemned because they will not run and the real problem is a simple capacitor. A motor may start but then slows down and stops and the first thing a tech does is condemn the motor. In reality, the first thing that should be checked is to make sure all the power is present and proper and then check the capacitor.

The capacitor has a rating in μF  but it also has a +/- % rating.  A lot of times the tech will use his capacitor checker and it will read 9.3 μF  for a 10 μF capacitor and he thinks it is good.  But what was the +/- rating on that 10 μF  capacitor?  If it was 10% then it was within the tolerance of the capacitor.  BUT, more and more today, those tolerances have become tighter and tighter.  A simple 10 μF capacitor today most likely has a +/-5% rating and in the scenario above, that 9.3  μF is out of the tolerance range for the motor.

Being able to accurately test capacitors (or any component for that matter) is essential to good diagnostics.  But just having the tool or meter is just the start, you have to know how to properly use it, but more so, YOU NEED TO UNDERSTAND WHAT IT IS TELLING YOU! As my simple scenario above said, the meter showed 9.3 μF , proving I knew how to use the meter, but if I didn’t take it that one step further to check the tolerance of the capacitor, I could very easily said it was good when actually, it was bad and condemned a motor, or worse, a compressor.

*(Though I have retired my Simpson 260  (and still have it), I do carry a simple analog meter with me as part of my tools.  I still use it on capacitors but it is also great for checking that nuisance “bouncing” switch that a digital meter isn’t quick enough to catch but you can watch the needle on the analog meter actually move when the switch opened and re-closes quickly — I’ll never get rid of my analog meter!)

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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