Capacitance testing with cheap Cen-Tech meter

Discussion in 'The Projects Forum' started by wayneh, Oct 4, 2014.

  1. wayneh

    Thread Starter Expert

    Sep 9, 2010
    If you have one of those $5 (often free) Harbor Freight DMMs like I do, you can use it to estimate the capacitance of capacitors over 5µF or so.

    By testing about a dozen capacitors I had laying around, I've found that mine has about 1.02MΩ of input resistance when set to the 20V scale. The math works out so that if you charge a cap and then measure the seconds it takes to fall from 10.00V to 3.75V, that is the capacitance in µF. You can use any two voltages you want as long as the lower one is 37.5% of the higher voltage.

    Below 5µF, it's to hard to manually time it.
  2. #12


    Nov 30, 2010
    Old trick: Choose a high ohms scale to get a small DC current available. Connect the test leads to a known good capacitor that is about the size of the suspect. Watch how fast the ohms reading rises. Reverse the leads and watch again.

    Short out the suspect to get it discharged and repeat the test you just did. If the ohms reading rises at about the same rate, it would seem the capacitor is "good".

    This is a very crude test, but it can get you by in some cases where it is obvious that the capacitor is not smoking. For instance, a PSC motor doesn't start. If the cap seems similar to a known good cap, you probably have a bad motor. It's not shorted because you can see the ohms reading rise. It's not open because it takes time for the ohms reading to go up. Therefore, it must be a capacitor (and not a resistor). If the rate of rise is similar, it's probably a good capacitor. If it was shorting under normal voltage conditions, it would either pop its internal fuse or start smoking.