Odd capacitor test results

Discussion in 'General Electronics Chat' started by fltchr, Apr 16, 2012.

  1. fltchr

    Thread Starter New Member

    Apr 16, 2012
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    0
    I'm testing a 50/12μF start/run capacitor out of an AC unit and got, what are to me, odd results.

    I tested it with a VOM, qualitatively looking at the rate of change in voltage and resistance. The VOM works at 0.5V. The voltage had the expected slow drop for both capacitors. The resistance went to ∞ immediately.

    I then redid the test with a 9V battery and got the same results, but in the 0.5-9V range.

    Finally, I redid the test with just the VOM and observed a gradual increase in resistance.

    I do not know what is going on. Could this be a temerature issue? I did the test in a garage at ~50°F. Did the 9V version of the test warm up the capacitor enough for it to function in the 0-0.5V range? I know this test is usually done on a hot summer day when the AC is broken.
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    What do you mean the VOM works at 0.5V?

    What do you mean a gradual increase in resistance?
     
  3. #12

    Expert

    Nov 30, 2010
    16,278
    6,790
    This type of capacitor has a very strong change in capacitance dependent on the voltage applied. The best way to test them is to apply line voltage and measure the current they pass.

    Xc = 1/ 2PiFC
    I = VACrms/Xc

    Always be aware that they could fail by exploding. Protect yourself!
     
  4. fltchr

    Thread Starter New Member

    Apr 16, 2012
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    0
    Thanks, I'll test it in the unit. If I get no results it'll narrow down the problem to a circuit board I think has gone bad.

    I'll also be sure to be a few feet away with my meter.
     
  5. #12

    Expert

    Nov 30, 2010
    16,278
    6,790
    I do this for my day job. Ask a lot of questions if you need to.

    You should be able to see the capacitor pass current during the start-up, with an amp-clamp meter, from several feet away. Diagnosis is usually not about how much. It's ususally about if it conducts at all. See a start surge and it's probably good.
     
  6. fltchr

    Thread Starter New Member

    Apr 16, 2012
    3
    0
    Thanks #12. The problem is that it does not turn on, at all. The power cord is good, the fuse is good, the switch is good, but nothing happens. I am trying to test everything before buying a ~$40 part. I think its the PWB display assembly but I don't have an extra on hand to test it out.
     
  7. #12

    Expert

    Nov 30, 2010
    16,278
    6,790
    Electronic controls on a window air conditioner?
    No compressor, no fan?
    You shouldn't even be checking capacitors.
    What is it using for a thermostat?
    Does it have a remote control?
    assuming you are in the USA?

    Talk to me. It will save you barking up the wrong tree.
     
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