How to confirm AC power capacitor OK?

Discussion in 'General Electronics Chat' started by wkyong, Jul 7, 2008.

  1. wkyong

    Thread Starter Active Member

    Apr 29, 2008
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    How to confirm AC power capacitor 100% OK?
    For an example a 200uF 250Vac power capacitor,
    I have came across by using multimeter to check its uF,resistance eventhough is ok,but when putting it to the unit,it didn't work.
    Any other methods can confirm it is 100% ok?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Check it's leakage rate.
    You'll need a 250v DC supply.
    It should have a leakage current less than 4.1mA in both directions.
    You measure it by using a 25k 5W resistor in series with the capacitor, and measuring the voltage across the resistor. Use Ohm's Law to calculate the current through the capacitor by I = E/R
     
  3. wkyong

    Thread Starter Active Member

    Apr 29, 2008
    33
    0

    It is an ac capacitor ,why use 250v DC instead of 250V AC supply?
    Thank you!!
     
  4. theamber

    Active Member

    Jun 13, 2008
    318
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    You need a capacitor analyzer to check it dynamically for all four failure modes:value, dielectric absorption (D/A), leakage and equivalent series resistance (ESR).
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Theamber gave you a very good (and correct) reply.

    Mine wasn't quite so accurate. If you wanted to test the leakage rate using a DC supply, you would actually need one capable of supplying 353.5VDC, as this is the peak voltage that would be seen with a 250VAC input.

    The acceptable leakage rate would then be 5.23mA.

    It would be very difficult to impossible to try to measure leakage current with only a resistor and a multimeter with AC voltage.

    AC electrolytic capacitors are basically two polarized electrolytic capacitors back-to-back (not quite, but that's the easiest way to explain them.)

    With ordinary polarized electrolytic capacitors, it is frequently possible to "re-form" them by slowly charging them to their rated voltage through a relatively large resistor over a period of time until their leakage current drops to within acceptable limits. You can't really do this with non-polarized caps.

    I usually wind up having to replace the starter capacitor on our air conditioning unit every 2-3 years. They fail due to the high operating temperatures they are exposed to.
     
  6. theamber

    Active Member

    Jun 13, 2008
    318
    0
    I did not know that but now I asume that has to be with high voltage ratings caps in order to disolve the carbonized leakage path that forms when arcing is produced through the dielectric.

    StgWooki is correct about the voltage.
    Leakage is a common defect in aluminum and tantalum electrolytic types. The cap. will have like a zener diode effect for example at low voltage potentials, below the "zener voltage", the capacitor will have little or no leakage. But as higher voltages are applied to the capacitor the "zener voltage" is exceeded and high leakage current flows.
    So it is essential to test the capacitor for leakage at its rated working voltage.
    At low voltage potentials the capacitor will show good most of the times.
     
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