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Electrolytic leakage

Discussion in 'Electronic Basics' started by Theo Markettos, Sep 12, 2003.

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  1. I'm trying to fix a switched mode PSU, which has the symptom that the output
    is dropping out under load at about 10Hz. As the PSU usually runs very hot
    (and has done for the 5 years I've had it working) this suggests that an
    electrolytic might have dried up. I've removed the main mains electrolytic,
    which is rated at 100uF 400V. AFAICS none of the caps show signs of damage.

    Is there any good test for leakage on such caps? My multimeter (Fluke 87)
    only measures capacitance up to 5uF. I tried it in a basic 555 timer
    circuit and the value seems to be in the right ballpark. The series
    resistance measures about 6.7M - is this too low? My previous meter only
    did up to 2M, so I haven't been able to measure this high in the past. I
    tried charging it up to 19.00V, and after 40 mins (/without/ anything
    connected in the interim) it was down to 18.35V. This sounds like there's
    not undue leakage, but are there better ways to test leakage than this - or
    other heat-related failure modes?

  2. Here's a good test to see if the cap is drying up. Charge it up to full
    rated voltage and leave it that way, still connected to a voltage source for
    some time (5 minutes at least, prolly more). Then, disconnect it and
    discharge it quickly with a small resistance or a short across the leads.
    Remove the short immediately, wait about 15 seconds and then measure the
    voltage across the leads with a high impedance meter. If the cap is partly
    dried out you should see the voltage slowly rise, as though the cap was
    recharging itself.

    This is caused by dried out sections of the cap acting as small capacitances
    with high resistances in series. If you charge them up, and then dischare
    the cap in much less then a time constant, they will still have most of
    their charge, and it will bleed away into the rest of the cap slowly. I have
    seen old high voltage caps recharge to many 10's of volts on their own.

  3. You want to test the ESR (Equivalent Series Resistance) of the
    capacitor There a re some simple schematics on the web, as well as kits
    and commercial test equipment.

    One popular meter in kit form is the Dick Smith ESR and Low Ohms
    Meter, available world wide.
  4. Thanks. I made the '99 cent ESR tester' (a square wave driver into the cap)
    - but the caps test out fine. Never mind :-|

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