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Verify a(maybe) dying diode caused by a bad capacitor???

Discussion in 'Electronic Repair' started by [email protected], Mar 12, 2007.

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  1. Guest

    I have just fixed an power supply by replacing a bulging capacitor.

    The capacitor is connected to diode. And the diode becomes hot and
    makes a black spot on the board since the faulty cap.

    I check the diode with a multimeter and it tests great.

    The repaired power supply( cap replaced) has been tested for several
    hours and no problem yet.

    Question: Is there any way to check whether the diode is working
    100% ?


  2. Diodes are cheap. It was stressed. Replace it.
  3. Meat Plow

    Meat Plow Guest

    Mostly they either work or don't. Short or open. Spend a buck and
    replace it.
  4. Guest

    It seems like the repair worked, but the hot diode concerns me. Your
    description is a little hard to understand. Did the diode get hot
    before or after the capacitor was replaced?

    In any case, the black mark by the diode isn't a good sign. Whether
    the mark was cause by the diode overheating or leakage from the
    bulging capacitor, I would replace it. As another poster tersely
    pointed out, diodes are inexpensive.

    For a definitive measurement, don't test the diode in place. Rather,
    unsolder one of its leads from the circuit board and then test it with
    your voltmeter's continuity checker.

  5. Jeroni Paul

    Jeroni Paul Guest

    By your descriptions it looks like your supply is working fine. It is
    normal for some components to warm up and if it were faulty you would
    see some problem with your supply. If it works, don't mess with it.
  6. If it tests good in-place, it won't test bad when removed. :)

    However, the inverse (??) may be true.

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  7. Arfa Daily

    Arfa Daily Guest

    I see lots of bulging caps in the power supplies of cheapo DVD players - I
    probably repair 2 or 3 a week on average. I have found it very rare indeed
    for the feeding diode to actually be faulty. Usually, it's just a case of
    replace the cap, and that's it. It is not at all uncommon for the diodes in
    these supplies to run very hot, and discolour the board. Sometimes, if there
    is room, I bend a small piece of metal around the body of the diode, with a
    bit of heatsink paste in there as well, to help cool it down a bit. Bear in
    mind if you do replace it that it will be a Schottky type ( I'm assuming
    that this is a switch mode PSU ??? )

  8. James Sweet

    James Sweet Guest

    Is the power supply being overloaded? If it's not and the output voltage
    is correct, I wouldn't worry about a warm diode.
  9. Eeyore

    Eeyore Guest

    Bad advice.

    Some components may have been stressed by the fault The charring near the diode
    indicates this.

    There's NO WAY to prove the diode hasn't had its reliability compromised through
    overheating so it should be replaced as a precaution.

  10. Eeyore

    Eeyore Guest

    Or a fast-recovery type.

    Using a standard diode will guarantee long term failure although it might work
    for a bit.

  11. Jeroni Paul

    Jeroni Paul Guest

    The charring proves that the diode has always run hot and it is likely
    the way it is meant to be. A board is not charred in just a few hours
    of operation.

    Messing and replacing these diodes is a good way to risk the power
    supply, and given that it runs hot I would be very careful with the
    type I replace it with, get it wrong and you may blow the supply again.
  12. JW

    JW Guest

    Unless the in-circuit test is turning on a junction of another
    semiconductor that's in the same circuit, and the diode is open.
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