Bench PSU working, but not right.

Discussion in 'General Electronics Chat' started by Mad Professor, Dec 27, 2012.

  1. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    Good day all.

    I have a 0-20v @ 20amp, bench power supply.

    Upto a few weeks ago this unit was working fine.
    But since then if you pull more then about 5amps from the unit, the step down transformer starts humming quite loud, and after a short time starts to get hot.

    I can not see any damaged components on the PCB's.

    I have removed the bridge rectifier (KBPC2502), and tested it with my DVM, and found that one of the four diode's has failed and gone open.

    So my 1st question is, as the bridge rectifier has failed would this be putting more strain on the step down transformer, Or am I still looking for more failed components.

    2nd question, If it is just the bridge rectifier at fault, can I replace the KBPC2502 with something like GBPC5008.

    Thanks for your time.

    Best Regards.
  2. bertus


    Apr 5, 2008

    The replacement of the KBPC2502 by the GBPC5008 should be fine.
    The average current of the GBPC5008 is higher, so should last longer 50 vs 25 Amps.

  3. tindel

    Well-Known Member

    Sep 16, 2012
    I'd sure be looking for more damaged parts... can you test with an ohm meter the resistance to critical points from the output of the rectifier. I.E. output of the rectifier to the main power output, to the ground, other housekeeping power rails, etc.

    Something caused that diode to short... possibly another short. It could be that the rectifier was a bit undersized for the time period that it has been used and was the first to give-up the ghost.

    I wouldn't worry too much about the transformer - you would have probably seen a lot of smoke if you torched it - unless there's a thermal fuse.
  4. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    Thanks for your replys.

    I had been running the bench power supply quite close to it's max 20amp limit, for an extended period of time.

    So I guess this was just to much for the 25Amp bridge rectifier.

    That is why I listed the GBPC5008 (50Amp) as a replacment.
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    A dead bridge rec usually happens when too much current goes through it. I would check short circuit limiting.
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    And heatsinking! Running a 25A rectifier at 20A for long periods is not great in the first place, then made even worse if it doesn't have excellent heatsinking.