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Wanting help fixing bench power supply.

Discussion in 'Troubleshooting and Repair' started by jman093, Jan 7, 2018.

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  1. jman093

    jman093

    11
    0
    Jul 6, 2014
    Hi. I'm seeing if I could get some help diagnosing my bench DC power supply that I just fried. It is a unbranded variable 30 volt / 5 amp CV and CC DC power supply. Model number is P305D. Cheaply made, I'm sure, by commie kids in China, but has worked great for years.

    The other night I was fixing some power tools and carelessly picked up the leads from the power supply when I meant to grab the leads from my DMM and touched them to an 18 volt battery I was resuscitating. There was a spark, and now something in the power supply is shorted. I'm actually not sure how this could have shorted something since the power supply's leads are meant to be shorted together to adjust the CC and a fully charged 18 volt battery is about 21.5 volts, well within the unit's 30 volt output capability. Still, whatever happened, it wasn't good.

    It now acts like the leads are constantly shorted together, even where there are no leads plugged into the unit. The voltage display always reads zero, and the amperage display always reads whatever the constant current is adjusted to, zero to five amps. So I assume there's now a short circuit internally somewhere that the current is going (and being displayed on the LCD display). Looking inside, nothing is visibly burned or looks fried.

    Forgive me, I really don't know a lot about electronics. There is a large heatsink inside with a couple very large things I believe are transistors that are the main controllers from the transformer? windings. They are getting quite hot when the unit is on. I thought about desoldering them and testing them, but I can see them being hot by nature though if the unit is shorted like it is and putting out some amperage (there's a huge heat sink on them for a reason) and the culprit is some diode or transistor on the unit's circuit board, but I don't even know where to begin there. Any help is appreciated.
     
  2. _Nate_

    _Nate_

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    Jun 12, 2017
    If this is DC, it would be a good idea to test all of the internal leads. For example, put leads at either ends of one of the "transistors" using a multimeter and see if you receive output. Try this with all of the components to pinpoint what is fried and replace it. Or, buy a new one.
     
  3. HellasTechn

    HellasTechn

    1,553
    215
    Apr 14, 2013
    Isnt there some model number or something ?

    Anyway i suggest that you check the power transistors that are most probably attached to a large heatsink. Some Power supplies use the 2n3055 transistors. They usually are the first to go when shorting the output. also you should check any voltage regulators or opamps like lm317, lm318.
    It is nearly impossible from distance to diagnose a device without some kind of circuit diagram.
    Should not be too hard to fix.
     
  4. kellys_eye

    kellys_eye

    4,275
    1,147
    Jun 25, 2010
    Any decent PSU will have protection from 'reverse feeding' a supply INTO it - this could be as simple as a transient absorber or a reverse connected zener across the output. Either way it's gone short - the current operating symptoms you describe clearly indicate this.

    Show a pic of the internals and we'll point to the suspect part.

    If it's the model I think it is there is also a 10A version and there are repair videos on Youtube that may assist.

     
    Last edited: Jan 7, 2018
    HellasTechn and Arouse1973 like this.
  5. jman093

    jman093

    11
    0
    Jul 6, 2014
    Thanks for the replies. Here's pictures.
    First shows what it's doing. Knobs currently set to 3.7amps. Should be reading zero though with no leads in it
    2nd is the large transistors on the heat sink. If those were fried, wouldn't it not push current at all and the display reading zero amps?
    3rd and 4th are the control board. I'm aware that red wire needs soldered back onto board. It came off just now when I was moving it around for pictures.
    5th is picture of the back side of the display/controls.

    Thanks again.
    IMG_1977.jpg IMG_2005.jpg IMG_2006.jpg IMG_2008.jpg IMG_2009.jpg
     
  6. kellys_eye

    kellys_eye

    4,275
    1,147
    Jun 25, 2010
    The large transistors are getting hot because the output is shorted and they are dissipating all the regulated voltage.

    You need to follow the wires from the output terminals (front panel) back to the circuit board and check for any components wired across the output terminals (diode) or where they connect to the main circuit board.

    Nice pictures, shame they aren't of the relevant area!
     
  7. jman093

    jman093

    11
    0
    Jul 6, 2014
    I figured that's why those transistors were hot. I really don't understand your second paragraph and have pictures of about the whole thing. I'm a newb.

    In any case, it's better to be lucky than good, and I think I got lucky. Knowing the little I know, I figured a "diode" was the issue somewhere and not a transistor, capacitor, resistor, etc. I googled pictures of diodes and the first one that looked like the picture I desoldered from the board to test, and lo and behold I think it is my issue. It shows 0 ohms continuity on my DVOM in both directions, which correct me if I'm wrong, means it's bad. Also, with the diode now completely missing from the board, the power supply is seemingly fully functional again. It must serve some purpose though and that leaves me with one more question.

    The numbers on the suspect diode are "IN5408" and below that "M10." Lots of results for replacement diodes come up googling "IN5408," but they all say they are 3A rated. This is a 5A power supply, so that can't seem right. Is the "M10" a special 10 amp version of this diode? Will a 3A diode be just fine? I'm not sure what to purchase here.
     
  8. kellys_eye

    kellys_eye

    4,275
    1,147
    Jun 25, 2010
    You found what I was proposing - a shorted diode across the output terminals (although the diode may well have been situated on the circuit board).

    The purpose of the diode is to prevent applying an external voltage, in the wrong direction (i.e. polarity), to the output terminals - under which circumstance the diode goes short circuit and effectively disables the PSU.

    As you found out, the PSU will work with the diode missing but you no longer have the reverse-polarity protection it affords. Use the correct replacement - it was designed that way for a purpose. The 1N540* series are common enough (sure that isn't a '3' and not an '8' though?).

    Either take care and carry on or fit a replacement and rest in the knowledge that your unit is fully protected.

    TBH I'm quite impressed with the build of that PSU - worth preserving in good order.
     
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