Increasing Load on PSU for short testing

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  • macattack600
    Senior Member
    • Aug 2011
    • 97
    • USA

    #1

    Increasing Load on PSU for short testing

    I am using atx psu 12v rail to find short on 12v rail motherboard. There is absolute short on this rail but 45 smd components in front of it. I started removing about 10 of the legs and gave up

    I believe the board has a internal short through the VIA and not a component but I want to confirm.

    I have now 1 12v car light bulb attatched to ground as the load the meter reads 2.0amps .


    question ----If I add another bulb and so on would that would raise the amperage by 2 ?

    Nothing is getting hot no components get warm, barley any voltage on the rail , the bulb in fact turns on so its completeting the circuit and not switching off the PSU

    I figure I need a heavy load to really feel this internal short to confirm.
    The more amps I tack on the more should be pulled through this heavy short right and I would be able to feel what is really getting hot, Providing i'm not going over say 12 or 16amps?
    Last edited by macattack600; 02-14-2017, 05:55 PM.
  • petehall347
    Badcaps Legend
    • Jan 2015
    • 4422
    • United Kingdom

    #2
    Re: Increasing Load on PSU for short testing

    quick tip ... transistors and diodes short in the main ... capacitors also short but not so often ... ic chips short also ... resistors burn up .
    get the schematic and look for culprits then test on the board . if showing short remove and test again .
    the way you are going you will make smoke and things will be worse than they are now ..
    you could always get really involved and start checking voltage drops across resistors and checking voltages at semiconductors to find the short .

    Comment

    • cpt.charlie
      Badcaps Veteran
      • Sep 2013
      • 270
      • Spain

      #3
      Re: Increasing Load on PSU for short testing

      If you connect another bulb in parallel yes, current should increase but you may ruin your board.
      Last edited by cpt.charlie; 02-14-2017, 05:59 PM.

      Comment

      • macattack600
        Senior Member
        • Aug 2011
        • 97
        • USA

        #4
        Re: Increasing Load on PSU for short testing

        so for every bulb it should raise up 2 amps ? I'm thinking about attaching a long LED rope light I need a very demanding load.
        Last edited by macattack600; 02-14-2017, 06:04 PM.

        Comment

        • macattack600
          Senior Member
          • Aug 2011
          • 97
          • USA

          #5
          Re: Increasing Load on PSU for short testing

          Originally posted by petehall347
          quick tip ... transistors and diodes short in the main ... capacitors also short but not so often ... ic chips short also ... resistors burn up .
          get the schematic and look for culprits then test on the board . if showing short remove and test again .
          the way you are going you will make smoke and things will be worse than they are now ..
          you could always get really involved and start checking voltage drops across resistors and checking voltages at semiconductors to find the short .
          I would like to learn what is thermal resistance short finding something to do with heating the board with heat gun and measuring resistance drop you know about this?

          Comment

          • cpt.charlie
            Badcaps Veteran
            • Sep 2013
            • 270
            • Spain

            #6
            Re: Increasing Load on PSU for short testing

            LEDs won't make a high load BTW, use a car lamp instead.

            If you had a miliohm meter it could be handy to approach to the fault.

            Comment

            • macattack600
              Senior Member
              • Aug 2011
              • 97
              • USA

              #7
              Re: Increasing Load on PSU for short testing

              Originally posted by cpt.charlie
              If you connect another bulb in parallel yes, current should increase but you may ruin your board.
              I also have a 50w 12v light bulb do you know if the amps i'm looking for will jump up by how much?

              Comment

              • cpt.charlie
                Badcaps Veteran
                • Sep 2013
                • 270
                • Spain

                #8
                Re: Increasing Load on PSU for short testing

                50/12=4.16A in theory, consider some extra resitance at the short and contacts so current will be a bit lower

                Comment

                • macattack600
                  Senior Member
                  • Aug 2011
                  • 97
                  • USA

                  #9
                  Re: Increasing Load on PSU for short testing

                  Great I have 2 I will report back later. I really don't think I'm going to burn this rail up i'm getting no action @ 2 amps I'll go 1 at time , it's a large smd internal VIA pad connecting 45 components to both sides. I have hunch its shorted inside but I need to know for sure.

                  Comment

                  • petehall347
                    Badcaps Legend
                    • Jan 2015
                    • 4422
                    • United Kingdom

                    #10
                    Re: Increasing Load on PSU for short testing

                    check voltages at semiconductors before adding more current and burning the board and more stuff .

                    Comment

                    • macattack600
                      Senior Member
                      • Aug 2011
                      • 97
                      • USA

                      #11
                      Re: Increasing Load on PSU for short testing

                      I have no voltage on the line I remove the resistor for voltage i'm missing per schematic. In diode mode--- red probe ground black on pad I get Dead short to ground and so on removing legs one by one stopped after 15

                      Cross reference working board gives 0.0500
                      Last edited by macattack600; 02-14-2017, 06:34 PM.

                      Comment

                      • cpt.charlie
                        Badcaps Veteran
                        • Sep 2013
                        • 270
                        • Spain

                        #12
                        Re: Increasing Load on PSU for short testing

                        Do you have a short in the general 12V rail (24 pin connector) or the short is related to 12V VRM rail?

                        Comment

                        • macattack600
                          Senior Member
                          • Aug 2011
                          • 97
                          • USA

                          #13
                          Re: Increasing Load on PSU for short testing

                          In VRM rail , when I remove the resistor everything behind it gives 12v the board is making what's necessary but stopping short .
                          Last edited by macattack600; 02-14-2017, 06:39 PM.

                          Comment

                          • macattack600
                            Senior Member
                            • Aug 2011
                            • 97
                            • USA

                            #14
                            Re: Increasing Load on PSU for short testing

                            The psu says the 12v is rated @9.2amps so I guess I cannot go over that right?

                            Comment

                            • budm
                              Badcaps Legend
                              • Feb 2010
                              • 40746
                              • USA

                              #15
                              Re: Increasing Load on PSU for short testing

                              BTW, the cold resistance of the bulb will be about 10~15 less than the hot resistance so watch out high inrush current.
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