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    What maybe causing this Chopper to short ?

    Hello everyone here .

    Am right now trying to repair a switching style power supply , this one converts low input current to pretty much 160 amps to be mainly used as a MMA welder .

    Anyways i checked it step by step , started with the primary side , the primary caps got 300 Vdc on them , now arriving to the chopper circuit witch consists of 2 power IGBTs , that are making the input signal chopped at a very high frequency with the command of a pulse width modulation chip " UC3842 " .

    So i checked the power transistors with a continuity tester , and they are both shorted , also found 2 shorted A3120 optoisolators , four 18 v zeners .

    I then changed them all with new parts , in the hope to save this unit but again all the parts that i' ve replaced have again fried .

    I guess something else is causing them to burn , i hope i get some ideas from people who encountered similar issue .

    Here is a Service manual of the SMPS welder :
    Attached Files
    Last edited by SM-Piyes90; 02-25-2014, 08:56 AM.

    #2
    Re: What maybe causing this Chopper to short ?

    perhaps pictures of the board would help , take it you have ~220vac mains.
    Perhaps use the dim lamp trick - a 75watt filament bulb in line wth the + mains
    saves blowing up components
    Please upload pictures using attachment function when ask for help on the repair
    http://www.badcaps.net/forum/showthread.php?t=39740

    Comment


      #3
      Re: What maybe causing this Chopper to short ?

      Originally posted by selldoor View Post
      perhaps pictures of the board would help , take it you have ~220vac mains.
      Perhaps use the dim lamp trick - a 75watt filament bulb in line wth the + mains
      saves blowing up components
      Hi thanks for the comment , ireally don't know about the light bulb trick , but i got this in mind

      You can take a look at the PDF service manual , it contains schematic and picture of the PCB .

      Comment


        #4
        Re: What maybe causing this Chopper to short ?

        That very good manual most of them are not very good
        9 PC LCD Monitor
        6 LCD Flat Screen TV
        30 Desk Top Switching Power Supply
        10 Battery Charger Switching Power Supply for Power Tool
        6 18v Lithium Battery Power Boards for Tool Battery Packs
        1 XBox 360 Switching Power Supply and M Board
        25 Servo Drives 220/460 3 Phase
        6 De-soldering Station Switching Power Supply 1 Power Supply
        1 Dell Mother Board
        15 Computer Power Supply
        1 HP Printer Supply & Control Board * lighting finished it *


        These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

        1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
        2 Hem Saw Computer Stack Board

        All of these had CAPs POOF
        All of the mosfet that are taken out by bad caps

        Comment


          #5
          Re: What maybe causing this Chopper to short ?

          One thing can be that the U3845 (as shown in page 9) DR+ has the output stuck HI instead of turning those two Optos on and off.
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment


            #6
            Re: What maybe causing this Chopper to short ?

            Check the secondary side rectifiers?
            Muh-soggy-knee

            Comment


              #7
              Re: What maybe causing this Chopper to short ?

              Originally posted by sam_sam_sam View Post
              That very good manual most of them are not very good
              Thanks ths one is for professional servicing.
              Last edited by SM-Piyes90; 02-28-2014, 08:24 AM.

              Comment


                #8
                Re: What maybe causing this Chopper to short ?

                Originally posted by budm View Post
                One thing can be that the U3845 (as shown in page 9) DR+ has the output stuck HI instead of turning those two Optos on and off.
                I have changed that too , but still the boom occurs

                Comment


                  #9
                  Re: What maybe causing this Chopper to short ?

                  Originally posted by ben7 View Post
                  Check the secondary side rectifiers?
                  ALL secondary side rectifiers were good and still good after chopper short .

                  I see that you guys think in most of the cases capacitors are bad , but how about the transformer how can i check if its good or bad ???

                  also how about ESR check for small value caps like plastic non polarized ones ??
                  Last edited by SM-Piyes90; 02-28-2014, 08:25 AM.

                  Comment


                    #10
                    Re: What maybe causing this Chopper to short ?

                    You need the ringer to test that power transformer, it may have shorted turn. The DCR of the transformer will be very low (<1 Ohm).
                    Never stop learning
                    Basic LCD TV and Monitor troubleshooting guides.
                    http://www.badcaps.net/forum/showthr...956#post305956

                    Voltage Regulator (LDO) testing:
                    http://www.badcaps.net/forum/showthr...999#post300999

                    Inverter testing using old CFL:
                    http://www.badcaps.net/forum/showthr...er+testing+cfl

                    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                    http://s807.photobucket.com/user/budm/library/

                    TV Factory reset codes listing:
                    http://www.badcaps.net/forum/showthread.php?t=24809

                    Comment


                      #11
                      Re: What maybe causing this Chopper to short ?

                      Originally posted by budm View Post
                      One thing can be that the U3845 (as shown in page 9) DR+ has the output stuck HI instead of turning those two Optos on and off.
                      Hi again am back topic .

                      Thanks BUDM for the assistance there also all the other members , thanks for the suggestions .

                      I have a few questions about how the PWM ic performs .
                      - Do you know an adequate method to check the output signal of that PWM IC ?
                      - And what is the nature of the signal at the IC output , is is square or sine ???
                      - Also what is the minimum frequency that needs to be applied to the IC in order to monitor the output .
                      Last edited by SM-Piyes90; 03-06-2014, 08:36 AM.

                      Comment


                        #12
                        Re: What maybe causing this Chopper to short ?

                        where do you buy your parts?
                        seen many repeat failures where counterfiet parts were the issue.

                        Comment


                          #13
                          Re: What maybe causing this Chopper to short ?

                          squarewave signal on the output pin of 3842 IC
                          "There is no shortcut to be successful. No pain, no gain."

                          Best Regards
                          Rudi
                          Thank You

                          Comment


                            #14
                            Re: What maybe causing this Chopper to short ?

                            Originally posted by kc8adu View Post
                            where do you buy your parts?
                            seen many repeat failures where counterfiet parts were the issue.

                            I buy them from a local electronics shop , but the transistor i used is a FAIRCHILD one took from another device .

                            After analysing the card more i found two bad plastic capacitors ( 0.1 uF 250 V-AC) that are directly connected in paralell to input mains , i was measuring only 3 nF on them , the main filter cap is a 68 uF , 400 V , measuring 56 uF and having a dissipation factor of 1.05 (DF) , these measurements were took with an LCR meter .

                            Any ideas ? could this be causing the chopper transistor to short ?


                            Originally posted by senz_90 View Post
                            squarewave signal on the output pin of 3842 IC
                            Thanks
                            Last edited by SM-Piyes90; 03-09-2014, 02:27 PM.

                            Comment


                              #15
                              Re: What maybe causing this Chopper to short ?

                              UPDATE :

                              I have changed the two main's 0.1 uF capacitors , and the unit got working again , but i get small problem on the two output sections :

                              First section is 5 v and 3.3 v given via voltage regulators, and they are outputing the right voltages .
                              Second sections is outputing 5.8 , 12 , 14 , 38 V dc , but i get higher results , for example the 12 and 14 v outputs gives me 22 v , 38 v output gives 65 .

                              Any one familiar with this scenario ??
                              Last edited by SM-Piyes90; 03-09-2014, 03:57 PM.

                              Comment


                                #16
                                Re: What maybe causing this Chopper to short ?

                                I can't see 14v or 38v rails on the schematic - do those rails have a feedback circuit? Perhaps there is something wrong with it.
                                "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                                -David VanHorn

                                Comment


                                  #17
                                  Re: What maybe causing this Chopper to short ?

                                  " i found two bad plastic capacitors ( 0.1 uF 250 V-AC)" What are the caps designator as shown in the diagram, I see C1 (1uF 275V X type) which is across the line. CI is the line filter cap, the unit will still function without it but it will inject the noise into the AC line.
                                  "the main filter cap is a 68 uF , 400 V" the schematic shows two (C3, C4) 680uF 400VDC caps.
                                  Last edited by budm; 03-09-2014, 08:55 PM.
                                  Never stop learning
                                  Basic LCD TV and Monitor troubleshooting guides.
                                  http://www.badcaps.net/forum/showthr...956#post305956

                                  Voltage Regulator (LDO) testing:
                                  http://www.badcaps.net/forum/showthr...999#post300999

                                  Inverter testing using old CFL:
                                  http://www.badcaps.net/forum/showthr...er+testing+cfl

                                  Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                  http://s807.photobucket.com/user/budm/library/

                                  TV Factory reset codes listing:
                                  http://www.badcaps.net/forum/showthread.php?t=24809

                                  Comment


                                    #18
                                    Re: What maybe causing this Chopper to short ?

                                    Sorry if i have mislead you , but am talking about another card witch i use for tryouts , It has the same problem as the one i mentioned earlier ; So when i replace the switching transistor , it shorts again.

                                    what i did next is checked the two interference suppression capacitors , and changed the main filter cap with another one but i doubt it was bad , what am sure of is that those two 0.1 uf suppressor caps were giving only 3 nF and after re[placing them the unit started to work , but i only get two correct outputs : 5v & 3.3 v , the other ( 12,14,5.8,38) voltages give respectively ( 22,22,7.6,64) , is there a possibility those outputs were meant to be regulated on another terminal ?? this card BTW belongs to an old satellite receiver .


                                    Here is the second card :
                                    Last edited by SM-Piyes90; 03-10-2014, 06:37 AM.

                                    Comment


                                      #19
                                      Re: What maybe causing this Chopper to short ?

                                      Probably better if you start a new thread for this other PSU. But have you checked the output capacitors? If they are dead, bad regulation problems can occur.
                                      "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                                      -David VanHorn

                                      Comment


                                        #20
                                        Re: What maybe causing this Chopper to short ?

                                        Originally posted by Agent24 View Post
                                        Probably better if you start a new thread for this other PSU. But have you checked the output capacitors? If they are dead, bad regulation problems can occur.
                                        I need ESR meter for that .

                                        I guess there is no need to start another thread .

                                        Thread can be closed .

                                        Comment

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