need some help with resistor values

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  • petehall347
    Badcaps Legend
    • Jan 2015
    • 4426
    • United Kingdom

    #1

    need some help with resistor values

    have a battery charger here that blew itself up like they do .only trouble is i cant find a schematic for it .its a 18v workzone one .so far there are 2 obvious blown resistors and maybe another one , plus the fet is totally shorted which i could do with a cheaper sub for as expensive to buy .
    here are some pics .will post more in next post .
    ah yes unknown resistors are R9 R5 R11.
    Attached Files
    Last edited by petehall347; 11-26-2019, 03:59 PM.
  • petehall347
    Badcaps Legend
    • Jan 2015
    • 4426
    • United Kingdom

    #2
    Re: need some help with resistor values

    more pics .oh the fet is 4n80
    Attached Files

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    • petehall347
      Badcaps Legend
      • Jan 2015
      • 4426
      • United Kingdom

      #3
      Re: need some help with resistor values

      just had a thought .wondered if i might pick up a plug in charger and simply connect the wires to charger socket to save money ?
      if so any special charger needed ? i am guessing not .only 2 connections to battery +-

      maybe this then i could put dc socket into the case of charger box . https://www.ebay.co.uk/itm/18V-1-5A-...kAAOSwZvRaYY-o
      Last edited by petehall347; 11-26-2019, 05:31 PM.

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      • R_J
        Badcaps Legend
        • Jun 2012
        • 9535
        • Canada

        #4
        Re: need some help with resistor values

        If R5 (gate resistor) is blown, it is likely that ic1 is also defective. R9 is likely a low value 1Ω or less, R5 could be around10~30Ω R11 seems to be 100 which would be 10Ω
        Last edited by R_J; 11-26-2019, 05:56 PM.

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        • petehall347
          Badcaps Legend
          • Jan 2015
          • 4426
          • United Kingdom

          #5
          Re: need some help with resistor values

          yes i wondered about the ic cant see what it is though yet .

          edit ...
          looks like 6B55322 . the 3 might be a 1 .
          Last edited by petehall347; 11-26-2019, 06:04 PM.

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          • petehall347
            Badcaps Legend
            • Jan 2015
            • 4426
            • United Kingdom

            #6
            Re: need some help with resistor values

            clear ish pic of ic
            Attached Files

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            • R_J
              Badcaps Legend
              • Jun 2012
              • 9535
              • Canada

              #7
              Re: need some help with resistor values

              The ic seems to be a Cr6855 60w current mode PWM control chip
              Last edited by R_J; 11-26-2019, 06:39 PM.

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              • petehall347
                Badcaps Legend
                • Jan 2015
                • 4426
                • United Kingdom

                #8
                Re: need some help with resistor values

                what frequency should it be at to gate ? am thinking i could remove the shorted fet to test the frequency and voltage ?

                edit think i found correct value at 65khz .
                Last edited by petehall347; 11-26-2019, 06:43 PM.

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                • petehall347
                  Badcaps Legend
                  • Jan 2015
                  • 4426
                  • United Kingdom

                  #9
                  Re: need some help with resistor values

                  wondering if this is correct mosfet ? https://uk.rs-online.com/web/p/mosfets/7925754/

                  Comment

                  • petehall347
                    Badcaps Legend
                    • Jan 2015
                    • 4426
                    • United Kingdom

                    #10
                    Re: need some help with resistor values

                    r11 is actually 10k and still good .its R4 i cant figure out .i see it starts with a 4 .
                    reads about 280k in circuit

                    edit .removed from circuit it measures 1.3 meg . which doesn't seem right as first digit is a 4
                    Last edited by petehall347; 11-27-2019, 08:23 AM.

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                    • petehall347
                      Badcaps Legend
                      • Jan 2015
                      • 4426
                      • United Kingdom

                      #11
                      Re: need some help with resistor values

                      i looked at the frequency and everything seems to be at 50hz which is of course the mains supply frequency.

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                      • stj
                        Great Sage 齊天大聖
                        • Dec 2009
                        • 30993
                        • Albion

                        #12
                        Re: need some help with resistor values

                        that's strange, it has a rectifier so even if the cap is fucked it should be 100Hz

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                        • petehall347
                          Badcaps Legend
                          • Jan 2015
                          • 4426
                          • United Kingdom

                          #13
                          Re: need some help with resistor values

                          Originally posted by stj
                          that's strange, it has a rectifier so even if the cap is fucked it should be 100Hz
                          might be my meter playing up .only way i can get any sense of frequency measurements is to use just one meter probe am sure it used to work fine if say i put red into live and black to neutral on the mains . i even did something silly and happened to have black probe in my hand and touched red to gate out on the ic . it sure felt like 50hz through my hand .
                          will have another play with it shortly .

                          Comment

                          • stj
                            Great Sage 齊天大聖
                            • Dec 2009
                            • 30993
                            • Albion

                            #14
                            Re: need some help with resistor values

                            probably read 50Hz in the air, radiating from all the wiring in the building.

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                            • petehall347
                              Badcaps Legend
                              • Jan 2015
                              • 4426
                              • United Kingdom

                              #15
                              Re: need some help with resistor values

                              ok new findings and things blowing up .
                              i replaced R9 with a 1k resistor just to have something to make up the circuit . checked frequency at gate out and now zero hz according to my meter had one probe on main cap minus and other on gate out pin of the ic . then a few seconds later 2 rectifier diodes blew up .didnt half make me bloody jump . bearing in mind the mosfet is removed . i have no idea if its because of that or not

                              edit
                              ok tacked in a couple of 1n4004 i think or might be 1n4148 plus botched 5 amp fuse wire accross the blown fuse .anyway that doesnt matter right now .back to 300v dc and zero hz .
                              parts count is adding up now . as need 4 diodes and a fuse including . am guessing rectifier was on its way out anyway .
                              Last edited by petehall347; 11-27-2019, 10:12 AM.

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                              • petehall347
                                Badcaps Legend
                                • Jan 2015
                                • 4426
                                • United Kingdom

                                #16
                                Re: need some help with resistor values

                                diodes marked M7 so am i right thinking they are 1N4007 ?

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                                • R_J
                                  Badcaps Legend
                                  • Jun 2012
                                  • 9535
                                  • Canada

                                  #17
                                  Re: need some help with resistor values

                                  R9 will be the source resistor for the fet, The ic monitors that line to control the current, If you used a 1K, and the fet turned on, the voltage developed across the resistor will blow CS input of the ic, Max input on that pin is usually 5~7 volts
                                  R9 is usually less than 1 ohm, Also if the fet shorted drain to gate, and blew R5, I can not see how the ic would handle 300vdc going to it.
                                  Last edited by R_J; 11-27-2019, 11:37 AM.

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                                  • petehall347
                                    Badcaps Legend
                                    • Jan 2015
                                    • 4426
                                    • United Kingdom

                                    #18
                                    Re: need some help with resistor values

                                    the fet is out of circuit .was hoping to see if the ic was working or not . can this be done without the fet in circuit ?

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                                    • R_J
                                      Badcaps Legend
                                      • Jun 2012
                                      • 9535
                                      • Canada

                                      #19
                                      Re: need some help with resistor values

                                      Without the fet, the ic will not see any voltage on the CS pin, it might try and output a few cycles, then shut down, it depends on how the ic operates.
                                      A similar ic would be the ON-Semi NCP1251, its pdf explains how it operates and it may be similar to the Chip-Rail ic works
                                      Last edited by R_J; 11-27-2019, 11:57 AM.

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                                      • petehall347
                                        Badcaps Legend
                                        • Jan 2015
                                        • 4426
                                        • United Kingdom

                                        #20
                                        Re: need some help with resistor values

                                        so are you saying a NCP1251 will be a direct replacement ?

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