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    #21
    Re: need help to test inverter transformer

    Originally posted by woaimi View Post
    well, i got the fuses, soldered it in and it blown instantly, so I followed your suggestion. the rectifier is good but I couldn't confirm the mosfets (following your ohm method) as each time i try i get different result.
    Always post your actual readings from your multimeter. We have had people state something was "good" only to find out they are mis-interpreting their results.

    Also post your multimeter make and model so we know what you are using.
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      #22
      Re: need help to test inverter transformer

      I'm using a cheapy Craftsman model 82141. I changed battery, set to 200 ohm the screen read "1" in open position and 00.3 when probes together. When I check the mosfets at 200 ohm setting I get out of range problem so I switch to 2000 ohm setting instead. Here what I've got from testing multiple times.
      Referring to the photo that retiredcaps circled the mosfets
      The top one (by itself)
      S-G: with red probe on S = 528 Ohm, with red probe on G = 1032 Ohm
      G-D: with red probe on G = 1695 Ohm, with red probe on D = no reading
      S-D: with red probe on S = 400 Ohm, with red probe on D = no reading

      The pair below
      The left one
      S-G: with red probe on S = 22 Ohm, with red probe on G = 22 Ohm
      G-D: with red probe on G = 427 Ohm, with red probe on D = no reading
      S-D: with red probe on S = 403 Ohm, with red probe on D = no reading
      The right one
      S-G: with red probe on S = 22 Ohm, with red probe on G = 22 Ohm
      G-D: with red probe on G = 425 Ohm, with red probe on D = no reading
      S-D: with red probe on S = 400 Ohm, with red probe on D = no reading

      On the rectifier: meter was set at diode mode

      Between (+) and the 2nd leg with red probe on (+) = no reading, with black probe on (+) = 548
      Between the 2nd and 3rd leg = no reading
      Between the 3rd leg and the (-) with red probe on (-) = 547, with black probe on (-) = no reading

      Comment

        #23
        Re: need help to test inverter transformer

        Originally posted by woaimi View Post
        The pair below
        The left one
        S-G: with red probe on S = 22 Ohm, with red probe on G = 22 Ohm
        The right one
        S-G: with red probe on S = 22 Ohm, with red probe on G = 22 Ohm

        On the rectifier: meter was set at diode mode

        Between (+) and the 2nd leg with red probe on (+) = no reading, with black probe on (+) = 548
        Between the 2nd and 3rd leg = no reading
        Between the 3rd leg and the (-) with red probe on (-) = 547, with black probe on (-) = no reading
        1) Multimeter tutorial. I have written about this before several times and just last week.

        2) A multimeter always gives a reading. There is no such thing as "no reading". If your multimeter is displaying "1" on the left hand side of the lcd, it is telling you the measurement is "out of range".

        3) For finding a short, we are only interested if the reading is below 30 ohms. We don't care if the actual reading is 200 ohms, 2000ohms, or 2M ohms. So on the lowest scale (200 ohms), if the multimeter shows a "1" on the left hand side of the screen, it is telling you the reading is > 200 ohms. That is, not shorted.

        4) Most people understand green means go and red means stop. However, if I say green means don't stop and red means don't go, it messes people up. This is why #3 above is so hard for people to understand.

        5) The "pair below" are shorted S-G (22 ohms is shorted). Desolder both and retest out of circuit.

        6) For the bridge rectifier test, which I have written many times, I prefer to see this test. Number the bridge rectifier pins 1, 2, 3, and 4. Measure the resistance between pins 1-2, 1-3, 1-4, 2-3, 2-4, and 3-4. Report all readings.
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          #24
          Re: need help to test inverter transformer

          please excuse my ignorant! I just wanted to make sure I get some real values other than 1 so I changed the mode setting. Anyhow, I got the mosfets desoldered will test them when i get home tonight.

          the rectifier resistance report, I start from left the right with 1 being the pin labeled with (-).
          1-2 = 453
          1-3 = 453
          1-4 = 453
          2-3 = 1.325M
          2-4 = 451
          3-4 = 453
          does that look right?
          many thanks

          Comment

            #25
            Re: need help to test inverter transformer

            Originally posted by woaimi View Post
            the rectifier resistance report, I start from left the right with 1 being the pin labeled with (-).
            1-2 = 453
            1-3 = 453
            1-4 = 453
            2-3 = 1.325M
            2-4 = 451
            3-4 = 453
            Those readings seem a bit strange. They indicate the rectifier is not shorted, but 5 of your readings are all 453 ohms. Desolder the mosfets first. With the mosfets out, retest the rectifier "in circuit" and report the readings again.
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              #26
              Re: need help to test inverter transformer

              with the 2 suspected mosfets removed this is what I've got from the rectifier
              1-2 =33K
              1-3 = 34.5K
              1-4 = 33K
              2-3 = 1.36M
              2-4 = 34.5K
              3-4 = 34.6K

              Comment

                #27
                Re: need help to test inverter transformer

                Originally posted by woaimi View Post
                with the 2 suspected mosfets removed this is what I've got from the rectifier
                1-2 =33K
                1-3 = 34.5K
                1-4 = 33K
                2-3 = 1.36M
                2-4 = 34.5K
                3-4 = 34.6K
                The readings above suggest your bridge rectifier is NOT shorted.

                Retest the mosfets now that you have removed them and post the readings.
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                  #28
                  Re: need help to test inverter transformer

                  should i be using the same "under 30 Ohm method" ?

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                    #29
                    Re: need help to test inverter transformer

                    Originally posted by woaimi View Post
                    should i be using the same "under 30 Ohm method" ?
                    Yes, measure the resistance between pins 1-2, 1-3, and 2-3.
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                      #30
                      Re: need help to test inverter transformer

                      mosfet #1
                      G-D = 1
                      G-S = 1
                      D-S = 265K

                      mosfet #2
                      G-D = 1
                      G-S = 1
                      D-S = 269K

                      Comment

                        #31
                        Re: need help to test inverter transformer

                        Originally posted by woaimi View Post
                        mosfet #1
                        G-D = 1
                        G-S = 1
                        D-S = 265K

                        mosfet #2
                        G-D = 1
                        G-S = 1
                        D-S = 269K
                        If that is 1 ohm, then both mosfets are definitely shorted and bad. If it is out of range, then the mosfet is good. So which is it?
                        Last edited by retiredcaps; 07-20-2012, 08:56 PM.
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                          #32
                          Re: need help to test inverter transformer

                          usually if something out of range I would be able to see the display flashes some value then go to 1. Not this case, number 1 remain the same as an open circuit. i'll probably replace them anyway. what's bad?

                          Comment

                            #33
                            Re: need help to test inverter transformer

                            Originally posted by woaimi View Post
                            usually if something out of range I would be able to see the display flashes some value then go to 1. Not this case, number 1 remain the same as an open circuit.
                            You are not correct in your interpretation. I think you need to re-read post #23 again.

                            If my explanation isn't clear, then try

                            http://www.sparkfun.com/tutorials/202
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                              #34
                              Re: need help to test inverter transformer

                              sorry for my previous senseless comment, i was typing my own thoughts (like thinking out loud). It wasn't 1 ohm, it was the 1 on the left side of the screen.

                              thanks for that very useful tutorial

                              Comment

                                #35
                                Re: need help to test inverter transformer

                                Okay, so now that you understand the mosfets are not shorted, there are 4 other diodes in the primary section (as outlined on the pcb by the dotted line) that are suspects in causing the fuse to blow.

                                Test all 4 in circuit first using the diode test function on your multimeter. Report all your readings. Again you should have 2 readings per diode. If any of them look suspicious, then desolder them to verify out of circuit.
                                Attached Files
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                                  #36
                                  Re: need help to test inverter transformer

                                  got the measurements includes in the photo. there was 1 (the one with 597 &1) initially was 1 & 1 in both direction. I desoldered it, retested still 1 &1, 15 mins later before i post result i retested again, that's when it gave me 597 &1
                                  Attached Files

                                  Comment

                                    #37
                                    Re: need help to test inverter transformer

                                    Originally posted by woaimi View Post
                                    got the measurements includes in the photo. there was 1 (the one with 597 &1) initially was 1 & 1 in both direction. I desoldered it, retested still 1 &1, 15 mins later before i post result i retested again, that's when it gave me 597 &1
                                    The above is odd behaviour. I would retest that diode again out of circuit maybe every 15 minutes and check the results again.
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                                      #38
                                      Re: need help to test inverter transformer

                                      done it many times since then and getting the same readings. there are only the caps and resistors are untested in the primary. Should those diodes (i think) on the secondary be concerned? These devices look like transistors with 3 leads but the board indicates with Dxxx.
                                      Attached Files

                                      Comment

                                        #39
                                        Re: need help to test inverter transformer

                                        Originally posted by woaimi View Post
                                        done it many times since then and getting the same readings. .
                                        What same readings 1 and 1 or 1 and 597 ?
                                        Please upload pictures using attachment function when ask for help on the repair
                                        http://www.badcaps.net/forum/showthread.php?t=39740

                                        Comment

                                          #40
                                          Re: need help to test inverter transformer

                                          Originally posted by woaimi View Post
                                          Should those diodes (i think) on the secondary be concerned? These devices look like transistors with 3 leads but the board indicates with Dxxx.
                                          Yes those are diodes some examples are here
                                          http://www.westfloridacomponents.com...ierDiodes.html

                                          Can you make out any part numbers on them?
                                          Please upload pictures using attachment function when ask for help on the repair
                                          http://www.badcaps.net/forum/showthread.php?t=39740

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