Newbie Testing P Channel Mosfet

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  • orientalsniper
    Badcaps Veteran
    • Apr 2013
    • 459
    • USA

    #1

    Newbie Testing P Channel Mosfet

    Hello all, I've read across different sites (badcaps included)

    To test a P-Channel mosfet, I have concluded the following:

    1) - plug to gate, + plug to source = open circuit reading
    2) + plug to drain, - plug to source = open circuit reading
    3) + plug to gate, - plug to source = open circuit reading
    4) + plug to gate, - plug to source = short circuit reading

    Now, the mosfet I'm testing is this

    I've performed the test to 3 same mosfet in a laptop motherboard and I've got different measurements, they are still soldered to the board, would that make any difference, or did I misunderstand the basic tests?
  • capkid
    Badcaps Legend
    • Oct 2010
    • 1339
    • United States

    #2
    Re: Newbie Testing P Channel Mosfet

    I'd de-solder them if your results are inconclusive.
    LG Plasma Mal-Discharge Correction Service

    Comment

    • eccerr0r
      Solder Sloth
      • Nov 2012
      • 8688
      • USA

      #3
      Re: Newbie Testing P Channel Mosfet

      Can't test these in circuit, there tends to be enough circuit stuff that will discharge the gate before you can make a good reading.

      To test a P-channel device basically you can do the exact same measurements as an N-channel, except swap the probes.

      In circuit there should still be no "shorts" unless the transistor is somehow on (which it shouldn't if it's not powerred up). If you see any shorts when testing in a dead circuit, likely that transistor is fried and something else killed it.

      Comment

      • momaka
        master hoarder
        • May 2008
        • 12170
        • Bulgaria

        #4
        Re: Newbie Testing P Channel Mosfet

        Originally posted by eccerr0r
        In circuit there should still be no "shorts"
        Not true.


        The lower drive MOSFETs on CPU and GPU VRM may (and usually do) appear shorted. It all depends on the GPU and/or CPU, though. Some GPUs/CPUs have low internal resistance, others not so much. So just beware when checking those. You'll be troubleshooting a problem that is not there.

        Comment

        • dj_ricoh
          Badcaps Legend
          • Jan 2013
          • 2073
          • uk

          #5
          Re: Newbie Testing P Channel Mosfet

          Originally posted by eccerr0r
          In circuit there should still be no "shorts".
          90% of all mosfet near cpu show short untill you take out the CPU
          Just cook it! It's already broken.

          Comment

          • orientalsniper
            Badcaps Veteran
            • Apr 2013
            • 459
            • USA

            #6
            Re: Newbie Testing P Channel Mosfet

            Thanks guys, so the general consensus is to desolder them and test out of circuit.

            Also can someone confirm the 4 steps test I'm doing is correct?

            Comment

            • capkid
              Badcaps Legend
              • Oct 2010
              • 1339
              • United States

              #7
              Re: Newbie Testing P Channel Mosfet

              Here's how I test N-channel mosfets with my Fluke 87. P-Channels will be the opposite:

              (1) + Drain, - Source = Open
              (2) - Drain, + Source = Reading around 0.5V
              (3) + Drain, - Source then quickly touch + to Gate and back to Drain = Voltage climbs steadily as mosfet is charging
              (4) Touch Gate and Source simultaneously with your finger(s) while maintaining probes in position to discharge = Open
              (5) Repeat step 3: + Drain, - Source then quickly touch + to Gate and back to Drain = Voltage climbs steadily as mosfet is charging

              You can also build your own cheap mosfet testers (see attachments).
              Attached Files
              Last edited by capkid; 08-10-2013, 01:13 PM.
              LG Plasma Mal-Discharge Correction Service

              Comment

              • redwire
                Badcaps Legend
                • Dec 2010
                • 3900
                • Canada

                #8
                Re: Newbie Testing P Channel Mosfet

                orientalsniper, I think your 4 steps are wrong.
                First, what do you consider a "short circuit"? For a proper test, I would use diode test on the multimeter and also ohms. Both give different and good information. Write down your readings.
                Second, the gate (on a good mosfet) will always read open circuit (your step 4 shows a problem) if the mosfet is out-of-circuit. Only N-ch mosfets used in car ignition modules have a zener diode connected D-G.
                Third, I have seen when testing a mosfet out of circuit, stray static electricity or multimeter can charge the gate, the mosfet turns on and testing says D-S leaky or shorted, when really the mosfet is just turned on. Shorting G-S ensures the mosfet is off for the D-S test.

                I do a multimeter test in-circuit and if I get suspicious readings, I pull it out and then test out-of-circuit.

                Most mosfets fail with a shorted D-S or G-S so I look for that first.

                Comment

                • eccerr0r
                  Solder Sloth
                  • Nov 2012
                  • 8688
                  • USA

                  #9
                  Re: Newbie Testing P Channel Mosfet

                  Ah that's true, it does depend on the circuit, the resistance is pretty low for the D-S junction for low voltage high current loads (I'm too used to the main switcher in on-line SMPS). In any case G-S/G-D should not be shorted even in circuit, and yes removing from circuit is needed for accurate measurement.

                  Comment

                  • orientalsniper
                    Badcaps Veteran
                    • Apr 2013
                    • 459
                    • USA

                    #10
                    Re: Newbie Testing P Channel Mosfet

                    Haha, let me see if I get that.

                    Basic test for P channel mosfet (assuming N channel would be opposite):
                    -G +S = Open
                    -G +D = Open
                    -D +S = Open

                    If they are shorted, mosfet is gone gone.

                    Comment

                    • dj_ricoh
                      Badcaps Legend
                      • Jan 2013
                      • 2073
                      • uk

                      #11
                      Re: Newbie Testing P Channel Mosfet

                      Originally posted by orientalsniper
                      Haha, let me see if I get that.

                      Basic test for P channel mosfet (assuming N channel would be opposite):
                      -G +S = Open
                      -G +D = Open
                      -D +S = Open

                      If they are shorted, mosfet is gone gone.
                      but you have to test it in diode mode to and see the value.
                      Just cook it! It's already broken.

                      Comment

                      • redwire
                        Badcaps Legend
                        • Dec 2010
                        • 3900
                        • Canada

                        #12
                        Re: Newbie Testing P Channel Mosfet

                        I made this table, I think it might help, or add confusion...
                        Attached Files

                        Comment

                        • trebo
                          Badcaps Veteran
                          • Feb 2010
                          • 514
                          • Wales, UK

                          #13
                          Re: Newbie Testing P Channel Mosfet

                          @redwire
                          thanks redwire
                          your post and table really help us to understand the testing method of the mosfet
                          can you just clarify for us, as to wether we short G-S (as in #8 post) or G-D (as in #12 post) to turn off the mosfet, or is it okay to use either method to turn of mosfet.???
                          and thanks to all who have contributed to this helpful thread

                          Comment

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