Need help with MOSFET tests and repairs

Discussion in 'General Electronics Chat' started by gamerguy, Mar 7, 2014.

  1. gamerguy

    Thread Starter New Member

    Mar 7, 2014
    Hello everyone,

    I am trying to find out the issue with my gaming graphics card which is not showing any display. I checked the voltage of both GPU and

    memory with the help of a multimeter(the card has built in point for checking voltages). They are running at optimum level. So I thought of checking the MOSFETs first.

    I could find 4 big MOSFETs and about 12 smaller ones.

    The bigger ones are P0603BD(x2) and P0903BD(x2) (both are n-channel).
    (These seem to be the datasheets for them P09 P06)

    Anyway, I checked the resistance between the source and drain(by multimeter) for all the four of them by connecting black to the Drain

    and red to the Source.

    When multimeter was set to 2000 Ohm,
    The P09s gave reading of 535 and 511 Ohm
    The P06s gave 130 and 002 Ohm

    When multimeter was set to 20k Ohm,
    The P09s gradually increased till 6.28k and 11.2k Ohm respectively
    The P06s gave 0.11k and 0.00k Ohm

    What do these results indicate? Is any of them dead? Do I need to run any more tests?
    Please let me know if any more details are needed as I am a total n00b in electronics. It's been a long time since I last studied them.
    Thanks in advance.
  2. wayneh


    Sep 9, 2010
    Without controlling the gate voltages, I don't think your tests mean anything. :(
    I'm not sure you can test on-board, except for MOSFETs that have failed to a dead short. You may need to remove them first.
  3. gamerguy

    Thread Starter New Member

    Mar 7, 2014
    Hi. Thanks. I'm extremely sorry for the late reply. I had some health issues.
    Anyway, if I manage to remove the MOSFETs from the card, what tests should I conduct to determine whether it is working or not?
  4. Flug540


    Mar 24, 2012
    I just built myself a simple MOSFET tester last night. Basically I'm supplying voltage to the gate through a switch and connecting an LED with a resistor in series to the drain. Simple and effective.

    Another thing I would check is big electrolytic capacitors. They tend to go bad with age.
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Do you have a multimeter?

    You could test:
    1. Body diode through a diode test from source to drain
    2. connect a power supply/battery with a lamp, LED/resistor or any other load in series with drain/source. Then control the gate with a for example a 9V battery. + to gate, - to source.

    I would connect a resistor of anything between 10k and 100k between gate and source while handling it. Don't leave the gate floating, it's ESD sensitive. Also, the gate, once charged needs to be discharged in order to switch the MOSFET off. You can short gate source (if you decide not to use a resistor)
  6. ian field

    AAC Fanatic!

    Oct 27, 2012
    You can precharge the gate capacitance with nothing more than a continuity checker. That will either turn the MOSFET on or off depending on polarity - that of course can be tested with the same continuity checker.

    Some continuity checkers are more sensitive than others - you can get a 2 LED type with a PTC resistor for indicationg any voltage from 4 - 450V, and contain a tiny 12V battery for continuity - the one I have shows a glow with about 22k resistor.

    With such a checker, any leakage is a fail - the MOSFET will be either on or off depending which way you charge the gate capacitance.

    Even my checker might not show "fail" gate leakage - but if you charge the gate to make the drain/source conduct and the gate charge leaks away - that's a fail.
  7. namar

    New Member

    Nov 27, 2014