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  1. #1
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    how to check mosfet

    Guys please help,
    DO you know how to check whether mosfet is good or not in circuit?
    Understand if it always short drain source then we can check it in circuit but sometime there is a case where mosfet switching is not function properly.

    Usually I will lift up it lead or remove it and check whether its swithing is ok or not.

    Do you have better idea or way to check this?

    Please help guys I am really beginner.

    •   Alt24th May 2007, 02:56

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    how to check a mosfet

    Checking if parts are working correctly in-circuit is possible .... but you need to know what kind of voltage/resistance/signal you have between two given nodes of the circuit when everything is working well. Otherwise the best way to be sure that the component is good is to remove it from the circuit and then test it.



    •   Alt27th July 2007, 09:00

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  3. #3
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    checking mosfet

    For N-Type, put a multimeter (set for continuity) in the drain to source of the FET and inject a voltage acros VGS. Drain and source should have very small resistance when you inject voltage at VGS..

    Quote Originally Posted by fazan83
    Guys please help,
    DO you know how to check whether mosfet is good or not in circuit?
    Understand if it always short drain source then we can check it in circuit but sometime there is a case where mosfet switching is not function properly.

    Usually I will lift up it lead or remove it and check whether its swithing is ok or not.

    Do you have better idea or way to check this?

    Please help guys I am really beginner.



    •   Alt30th July 2007, 05:13

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  4. #4
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    how to check mosfet

    i dont suggest testing a MOSFET directly while it is in a circuit... if you try removing its pins and testing then you are in for trouble... especially if it involves your GATE terminal(it is very susceptible to ESD)....

    maybe you can check whether the correct voltages and current without disturbing the MOSFET are present in the circuit while it is working..



  5. #5
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    mosfet checking

    If a FET fails it most commonly give a short circuit between Source and Drain. (in power applications) You can messure the resistance between these pins. If it is lower than a MEG it is probably dead.

    goodluck..

    PS: don't forget to power down your device.. :D



    •   Alt1st August 2007, 08:47

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  6. #6
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    check mosfet

    check impedance between drain and source as many suggested. You can also check for diode. Since you have NMOS in your circuit, use multimeter in diode mode, place (-) at drain and (+) at source.

    Just an idea. I assume drain of Q2 is Vsupply while source of Q1 is GND. Remove the gate resistor R22 and R21, and then short gate and source together by wires. Apply a small voltage with current limit at drain of Q2 and measure the voltage at S(Q2).



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