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Can MOSFETs be tested with OhmMeter?

Discussion in 'Electronic Basics' started by Guest, Oct 16, 2004.

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  1. Guest

    Guest Guest

    We can test normal PNP and NPN transistors with OhmMeter.
    Can we test (how) MOSFETs in the same way?
     
  2. Phil Allison

    Phil Allison Guest

    ** You can certainly test for shorts and opens. The drain and source pins
    have ( effectively ) an internal diode connected between them - so you get
    the same result as with a diode. The gate should test open to the other
    pins.

    Testing mosfet operation is not so easy as most mosfets need 4 volts or
    more to bias the Gate so the mosfet will conduct from Drain to Source.




    ............ Phil
     
  3. D MARSHALL

    D MARSHALL Guest

    and dont forget before you test them
    with the meter short all pins first to get good
    reading.
     
  4. **Clip your meter probes to the source and drain. Place a finger between
    gate and drain, then source and drain. Reverse the meter leads and perform
    the above procedure. This is a very basic go, no-go test. Personally, I use
    my Atlas tested, from Farnell. Easy, fast and helpful.
     
  5. Mike

    Mike Guest

    Easy,

    For n-channel mosfets.

    As there is a diode from source to drain, first put some -ve bias
    on the gate so connect meter with +ve on ground (source) and -ve on gate
    to impress a bias on the gate and it will stay for a while if
    the device and air is dry as its a nice capacitor.

    Then, without touching gate, remove leads and connect across
    source and drain, first one way then the other, in one direction
    it will be a diode in the other it should be off.

    Then put +ve bias on gate in reverse of above procedure. Then
    measure again, this will show a lower than normal diode forward
    drop and conduction in the other direction as well if the mosfet
    is OK. On some good mosfets as little as 1.5v gate will show
    only a few ohms across source/drain - if you are fast enough.
    If charge bleeds quickly then gate might have some damage or
    device or air is moist. On good mosfets you can hold a gate
    charge for several minutes.

    On slightly bad mosfets and after putting a +ve bias, reversing
    the source drain with a meter will drain some charge from
    the gate - I consider this slightly marginal as it doesnt seem
    to happen on a new device straight out of the tube but often
    appears to happen to devices that have been close to SOA for
    a while...

    Caution, as mosfets dont need much to blow a gate, 20-30v is all
    it takes at ultra low current to pop a gate !

    You can also measure between gate and source or drain, in either
    direction, should be nothing if gate hasnt popped its insulator.

    Reverse above for p_channel.
     
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