testing a FET

Discussion in 'General Electronics Chat' started by johndoe45, Mar 18, 2010.

  1. johndoe45

    johndoe45 Thread Starter Member

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    i am testing a FET

    here is spec sheet

    http://forum.allaboutcircuits.com/attachment.php?attachmentid=17300&d=1268639919

    now here is what one website says.

    using the diode function on your DMM.
    probe with - on the drain and + on the gate. this should arm the FET.

    then keep - on drain and put + on source. multimeter should beep.

    my FET passed this SO FAR.

    now it says to test the FET's ability to switch off touch all three pins with finger. then put - on drain and + on source again and should get a high reading.
    well my FET still beeps. is the FET defective????
  2. retched

    retched AAC Fanatic!

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    Your finger could be defective. :)

    Does your DMM have a npn or pnp test area/function?
    musicinhills likes this.
  3. johndoe45

    johndoe45 Thread Starter Member

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    lol

    i don't think so
  4. retched

    retched AAC Fanatic!

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    ok, try this instead:
  5. retched

    retched AAC Fanatic!

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    If you are getting continuity between drain and source, its a gonner.
  6. johndoe45

    johndoe45 Thread Starter Member

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    gate has infinite resistance to both drain and source

    and no continuity between drain and source

    so i'm guessing it is still good?

    i don't understand this part of third paragraph
    Forward biasing GS with 5V and measuring DS in forward bias should yield very low ohms. In reverse bias, it will still act like a diode.
  7. retched

    retched AAC Fanatic!

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    It is still good.

    The third paragraph is explaining how to test protected FETs as diodes.
    No continuity, no problem. If you had continuity, then you would do a diode check. If you get the diode drop, its still good. If no diode drop, its failed shorted
    johndoe45 likes this.
  8. johndoe45

    johndoe45 Thread Starter Member

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    oh ok. thanks a lot. spent thirty minutes reading up on it and you helped me in only about 5 minutes. :)
  9. retched

    retched AAC Fanatic!

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    Any time I can help, i'll try.
  10. johndoe45

    johndoe45 Thread Starter Member

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    i can't figure out if it is not switching off???

    here is what i did. hold black probe at drain whole time. touch gate with red and then touch source with red while touching gate and drain with finger. a reading shows.
    now to switch it off do i keep my fingers on the drain and gate???

    cause i read you have to touch gate once with red and then touch source again. but doesn't specify if i keep my fingers on drain and gate.
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