NMOS reversed drain-source

Discussion in 'General Electronics Chat' started by m1ch43l, Aug 25, 2013.

  1. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    Hello,
    I've reversed the roles of the drain and source pin.
    What would happen if the source pin of a NMOS were to be connected to +ve of a load or the rail voltage and the drain pin connected to the + of a load or to ground, then driven with a PIC?

    I can't seem to shut them off in their proper configuration: drain to -ve of load and source to ground. A pull-down res doesn't help either. What possibly happened?

    thanks.
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    So that's why it cant turn off once started...
     
  4. crutschow

    Expert

    Mar 14, 2008
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    3,229
    Show a schematic of the "proper configuration" that doesn't work.
     
  5. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    The image labelled test was what i did first. Don't yell. I just wanted to know what happens when a NMOS is used this way. Saw in a schematic.

    The second image represents the "normal configuration" that I refer to.
    Seems once I signal the FET it remains on. I did try to ground the gate, as well as using sub-zero voltage but once I turn on, it remains on.

    Testing the D-S junction's resistance, it is high enough, G-S is ok as well as D-G. This was just to make sure the junction was undamaged.

    This only leaves the body diode as stated above by the first respondent. I however am yet to understand what problems with the body diode.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    In the second image, if you ground the gate to the source then the MOSFET should turn off. If it doesn't there's something wrong with the MOSFET.
     
  7. m1ch43l

    Thread Starter Member

    Aug 16, 2012
    62
    1
    Ok. Ditching mos for a new one. At least I've now known what happens. wont bother with the why:cool:
     
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