Smoked wiper pot

Discussion in 'General Electronics Chat' started by gauss77, Mar 22, 2013.

  1. gauss77

    Thread Starter New Member

    Mar 22, 2013
    4
    0
    Hi,

    I put together a little JFET curve tracer for my scope in XY mode but managed to burn the pot wiper to control the Vgs level.

    Wondering if anyone can see an obvious flaw in my design?

    Thanks,
    G

    ***EDIT***

    Shoot, I think I should have wired the 18Vdc supply in reverse polarity as the gate actually needs to be at a lower potential than the source...

    MPF102 Igf(max) = 10mA so I can see why this would cause the JFET to fail but why did this cause the pot wiper to fail?

    Sources:
    18Vdc @ 1A wallwart (DC)
    7.5Vrms @ 200mArms wallwart (AC)

    http://imageshack.us/photo/my-images/845/octopuscircuit.png
     
    Last edited: Mar 22, 2013
  2. gauss77

    Thread Starter New Member

    Mar 22, 2013
    4
    0
    I didn't think that the gate current would be that large as the gate has such a high input resistance, adding to my confusion.
     
  3. gauss77

    Thread Starter New Member

    Mar 22, 2013
    4
    0
    Ah, forward biasing this PN junction was not a good idea when cranking the pot wiper more positive... Too much current and now I have a burnt pot and bum JFET.

    I figured this one out. Damn.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Are you certain you have the pinout on the JFET correct? If you had the gate and the source swapped, that looks like trouble for the pot.
     
  5. gauss77

    Thread Starter New Member

    Mar 22, 2013
    4
    0
    Yeah, it was wired correctly.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    A lot of circuits similar to yours place a resistor that would represent the minimum acceptable either before the power feed to the pot or in series with the pot wiper.
     
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