protection diode for mosfet

Discussion in 'The Projects Forum' started by 0001T, May 7, 2008.

  1. 0001T

    Thread Starter Member

    Feb 28, 2007
    15
    0
    Hi, I am making use of a power mosfet (RFD16N05) as a switch to heat up a DUT (12-15V, 2.5A max). Would it be necessary to connect a protection diode across the DUT to prevent damages to the mosfet?

    If the diode is required; how do i size it?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What's your DUT? If it's inductive, then yes - use a protection diode.

    If non-inductive, then no.
     
  3. 0001T

    Thread Starter Member

    Feb 28, 2007
    15
    0
    thanks for the reply.
    DUT is actually a metallic wire.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Then it's inductive, and you should use a protective diode.

    Also consider using a small capacitor in the neighborhood of 100pF-500pF to give the diode time to conduct. Switching is pretty quick, but it's not instantaneous. The capacitor will slow the rise time of the voltage spike.
     
  5. 0001T

    Thread Starter Member

    Feb 28, 2007
    15
    0
    thanks once again
     
  6. Digi Dave

    Member

    Apr 25, 2008
    26
    0
    I would use a Shottky diode such as the 1N5820,as they are fast and have low forward volt drop>>

    Any of these
     
  7. 0001T

    Thread Starter Member

    Feb 28, 2007
    15
    0
    i have wired the diode to across my DUT (wire) and circuit is running okay.
    however, my operation is cyclic in nature...i realised that after a certain cycles (observed to be about 15 cycles), there seems to be some residual current coming from the supposingly 'open' mosfets. i have LEDs connected parallel to the DUT as well...so i observed that the LEDs are still dimly lighted even though the mosfet gate voltage is cut. what could be the cause of this? it seems like the mosfets are behaving somewhat like a capacitor. what should i do?

    the mosfets are controlled by a basic stamp microcontroller, and a 10k ohm resistor is wired in series from the I/O of the basic stamp to the gate of the mosfet.
     
  8. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    The diode could have a high leakage and you could also lower the gate resistor and see what happens.
     
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