Alternative to pull down resistor?

Discussion in 'General Electronics Chat' started by kenw232, Apr 26, 2015.

  1. kenw232

    Thread Starter Active Member

    May 18, 2009
    I have a gate of an SCR that should be pulled to ground when off. But if I connect it to ground via a pull down resistor I will fry the gate from excesssive backemf from a high voltage pulse coil. I cannot stop the backemf, flywheel diode does nothing. So I appear to be stuck with a floating gate.

    Are there any alternatives to pull my gate down or make sure it is otherwise off and does not have any parasitic capacitance which might leave it high? Its currently being turn on and off by an ardunino digital pin out and a 100 or so ohm resistor in series.
  2. #12


    Nov 30, 2010
    An SCR is not a CMOS device. It requires current to turn it on. Just don't apply enough current to turn it on when you are not applying the turn-on voltage.
  3. SamEricson


    Apr 25, 2015
    Enhancement N or P channel power mosfet can be used as a pull down to sink the voltage.
  4. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    Another option would be to use a gate pulse transformer. The secondary would keep the gate at cathode potential unless pulsed by energy to the primary.
  5. Roderick Young


    Feb 22, 2015
    I'm having trouble visualizing how the high voltage pulse can get to the gate of your SCR, sorry. Can you sketch a schematic of your concern? The pull down resistor from gate to cathode can be a large value, like 100k, and you could put a small value capacitor in parallel with that for additional noise immunity.