What is the purpose of the resistor between the gate and source of an N-FET?

Discussion in 'The Projects Forum' started by imbaine13, Jan 10, 2014.

  1. imbaine13

    Thread Starter Member

    Oct 6, 2013
    Hello everyone,

    I can't seem to find it (the purpose of R1). What if I used a zener diode between the gate and source of an N-FET, and eliminated R1, would the circuit not work? I read that this resistor is to discharge the charges stored on the capacitor in the MOSFET, how true is that, and is it really necessary?
    I know that sometimes R1 is used in series with another resistor to form a voltage divider to turn on the mosfet. Could it be a pull down resistor?
  2. iimagine

    Senior Member

    Dec 20, 2010
    Depends on the input signal. If it is a push/pull type like the output from a 555 timer then R1 is not needed, otherwise R1 is necessary.
  3. wayneh


    Sep 9, 2010
    R1 may be optional but is a good idea to protect the high current circuit from a failure (to "open") of the control signal. R1 pulls the gate charge off and turns the MOSFET off in that case. Obviously it's mandatory if the control signal does not sink enough current to pull out the gate charge on each cycle.
  4. Papabravo


    Feb 24, 2006
    It creates a known level on the gate in the absence of an input signal. The gate is a high impedance node, and when left floating, can assume any voltage due to parasitic coupling, noise and other factors. The resistor prevents the bad stuff from affecting your circuit.
  5. AfdhalAtiffTan

    Senior Member

    Nov 20, 2010
    Depending on how the circuit is viewed, usually it is for turn-off enhancement or like Papabravo said, to make sure the fet turns-off just in case if the gate left floating.

    Operating mosfet in linear region (i.e. not digital 1's and 0's) will generate a lot of waste heat, a 'mosfet driver' could cure the loss.

    Zener could be use to clamp the gate voltage so that it doesn't exceed its breakdown limit, for instance, common number would be 12V.

    Logic level fet would have different limit.
  6. RichardO

    Late Member

    May 4, 2013
    Since the signal going to the gate is labeled Digital_Out I am guessing that the reference is to an input/output port on a microcontroller. When a microcontroller is first powered on or when it is reset all input/output pins are set as inputs. In the case of a MOS-FET, that would allow the gate to float until the software sets the pin to be an output. A resistor like the one shown holds the transistor off until the software has everything set up.
  7. DickCappels


    Aug 21, 2008
    It also suppresses parasitic oscillation and ringing by lowering the Q of the gate circuit.
    Roberto Gatti likes this.