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Help with mosfet and resistors

  1. Feb 26, 2008 #1
    Hi I am trying to figure out how to determine the size of the resistors I need to put in line of the gate wire to turn on and off the mosfet in a circuit for an airsoft gun
    I have several mosfets laying around the 1 I want to use is a 50a 100v
    that said what size resistors do I need to put in line for the gate 100ohm ?watt and what watt do I need to use for the discharge a 30kohm ? watt
    like they have in this schematic

    http://www.airsoftmechanics.com/home/content/view/23/30/1/6/ [Broken]

    any help will be greatly appreciated
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Feb 27, 2008 #2


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    The wattage of the resistor can be computed from the two equations E=IR and P=EI.
    In this case E would be your battery voltage.
    In selecting the resistor the common practice is to double the computed wattage to allow for environmental effects.
  4. Feb 27, 2008 #3
    first I would like to say thanks for replying
    next is there a way you could explain that in a more simple way I am just starting out in electronics and know very little I need it in layman's terms please
  5. Feb 27, 2008 #4


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    R = Resistance
    I = Current
    E = Voltage (or EMF)
    P = Power (or Watts)

    You probably don't know the current.
    The battery voltage you can get from the battery.
    The resistance you stated.

    A little algebra and the two equations will let you solve this without knowing the current.
  6. Mar 1, 2008 #5
    The best way to go about it is to see the gate waveform on an oscilloscope. Then you tweak the value of 100ohm and see if any ringing occurs at all. The 30k discharge resistor depends on the gate capacitance and how quickly you want that to be discharged (A rough value is 1600pF but it varies from MOSFET to MOSFET so please look up the datasheet). Time constant= RC.
  7. Mar 1, 2008 #6


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    Not that I actually worked this out, but I suspect that the most current will flow when the trigger switch is pushed putting the battery voltage across the 30k resistor.
    The gate charge works out to < 20nJoules and the repetition rate is essentially 0.
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