pulse circuits with MOSFET

Discussion in 'The Projects Forum' started by yerff, Sep 10, 2013.

  1. yerff

    Thread Starter New Member

    Sep 9, 2013
    3
    0
    Hello all,

    I'm new to circuit and in fact I'm more of a mechanical student. I have a problem
    to an experiment which requires me to continuously apply square pulse of between 10-35V, where the rise time of the pulse should be faster than 50us, dwell time of 250us and regardless of fall time.

    I was told I can do it with a mosfet, so I bought some fujielectronic MOSFET, 2SK3362-01 and tried to give it a shot.

    I attach the simple connection schematic I tried, The drain is connected to a Agilent powersupply, 25V. When I turn on the function generator, it seems like the voltage rise is slower than I what I need, and if I increase the frequency, the voltage can't even rise to 25V, could it be due to the power supply? Or is there something wrong with my circuit??:confused: Please help.

    thanks,
    Yerff
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,990
    3,226
    What is the purpose of the 100 ohm source resistor?
     
  3. yerff

    Thread Starter New Member

    Sep 9, 2013
    3
    0
    Hi Zapper,

    It is a pull down resistor, even if I take it off, the circuit output still look the same.
     
  4. Experimentonomen

    Member

    Feb 16, 2011
    331
    46
    Because the falstad simulator does not include the gate charge of a mosfet which just like a capacitor stay charged = fet remains on without the pulldown.
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,523
    @Yerff and Experimentomen, not the pull down resistor, but the one between source and ground. With a resistor in that position it makes your mosfet essentially a high side switch. It starts to turn on fast then slows down as the source voltage rises, when you add a component between source and ground. What you are wanting to do is called a 'low side switch', the source should be connected directly to ground for that.
     
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