Strange problem with mosfet circuit

Discussion in 'The Projects Forum' started by deadlydentures, May 2, 2007.

  1. deadlydentures

    Thread Starter New Member

    May 2, 2007
    3
    0
    So I want to drive this tiny 12v DC motor I have to just mess with some PWM i'm generating. There should be a schematic attached with this.

    The problem is this. The voltage at the source of the mosfet never exceeds 7'ish volts. Strangly enough, when I lower V1's voltage, it had no impact on the source voltage. Only when V1 drops below 7v does Vsource decrease.

    I then switched the iRF540 out for a PN2222, and had exactly the same results, except for the fact that it oscillated at 53MHz, which is when i added the cap across the motor. 0.1u might be a bit high, but its all I had on me at the time.

    I removed the diode thinking maybe I was exceeding the breakdown voltage, but the circuit still performed the same ( i assume the cap was supplying the current when the mosfet was off.)

    Thanks in advance :)
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Your schematic shows MOSFET source connected to battery +. That won't work too well with an N-channel. (Also won't work well with NPN emitter connected to +.)

    Try source to battery negative, with drain connected to motor, and other side of motor connected to battery +. (Or with NPN emitter to ground and collector to motor.)
     
  3. deadlydentures

    Thread Starter New Member

    May 2, 2007
    3
    0
    crap, that schematic is wrong. The source and the drain are switched. Its been edited.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Your gate voltage has to be higher than source voltage for your MOSFET to conduct. Using your existing set-up, source voltage will never be as high as gate voltage.

    Try source to battery negative, with drain connected to motor, and other side of motor connected to battery +. (Or with NPN emitter to ground and collector to motor.)
     
  5. deadlydentures

    Thread Starter New Member

    May 2, 2007
    3
    0
    haha, i see, sorry for making you repeat yourself :)

    thanks!
     
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