Strange problem with mosfet circuit

Thread Starter

deadlydentures

Joined May 2, 2007
3
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 :)
 

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thingmaker3

Joined May 16, 2005
5,073
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.)
 

thingmaker3

Joined May 16, 2005
5,073
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.)
 
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