trouble with mosfet switching circuit

Thread Starter

mikewax

Joined Apr 11, 2016
184
hello guys i made a circuit where every time i press a button, the uC sends pwm to a pnp for 3 sec., then pulls the base low again, and stays low until i press the button again. The point is that when the base is pulled low, the gate is pulled low. And when the base gets 3.3V, 75% pwm, the gate gets 5V, 75% pwm, the LED lights up for 3 seconds, then goes dark again.
And it works as expected. BUT when i substitute the LED,500ohm combination with a single 2.7ohm, the behavior changes. In the that case, the NFET will turn on 75%, but will not turn off after 3 seconds. it stays on until i unplug the power. I don't know why it will obey the uC when i have a LED, but not when i have a larger load.
circuit2.png
 

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Dodgydave

Joined Jun 22, 2012
11,284
replace the 33K to 10K. and replace the 100K gate resistor to 1K, or better still remove the pnp and drive the fet directly.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
12,345
Replace the 100K gate resistor to 1K, or better still remove the pnp and drive the fet directly.
Changing the 100k to 1k will mean that there will never be enough voltage to turn on the FET because of the 33K.
However I agree with the second part. The PNP adds nothing to the circuit.
 

Thread Starter

mikewax

Joined Apr 11, 2016
184
Changing the 100k to 1k will mean that there will never be enough voltage to turn on the FET because of the 33K.
However I agree with the second part. The PNP adds nothing to the circuit.
damn you guys are right. i miscalculated the gate threshhold voltage in an attempt to minimize RDSon.
don't know how i got the wrong numbers.
THANX i'll rewire it and try again.
 
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