I'm building an opamp based device to drive the cooling fan in my car. The circuit samples the OEM sensor voltage and as it drops with temperature, the fan speed increases. Today I tested the circuit with the motor attached and the MOSFET I'm using very quickly reached over 240C (read with my laser thermometer), it's power lead changed in color, as well as the small heatsink it was sitting on changed in color from black to something lighter I then quickly cut the power to the circuit which was otherwise working fine (from the wave generation part). I also measured the current going to the fan and my Fluke T5-600 read around 7A at slow speed and then it decreased to less then an amp when reached full speed. The measurement was taken with the meter "around" the wire, not inline. So one question I have is can I trust the current reading from a PWM lead with the tester "hugging" the wire? Judging by the low reading, it was where it needs to be.

Some facts:

- Motor resistance is 8.7ohm.
- V+ on the schematic is from car's battery when running, so it can be assumed at +14V.
- The MOSFET I'm using is 33N10, which by the spec looks like should be up to the job (but I'm learning, so might be missing something).
- Please disregard the diode markings on the attached schematic, I'm using some other 8A rated diode.

The main question I have is why the MOSFET heated up so much? Is it normal with these currents and I just need a bigger heatsink?

Thanks!

 

Are you supplying at least 10V to the gate? That is what is needed to turn it on fully. The datasheet does not even characterize any lower gate voltage.

Bob

 

Yes, it should be the same as V+.

 

And you are using PWM? At what frequency? And what is driving the gate?

Bob

 

Hello,

I am closing this thread as it violates AAC policy and/or safety issues.

Quote:
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