I'm building a simple temperature controlled NMOS switch for regulating the speed of a fan.

It's a 12V fan and the controller works great (as long as the transistor is in it's active region.)

This is the circuit I'm using, where R2 is the temperature sensor:


However, the fan only turns on at a voltage of ~3V or higher, and makes squeaking noises on anything lower than that. The voltage output vs. temperature (C) looks something like this:


Ideally I'd like it to "cut off" completely when reaching an output below 3V with a result like this (in red):

Preferred voltage/temp

I want to remove the exponential behaviour when exiting the active region.

Is there a way to "reverse clamp" the gate voltage, immediately bringing it to 0 if it's too low, or is there some other solution to this problem?

  • \$\begingroup\$ Only removing/changing that part of the curve will be very hard. The most common way to control a fan based on temperature is two "hard" decision points, one where the fan switches on (for example 50 C) and a point where it switches off (for example 45 C). That can be done with several circuits. Search for "temperature control fan circuit". There are also circuits that will regulate the fan's speed based on a temperature. Making sure that the fan will not stall due to low voltage makes that a bit more complex. \$\endgroup\$ – Bimpelrekkie Oct 8 '20 at 9:48
  • \$\begingroup\$ 'I'm building a simple temperature controlled NMOS switch for regulating the speed of a fan' Really ? Switches don't regulate anything. Re-evaluate and edit ! \$\endgroup\$ – Graham Stevenson Oct 11 '20 at 0:29
  • \$\begingroup\$ @GrahamStevenson Have you heard of a bang-bang regulator? Use in almost every home appliance like ovens and refrigerators. Those works by switching cooling/heating on/off at a certain temperature, making the switch regulate the temperature. Using the active region as a simple speed control is an added bonus. \$\endgroup\$ – Pythe1337N Oct 12 '20 at 7:06
  • \$\begingroup\$ Actually, NO, I've never heard of that. I can imagine that such an arrangement is ON/OFF/ON/OFF etc. Not a LINEAR circuit at all. Yes, it can work but is somewhat 'clumsy'. \$\endgroup\$ – Graham Stevenson Oct 12 '20 at 8:09

I've found the source for the squeaking noises. It was caused by power supply noise propagating through the voltage divider and making the mosfet 'flicker' when approaching it's cut off voltage.

Adding a capacitor to the gate pin of the transistor solved the issue. I hade a 10u cap laying around and that's what I used. Just leaving it here in case somebody else decides to build this.


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