What is a good (cheap) practice to manage a fan with a thermistor?

Here is what I think to do:


simulate this circuit – Schematic created using CircuitLab

This should work as temperature change to speed up or down the fan, right?

I do not want to make a IC controlled fan controler for it. I need to work in very tiny box beetween actually existing fan wires (it makes too much noise) so the idea is to lower its speed by few % only.

I may be able to work around with a mosfet "classical" fan drive schematic but then I need to deport that elsewhere in the box.

I mean something like this:

enter image description here

Is first schematic useable or I need to work around second one (with the mosfet?)

  • \$\begingroup\$ Where is the thermistor relative to the fan or whatever it is you are trying to cool? \$\endgroup\$
    – SteveSh
    Jan 2, 2020 at 19:20
  • \$\begingroup\$ Have you attempted a loop analysis? This is a closed loop system with the feedback path being a thermal one through the fan's airflow to the thermistor (and whatever is causing the thermistor to heat up). \$\endgroup\$
    – SteveSh
    Jan 2, 2020 at 19:23
  • \$\begingroup\$ @SteveSh: it should be quite close (maybe 5mm) \$\endgroup\$
    – user201301
    Jan 2, 2020 at 19:33
  • 2
    \$\begingroup\$ I did 10k for $2 with therm. bridge ,NPN , trimpot and LM317. tuned for 45~55'C 0 to max RPM \$\endgroup\$ Jan 2, 2020 at 20:12
  • 1
    \$\begingroup\$ Insufficient conditions to oscillate like hysteresis. Also Cap response is ~ 1ms but T is not very accurate with Vt tolerances \$\endgroup\$ Jan 2, 2020 at 20:21

1 Answer 1


so Idea is to lower its speed by few % only

The first circuit will work with the correct resistance values, but wastes power. A few % will make little difference to the cooling, so remove R2 (the thermistor?) and adjust the resistance of R1 to drop the fan speed by a few percent. So long as the resistance is low enough for the fan to start up it should be fine. You could also try it without the capacitor.

If you need temperature control then combine that with the second circuit by putting the resistor across the FET Drain-Source. This will make the fan run a bit faster when temperature exceeds a threshold. The FET won't dissipate much power so it can be quite small.


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