So on my 3D printer from Prusa they use 5V fans and monitor the fans by the tacho signal.

I want to replace the hotend with a water cooled version and still want to have the safety feature to turn off the hotend heater when the fan is not running. Well, in my case the water pump.

The water pump is a 3pin water pump as used for PC water cooling running on 12V.

So 2 problems I have to solve, one is the difference in power for the fan voltage and the 2nd is the difference in voltage for the tacho signal.

The idea I have is to use a DC-DC step down module to convert the printer main power 24V to 12V. Then use a MosFET to use the 5V power output from the printer to switch the 12V with the MosFET.

For the tacho signal I was thinking of a simple resistor with Zener diode to convert the 12V signal to a 5V (5.1V to be exact) signal for the printer input.

Has anybody a better idea?

Maybe with diagram and recommendations for the parts?

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  • 1
    \$\begingroup\$ Waiting for the diagram you mention to understand exactly what you mean. \$\endgroup\$
    – Solar Mike
    Commented Nov 18, 2018 at 12:58
  • \$\begingroup\$ FYI "MOSFET" is written in all capitals (or all lowercase if you're being lazy). \$\endgroup\$ Commented Nov 29, 2019 at 12:06

1 Answer 1


If you need to use low side switch for the fan, the tacho signal will not go to 0 V. There are basically two choices.

  • Change to high side switch, or
  • Add a comparator with open drain output to monitor the tacho.

I recommend the first choice.

There are also a couple of other issues with your design. P-FET doesn't work well the way you have connected it. Use N-FET, or change to high side switch where you can use that P-FET.

Also, the fan is an inductive load. You should add a diode parallel to the fan to offer a path for the current when the FET is not conduting.

The tacho signals are usually open drain, so you need to have a pull-up resistor for it.


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