I would like to a build a temperature sensing variable speed fan controller for my truck. Currently using a purchased 2 speed on/off relay based controller, but my fans are overloading the whole circuit when the controller is powered on during a high speed temperature scenario. On low speed, I have the fans wired in series to spin both slower - works great. Each fan initially draws 100 amps at full 12v. On the low speed (6v/series), they cruise about 20a together. On high speed, they settle in around 25a each.

I am somewhat experienced with arduino and using a few references to build in specific features I want, but here is the main example I am going by. https://circuitdigest.com/microcontroller-projects/automatic-temperature-controlled-fan-project

My question is around the type and size of transistor that I would need to handle the amount of current these fans draw. Some examples have MOSFET while others have FET or even standard PNP. I am pretty new to working with transistors and can't find any references that make sense. Would love to get a reference to learn this calculation and appropriate type.

Edit: removed picture with incorrect wiring diagram.

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    You would typically have a bank of parallel power MOSFETs with a big heatsink.
    – Majenko
    Dec 25, 2020 at 20:16
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    the arduino cannot induce current flow into the base of Q1 because the arduino ground is not connected to the emitter of Q1 ... therefore Q1 will not turn on
    – jsotola
    Dec 25, 2020 at 22:09
  • @jsotola I didn't catch that before linking to that picture. Thanks for pointing out, though it's not my question. Removed the picture.
    – WMIF
    Dec 26, 2020 at 16:26
  • The easiest might be to use a solid state relay. There are devices for DC and they often can be controlled directly with 3.3V or 5V. In any case, thermal management will play an important role
    – Sim Son
    Jan 1, 2021 at 0:54

1 Answer 1


Doing a quick search for high-current MOSFET transistors, I ran across the following:


The important spec here is the Rds ON (max) of 11mOhm @ 60A, 10V

At your peak current of 100A, you would drop 1.1V across the transistor, for a power dissipation of 110W.

The advertised P(max) is 1150W, so assuming the junction temp can be held to under 175C (and with an entire metal truck to use for a heat sink, this shouldn't be a problem) you should be OK with this device. In the 'cruise' configuration with 25A motor current, you should have no problem controlling the fans.

YMMV ;-)

  • Your answer is fine, but as the OP mentioned to have little experience with transistors, I want to add that this mosfet can't be controlled with qn arduino's logic level and needs an additional drive circuit
    – Sim Son
    Jan 1, 2021 at 0:53
  • Good point - thanks for the addition!
    – starship15
    Jan 1, 2021 at 15:49

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