Variable Fan Controller - Properly Sized Transistor 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.
Doing a quick search for high-current MOSFET transistors, I ran across the following:
https://www.digikey.com/en/products/detail/ixys/IXFK170N20T/2126313
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 ;-)