Hi, my computer has 4 fans in it running on 12v from the PSU.
How can I control their speed?
Maybe a potentiometer?
Electronic – Simplest way to control the speed of 4 identical fans
fanmotor controllervoltage-regulator
Related Solutions
Your math is wrong, as you might have seen for yourself if you had properly carried the units thru your calculations. You have 20 k interrupts per minute, which is 333 per second, which can be easily handled by even a modest microcontroller.
PWM is used because it wastes much less heat than "linear" methods like that shown in your schematic. A TO-92 voltage regulator and a DIL8 microcontroller is all you need to read the voltage from a 10kohm pot and then generate a PWM signal to put through a power MOSFET on the low side of the fans. You could even substitute a 555 and associated passives, but the MCU will give you better control.
simulate this circuit – Schematic created using CircuitLab
Best Answer
As peril brain already pointed out in the comments, a simple resistor in series is the easiest solution.
Note however, that speed regulation with a resistor is poor if you want to achieve very low speeds: fans require more voltage to start turning than to keep turning. (They need to overcome static friction.)
Most intelligent fan controllers therefore start up the fan with a "burst" of higher voltage and then reduce the voltage. Obviously this in not something you can do with a simple resistor.
I had fans that could not be made to turn slower than half speed with a simple resistor. Else they would not start. With others this was no issue at all. It really depends on how much friction your fan has.