Quote Originally Posted by Alexandr0s View Post
The Mosfet will be fitted with a big heatsink, because it's getting rather hot right now.
this shows, the mosfet is not switching off completely! ran into the same issues when i played around with PWM. fortunately i had an oscilloscope at hand which proved my doubts.

The potentiometer will be replaced, because it currently lets the fans run at 100% when it's only at 10% voltage.
as the mosfet is not switching off completely, there is no way to control the fans down to lower speed.
also different fans behave differently. depends an load and the fans internal circuitry. there already transistors build into the fan which are switching the coils to allow the fan to rotate. this is by design of the brushless motor used in the fan. guess this circuitry gets messed up if fed with PWM instead of a continuous voltage.

i finally gave up on PWM and decided to go the linear voltage control route.
parts already arrived, but unfortunately i'm too busy right now. hopefully i can start my project during the next few weeks ...

anyway, congrats on your first PWM controller!