when switched ON there is of course
current flowing through the mosfet (lets say 2A just for fun) but as the mosfet has a very low Ron (resistance when switched ON) there are
just a few mv (may be 20 milli volt)
across the mosfet leaving almost all 12V for the fan. now, the power the mosfet has to transform into heat is
0,02V * 2A = 0,04W which is almost nothing!
thats why - if PWM is working as expected - the mosfet should not even be warm!
in your circuit, unfortunately the mosfet is not switching off completely, thus there is a small amount of current flowing even when you dial in lets say 10% pwm. this results in having some voltage across the moset and some voltage on the fan. add the current which flows through the fan now an the mosfet starts to heat up, while the fan refuses to regulate to lower rpm.
cant tell you the reason for this behavior, its just the explanation for the unexpected heat on the mosfet. may be another mosfet type, or different gate voltage might help. but as said before, i stopped playing with PWM and moved on to linear voltage control.
getting rid of the heatsink would be the main advantage of PWM to me. if i need a heatsink for the mosfet i can even use linear voltage control which requires a heatsink by design. and of course i even wont run into rpm reading problems or unwanted switching noise with linear voltage control.
give me some time. will post my solution as soon as its ready. some of my ideas are:
- microcontroller based
- usb interface to system
- read tempsensors already available in the system
- read external DS1820 sensors
- communicate with rivatuner
- control fans via motherboard pwm headers
- low power (1.5A) and high power (3A) outputs (although i don't think anybody needs 3A)
and much more
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