Originally Posted by RatDog
Martin, do you have any experience with the number of PWM fans you could run off the CPU header. Just the PWM signal, not the +12.
Ratdog, yes, its possible, the PWM signal from the mobo header (pin 4) can be shared on number of fans and all fans (assuming exact same fans) will run at the same power or very similar speed if they have the same obstruction/load level. Thats the other advantage of using pwm fan.
Of course the RPM sensor on each fan should be connected at different mobo headers since these rpm signal can not be join together. In your situation, I assume they're mounted at the same radiator, hence all will have the same load/obstruction level, then just pick one of the fan's rpm signal and connect it to one of the mobo header at pin 3 (rpm reporting), imo this single fan's rpm speed should be enough to represent all other identical fans.
About how many fans can be connected at single PWM signal source from the mobo or from controller, 5 to 10 should be fine, as long all those fans maker are following the standard PWM specification released by formfactors.org. Just try it out and its quite safe as long you don't mis-wired them to see if its capable of drive all of your fans.
Should you have any problem on driving too many fans, PM me, I will help you on another circuit to boost/amplify the PWM signal, so it will be powerful enough that capable to drive probably 30 to 40 fans.