After reading everything I could find on the subject, it seems that the 4th pin contains the raw PWM signal, and the fan is expected to contain the mosfet and bypass diode required to use the PWM signal to regulate the voltage down from the 12v supply. It seems that I can add another hwmon controller described here:
http://www.lm-sensors.org/wiki/HardwareHacking
The motherboard I am thinking of buying has one 4 pin and two 3 pin fan headers, and the case has 3 fans, so what I think I will do is break the PWM pin out of the 4 pin and split it out to all 3 fans. The motherboard might not be able to sink the combined current, so I will probably add a transistor to boost the signal.
That should let me control the speed of multiple fans, just not independently of each other. Now I'm just having a hard time finding 4 pin fans.
Do you mean PWM (Pulse Width Modulated)? – Moab – 2011-01-12T20:07:17.597
Yes, I want to be able to control the fan speed with PWM. – psusi – 2011-01-12T20:43:28.053