PWM and non-PWM Fans

[WildW]

I changed the cooler on my graphics card to a quieter Zalman one. It would be nice if I could connect the fan to the graphics card fan header to let it control the speed, but the original fan was a 4-wire (presumably PWM) type, and the new cooler has a 3-pin fan.

Is there a way to connect the 3 wire fan to the 4 wire fan header, including speed control, without drawing too much current, burning something out, etc?

Could I. . .

1) Unlikely - Just run the fan on the ground and PWM lines?

2) More work - a circuit to switch power to the fan with the PWM line controlling a transistor?

3) Something else?

4) No way no how?

 

[Modelworks]

You could use a transistor + resistor connected to the PWM wire to control speed. The PWM line itself is very low current ~10ma range with +5V peaks. If you try to power anything off that line it will not work.

So it would work like +12V to transistor controlled by +5V PWM line with fan wire + connected to transistor.

PDF on how to do it
http://techhouse.brown.edu/~dm...ransistor.switches.pdf

 

[PM650]

You will not be able to measure the fan rpm (accurately) using direct pwm control on a regular fan - feeding a pwm signal to a 3-wire or 2-wire fan also pulses the electronics for motor control & rpm reading, causing it to be reported wrong. This could potentially cause a problem if the card has any sort of fan-stop protection (unlikely).

Also, using pwm control on a 2/3-wire fan limits the lowest speed the fan can spin at, as compared to a pwm enabled fan; this is one reason why 4-wire pwm fans are utilized. If this is a problem, you would need to raise the minimum fan speed on the vid card.

Potential issues asside, the transistor switch will work - as long as you use the NPN circuit shown. The signal from the pwm control pin is high duty cycle for high speed, and low duty cycle for low speed. Using the PNP circuit will reverse this - a low duty cycle will cause the fan to run at high speed and vice versa.