avrfanspeed
I have a circuit already built. A 3-wire fan which a wire is connected to 12V, a wire is ground, and a wire is connected to a pin of an AVR microcontroller. How can I drive this fan and control its speed?
On the fan is written Brushless DC fan and it is like PC fan.
The two output pins (2,3) are open collector NPN transistors. These will be switched on and off at a rate that sets the speed of the fan. So your guess is correct.
Coils can produce some spiky voltages so I wouldn't recommend directly connecting these pins to the input of a microcontroller. A simple resistor/transistor should do.
A quick search led me to this:
Sense (tachometer) provides two pulses per revolution of fan. Output is open-collector and main board must have pull-up resistor to 12V.
Signal Control (PWM) is input for PWM pulses. Base frequency is 25kHz and it is acceptable from 21kHz to 28kHz. Input has TTL level and includes pull-up resistor to 5V or to 3.3V in new constructions. Signal is not inverted and 100% PWM means maximal revolutions of fan. Motherboard has open-collector type output. This construction guarantee, that with disconnected PWM signal will runs fan with maximum revolutions.
So in theory, you could use a 555 timer that produces a PWM signal based on a potentiometer input. Or simply anything capable of doing 25kHz PWM.
An example of the Triple-Five-Timer being used in such a way below. (Note that it's not from my head, found it here. Hope I didn't mess something up.) Also note, that I did not bother calculating proper values for that 25kHz operation.
simulate this circuit – Schematic created using CircuitLab
Basically you adjust the discharge/charge rate of the C capacitor by sweeping the R1 pot. All there is to it. The rest is just use of the 555 magic.
That said, I'd consider replacing the transistor. That is IMO the most relevant solution.
Best Answer
Since you did not identify the fan in question, we will assume that it is a typical 3-wire example. One wire (typically black) is circuit ground. One wire (typically red) is the +12V power to the motor, and the third wire (perhaps yellow?) is likely the tachometer feedback from the fan. Your controller can monitor the speed of the fan using the pulses from the tachometer feedback signal from the fan. And your controller can change the speed of the fan by pulse-width modulating the power it is sending into the fan.