3-wire fan

Today, 3-wire fan are very common in computers. Two pins are for the power supply and one is for the speed feedback as an open collector circuit.
If I want to use a PWM to control fan speed and still have speed feedback, how do I connect the driving transistor? On the +V side or on the -V side of the fan?

Here is a circuit of the internal circuit of a 3-wire fan taken from a web site

**broken link removed**

If I connect, let's say, +V do +5Vdc and -V to the collector of an NPN transistor (whose emitter is to GND) driven by the PWM of a microcontroller, even if I connect "tach out" to an input, the micro will not be able to read the speed beacause the hall-effect switch will be continuosly powered on and off by the pwm.

Maybe that schematic is wrong but I'm not able to understand how to control fan speed with a PWM and still read the speed pulses.

Maybe you need to add an RC circuit to smooth out the pwm signal so you get filtered DC

Yes, filtering could be an idea, even if the fans use high current so filtering should use low R and high C.
But my question is more related to "how does a PC controls the fan speed and how the speed is read using the tachometer output".

I've just found an IC that should be in computer fan (**broken link removed**) and attached you may find the schematic.



As you see, it seems that the fan should always be powered to have the IC working correctly and giving meaningful output with the open collector output.

You could use the PWM signal to control a simple switching regulator. Google for buck voltage regulator. All you need is capacitor, inductor and diode.

The controller chip I use, ADT7460, measure the tach only when the PWM signal is on. With a fairly slow PWM, it can check RPM and PWM at the same time. It talks about the method in the datasheet.

Thank you Mark!
This approach really make sense and answers to my doubts.
I'll search the datasheet to learn more.