Donwill coughed up some electrons that declared:

I recently purchased a small fan (40mm square, 12v dc) to mount in a
temperature sensor housing in order to ventilate the sensors. I assume
the fans are normally used in a computer.

There are three wires coming out of it, red, black, and yellow I assume
the red and black are + and - 12v. What is the yellow for? is it some kind
of pulsed output to enable one to control the speed? if so how is it
usually done? some kind of diode pump arrangement perhaps?

Don

It is the tacho line - open collector arrangement I believe.

So feed it with a small current from a pullup resistor and you should get
pulses proportional to RPM.

There's a 4 wire fan too - the 4th wire controls fan speed.

There are some diagrams here that may be of interest:

http://www.bearblain.com/fan_speed_control.htm

Cheers

Tim