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