On Sat, 15 Nov 2008 09:57:51 -0800 (PST), wrote:

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

If you can get hold of Wireless World magazine for (ISTR) September
1967 there's an article on speed control for small dc motors.
The idea is that you apply variable pulse-width full voltage to the
motor, and between pulses measure the back-emf of the motor to measure
its speed. The b.e.m.f. is used to control the speed to pretty fine
limits, taking into account varying load on the motor.
The idea was based around a controller for model railways :-)

--
Frank Erskine