In message , Derek Geldard
writes
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

Thanks for your help

I don't know how the fan would behave, PC fans have brushless solid
state DC motors they might need almost the full 12 v to run the
electronics. Also 0.15A will be the average, they might draw their
current in short heavy pulses.

However 50 ohms would drop 7.5volts at 0.15 A.

Might be cheaper to get a 50 ohm and a 25 ohm resistor and try it with
either one in circuit, or both in parallel (= 17 ohms) and see how it
goes.

Prolly better to knock up a cheap and nasty controller using a voltage
regulator IC with the pot controlling the voltage

--
geoff