On Sat, 15 Nov 2008 19:01:39 +0000, Derek Geldard wrote:

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.



The "proper" way to speed control these is using PWM. The OP may be able
to get a PWM controller for a case fan and just run everything off 12v as
its supposed to. The problem with using a reduced supply voltage is that
the torque falls off drastically. When used as a fan this *appears* to be
ok, but the airflow is actually far less than the speed would suggest.
Using the fan as a stirrer is more likely to just stall it. Note that PWM
can run the fan much slower (although it is much noisier) than a dropper
resistor.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
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