[bigben2k]

Thank you aabtek!

It's very simple, really.

If you run a 12V fan at 12 volts, then it'll draw the maximum # of watts.

If you use a rheostat (a rose by any other name...), and you run the fan at full speed, then the fan still gets 12 volts, and there is no voltage drop at the rheostat.

If you run the fan at 7 volts (by adjusting the rheostat), then the rheostat is dropping 5 volts (12 - 7 = 5). The rheostat basically adds resistance to the circuit, in just about the same amount as the fan (a bit less since it's actually 5 and 7 volts), but overall, the total resistance almost doubled, so the total current is a bit more than half of what it was.

V=R*I

P=V*I

P=R*I*I

If the current is about half, and the resistance is almost doubled, then the total power is about half of the fan at full speed.

So the Rheostat needs to be rated for at least half (edit: no 1/4, see below) of what the fan is rated at.

A 15 Watt Rheo could power 30 Watts worth of fans. It's overkill.

A 3 Watt rheo won't cut it (edit: yes it will, see below) for a 10 Watt fan.