Originally Posted by iCrap
Pump won't even start up with a 100ohm resistor on it...
What the heck? Adding a resistor won't filter jack, it will drop the voltage making the pump run slower. That's one way to lower the noise footprint but you're better off using a fan controller that can support 20w loads so you don't lose out on the max performance. PSU's already have ripple suppression. Go read stuff at jonnyguru.com and you'll see what I mean.
For this you need a few equations with the following parts.
I = current (Amps)
U = voltage (Volts)
R = Resistance (Ohms)
W = Power draw (Watts)
U = I * R, That is voltage dropped across a electrical part is the current times the resistance.
We know voltage and current and can calculate the "static" electrical resistance of the pump.
12 = 1.5 * R -> 12/1.5 = 8 Ohm.
Resistors in series have to share a voltage drop, thus adding a resistor in series with the pump reduces to voltage for the pump. The equation for this is below along with the calculation for the drop across the pump when using a 4.4 Ohm resistor.
(Rpump/(Rpump + Rseries)) * U = voltage drop across the pump. (8/(8+4.4))*12 = 7.75 Volt.
7.75 Volt is a bit less than what you wanted, but maybe he thought you wanted 10 Watts.
One equation for calculating power draw is:
W = U * I
The trick is that the current (I) depends on the resistance of both the pump and the series resistor.
I = U / (Rpump + Rseries) -> I = 12 / (8 + 4.4) = 0.97 Amps
Going bach to the power consumption again.
W = U * I -> W = 7.75 * 0.97 = 7.5 Watts.
Looks a little low to me, but I am tired and calculated with the resistance expected from your 18 watts number and it has been a while since i calculated these kind of thing with motors.
The horrible realisation you should have from these equations is when you use them on the resistor, as it is taking on a voltage drop of 4.25 volt at 0.97 Amps, that is more than 4 Watts. Usually a resistor can handle around 0.2 Watts without burning to a crisp.
Maybe I should just have started by writing, you should not use a resistor to drop voltage across that pump, it has way to high a power draw.
See if you can find a voltage controller capable of handling 1.5 Amp, as you will need something with a solid heatsink or PWM based techniques.