Quote:
Originally Posted by mans
what size resistor would I use? how many ohms?
the motor sucks about 7 amps when on full power, I'd like to cut that in half
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Hi Mans,
If you assume about 14 volts and you know 7 amps, then the motor looks like a
2 ohm resistor, using 100 watts. If you add a 2 ohm resistor in series with the motor then the total resistance looks like 4 ohms and at 14 volts the current will be 3.5 amps. the total power will be 50 watts half through the motor and half through the resistor. So the resistor needs to be rated more than 25 watts. The motor will run a little slower than 1/4th as fast as full speed.
Power = Volts X Amps
Ohms = Volts / Amps
But you said you wanted just a little bit of air flow. Some fan motors will run at much lower speeds than they can start at. You need to start on a higher setting then lower the speed. If you want to run a lower speed than the speed you said used 1 amp, say you want to try 1/2 amp. If the voltage is 14 volts and the current through the motor is 1 amp then the total resistance is 14 ohms. The motor resistance is 2 ohms, so the resistor used is 12 ohms. To go to 1/2 amp the total resistance will be 14 volts / .5 amps = 28 ohms. so you need to add a 14 ohm resistor. The power coming out of the resistor is 3.5 watts.
-mort