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Originally Posted by sanddrag
If I was to just put a resistor in series with it (provided I could get a resistor rated for that much power), how would I calculate what resistance (in ohms) I would need to take the 12V down to oh, say, 6V? I should know this but I don't remember.
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Kirchoff's Voltage Law: sum of the voltage drops around a loop is equal to the sources
You have a 12V source and two drops. You can get the voltage across the resistor with Ohm's Law: V=IR. Whatever's left is the voltage across the motor.
12 = IR + V (V = motor voltage)
Notice that the voltage will change with the current.