Thanks crashwg!
So here's what I have for calculating what resistor.
The fan in question is 12 volts, 0.27 amps.
1st we discover the wattage:
volts multiplied by amps
3.24 watts
2nd we discover the fans resistance:
volts multiplied by volts
divided by watts
44.4
3rd we note the desired voltage, and the difference in voltage, in this case I want to turn the 12 volt fan in to a 6 volt fan, so the desired voltage is 6, and the difference in voltage is 6.
4th we calculate the value of the resistor:
volts minus desired voltage
multiplied by fan resistance
devided by desired voltage
also 44.4
So I'll need a resistor that is rated about 44.4 OHMs
(That's an ironic integer, does that mean to make a fan run at half speed you need to buy a resistor that is equal to fan resistance?)
5th we calculate power rating (the minimum wattage of the resistor to avoid overheatring it) :
difference in voltage multiplied by difference in voltage
divided by resistor value
equals minimum wattage of resistor
0.81 watts
So, to turn my 12 volt fan, that is 0.27 amps, into a 6 volt fan I will need a resistor that is about 44.4 OHMs and more than 0.81 watts
OHM's law is really complicated!
