reducing electric fan speeds

krsmayo · May 20, 2010

Can anyone help me out with what resistor i should use and where to get one from.
My new electric fan thermo switch has two settings 88-83, 92-87
for the 88-83 i want the fan to spin slower prob 3/4 speed.
the fan is 12v 120w around 10A

i think i worked it out as 0.6ohm i need to put in, but i'm not sure what resistor to use ie size and type

richb1967 · May 20, 2010

Hello KRSMAYO,

If you put a 0.6 ohm resistor in series with the fan motor you have, the current will drop to approx. 6.66 amps from 10 amps you stated. The motors resistance is 1.2 ohms if the figures you stated are correct. To slow the speed to 3/4 the existing speed, you need to increase the resistance to 1.6 ohms, so the in-series resistor you require should be 0.4 ohms, but must be capable of carrying the current and dissipating the power (they are not cheap). Also, the voltage across the circuit will be nearer to 14Vdc whilst the engine is running.

krsmayo · May 20, 2010

Any idea where i could get one from

Sean · May 20, 2010

krsmayo said:
Any idea where i could get one from

do a search for metal clad resistor - plenty of places sell them

note it's not the same as a metal film resistor

krsmayo · May 21, 2010

richb1967 · May 21, 2010

try rswww.com

Sean · May 21, 2010

krsmayo said:
something like this, just i can't find anything lower than 1R
Vishay | Passives | Resistors | High Power | Aluminium Housed, Wirewound - 5 to 600W |RH0501R000FE02

don't forget you can put them in parallel to create a lower resistance value

therefore 2x 1ohm resistors in parallel will give .5 ohms overall resistance

reducing electric fan speeds

krsmayo

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