Hi all  I am going to buy a resistor and just wanted to make sure I got my calculations correct. I want to cut the amps it uses in half but keep the same voltage:
Appliance is 12V @ 4 Amps fan, R=V/I = 12/2 = 6 Ohms.
Is this correct?
Thanks!

Hi all  I am going to buy a resistor and just wanted to make sure I got my calculations correct. I want to cut the amps it uses in half but keep the same voltage:
Appliance is 12V @ 4 Amps fan, R=V/I = 12/2 = 6 Ohms.
Is this correct?
Thanks!
It's not quite clear what you are doing here. If you put the resistor in series with the fan, then you will have the voltage across the resistor and half across the fan. So the fan will only be getting 6V. Will it still work at that voltage?
In that case you have 6V across the resistor at 2A = 3 ohms.
Note that this means the resistor is dissipating 12W  it will get quite hot.
Ah... not quite. You have halved the voltage and the current so the power will be a quarter!
But the motor probably wont have a linear voltagecurrent relationship.... Maybe you need to hook the fan to a variable supply voltage and see how it behaves.
By cutting "it" in half, I assume you mean power.
It's a somewhat complicated relationship because the resistor cuts the voltage across the fan (because it makes a "voltage divider" circuit) as well as the current through the fan (because it adds to the load).
You would want a 1.25Ω resistor (more accurately, 1.2426407Ω).
Original fan power consumption would be 48W (that is, 12V * 4A = 48W).
Fan resistance is 12V / 4A = 3Ω, and by adding the resistor, total resistance would be 4.25Ω, and current would drop to 2.82A (that is, 12V / 4.25Ω = 2.82A.
The voltage drop across the fan would be 8.47V (that is, 12V * (3Ω/4.25Ω) = 8.47V).
The fan power consumption would be 23.89W (that is, 8.47V * 2.82A = 23.89W), which is about half (49.83%) of the original 48W.
Someone check my work, please.
A 9Ω resistor would be far too drastic. Total resistance = 3Ω + 9Ω = 12Ω, current = 12V / 12Ω = 1A, voltage drop across fan = 12V * (3Ω/12Ω) = 3V, and fan power consumption = 3V * 1A = 3W = 6.25% (or 1/16th) of original 48W.
If we assume that the motor just acts like a resistor in the circuit, then its resistance is 3 ohms. (Power = 12^2/R=48, R=3)
To cut the motor's power in half consider that the power in the motor after the circuit modification is
I^2*R(motor)=24, then
(I^2)*3=24 and I= square root of 8 = 2.83 amps.
To get a current of 2.83 amps at 12 volts requires a total circuit resistance of 12/2.83=4.24 ohms.
The value of the resistor is 4.243=1.24 ohms
This agrees with what jrmonroe calculated.
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