paero-RCU

My Feedback: (14)

Ok...I have a flight box with two elec fuel pumps. One for fixed wing and one for heli fuel. The one pump (sonictronics) had the motor burn out. Had one that fit, but it must have been either 6 or 7.4 volt. Runs really really fast on 12 volts and I'm sure it won't last real long.

Need to know type and rating of a resistor to put in line to reduce the voltage.

Thanks in advance for your help

jdetray

Simply sticking a resistor into the circuit is not a great way to reduce the voltage. You will need a small resistor with a pretty large power rating.

Calculations
You can't do this properly without knowing how much current your fuel pump draws. The instructions for the pump may tell you. I'm going to assume it draws 2A.

Assuming you want 6V to run the pump, you need to drop the voltage from 12V to 6V, a difference of 6V.

Using Ohm's Law R=E/I, 6V/2A = 3 Ohms. So you need a 3 Ohm resistor.

Now we need to determine the power rating of the resistor. Using the formula P=EI, 6V x 2A = 12 Watts. So you need a 3 Ohm resistor with a minimum power rating of 12W. If you use a resistor with a smaller power rating, it will get very hot, maybe hot enough to fail.

All of the above is correct only if your pump draws 2A at 6V. If the pump draws a different amount of current, the specifications for the resistor will be different.

A better way to reduce the voltage is with a voltage regulator circuit.

- Jeff