# How to choose the size of resistor

Discussion in 'Homework Help' started by ericyan, Oct 7, 2004.

1. ### ericyan Thread Starter New Member

Sep 16, 2004
6
0
Hi

I want to control a 3W DC pull solenoid by 12Voltage.

by Power Equation, P=IV
P=3; V=12, then I=0.25

by Ohm's Law, V=IR
V=12; I=0.25, then R=48ohm

R=48ohm
-----^v^v------(3W)---------
| |
| |
<----------12V--------------->

but the pull force of solenoid reduced too much. How can a choose the size of resistor without affacting the performance of solenoid?

Thanks a lot.

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

If the solenoid is designed for 12 volts, then no power limiting resistor is necessary. When energized, the solenoid winding will self-generate a magnetic field that will control the current through the device - like back EMF in a motor. Just wire it up and there you are.

3. ### ericyan Thread Starter New Member

Sep 16, 2004
6
0

But, if i don't insert any resistors to the circuit, the temperture of the solenoid/its coil is rasied quite high in a short period. Will it quickly overheat or burn out later?

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

Sorry to be slow getting back.

You may have an intermittent duty solenoid. It will burn up if left energized for long periods. Check the specifications. A continuous duty soleniod will also get warm, but will not burn up.

The problem with trying to limit current in an intermittent duty solenoid is that either it will not pull in, or will not hold. Usually they are limited to something like a 10% duty cycle, or no more than a certain time on before they have to turn off and cool down. If this is your problem, look for a replacement rated for continuous operation.