Advice on rheostat
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Senior User
Joined: Jan 2003
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From: Sebastopol, CA
Advice on rheostat
I'm technically incompetent when it comes to electricity. I'm installing a quad A pillar gauge setup and I want to be able to control the brightness of the lights on the gauges. The only rheostat I have been able to find is rated at 3 amps at my local Radio Shack. Will this handle the power requirements for the 4 gauges listed above and two additional gauges that I want to control, gas gauge and air pressure gauge for air bag system? Obviously only the lights will be run through the rheostat.
If the 3 amp is not enough I'd appreciate a source for something that is adequate.
Thanks for your help.
CLPF350
If the 3 amp is not enough I'd appreciate a source for something that is adequate.
Thanks for your help.
CLPF350
Post Fiend
Joined: Apr 2002
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From: Central Kali
You need to find out how many fractions of an amp that one lamp draws. Say it is .1 amp for an example. You have 5 lamps so the total is .5 amps.
.5 times 14 volts is 7 watts. The total resistence is 28 ohms. If you dimmed the lamps to 7 volts, you would need about 28 ohms in the reostat. The total current would then be .25 amps.
The reostat wattage would be .25 times 7 volts = 1.75 watts.
At a lamp voltage of 10 volts, the current draw is 10/28=.357 amps. The reostat needs to drop 4 volts. It's resistence would be 4/.357=11.2 ohms. The wattage is 4 times .357 =1.42 watts.
If the lamps were dimmed to 4 volts, the wattage would be 1.42 and the resistence would be 70 ohms.
Hope this was not confusing. The key is how much current each lamp draws. I would use a 100 ohm reostat. Allied Electronics and Newark carries wire wound reostats in different wattage sizes.
.5 times 14 volts is 7 watts. The total resistence is 28 ohms. If you dimmed the lamps to 7 volts, you would need about 28 ohms in the reostat. The total current would then be .25 amps.
The reostat wattage would be .25 times 7 volts = 1.75 watts.
At a lamp voltage of 10 volts, the current draw is 10/28=.357 amps. The reostat needs to drop 4 volts. It's resistence would be 4/.357=11.2 ohms. The wattage is 4 times .357 =1.42 watts.
If the lamps were dimmed to 4 volts, the wattage would be 1.42 and the resistence would be 70 ohms.
Hope this was not confusing. The key is how much current each lamp draws. I would use a 100 ohm reostat. Allied Electronics and Newark carries wire wound reostats in different wattage sizes.
Thread Starter
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Senior User
Joined: Jan 2003
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From: Sebastopol, CA
Mike W:
Dave F:
Thanks for your help. Since making the original post I found another thread that covers splicing into the gauge lighting at the light switch. That approach is certainly simpler than adding another rheostat. I'm assuming from your replies that the Ford Rheostat will handle the additional load.(?)
CLPF350
Dave F:
Thanks for your help. Since making the original post I found another thread that covers splicing into the gauge lighting at the light switch. That approach is certainly simpler than adding another rheostat. I'm assuming from your replies that the Ford Rheostat will handle the additional load.(?)
CLPF350
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Thread Starter
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Senior User
Joined: Jan 2003
Posts: 171
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From: Sebastopol, CA
Duderoy:
Many thanks for the advice. By the way I figured out that I can take 3 ring connectors, use a pop rivet tool to secure them to each other and have a good electrical connection for the additional circuit.
CLPF350
Many thanks for the advice. By the way I figured out that I can take 3 ring connectors, use a pop rivet tool to secure them to each other and have a good electrical connection for the additional circuit.
CLPF350
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