Dimmer switch

I'm wiring the dash in my 32 and needed a dimmer switch for the gauge lights. Went to good ol Radio Shack and picked up a dimmer (or rather, potentiometer in Radio Shack speak) that should work. There are 3 tabs coming off the component, kinda like a W. Anyone know which might be the 12v "in" and which would be the "out" and is one ground? I would think the component would be grounded by its mounting...... but I'm wrong quite abit Advise welcome!

Shoebox

 

Roadrunner,
The part # is 271-265, a 25 ohm rheostat, 3 watt, 20% tolerance, whatever that means.

Thanks for the help,

Shoebox

 

How many dash lights do you have, what bulb number are they? You could look up the resistance and calculate it all...or you could just hook it up like he said, and see what happens.

Anyways, it sounds like you bought the right part.

 

Tried that once, burned it up. Calculating resistance is easy-if they are in series and they aren't. Look at a regular light switch and you'll see kinda big looking reostat in the fron for the dash lts. Can't you use a regular light switch?

 

Calculating resistance with them in parallel is easy too, if you can add fractions.

 

Usually on a 3 post the center terminal is the output, it comes from the movable arm. either one of the other post depending on which way you want it to turn, brighten or dim is your input. Take a ohm meter and check it. As to resistance it depends on if it is wired in series or parallel. Series is if one lamp goes out they all go out. You can use oms law to determine your total draw. DO NOT GROUND IT. Can you say Direct Short!!!!!!
Here is a link on how to determine resistance in a parallel circuit. http://www.tpub.com/neets/book1/chapter3/1-26.htm

 

The equations are:

Series Resistance:

RT=R1+R2+R3+R?

Parallel Resistance:

RT= 1/ 1/R1+1/R2+1/R3+1/R?

 

Thanks guys for the responses! I can't believe I mentioned grounding anything I have 5 SW gauges, each with one little bulb. I am using a headlamp switch, but its strictly a 2 position park, headlights, unit with no dimmer function so I'm feeding from this, through the dimmer to the gauge lamps. When I first posted this I was at work, now I'm home, guess I'm going to do a bench test.

Shoebox

 

would you actually run the lights any less than 100%? I never do, neither does anyone I know.
If thats the case, just wire them in full on.

 

I have the dimmers on the late models set about 1/3 to 1/2 way up. But on the old cars...it's hard to see the gages with them on full bright. I guess some aftermarket gages are pretty bright though.

If you rewire the lights so they are paired in series, it'll drop the brightness about half.