Why a dash dimmer switch has three prongs?

[sifan]

I am looking for a used dash dimmer switch to adjust the gauge lighting. I thought the switch should have only two prongs. But, the modern ones all have three prongs. What does the third prong do?

 

[johnd]

On my car at the end of the reostat travel is a switch that turns on the dome light. I'm guessing that is what the third terminal is for. Can you just use two of the prongs? Sometines using push on terminals you can adapt in this way.

 

[sifan]

http://www.autoshop101.com/forms/h3.pdf

Based on the above site I believe the two-prong switch is a rheostat, the three-prong one a potentiometer. Is there an advantage of using either one of them?

For a rheostat one prong connects to the power source, the other to the gauge, right? For a potentiometer one connects to the power source, one to the ground, and one to the gauge, right? For the potentiometer the prong connecting to the ground is the one that lie 90 degrees to the other two prongs, right?

 

[johnd]

Never say never but up to now, at least, I have never seen a dash dimmer that wasn't a reostat. The idea is to vary the current to vary the lamp brightness. The purpose of a potentiometer is to produce a varying voltage and they usually deal with miniscule currents. It is possible that a potentiometer could be used to control the base of a transistor, using the transistor as the current controling medium but I can't see a car manufacturer doing that. They never do anything that requires 2 parts when one will do.

 

[sifan]

I guess I will try a 3-prong rheostat and play with it. Thanks.

 

[brianl703]

Some old dash dimmers are pulse-width modulated controls, and just about all recent ones are.

If one of those has three terminals, one terminal will be 12V in, one will be PWM out (to the gauge lights), and one will be ground.

It'll also have electronic circuitry in it so you'll know it's not just a rheostat.