How Does a Car Dimmer Switch Work?

Most cars actually have two dimmer switches which work in different ways. One switch inside the car controls the dashboard lights and instrument patterns, adjusting them from a bright glow to a dim smolder. Another dimmer switch controls the headlights, switching them between extremely bright high beams, bright low beams and dimmer parking lights. Some cars also have a third dimmer switch which controls the brightness of in-cabin lights, allowing passengers to turn on low lights when the car is driving so as not to disturb the driver.

The instrument panels in a car are usually controlled by a variable resistor, or varistor. A resistor is a device which resists the flow of electricity. This resistance slows down the flow of electric current. When a resistor is stuck into a light circuit, it decreases the brightness of the bulb, since less electricity flows to it. A variable resistor has a ribbon of resistive material between two contacts. When the dimmer is turned down, the contacts slide apart. The electricity has to slide through more resistive material, so the overall resistance increases, dimming the lights. Adjustable cabin lights use the same system.

Headlight dimmers work in a different way. The dimmer switch actually controls two separate circuits. When the normal lights are turned on, the dimmer sends a small current to an electric switch called a relay. This current closes the switch, turning on the normal headlights. When the dimmer switch is turned to high beams, it triggers a different relay, turning on the bright high beam headlamps. For parking lights, the normal driving lights are used, only with an added resistor to dim the light.