my Old Workshop

Do dimmer switches save electricity?

With a headline like that, you’d think we were going to shock you and say “No!” But the answer is they do save electricity.

So why ask the question? Well, dimmers are related to old-fashioned rheostats, big industrial-type switches that controlled electricity. In order to allow less energy to reach the bulb or device, they used a variable resistor. A resistor doesn’t conduct electrical current very well, and as the current passes through it, it loses energy in the form of heat. With a rheostat, a contact was attached to the knob or lever. When you turned the knob, the contact point moved from one end of the resistant material to the other, lengthening or shortening the distance the current would have to travel through it.

As you can see, this would waste a lot of electricity, not to mention giving off a lot of extra heat. The less current to the device, the more electricity you’d waste.

Most household dimmer switches continue to use a variable resistor for a small part of the job, but they use an entirely different technique to dim the light. Instead of burning off excess energy, dimmer switches shut the circuit rapidly on and off, reducing the total amount of energy flowing to the light. The more dim you have the light, the longer the circuit will stay off in each cycle. The handy dandy device that handles this is a semiconductor called a triode alternating current switch or triac. The variable resistor’s still there, but it’s simply controlling the voltage that triggers the triac.

Over time, the semiconductors have improved in efficiency, saving more electricity and generating less heat. So if you’re dimmed 75 percent, you’re saving close to that amount of electricity. Of course, running a 100-watt bulb dimmed to orange filaments is inefficient in terms of the light you’re getting for the wattage spent; you’d get more light from a nightlight bulb. But it looks nice.

Be aware that dimmer switches will lose some energy in the form of heat through the faceplate, and they’re rated for the number of watts they can handle.