LED"s on a dimmer continued info


Senior Member
I thought this was interesting enough to be a post by it's self. I found this the other day while looking for some data on LED's.

Domestic dimmer switches work in one of two different ways.

The old way used a variable resistor to reduce the amount of voltage available for the bulb. As the user (wanting to dim the lights) turned the dial of the dimmer, the resistance increased. This caused some of the electricity supply to be lost as heat in the resistor leaving less available for the bulb (which therefore glowed less brightly).

Modern dimmer switches use electronics to increase efficiency and safety. The alternating current (AC) of mains electricity has a varying polarity moving from a positive voltage to a negative voltage 50-60 times per second. The modern dimmer switch simply turns off the power to the lighting circuit every time the electricity supply changes polarity - with the time spent in the off-state controlled by the dimmer switch.

When the lights are very dim, the power to the lighting circuit is kept off for a (relatively) long time, and when the lights are bright, the power to the lighting circuit is on almost all the time. The human eye cannot detect the lights strobing on and off more than 100 times per second and so the lights appear to just change in brightness.

So with the modern approach this type dimmer is effective in dimming LED's.

Lutron now makes a dimmer specificially for dimmable LED/ CFL (it must say it on the bulb.) You have to do a final adjust with a dial on the front of the dimmer. I've installed a dozen or so already and had one customer say when his stereo was on it strobed the lights . I told him to readjust the dial and see if that get's rid of the strobe, haven't heard back yet.


Senior Member
Delmarva, USA
Dimmers operate by cutting portions of the sine wave out.

Reducing the average voltage more than reducing the peak voltage.

This is one reason why incandescent bulbs tend to "sing" when connected to a dimmer ....