But Will It Dim?

Here's one LED professional's real-world experience with dimming LEDs.

Last year, when I purchased my new house, one of the first things my wife Gabi and I did was to replace the old, inefficient incandescent and fluorescent lamps with LED lamps. My kitchen originally came equipped with four PAR38 halogen lamps controlled by a standard Leviton wall dimmer. Back then I replaced the kitchen halogens with Philips LED equivalents. You might recall from my previous article, The Lighting Bug, that I had some concerns about the compatibility of the Philips LED fixtures with the triac-style dimmer. I feared that an incompatibility could damage those relatively expensive Philips LED lamps -- they cost $29 each at the time. I installed the LED lamps, tested the dimmer and system, and everything seemed to work fine so I left well enough alone.

The basis for my nagging concern was my experience at LED Specialists developing dimmer-compatible LED lighting for a customer. That customer engaged us to design an AC LED driver with the proviso that the driver operate with a standard dimmer. We tested numerous types and styles of dimmers with our driver and came to the realization that 100% compatibility with LEDs is impossible. There is a subset of dimmers that are essentially simple triacs.

A triac is a semiconductor device that acts as a switch, much like a diode. But unlike a diode, a triac can conduct current in either direction, making it an ideal device for dimming (incandescent and halogen) lighting systems operating on AC voltage.

Every real-world component has its idiosyncrasies, and the triac is no exception. A triac requires a minimum amount of current, known as its holding current, flowing through it in order for it to stay "on." Incandescent and halogen lamps have no problem sucking up current, but LEDs require much less. Whereas the incandescent will keep the triac conducting, LEDs often cause the triac to turn off prematurely. Turning off prematurely can also lead to oscillations where the voltage is high enough to turn the triac on again. Therein lies a quandary. We opt for LEDs to save current (energy), but the fickle old triac thirsts for more power.

Fiat dark

And lo and behold, last week my kitchen plunged into darkness. My poor old triac had given up the ghost. Fortunately it was the dimmer and not the significantly more expensive LED lamps.

I rushed to Home Depot and selected a new dimmer, a Lutron Skylark Coutour Model CTCL-153PDH-IV forward phase control dimmer. The packaging claims the device is compatible with an LED lamp load of up to 150 Watts. Given my kitchen's four 19.5 Watt Philips PAR38 lamps, my total load is 78 Watts, well below the maximum. Lutron also posts compatibility information online. My Philips PAR38 LED lamps are compatible!

The dimmer is now installed, and my kitchen has light. The dimming range now is significantly greater than it was with the old triac. Full bright is the same as before, and maximum dim is substantially less. I estimate max dim at under 10% of full bright compared with 40% with the old dimmer. So hopefully, I still have 21.8 years of life left on my 22.8 year LED lamps.

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