The Weak Spot

The light-emitting diode (LED) offers energy savings, lower maintenance requirements and other advantages compared to incandescent sources. However, many LED products perform poorly with existing dimmer controls.

Most incandescent dimming installations use phase-control techniques. When dimming incandescent lamps, the dimmer’s design determines the resulting performance. When dimming LED products, performance is determined both by the design of the dimmer and the LED product’s driver circuitry, which varies by manufacturer. As a result, while the LED product and a paired control device may be capable of dimming, they may not be compatible with each other in a specific combination, leading to performance issues.

Testing standards have not defined a typical LED load against which dimmers can be tested; dimmer manufacturers must test their dimmers with individual LED products. While manufacturers have developed dimmers specifically for dimmable LED products, determining compatibility becomes problematic with older phase-cut dimmers designed solely for incandescent lamps.

In its October 2013 publication, “Dimming LEDs with Phase-Cut Dimmers: The Specifier’s Process for Maximizing Success” (updated August 2014, available at, the U.S. Department of Energy (DOE) provides some guidance for evaluating compatibility and minimizing any related problems. The following advice (in decreasing order of confidence) can be applied individually or in combination:

1. Conduct a full mockup if possible. This involves evaluating and testing all LED sources and dimming controls in a mockup installation. This is recommended for existing and new dimmer installations. Even when products are identified as compatible, there is no guarantee that a specific pairing will satisfy expectations at every dimmer setting.

2. Rely on proven LED-dimmer combinations based on experience not older than six months; LED product or dimmer characteristics change.

3. For wallbox installations, specify a new forward-phase line-voltage dimmer and LED product with a light engine (LED modules and control gear) that are both compliant with the NEMA SSL-7A standard. This ensures the pairing will perform reliably and can be used to determine maximum dimmer load and minimum dimmed level, though it does not address every potential performance variation.

4. Specify a combination of LED sources and dimmers recommended by either the LED source manufacturer, the dimmer manufacturer or, ideally, both. Check the manufacturer websites for dimming reports for specific combinations, compatibility lists and general guidance.

5. Specify a three-wire dimmer, and derate the dimmer’s capacity. While architectural dimmers are three-wire, not all wallbox dimmers are.

6. Specify either forward-phase or reverse-phase as the LED product manufacturer recommends, and derate the dimmer capacity.

As products are modified, discontinued or substituted, compatibility will need to be re-evaluated and confirmed.

In a May 2014 technical white paper, “Controlling LEDs,” by Lutron Electronics, the company identifies several factors worth considering:

1. Note the dimming range for the specific source. While incandescent lamps dim smoothly to off, LED dimming range depends on driver design. Some LED drivers dim to 1 percent, while others may be limited to another level. Match the dimming range to the application.

2. The LED product should not produce flicker at any time. A well-designed dimmable LED driver should not produce flicker. The product also should not “shimmer” i.e., pulse in intensity at medium to low dim levels.

3. The LED product should not “pop on.” After dimming to a low level and switching, some LED products will not reactivate until the dimmer’s slider is moved up 20–40 percent. This can be particularly problematic in three-way installations where the lights are controlled from more than one location.

4. The LED product should not exhibit “dead travel.” When the dimmer is adjusted, there should be a corresponding change in light level.

5. Neither the LED product nor the dimmer should produce noise.

Lutron also points out that, unlike incandescents, LED lighting does not become warmer in color appearance during dimming, so it may fail to meet the expectations of some users. Some LED product manufacturers mix white, amber and other colors to imitate this effect. Because these manufacturers use different methods with varying degrees of effectiveness, mixing manufacturers may result in inconsistent performance.

That last bit of information extends generally to dimming performance as well. Different products may behave differently on the same dimmer even if they are compatible.

Just remember, LED lighting is inherently compatible with dimming for many benefits, but not all LED products and dimmers can be used together. Ensure compatibility and good performance.

About the Author

Craig DiLouie

Lighting Columnist
Craig DiLouie, L.C., is a journalist and educator specializing in the lighting industry. Learn more at and .​

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