The Challenge of Dimming LEDs

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One of the major issues that continue to plague light-emitting diodes (LEDs) is the challenge of dimming. Although some LED light sources dim as well as incandescent lamps, exhibiting no adverse characteristics during the dimming process, others display very strange dimming behavior. These LED light sources may:

  • Dim erratically
  • Change color as they dim
  • Have a limited dimming range
  • Cause flicker or noise
  • Produce no light, or
  • Refuse to dim at all.

Incompatibility is the root of the problem

LEDs are dimmable by nature, but unfortunately not with every driver or dimmer on the market. Whether or not the LED dims is not the issue. Instead, the dimmable nature involves how well LEDs dim, with considerations including color, smoothness, the time it takes to dim and how evenly light is dispersed. It is basic compatibility, resolving the new and the old lighting technology, what causes the most trouble.

The differences between LEDs and incandescent lamps cause dimming complications. The problems are typically attributable to incompatibility between the LED source and the phase-cut dimmer rather than due to an LED shortcoming. This incompatibility has pushed LED manufacturers and others in the industry to aggressively focus on solutions.

Several different dimming techniques are in use to separate the dimming signal itself from the voltage provided to the light source. The majority of dimming systems still apply phase-control techniques that were used with incandescent lamps.

Dimmers were designed to drive an incandescent bulb’s simple resistive load. These dimmers use a triode alternating current switch (or TRIAC, a small semiconductor device similar to a diode or transistor) to phase-cut the AC waveform. This phase-cutting is not necessarily clean or consistent. In some solutions, it is performed at the leading edge of the waveform, and with others, at the trailing edge. LED manufacturers are attempting to deal with the tradeoffs necessary to be compatible with these TRIAC dimmers. Glitches occur inadvertently as LED circuitry attempts to deal with the inherent variables.

Safety is a major consideration

LED manufacturers are also intent on providing bulbs that work safely with dimmers. They’ve learned from the lessons involving compact fluorescent lamps (CFLs) and dimmers. CFLs, when exposed to a dimmer’s electrical fluctuations, consume up to five times the current as they do without the dimmer. As a result, bulbs overheat and fire can occur. LED lamp manufacturers are faced with trying to create a bulb that achieves rapid turn-on, with no warm-up time to full brightness, no toxins and, above all else, one that operates safely when used with dimmers.

On the road to a solution

Manufacturers and industry efforts such as ENERGY STAR® are working to solve dimming challenges. For example, on Sept. 30, 2014, the ENERGY STAR Program Requirements Product Specification for Lamps will replace the existing Integral LED Lamps Version 1.4.

These new product specifications recommend, but do not require, different dimming technologies be included in tests performed. Dimmer compatibility testing will, under the new specifications, involve dimming performance in terms of maximum and minimum light output, flicker and audible noise.

At this time, no mandate exists for all LED designs to be dimmable; only those bulbs intended for and identified as being usable in dimming applications are regulated. The anticipation is that vendors will test and list compatible dimmers on their websites or on packaging in an attempt to remain competitive.

LED and dimming challenges will no doubt be solved, and manufacturers are making great progress in addressing the challenges. However, for the moment, issues such as incompatibility and safety are still very big concerns.

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