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DOE’s Jim Brodrick on Phase-Cut Dimming of LEDs

Republication of Postings from the U.S. Department of Energy (DOE) Solid-State Lighting Program

by Jim Brodrick, U.S. Department of Energy

Solid-state lighting has proven that it can deliver high-quality light with substantial energy savings in many applications. But dimming can still be an issue, as CALiPER testing and GATEWAY demonstrations have shown. This concern was reiterated by utilities and energy efficiency programs at a special SSL roundtable convened last year by the U.S. Department of Energy (DOE). In response, we’ve published a new report, Dimming LEDs with Phase-Cut Dimmers: The Specifier’s Process for Maximizing Success.

Based on CALiPER and GATEWAY results, the report focuses on the most common type of dimmer used in architectural installations: the phase-cut dimmer, which differs from dimmers that separate the control signal from the AC mains voltage provided to the light source. While progress has been made in the dimming capabilities of LED luminaires with carefully matched dimming systems, challenges remain with compatibility and performance of LED replacement lamps and dedicated LED luminaires on existing phase-cut dimming systems.

The new report offers general guidance intended to reduce the likelihood of compatibility-related dimming problems, and also provides specific, step-by-step procedures for designing phase-controlled LED dimming on new and existing projects — plus real-world examples of how to use those procedures. It reviews phase-cut dimming technology and describes how differences between LEDs and the incandescent lamps these dimmers were designed to control can cause problems in dimming LEDs. Such problems are usually due to incompatibility between the LED light source and the dimmer, rather than to any shortcomings in the LED source. But compatibility alone doesn’t guarantee good dimming quality — such as smoothness, lack of flicker, or a specific minimum dimmed level. Those characteristics are best evaluated by observation, which is why mockups are strongly recommended where at all possible.

The step-by-step procedures outlined in the report describe the type of product research that can be done by the specifier to improve the odds of achieving good dimming performance. This research and documentation must be performed for every dimmed load on a project, which is not a simple undertaking and requires considerable time and effort.

Recommended procedures are provided for both new construction and major remodel projects, and for existing installations. Examples for applying these procedures are provided for a GATEWAY demonstration where LEDs and a new lighting control system are being specified for the Burden Museum in Troy, NY; and a project where halogen MR16 and PAR38 halogen lamps controlled by an architectural dimming system are being retrofitted with LED lamps at a church in New York’s Hudson River valley. These projects are both currently in construction or in progress, so project performance results are not yet available. However, both are useful as real-life illustrations of the process for evaluating LED lighting products in conjunction with phase-cut dimmers.

The designer, engineer, or specifier who sets out to develop a dimmable LED system should be aware of the effort that’s presently required to ensure satisfactory dimming performance. One viable option that should be considered is to avoid the need for dimming LED products, if possible. If lower light levels can be alternatively achieved through a wise design using multiple lighting switch groups, the results will be predictable and easier to design and implement. Where dimming is needed for mood, specific activities, daylight compensation, energy savings, or other reasons, the specifier needs to realize not only that the performance of the LED product is a function of its driver rather than the LED package, but also that the dimming performance is dependent on both the specific design of the LED source and the specific dimmer. It’s also important to be familiar with the detail-oriented process required for producing dimming that meets client expectations.

Excellent-quality LED dimming with phase-control dimmers can be achieved, but it requires full-scale dimming mockups or diligent research by the specifier into compatibility for every combination of dimmer and LED product. And because LED products continue to evolve so quickly, it’s important to check for any product changes between the time of the mockup and initial specification and the time the products are delivered for installation. Updates to the product could affect dimming compatibility and performance. For wall-box dimming in smaller-scale applications, specifying three-wire dimmers can reduce some erratic dimming behavior; and for two-wire dimming applications, NEMA SSL-7A-compliant dimmers and LED sources can increase the chances for success, because it guarantees some level of performance from pairings of compliant products. In the absence of industry-standard dimming guidance, the new report provides a conservative means to derate phase-control dimmers, thereby protecting them from overloading by LED sources but not implying a level of dimming quality.

Some LED products are already showing dramatically improved dimming on existing phase-cut dimmers, relative to their predecessors. With time, LED sources, their components, and dimming systems will evolve and improve; but until then, this report can serve as a useful reference. You can find it online at www.ssl.energy.gov/gatewaydemos_results.html.

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