Energy-efficient lights to gain from incandescent ban

The 100-year-long reign of the incandescent light bulb is about to end. Rather than bemoaning its death, lighting-circuit designers would do well to see the opportunity in offering a light with instant-on, that dims without flicker, and that is reliable and cost-effective.

Margery Conner, Technical Editor -- EDN, May 26, 2011

Nevertheless, both state and national governments, as well as consumers’ preference for saving money in the face of rising energy costs, are signaling the end of the line for common incandescent light bulbs. Herein lies an opportunity for engineers in creating lights that not only replace incandescents but also enhance the home or commercial environment through automatic energy savings and create a pleasant lighting environment.

Lighting technologies such as LED, fluorescent, and halogen are vying to become the new ubiquitous light source. The challenge in the near future is to provide a lighting experience that matches consumers’ expectations for how a light should work. Consumers don’t necessarily want incandescent lights but rather lighting “experiences” that match their expectations—lights that come on instantly; work with currently installed light switches, including TRIAC (triode-alternating-current)-based dimmers; deliver a warm- to bright-white light; cost-effectively save energy, and have lifetimes of more than 10,000 hours.

As a recent EDN article notes, Avnet illumineer George Kelly believes that our preference for warm colors dates back to prehistoric times when firelight was the only option for light at night (Reference 2). Blue light is more prevalent during the day when the sun is high, whereas redder, warmer light is a signal that the day is winding down and it’s time to relax. In other words, the goal of indoor lighting should be to as closely as possible match the black-body curve rather than simply to meet a color temperature or CRI (color-rendering index).


Another recent EDN article suggests that lighting can influence sleep (Reference 4). The circuit in the article uses one cyan LED and one royal-blue LED to vary the current between them to achieve 32 shades of blue. According to the article, “When coach cars of long-range trains comprised compartments for six to eight passengers, the passengers could choose either ‘white’ or deep-blue light. The blue light helped passengers sleep, even when they were not in full darkness.”

This brief description is a bit hazy, but it implies that European trains once offered a blue light as a soothing nighttime color that would aid in sleep. Although the approach of using light to influence sleep is correct, the color is wrong. We now know that it’s just the opposite: Blue light suppresses the production of melatonin, a hormone that helps induce sleep and, hence, drowsiness. Blue light of approximately 460 to 480 nm suppresses melatonin, an effect that increases with increased light intensity and length of exposure. Until recent history, humans in temperate climates were exposed to few hours of blue daylight in the winter; their fires produced predominantly yellow light (Reference 5). In addition, blue light also has a strong link to the setting of circadian rhythms, also necessary for healthful living (Reference 6).

A link between lighting and insomnia may also be possible. Seth Roberts, a psychology professor at the University of California—Berkeley, has explored the connection between lighting and insomnia, using himself as a guinea pig and referencing related research studies. He concludes that people who experience bright sunlight early in the day and no fluorescent lights just before bedtime have better sleep patterns (Reference 7).

Lighting technology has so far been developing along a drunkard’s walk of innovation: We started out with the incandescent light bulb, which seemingly by chance uses a filament that mimics the yellow-red tones of white light and is a good stand-in for the burning embers of a prehistoric community fire. We then moved to fluorescent light, which in some instances has a distinct blue bias in its color temperature—one of the worst color choices for nighttime lighting if you’re interested in sleeping shortly afterward.

One of the newest lighting technologies, LED-based solid-state lights have emerged as energy-efficient lighting that’s easily controllable over a local network and lend themselves to intelligent-building environments that automatically adjust to a building’s occupancy and use. However, the cheapest white LEDs, which commonly available LED lights currently use, have a blue hue. Designers of solid-state lights and lighting networks are learning about matching the right LED to the right use and can justify using the more expensive but also more congenially red-hued warm-white LEDs.

LED manufacturers are responding to the lighting requirement for warm lights through a variety of approaches. Cree’s TrueWhite modular lighting adds two small red LEDs that kick in depending on the current to the white LEDs. A bulb from system manufacturer Pharox, on the other hand, uses a matrix of discrete LEDs that balances continuously on white and red LEDs.


Most discrete white LEDs comprise a blue LED covered by a dollop of phosphor that emits white light when the blue LED’s light strikes it. It’s difficult to decipher the technology that many white LEDs use, and manufacturers are not always forthcoming about what’s inside the seemingly discrete LED packages. One way to check out LEDs’ warm-light performance is to look for peaks in the light-power-versus-frequency charts for a part. Avnet’s Kelly points out, for example, that Seoul Semiconductor adds red LEDs to its warm-white Acriche A4 ac LEDs to achieve a high CRI; the spike at 620 nm in the A4’s spectrum provides evidence of this approach (Figure 3).

Another technological hurdle to replacing the incandescent light is the requirement that replacement lights be compatible with the more than 150 million currently installed TRIAC-based dimming switches. Joel Spira, cofounder and former chairman of Lutron Electronics, invented the solid-state dimmer switch in 1959 (Reference 8). Now, at least 150 million dimmers are in use worldwide—most likely the reason for the Energy Star requirement that future CFLs (compact fluorescent lights) and LED lights must be compatible with the installed base of dimming switches. This compatibility requirement is difficult to meet because there is no universal specification for the performance characteristics of dimming switches. Therein lies the rub for LED-light designers. TRIAC dimmers are simple in their essence: They stop the ac line from reaching the load during part of the cycle. Less power means less light. This approach is fine when you’re dealing with a purely resistive load, such as an incandescent light, but when you’re dealing with an LED that expects constant current, handling the chopped line input from the dimmer can prove challenging. Two characteristics are their triggering voltage, or the minimum amount of line voltage it takes to cause the TRIAC to fire, and their holding current, which is the minimum current necessary to make the TRIAC remain on.


“Here’s where we get into the confluence of the product design and the requirements from the load,” says Shearer. “For example, if you buy the cheapest possible dimmer, it will probably have a higher holding current. Keep in mind that you’ve got the latching or firing current, and you’ve got the holding current. So this cheaper dimmer will have a higher holding current of, say, 50 mA. At 50 mA, you’ve got 6W on a 120V line. Just to keep the TRIAC on, you’re losing 6W.” A more costly dimmer may have a substantially lower holding current, he adds.

Another way to solve the load problem is to decrease the LED light to only 10 or 20% of full intensity. If you don’t go too low in light level, then the light itself still draws enough current to keep the TRIAC in a good state, Shearer explains. EDN’s LED-light tear-downs show that many LED lights dim to some fraction of the light and then abruptly shut off but still draw current from the line.

You can reach Technical Editor Margery Conner at 1-805-461-8242 and margery.conner@ubm.com.

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References

“Public Law 110-140, Energy Independence and Security Act of 2007,” Dec 19, 2007, US Government Printing Office. Conner, Margery, “LED bulbs reveal different design approaches,” EDN, April 21, 2011, pg 47. “Black body,” Wikipedia. Stofka, Marian, “Finely tune the hue of blue-light sources,” EDN, March 17, 2011, pg 64. “Melatonin,” Wikipedia. Eskow, Cary, “Light Matters,” Avnet Electronics Marketing. Roberts, Seth, “Strong Light and Cancer,” Seth’s Blog, Oct 10, 2010. “50 years of history and innovation,” Lutron Electronics.

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