LED illumination for building interiors and outdoor public spaces continues to gain serious traction, opening up new opportunities for electronic designers. For example, National Semiconductor’s LM3445 and NXP’s SSL2101 monolithic controllers for high-brightness (HB) white LED (WLED) building lighting accommodate legacy triac dimmers while providing wide-range dimming and power factor correction (PFC).
Meanwhile, Texas Instruments and Microchip both provide online resources for designers who want to develop custom microcontroller dimming applications. Every company that makes ICs for driving LEDs now offers chips that provide for pulse-width-modulation (PWM) control among their products.
CITY LIGHTS
Outside the building, things are changing fast. Mark McClear, director of business development, solid-state lighting, at Cree, observes that two years ago, it simply wouldn’t have been practical to use LEDs for street lighting. A year ago, the idea was barely into the trial stage. “A few cities would replace the fixtures on one block of one street,” he says. This year, it’s full steam ahead around the world (Fig. 1).
Last summer, the city council of Anchorage, Alaska authorized the replacement of 16,000 streetlight fixtures—a quarter of all the streetlight fixtures in the city—with LED luminaires. The city is investing $2.2 million in the plan. The new streetlights will consume half the energy used by the current fixtures, leading to a potential savings of $360,000 each year. According to Cree, the LED fixtures, based on Cree XLamp LEDs, typically last up to seven times longer than high-pressure sodium fixtures, allowing the city to better utilize maintenance resources.
One interesting aspect of Anchorage’s situation is that 85 days a year have fewer than eight hours of sunlight, which suggests that the payback in power costs increases closer to the Earth’s poles. However, it ignores the corollary that Anchorage must also have 85 days in a year with fewer than eight hours of darkness.
Down south, around latitude 47N, the city of San Jose, Calif., just announced plans to replace its 62,000 streetlights with new LED versions that will “use state-ofthe- art technology to vary their intensity and timing.” San Jose plans to convert 100 lights this spring and is seeking $20 million from the stimulus package to install 20,000 new lights. The intent is to have all of the city’s streetlights changed by 2022.
According to the San Jose Mercury News, the city’s street lighting bill is nearly $4 million a year. Last year, costs rose so high that the city turned off 900 streetlights to save money. Although the new lights will have a high acquisition cost, the city estimates it could recoup that within five years. That’s because, says the newspaper, the current lights need to be changed every few years, and often the city doesn’t know a light is out until it gets a call from a resident.
“Under the new system, the lights will last 10 to 15 years, and they will alert the city automatically when they are out,” the Merc reports.
GET THE YELLOW OUT
The LED lighting will replace sodiumvapor lights used in cities at the southern end of San Francisco Bay for the sake of the optical telescopes at Lick Observatory on nearby Mount Hamilton. (With just two spectral lines, the loom of sodium lighting can be dealt with using optical filters.) While the accommodation had the support of the astronomers, the sodium lights have been unpopular with the general citizenry. The complaint is that they’re too easily confused with traffic signals and distort the colors of cars and painted curbs.
That’s where the San Jose technology goes beyond merely swapping light fixtures. The plan may even give the astronomers control over the lighting when they want to take a picture. Well, maybe not initially, but all of the lights will be radio-controlled, and the first concession to the astronomers’ need for “dark skies” will be selective dimming, cutting them back, say, by 30% from 3 to 5 a.m., when Lick’s telescopes are most active (see “LED Lighting And Light Pollution").
In another Silicon Valley wrinkle, to help power the streetlights, the city plans to set up solar panels along light poles, on top of buildings, and on canopies over sidewalks. In time, the goal is to power these lights entirely from renewable sources.
More than doubling San Jose’s volume of replacements, the city of Los Angeles, Calif., announced in February plans to replace 140,000 existing streetlight fixtures in the city with LED units over the next five years. The city expects electricity savings of at least $48 million over seven years and a reduction in carbon emissions by approximately 40,500 tons a year.
McClear says this fresh activity is possible partly due to tighter binning and improvements in the Color Rendering Index—getting a broader range of photon energy levels out of the phosphors so that colors appear more natural. Yet even more important may be what he calls “managing photons,” making sure light generated inside the lamp exits the lamp and can be directed so that it illuminates what it was intended to illuminate, instead of radiating off into space. Managing photons translates into more lumens/watt and better energy efficiency.
FIXTURES, NOT BULBS
Surprisingly, McClear thinks we shouldn’t expect to see much direct replacement of old, Edison-based bulbs or fluorescent tubes with direct LED equivalents. The implication for dimming designs is that even though chip companies are now focusing on dimming room lighting using legacy triac dimmers, this doesn’t mean just changing out old bulbs and tubes. Instead, it means replacing existing can and overhead fixtures at the junction box in the ceiling while retaining the existing wiring and controls upstream from that.
The caveat reflects Cree’s roadmap for 2008 through 2012. In part, it looks at three types of LED replacements for screw-in and plug-in form-factor incandescent bulbs of the type normally used in recessed can fixtures and flood/spotlight track lighting: the A19 Edison-base “light bulb,” the PAR38 Edison-base “flood,” and the MR17 pin-socket compact flood (Fig. 2).
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