Electronic Design

Cost-Effective LEDs Fit Snugly In Today's Energy-Conscious World

Know the basic concepts of LEDs and their benefits, and learn how to maximize their use in various applications.

For many applications, LED lamps are superior to incandescent lighting. So why is it that in tens of millions of switches, indicators, control panels, signs, annunciators, displays, decor lights, and dozens of other applications, design engineers still specify incandescent technology? It might be that they're just a few years behind what's really happening in LEDs.

Although recent advances in LED technology have dramatically broadened the applications for these rugged little light sources, it wasn't so long ago that red was the only "daylight-visible" LED color. This wasn't the only characteristic limiting their use, either.

Unlike incandescent bulbs, which give off the full spectrum of light in a spherical pattern, LEDs emit a focused beam of a single wavelength (color) in only one direction and in a variety of angles. For many applications, such as indicators or switch illuminators, this isn't a problem. But it took the development of multichip arrays and high-flux LED chips to begin to achieve the effect of an incandescent filament.

Major advances in LED technology have taken place in recent years. For example, look at the new "doping" technologies that increase LED light output by as much as 20 times over earlier generations while allowing the production of daylight-visible LEDs in virtually any color of the spectrum. In addition to red, yellow, and amber/orange, LEDs are now available in many colors, from leaf green to ultra blue. Even white light, long thought to be impossible to achieve, is now available in three different LED shades (see "What Can White-Light LEDs Illuminate And How Can They Be Used?," p. 94).

The efficiency of LEDs is most apparent in applications that require color. Light from a typical incandescent bulb must be filtered so that only light from a particular part of the spectrum (for example, red, amber, or green) is visible. While LEDs deliver 100% of their energy as colored light, incandescent bulbs waste 90% or more of their energy in light blocked by the colored lens or filter they use. Incandescent bulbs also waste 80% to 90% of their energy on heat generation to reach the intensity (Kelvin scale) for which they are designed.

LEDs are able to provide a more color-controlled intense light. For instance, observe the color difference in the center brake light on many modern cars. One third of all center brake lights are red LED clusters. Many automobile manufacturers frequently use LEDs because the center light is often inaccessible, and replacement is essentially impossible. The next time you're in traffic, look around and notice how much more vivid the red is on this light than it is on standard, filtered incandescent taillights.

Once considered a fairly marginal light source, LEDs aren't marginal anymore. In many applications, LEDs exceed the energy available from incandescent bulbs and offer significant additional benefits. Consequently, LED clusters and lamps are as friendly to the environment as they are to the operating budget.

LED Options Abound
LEDs with standard light bases used to be the only competition for tiny miniature and subminiature bulbs, called the T1 or 3-mm "grain of wheat" bulbs. Today, LED lamps come in a wide variety of standard light bases, in sizes ranging from the grain of wheat T1 3-mm to medium-screw 25-mm bulbs and larger. LED lamps are rugged, durable, and daylight-visible. As a blessing to overworked maintenance personnel, they also have a life span far exceeding that available from current incandescent technology.

Previously, the cost of the incandescent bulb itself was inconsequential. The real cost came in lost production and wasted labor and energy. More significantly, a failed light can bring an entire machine down, or even idle an entire line of skilled workers. Also, consider the labor costs associated with maintenance workers who could spend time on more productive projects than changing light bulbs.

Depending on what it costs you to have a machine down and pay an operator and maintenance worker—even if it's just for a quarter of an hour—the real cost of relamping incandescent bulbs can be astronomical.

The basic LED consists of a semiconductor diode chip mounted in the reflector cup of a leadframe. This frame is connected to electrical wires and then encased in a solid epoxy lens. LEDs emit light when energy levels change in the semiconductor diode. This shift in energy generates photons, some of which are emitted as light. The specific wavelength of the light depends on the difference in energy levels, as well as on the type of semiconductor material used to form the LED chip.

LEDs are available in both visible and infrared wavelengths. Infrared LEDs reach wavelengths of 830 to 940 nm and higher. Visible colors include red, blue, yellow, orange, amber, green, blue/green, and white, and fall into the 400- to 700-nm spectral wavelength region. The colored light of an LED is determined exclusively by the semiconductor compound used to make the LED chip, which is independent of the epoxy lens color.

Molding different LED chips within a common housing creates multicolor LEDs. Applying positive and negative voltages activates each color.

LED lenses are available in several different configurations:

  1. Clear lenses. With no tint or diffusion, these produce the greatest light output and narrowest viewing angle. They're designed for applications that require very high intensity but need to be colorless in the off state.
  2. Tinted lenses. These are used to indicate what the LED color will be in the on state.
  3. Diffused lenses. Tiny glass particles embedded in their epoxy spread the light to a viewing angle of approximately ±35° from the center. These LEDs are often used for applications in which the LED protrudes through a hole in the front panel of electronic equipment.
  4. Non-diffused lenses. Without any glass particles in the epoxy, these produce a narrow viewing angle of ±12° from the center. Often, they're used in backlighting applications in which the LED is focused on a translucent window in the front of a panel.

Other viewing factors include the LED's shape and size, as well as the distance from the LED to the nose of the epoxy lens. The closer the LED is positioned to the nose of the epoxy lens, the wider the viewing angle.

LEDs use a fraction of the power (80% to 90%) required by conventional filament bulbs. Solid-state design enables LEDs to withstand shock, vibration, frequent switching (electrical on and off shock), and environmental (mechanical shocks) extremes without compromising their famously long life—typically 100,000 hours or more.

Incandescent lights, meanwhile, heat a metal filament that radiates light inside a glass bulb. Their radiated white light consists of a wide spectrum of electromagnetic radiation. They generate high-intensity light for a short operating lifetime and are susceptible to damage from shock, vibration, and temperature extremes.

Let's take one example. In 1995, New York State Electric & Gas Corp. (NYSEG) entered into a joint testing agreement with LEDtronics. The basic agreement called for NYSEG to test the LED lamps in a specific substation and report on their use shortly after installation, and again at the end of the test period. LEDtronics supplied the bulbs, which were installed in February of 1996. The power station used over 100 bulbs.

Each incandescent bulb in the station was turned on and off and then back on once before removal. The result was that 20% of the incandescent bulbs failed. The same process was used with the LED lamps, which produced no failures.

As of December 1997 (22 months later), NYSEG experienced no LED lamp failures at the station, even though the bulbs had been turned off and on several times during the period. (NYSEG recorded all of the incandescent bulb failures under normal operations at a comparable station and found an approximate 25% failure rate.) The LED bulb life, brilliance, and viewing angle also proved to be acceptable to NYSEG field personnel.

NYSEG believes that even though the initial installed cost for LED lamps is much greater than incandescent bulbs, over the life of any substation, LED lamps for NYSEG's purposes will cost much less. There are fewer storage needs, replacement time is reduced, and much less current drain is placed on the substation batteries. Additionally, there should be less frustration during testing periods where bulbs are a key to determining which devices are in or out of service.

Here's how to calculate the cost savings attributed to using LED lamps. First, choose the value that most closely represents your per-hour cost of having a machine down. Then, figure in the approximate cost per hour of your maintenance labor rate and the cost per hour of the machine operator (see the table). The results are the approximate savings an LED can offer in its 10-year life span.

If you want to do the math based on your own figures, here's our formula:

(M+L+O) × T × 35 = Cost of conventional lamps for 10 years

where:

M = the cost of machine downtime per hour
L = the cost of maintenance labor per hour
O = the cost of operator labor per hour
T = the time it takes to change a bulb. (We used 0.25 hours, by the way, but the actual time could be different.)

To get the total costs, you'll also have to add in the number of LED lamps that you are replacing and the energy savings that you will achieve over ten years (average life of the LED lamp). In this case, you add in 35 incandescent lamps, which is the number you'll need over 10 years. The aforementioned formula then becomes:

(M+L+O) × T + S = Cost of a StackLED for 10 years

where:

S = LEDtronics StackLED (see the figure).

Of course in this case, you only add one LEDtronics StackLED instead of 35 incandescent lamps.

The rise in ambient temperature affects an LED-based lamp's longevity. The operating current (or power dissipation) must then be adjusted.

For reference purposes, we will use as an example the LEDtronics BF321CR3K-24V-BP, which is designed for a 24-V nominal and 28-V maximum ac or dc operating voltage. All components are designed to operate in the 35° to 45°C range with an ambient temperature of 25°C.

Let's examine the case where an LED-based lamp is operating in an environment where the ambient temperature is above the temperature range specified for operation.

Maintaining Longevity
Since LEDs are current driven, each degree the ambient temperature rise affects the LED's (including its components) operating temperature. In order to maintain the LED's projected longevity (100,000 hours), the LED and its components should be redesigned to operate within the nominal desired operating temperature range of 35° to 45°C. This can be accomplished by derating the operating current of the LED by a 0.3-mA per degree rise in ambient temperature above the nominal (25°C).

For instance, when the ambient temperature is specified around 25°C but rises to 55°C, the LED experiences an increase of 30°C in its operating temperature. As a result, the LED's operating current range should be reduced by 10 mA (+70° to -40°C ∞ 0.3) to compensate for the rise in ambient temperature and to maintain the LED's expected longevity.

Similarly, the operating current of the LED-based lamp should be derated to compensate for the rise in ambient temperature, at an ambient temperature of 55°C and an operating temperature 70°C. Otherwise, the LED is operating at a current 10 mA higher than it was designed to accommodate. (The maximum current for most LEDs is 20 mA.)

Operating an LED-based lamp at an operating temperature higher than its intended specifications adversely effects its longevity. But without extensive life-cycle testing on the LED lamp, it's impossible to determine the precise number of hours by which the lamp's longevity will be reduced.

An Expanding Market
Many factors are pushing LED lamps into more and more general lighting applications. Lower LED chip costs, brighter high-flux LEDs, reliable light-source requirements, and energy saving concerns, to name a few, are opening up new markets and applications daily.

Even though LEDs have been around for over 30 years, many designers perceive them to be a new lighting technology. As with any new technology, a basic understanding is required in order to let them properly work for you. When designing or purchasing an LED lamp, remember these basic LED concepts and you'll design or purchase a device that will outlast an incandescent bulb by ten- or twenty-fold.

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