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Why LED's? |
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| LED's (Light
emitting diodes) have begun to light the future with their
capabilities of reducing power consumption by 80% over
conventional lighting. Their "light" span is a minimum of 10
years for commercial usage, and a minimum of 25 years for
residential applications. Besides eliminating the environmental
hazards of mercury in fluorescents, LED's quietly illuminate
with the closest look of natural daylight. |
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About LED Technology |
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| Light emitting diodes (LEDs) were
first developed in the1960s, but only in the past decade have
LEDs had sufficient intensity for use in more than a handful of
lighting applications (Stringfellow and Craford 1997), and
specifiers are confronted with an increasing number of lighting
products that incorporate LEDs for certain applications.
Primarily, these applications have taken advantage of the
characteristics of LEDs that have made them most suitable for
indication, not illumination (Bierman 1998). |
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What determines the color of an LED? |
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| The material used in the
semiconducting element of an LED determines its color. The two
main types of LEDs presently used for lighting systems are
aluminum gallium indium phosphide (AlGaInP, sometimes rearranged
as AlInGaP) alloys for red, orange and yellow LEDs; and indium
gallium nitride (InGaN) alloys for green, blue and white LEDs.
Slight changes in the composition of these alloys changes the
color of the emitted light. |
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What are the electrical characteristics of LED's? |
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| Individual LEDs are low voltage
devices. Single indicator LEDs require 2 to 4 volts of direct
current, with current in the range from 1 to 50 milliamperes. An
illumination-grade LED containing a single semiconducting
element requires the same voltage, but operating currents are
much higher, typically several hundred milliamperes. A device
containing multiple elements connected in series will require
higher voltage corresponding to the larger number of individual
elements in the device. LEDs require a specific electrical
polarity. Applying voltage in reverse polarity can destroy them.
Manufacturers provide specifications about the maximum reverse
voltages acceptable for LED devices; 5 volts is a typical
maximum rating. |
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Why
is heat sinking important for LEDs? |
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It is common to refer to
LEDs as "cool" sources in terms of temperature. This is
because the spectral output of LEDs for lighting does
not contain infrared radiation, unlike incandescent
lamps that produce a large amount of infrared (of
course, some LEDs for manufacturing purposes are
designed to produce infrared energy, but these are not
considered in this publication). LEDs are also often
considered "cool" because they generate light through a
mechanism other than thermal excitation of a substance,
such as the tungsten filament in an incandescent lamp.
Although LED lighting systems do not produce significant
amounts of radiated heat, LEDs still generate heat
within the junction, which must be dissipated by
convection and conduction. Extracting heat from the
device using heat sinks and by operating LEDs in lower
ambient temperatures enables higher light output and
longer life of the device.
The need to ensure heat sinking with LED systems is also
important to consider when these systems are installed
in applications. There must be sufficient means to
conduct the heat away from the system, or ventilation to
cool heated surfaces by convection. Locating an LED
lighting system in an insulated and relatively small
space will likely result in rapidly increased junction
temperature and suboptimal performance. |
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