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What is an LED?
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LEDs(light emmitting diodes) are semiconductor diodes that permit current to flow in only one direction. The diode is formed by bringing two slightly different meterials together to form a PN junction. In a PN junction, the P side contains excess positive charge("hole"), while the N side contains excess negtive charge(electrons).
When a forward voltage is applied to the semiconducting element forming the PN junction, electrons moves from the N area toward the P area and holes moves toward the N area. Near the junction, the electrons and holes combine. As this accurs, energy is released in the form of light that is emmitted by the LED.
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What determins the color of an LED?
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The mererial used in the semiconducting element of an LED dertermins its color. The two main types of LEDs presently used for lighting systems are AlGaInP alloys for Red and Yellow LEDs; and InGaN alloys for Green, Blue, White LEDs. Slight changes in the composition of these alloys changes the color of the emmitted light.
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Why is it important to control the current through an LED?
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According to the voltage-current relationship figure, a slight change in volatage can result in very large changes in current. Since the light output of an LED is proortional to its current, this can result in unacceptable variation in light output. If the resulting current exceeds limits recmmended by the manufacturer, the long-term performance of the LED can be affected, result in shoter useful light.
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Can LEDs be dimmed?
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The forward current is proportional to the light output of an LED over a large operating range, so dimming can be achieved with reductions in the forward current.
Because LEDs can be rapidly swithed on an off with no harmful effects, dimming can also be accomplished using a method called PWM(pulse width modulation). By adjusting the relative duration of the pulse and the time between pulses, the apparent intensity of the LED can be dimmed.
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Does dimming LEDs decrease their lamp life?
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Life and light output degradation of an LED are determined largely by the junction temperature, with higher temperatures resulting in reduced light charaxteristics. Since dimmering, either by reducing current or by PWM, results in lower overall junction temperatures, it will have no negtive impact on LED life; it might even extand life.
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Why is heating sinking important for LED?
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LEDs generate heat within the PN 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 light of the device.
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