LED stands for Light Emitting Diode. An LED is a semiconductor chip that converts electrical energy into light. The conversion of energy into light happens on the quantum level within the molecular makeup of the semiconductor chip. The process begins with the chip acting as a diode with two terminals, a P (Positive hole carrier) and N (Negative electron) region in its basic structure, which allow the chip to conduct in one direction for operation. In addition, there are added chemical layers called epitaxy layers that enhance the ability of the device to emit light (Photons). As electrical energy passes through the P and N regions of the LED, electrons move to higher energy levels called band gap potentials. To meet the conservation of energy law, the electron's excess energy, gained while moving energy levels, will then produce a photon that our eye will perceive as light. At this point, the band gap potentials equal the energy of the photon created when the electron that was moving energy levels comes back to the ground state.
The colour of the light emitted directly relates to the size of the band gap potentials or the amount of energy the photons produce.
White LED Characteristics
a) Structure of WLED consisting of a GaInN blue LEDchip and a phosphor encapsulating the die. b) Wavelengthconverting phosphorescence and blue luminescence.
Since different colours occur at different band gap potentials, or energy levels, this explains why different colour LEDs exhibit different forward voltages to operate. Recent advances in LED technology have led to brighter LEDs due to higher quantum efficiencies and higher chip extraction efficiencies Another recent development of a blue color LED has led to RGB (Red Green Blue) white lighting as well as Phosphor on Blue to form white LEDs. The technique of Phosphor coating on Blue has shown that in the near future, white lighting from solid-state sources is a possibility, which has led to a lot of excitement.
White Light Emitting Diode (WLED) is a device that emits white light when an electrical current passes through it in.
where Iph is photo current in ampere, Is is reverse saturation current in ampere(approximately 10-8/square meter), V isdiode voltage in volt, and m is diode ideality factor (m = 1 for ideal diode). Thermal voltage can be calculated with the following equation:
where VT is thermal voltage; its value about 25mV at 25°C kis Boltzmann's constant. The value of Boltzmann's constant is approximately 1.3807 x 10-23 joules per kelvin (J · K-1). T istemperature in kelvin and q is charge of electron which valueis1.6 x 10-19 coulombs.
White light is composed of various colors of the wholerange of visible spectrum. The first commercial WLED basedon Phosphors was produced by Nichia in Japan. The structureof WLED is shown in Fig. 3 which was also the first tomanage to make the blue LED. Nichia used a blue lightemitting Gallium Indium Nitride and coated the chip withyellow fluorescent phosphor.See figure given below.
FIG.shows the WLED spectrum, the color of WLED isdetermined by the band gap of the semiconductor used tomake them. White light contains all colors and cannot be directly created by a single LED. The most common form ofWLED really is not white. It is a Gallium Nitride blue LEDcoated with a phosphor that, when excited by the blue LEDlight, emits a broad range spectrum that in addition to the blue emission, makes a fairly white light. The actual light has ablue cast and is similar in color to a mercury vapor street lamp. On the curve of Fig. 4 shows that the peak at the leftside is the shortest wavelength blue light from the LED. The lump of emission to the right side is the longer wavelengthemission of the phosphor. There are other types of WLED that are made from several different LED chips of different colorsassembled into one package. These have not been particularlysuccessful as they tend to change color depending on viewingangle and their color balance is not really good at best.
The technical data of white led is given below in which it is mentioned that in which voltage range it work and upto which current .by which you simply find what is the value of registor used in ciruit of white led.
In the past whenever something had to be lighted, we have used yellow LEDs(Light Emitting Diodes), as a substitute for white light. But now white LEDs are available. We can finally get rid of those pesky yellows.
The value of zener diode and the resistor, can be changed, a value of 470 Ohm/5W for the resistor and 3,3 Volt for the zener to be OK. You can also put 4 or 5 normal diodes in series instead of the standard 3 - but it may be hard to fit them.
Bright LEDs For Outdoor Applications
The first LEDs bright enough for use in outdoor applications were made of aluminium-gallium arsenide (AlGaAs). These red LEDs appeared as high mount-stop lights on automobiles and in a limited number of traffic lights. The recent advent of efficient green, blue and white LEDs may lead to more applications. Aluminium-gallium-indium phosphide (AlGaInP) and indium-gallium-nitride (InGaN) LEDs have succeeded AlGaAs as the brightest available LEDs. AlGaInP LEDs range in color from red to amber and produce about 3 lumens with efficacies greater than 20 lumens per electrical watt, although green and yellow AlGaInP LEDs have much lower efficacies. Hewlett-Packard plans to release AlGaInP LEDs with a light output of more than 10 lumens per LED.
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