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How does an LED Work
Written By: John R. Sewell

Light Emitting Diodes or as we know them LEDs have been commercially available since the late 1960s. Based on strict physics of electroluminescence, the light you visibly see is the release of energy as photons. The output color is a function of the band gap energies in the material forming the traditional diode P-N junction. 3mm Green LED, Diffused, Pack of 100 Early LEDs released their energy in the Infrared region of the spectrum at wavelengths greater than 760nM. As you would expect, LED are true Diodes in the sense that they conduct in one direction and block in the other, just like any other P-N junction Diode. In a normal Silicon or Germanium junction diode the recombination of Electrons and Holes (actually a missing electron in the valance band of the doped material) give up or releases energy but this release is not in the form of a photon. The use of other materials cause a more direct Band gap energy (Photon) release in the Visible, I/R & UV spectrum. Combinations of Indium, Gallium, Arsenic, are used in LEDs today. The technology has advanced nearly exponentially over the past years, bringing us LEDs of most any color. 20 lm 1 Watt BLUE Star LED As the technology advanced so did the light output. Early LEDs were no match to incandescent lamps, except in a few places, because light output was just a few MCD (Millicandela). Modern LEDs are now available with light outputs above a candela although these “super” LEDs are mostly rated by Lumens.

While there is no production of a “white” LED, through technology we create White LEDs by use of a fluorescent coating on a Blue LED die. The coating layer “up” converts some of the blue to a yellow and the mixture appears as white. By varying the coating we get warm white and cool white. In some general lighting applications the use of Red, Green & Blue LED mounted together will produce the perception of white to the eye.

LEDs are what we call a “Current” mode device where excitation is by current magnitude. Most signal LEDs in the 3mm-10mm dia. size are rated for use at 20mA (.020A) continuous with pulses to 50mA. LEDs must be treated with care not to exceed the current limits shown on the Specifications sheet. Over drive equals overheat which degrades color, light and lifetime. To use an LED you should always have a current limiting component in series with it. This is usually a simple resistor. To figure the resistor you need to know your Bi-Pin G4 LED Lamp, 12V White available supply voltage, the forward voltage of the LED and number of LEDs in series. Ohm’s law prevails here; so: Vs-(Vf1 + Vf2…Vfn)/If = Rs where Vs=Supply Voltage, Vf1+Forward Voltage of LED #1 Vf2 = Forward Voltage LED #2 etc, Rs=limiting resistor value (pick the next higher 5% resistor. Say you get Rs = 1082 ohms you pick 1100 ohms.). Vf is related to color so Red LEDS range 1.5-2.0V Green: 1.8-3.8V, Blue: 2.5-3.8V, Yellow: 2.1-2.5V. The thing to remember is that you don’t need the precise value but you should use worst case Vf. You might get some Reds at 1.5V other at 2.0, but what is critical is that if you use more than 1 led, that your supply Vs exceeds the total Vf of your LEDs. If Vf is greater then they won’t light. Some common LED current specifications are 20mA, 250mA, 350mA, 700-750mA. The newer Super Lumen LEDs are also rated in Watts. Some common Watt values are: 1W, 3W, 5W, 10W, 30W and even 50W+. Most of the high rated ones are really multiple die mounted in single case and connected in series. Heat is the biggest destroyer of LEDs so generally, these super power LEDs require a heatsink & sometimes even a fan.

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