LED & LED Displays
Light Emitting Diodes or LEDs have been commercially available since the late 1960s. Today they are readily available as singles or as Bulk LEDs in packages of 100 & 1000. We stock LED of many colors & Function. Based on electroluminescence, the light is the release of energy as photons. The output color is a function of the energies in the material forming the traditional diode P-N junction. LEDs are true diodes in the sense that they conduct and block just like any other Diode. Combinations of Indium, Gallium, Arsenic, are used in LEDs of today bringing us most any color. Modern LEDs are now available with light outputs above a candela although these LEDs are usually rated in Lumens. There is no “white” LED, through technology we create White LEDs by use of a fluorescent coating on a Blue LED.
The coating layer “up” converts some of the Blue to a yellow and the mixture appears as white. In general lighting applications the use of Red, Green & Blue LEDs 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 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 in the specifications. Over drive= overheat which degrades color, light and life. You should always have a current limiting component in series with the LED(s). This is usually a simple resistor. To figure the resistor you need to know your supply voltage, the forward voltage of the LED & 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: 1.8-3.8, Blue: 2.5-3.8, Yellow: 2.1-2.5. 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, what’s 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. Common LED current specifications are 20mA, 250mA, 350mA, 700-750mA. The newer Super Lumen LEDs are also rated in Watts (W). Common Watt values are: 1,3,5,10, 30, & even 50W+. Most of the high rated ones are really multiple die mounted in single case & connected in series. Heat is the biggest destroyer of LEDs so all these super power LEDs require a Heatsink & sometimes a Fan.
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