LED (Light Emitting Diode)

 

·         Light Emitting diode: LED working principle

            Construction:

            It is a light emitting semiconductor device with a single p-n junction intended to convert electric energy into light energy.

            It is fabricated using gallium phosphide (GaP), gallium arsenide (GaAs) and silicon carbide(SiC).

            Depending on the material used in LED, the colour of light vary.

            Red light- GaAs, GaP     Yellow light-SiC

            LEDs are available in round tops as well as square tops with different sizes in millimeters.



            Operating Principle:

            If an ordinary diode is forward biased, free electrons cross the junction and fall into holes.

            As these electrons fall from higher energy level to a lower energy level, they radiate energy.

            In ordinary diodes, this energy is radiated in the form of heat.

            But in LED, energy is radiated in the form of light.            




Symbol of LED



Specification Parameters:

            Current rating and power (in mW), size (in mm)

            Appearance- round capped or square shaped

            Colour – peak wavelength in nm

            Luminous intensity in millicandela

            Typical Voltage drop from 1.5 to 2.5 volts

            Current between 10 and 50 mA

            Power rating 30 to 40 Watt

            Current limiting resistor of 220 ohm or 330ohm is connected in series with LED.

            For selection of any LED for particular application , the driving voltage and current flowing through it are to be calculated.

I-V Characteristics for LED



Typical LED exhibits Turn ON Voltages between 2.5 to 3 Volts, breakdown voltage at approximately 8 volts and series resistance 60 ohm estimated from the I-V graph.

In the Reverse biased mode, there is no current flow through LED.

Applications:

By using compounds of gallium, arsenic and phosphorous, LEDs that radiate red, green , yellow produce visible radiation are used in instrument- displays, calculators, digital watches, panel indicators etc.

Now-a days, LEDs are also used for decorative purposes like running lights, traffic signals, display indicators in bank or railway etc.

·       You Tube link of LED : https://youtu.be/iY7g0A6P4EY

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