How Does Light Emitting Diode Works?

25 Oct 2019

LEDs form an inevitable part in the modern electronics as simple indicators to optical communication devices. Light Emitting Diodes exploit the property of the p-n junction to emit photons when it is forward biased. LEDs are specially made diodes to emit light when a potential is applied to its anode and cathode.

What makes LED ideal?
LEDs are extensively used in electronic circuits because of its advantages over bulbs. Some important features that make LED ideal in electronic circuits are

LEDs are encapsulated in plastic or resin cases so that they can withstand mechanical shocks.
Unlike bulbs, LEDs do not generate heat and power loss through heating is practically nil.
LEDs require very low current and voltage typically 20 milliampere current and 1.8 volts. So these are ideal in battery operated circuits.

What is inside an LED?
Inside the casing of an LED, there are two terminal posts connected by a small chip made of Gallium compound. This material exhibits the property of photon emission when the p-n junction is forward biased. Different colours are produced by dopping the base material with other substances.

LED technology follow some physics
Brightness is an important aspect of LED. Human eye has maximum sensitivity to light near 550 nm region of yellow – green part of the visible spectrum. That is why a Green LED appears brighter than a Red LED even though both use same current. The important parameters of LED responsible for its performance are:

Luminous flux
Indicates the light energy radiating from the LED. It is measured in terms of Lumen (lm) or Milli lumen (mlm)
Luminous intensity
The luminous flux covering a large area is the luminous intensity. It is measured as Candela (cd) or milli candela (mcd) Brightness of LED is directly related to its luminous intensity.

Luminous efficacy
It is the emitted light energy relative to the input power. It is measured in terms of lumen per watt (lm w).
Forward current, forward voltage, Viewing angle and Speed of response are the factors affecting the brightness and performance of LEDs. Forward current ( IF ) is the current flowing through the LED when it is forward biased and it should be restricted to 10 to 30 milli amperes other wise LED will be destroyed.

Viewing angle is the off – axis angle at which the luminous intensity fall to half its axial value. This is why LED shows more brightness in full on condition. High bright LEDs have narrow viewing angle so that light is focused into a beam. Forward voltage (Vf) is the voltage drop across the LED when it conducts. The forward voltage drop range from 1.8 V to 2.6 Volts in ordinary LEDs but in Blue and White it will go up to 5 volts. Speed of response represents how fast an LED is switched on and off. This is an important factor if LEDs are used in communication systems.

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