LEDs having a full meaning of light emitting diode is a special type of diode that converts electric energy into light. Light emitting diodes converts electric current either AC or DC into light rather than other diodes which converts electric current into heat instead when connected in circuits.
Light emitting diodes are just like normal diodes which allows current to flow in only one direction and blocks current in the reverse bias.
In normal diodes like rectifiers, tunnel diodes, zener diodes and shocktty diode when conducting in forward bias(for zener diodes both forward and reverse bias) allows current to flow in one direction but due to the crossing of electrons from one barrier to the other (holes and electrons meets at a junction) heat is produced contrary to this instead of producing heat LEDs produces light and also accompanied with heat when much current flows through them.
Since the invention of LEDs filament bulbs and incandescent lamps have diminished in the electronics market. Thus filament and incandescent lamps consumes power and are less effecient, most of the converted energy results to heat.
LEDs has two terminals for connecting to circuits namely, cathode and anode it circuit symbol is shown below with just a diode symbol with arrows pointing out.
Circuit symbol of an LED.
Working principles of LEDs.
When Light emitting diodes are connected to a power source in reverse bias no current flows through it and no light is produced.
If connected in forward bias current flows in it and light is produced.
LEDs are made from junctions of two different semiconductors which are the N-TYPE and P-TYPE, usually made from silicon- germanium and indium phosphide and gallium antimonide.
When current is passed to the junction diode electrons flows and cross the barrier between the P and N junction and combine with holes during this process energy is released in form of light whose color depends on the semiconductor material. This process is termed electroluminance.
Color properties of LEDs.
The type of color LEDs produces depends on the energy band gap of the component.
LEDs are basically made from aluminium-gallium-arsenide (AlGaAs). In it purest state or form electrons are strictly bond to their atom and can't conduct electricity.
To make it conductive, it's doped with a foreign element.
The impurities can either increase the amount of electrons in the semiconductor to form an N-type or reduce the electrons by sucking it to itself (chemical reaction) to form a P-type semiconductor.
When current is apply to the N-type electrons will flow from the semiconductor to the negative terminal of the DC power, while in a P-type semiconductor electrons ow from the power source to the semiconductor then back to the positive terminal of the power source.
When current is applied to an LED, electrons jumps from the highest or last orbital of it atom to the lowest orbit near the nucleus emitting or releasing a quanta of light wave.
The farther the orbital from each other(i.e semiconductors with high number of orbits) the stronger the intensity of light emitted.
As explained earlier the color and intensity of light of an LED depends on the semiconductor atomic structure. Different wavelengths
Involved in the process determines the different color produced from the LEDs.
• Infrared light is produced from an LED fabricated from galium arsenide.
•Blue LEDs requires little voltage about half a volt light up brightly because of it semiconductor nature.
Blue LEDs are made from aluminium germanium nitride.
•Red or yellow light is produced from Gallium arsenide phosphorus.
•Green or red light is produced from gallium phosphorus.
•White color is produced from the mixture of red, blue and green light LEDs manufactured on the same block of semiconductor by the process of lithography and covered with lense to produce a monochromatic beam of white light.
The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light.
LED in circuits.
To check the polarity of an LED you can use a simple multimeter set to continuity or diode, and place it probes at the terminal of the LED, if connected in correct polarity it should glow, other wise change the probes terminals across the LED.
If the LED is burnt or shorted it won't glow.
When connecting an LED in circuits it's important to reduce the current flowing through it, because excess current will make it to short and burn. At most a current of 0.5A and a voltage of 3V is enough to power an LED.
To avoid burning the LED by excess current we have to connect a resistor in series with it.
Applications of LEDs.
LEDs are one of the latest semiconductor components invented. They have much uses in electronics, automation and wide field of technology.
The big banners you see at night along the rail way or main roads looks luminous, LEDs are actually the one giving them a bright colors and luminousity.
They're use as indicators, projectors lighting and all devices that produce visual effects.
Smartphones LCDs and OLED TVs are illuminated by LEDs.
Light emitting diodes are use as LED display in calculators and petrol filling machines in petrol stations.
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Power Electronics