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Light Emitting Diodes 1
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. The history of LED date backs to 1907 when
Captain Henry Joseph observed the property of electro-luminescence in Silicon Carbide. The
first LED was designed in 1962. It was developed by Holonyak worked at General Electric
(GE). It was a GaAsP device. The first commercial version of LED came in the market during
1960s. LED industry became a boom during 1970s with the introduction of Gallium Aluminium
Arsenide (GaAlAs). These LEDs are high bright types and are many times brighter than the old
diffused types. Blue and White LEDs was introduced in 1990 which uses Indium Gallium
Nitride (InGaN) as the semiconductor. White LED contains a blue chip with white inorganic
Phosphor. When blue light strikes the phosphor, it emits white light.
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
1. LEDs are encapsulated in plastic or resin cases so that they can withstand mechanical shocks.
2. Unlike bulbs, LEDs do not generate heat and power loss through heating is practically nil.
3. 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.
LEDs 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
1. Luminous flux
Indicates the light energy radiating from the LED. It is measured in terms of Lumen (lm) or
Milli lumen (mlm)
2. 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.
3. 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
otherwise 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.
Is LED requires a Ballast resistor?
LED is always connected to the power supply through a series resistor. This resistor is called
as” Ballast resistor” which protects LED from damage due to excess current. It regulates the
forward current to the LED to a safer limit and protects it from burning. Value of the resistor
determines the forward current and hence the brightness of LED. The simple equation Vs – Vf /
If is used to select the resistor value. Vs represent input voltage of the circuit, Vf the forward
voltage drop of LED and If, the allowable current through the LED. The resulting value will be
in Ohms. It is better to restrict the current to a safer limit of 20 mA.
The table given below shows the forward voltage drop of common LEDs.
Red
1.8 V
Orange
2V
Yellow
2.1 V
Green
2.2 V
Blue
3.6 V
White
3.6 V
A typical LED can pass 30 –40 mA safe current through it. Normal current to give sufficient
brightness to a standard Red LED is 20 mA. But this may be 40 mA for Blue and White LEDs.
Current limiting ballast resistor protects LED from excess current that is flowing through it. The
value of the ballast resistor should be carefully selected to prevent damage to LED and also to
get sufficient brightness at 20 mA current. The following equation explains how a ballast
resistor is selected.
R=V/I
Where R is the value of resistor in ohms, V is the input voltage to the circuit and I is the
allowable current through LED in Amps. For a typical Red LED, the forward voltage drop is
1.8 volts. So if the supply voltage is 12 V (Vs) , voltage drop across the LED is 1.8 V ( Vf ) and
the allowable current is 20 mA ( If ) then the value of the ballast resistor will be
Vs – Vf / If = 12 – 1.8 / 20 mA = 10.2 / 0.02 A = 510 Ohms.
But 510 ohms resistor is not usually available. Therefore 470 ohms resistor can be used even
though the current through the LED slightly increases. But is advisable to use 1 K resistor to
increase the life of the LED even though there will be a slight reduction in the brightness.
Following is a ready reckoner for selecting limiting resistor for various versi ons of LEDs at
different voltages.
Supply voltage Red
Orange
Yellow
Green Blue
White
12 V
9V
6V
5V
3V
470E
330 E
180 E
150 E
47 E
470 E
330 E
180 E
150 E
47 E
470 E
330 E
180 E
150 E
33 E
390 E
270 E
120 E
68 E
-
470 E
330 E
180 E
180 E
56 E
390 E
270 E
120 E
68 E
-
Added colours
An LED that can give different colours is useful in some applications. For example, an LED
could indicate all systems OK when it becomes Green and faulty if it becomes Red. LEDs that
can produce two colours are called Bicolour LEDs. A bicolour LED encloses two LEDs
(usually Red and Green) in a common package. The two chips are mounted on two terminal
posts so that the anode of one LED forms the cathode of the other. Bicolour LED gives Red
colour if current passes in one direction and turns Green when the direction of current is
reversed. Tricolour and multicolour LEDs are also available which have two or more chips
enclosed in a common package. The Tricolour LED has two anodes for red and green chips and
a common cathode. So it emits red and green colours depending on the anode that carries
current. If both the anodes are connected to positive, both the LEDs lights and yellow colour is
produced. Common anode and separate cathode type LEDs are also available. Bicolour LED
glows in different colours ranging from green through yellow orange and red based on the
current flowing through their anodes by selecting suitable series resistor to restrict anode
current. Multicolor LED contains more than two chips-usually red, green and blue chips- within
a single package. Flashing type multicolor LEDs are now available with two leads. This gives a
rainbow colour display which is highly attractive.
Infra Red diode – The Source of Invisible light
IR diodes are widely used in remote control applications. Infra red is actually a normal light
with a particular colour which is not sensitive to human eye because its wave length is 950 nm,
below the visible spectrum. Many sources like sun, bulbs, even the human body emit infra red
rays. So it is necessary to modulate the emission from IR diode to use it in electronic
application to prevent false triggering. Modulation makes the signal from IR LED stand out
above the noise. Infra red diodes have a package that is opaque to visible light but transparent to
infra red. IR LEDs are extensively used in remote control systems.
Photo diode – It can see light
The Photodiode generates current when its p-n junction receives photons from visible or
infrared light. The basic operation of a photo diode relies on the absorption of photons in a
semiconductor material. The photo-generated carriers are separated by an applied electric field,
and the resulting photocurrent is proportional to the incident light. The velocity at which the
carriers move in the depletion region is related to the strength of the electric field across the
region and the mobility of carriers. A photon that is absorbed by the semiconductor in the
depletion region will cause the formation of an electron- hole. The hole and electron will be
transported by the electric field to the edges of the depletion region. Once the carriers leave the
depletion region they travel to the terminals of the photo diode to form a photo current flowing
in the external circuitry. In most circuits the photo diode is reverse biased, so that charge is
carried by extrinsic charge carriers. The response time of a photo diode is typically 250 nano
seconds.
LASER Diode – Pointing a beam
A laser diode is similar to an ordinary transparent LED but produces Laser with high intensity.
In the laser beam a number of atoms vibrate in such a fashion that all the emitted radiation of a
single wave length is in phase with each other. Laser light is monochromatic and passes in the
form of a narrow pencil beam. The beam of typical laser diode is 4 mm x 0.6 mm which widens
only to 120 mm at a distance of 15 m. Laser diode can be switched on and off at higher
frequencies even as high as 1 GHz. So it is highly useful in telecommunication systems. Since
the laser generates heat on hitting the body tissues, it is used in surgery to heal lesions in highly
sensitive parts like retina, brain etc. Laser diodes form important components in CD players to
retrieve datas recorded in compact discs.
D. Mohan kumar