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 technology 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.
LEDs are extensively used in electronic circuits because of its advantages over bulbs. Some important features that make LED ideal in electronic circuits are:
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.
Inside the Light Emitting Diode
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:
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.
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.
|1.8 V||2 V||2.1 V||2.2 V||3.6 V||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 versions of LEDs at different voltages.
|12 V||470 Ω||470 Ω||470 Ω||470 Ω||390 Ω||390 Ω|
|9 V||330 Ω||330 Ω||330 Ω||330 Ω||270 Ω||270 Ω|
|6 V||180 Ω||180 Ω||180 Ω||180 Ω||120 Ω||120 Ω|
|5 V||180 Ω||150 Ω||150 Ω||150 Ω||68 Ω||68 Ω|
|3 V||56 Ω||47 Ω||47 Ω||33 Ω||–||–|
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.
IR diodes are widely used in remote control applications. Infrared 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.
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.
A laser diode is similar to an ordinary transparent LED but produces Laserwith 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.