A light-emitting diode (LED) is a diode that gives off visible light when forward biased. Light-emitting diodes are not made from silicon or germanium but are made by using elements like gallium, phosphorus, and arsenic. By varying the quantities of these elements, it is possible to produce light of different wavelengths with colors that include red, green, yellow, and blue. For example, when a LED is manufactured using gallium arsenide, it will produce a red light. If the LED is made with gallium phosphide, it will produce a green light.
When light-emitting diode (LED) is forward biased as shown in Fig. 7.2 (i), the electrons
from the n-type material cross the pn junction and recombine with holes in the p-type material. Recall that these free electrons are in the conduction band and at a higher energy level than the holes in the valence band. When recombination takes place, the recombining electrons release energy in the form of heat and light. In germanium and silicon diodes, almost the entire energy is given up in the form of heat and emitted light is insignificant. However, in materials like gallium arsenide, the number of photons of light energy is sufficient to produce quite intense visible light.
Fig. 7.2 (ii) shows the schematic symbol for a LED. The arrows are shown as pointing away from the diode, indicating that light is being emitted by the device when forward biased. Although LEDs are available in several colours (red, green, yellow and orange are the most common), the schematic symbol is the same for all LEDs.
There is nothing in the symbol to indicate the colour of a particular LED. Fig. 7.3 shows the graph between radiated light and the forward current of the Light-Emitting Diode. It is clear from the graph that the intensity of radiated light is directly proportional to the forward current of Light-Emitting Diode.
The forward voltage ratings of most LEDs is from 1V to 3V and forward current ratings range from 20 mA to 100 mA. In order that current through the LED does not exceed the safe value, a resistor RS is connected in series with it as shown in Fig. 7.4. The input voltage is VS and the voltage across LED is VD.
Example 7.1. What value of series resistor is required to limit the current through a LED to
20 mA with a forward voltage drop of 1.6 V when connected to a 10V supply ?
Note that resistor RS is also called current-limiting resistor.
Example 7.2. What is current through the LED in the circuit shown in Fig. 7.5 ? Assume that voltage drop across the LED is 2 V.
The light-emitting diode is a solid-state light source. LEDs have replaced incandescent lamps in many applications because they have the following advantages :
(i) Low voltage
(ii) Longer life (more than 20 years)
(iii) Fast on-off switching Protecting Light-Emitting Diode against reverse bias. The LEDs have low reverse voltage ratings. For example, a typical Light-Emitting Diode may have a maximum reverse voltage rating of 3V. This means that if a reverse voltage greater than 3 V is applied to the LED, the LED may be destroyed. Therefore, one must be careful not to use LEDs with a high level of reverse bias. One way to protect a LED is to connect a rectifier diode in parallel with LED as shown in Fig. 7.6. If reverse voltage greater than the reverse voltage rating of LED is accidentally applied, the rectifier diode will be turned on. This protects the LED from damage.
A LED that emits one colour when forward biased and another colour when reverse biased is called a multicolour LED. One commonly used schematic symbol for these LEDs is shown in Fig. 7.7. Multicolour LEDs
actually contain two pn junctions that are connected in reverse-parallel i.e. they are in parallel with
anode of one being connected to the cathode of the other. If positive potential is applied to the top terminal as shown in Fig. 7.7 (i), the pn junction on the left will light. Note that the device current passes through the left pn junction. If the polarity of the voltage source is reversed as shown in Fig. 7.7 (ii), the pn junction on the right will light. Note that the direction of device current has reversed and is now passing through the right pn junction.
Multicolour LEDs are typically red when biased in one direction and green when biased in the
other. If a multicolour LED is switched fast enough between two polarities, the LED will produce a
third colour. A red/green LED will produce a yellow light when rapidly switched back and forth
between biasing polarities.
The LED is a low-power device. The power rating of a LED is of the order of milliwatts. This means that it is useful as an indicator but not good for illumination. Probably the two most common applications for visible LEDs are (i) as a power indicator (ii) seven segment display.
(i) As a power indicator. A LED can be used to indicate whether the power is on or not. Fig. 7.8 shows the simple use of the LED as a power indicator. When the switch S is closed, power is applied to the load. At the same time current also flows through the LED which lights, indicating power is on. The resistor RS in series with the LED ensures that current rating of the LED is not exceeded.
(ii) Seven-segment display. LEDs are often grouped to form seven-segment display.
Fig. 7.9 (i) shows the front of a seven segment display. It contains seven LEDs (A, B, C, D, E, F and
G) shaped in a figure of *8. Each LED is called a **segment. If a particular LED is forward biased,
that LED or segment will light and produces a bar of light. By forward biasing various combinations
of seven LEDs, it is possible to display any number from 0 to 9. For example, if LEDs A, B, C, D and G
are lit (by forward biasing them), the display will show the number 3. Similarly, if LEDs C, D, E, F, A
and G are lit, the display will show the number 6. To get the number 0, all segments except G are lit.
Fig. 7.9 (ii) shows the schematic diagram of seven-segment display. External series resistors are
included to limit currents to safe levels. Note that the anodes of all seven LEDs are connected to a
common positive voltage source of +5 V. This arrangement is known as *common-anode type. In
order to light a particular LED, say A, we ground the point A in Fig. 7.9 (ii). It forward biases the LED
A which will be lit.
Reference: Principles Of Electronics By V K Mehta And Rohit Mehta
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