Question: Q. 8. Draw the $I-V$ characteristic of an LED. State two advantages of LED lamps over conventional incandescent lamps. Write the factor which controls (a) wavelength of light emitted, (b) intensity of light emitted by an LED.
U] [OD III Comptt. 2017]
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Solution:
Ans. I-V characteristics
Two advantages
Factors
Advantages (any two)
(i) Low operational voltage.
(ii) Less power consumption.
(iii) Long life
(iv) Ruggedness
[or any other]
(a) Energy band gap controls the wavelength of light emitted.
(b) Forward current controls the intensity of emitted light.
Detailed Answer :
(i) Characteristics of LED
| Typical LEDCharacteristics | |||
|---|---|---|---|
| Semiconduc- tor Material |
Wavelength | Colour | $V_{f} @ \mathbf{2 0}$ $\mathbf{m A}$ |
| GaAs | $850-940 \mathrm{~nm}$ | Infra-red | $1.2 \mathrm{~V}$ |
| GaAsP | $630-660 \mathrm{~nm}$ | Red | $1.8 \mathrm{~V}$ |
| GaAsP | $605-620 \mathrm{~nm}$ | Amber | $2.0 \mathrm{~V}$ |
| GaAsk | $585-595 \mathrm{~nm}$ | Yellow | $2.2 \mathrm{~V}$ |
| AlGaP | $550-570 \mathrm{~nm}$ | Green | $3.5 \mathrm{~V}$ |
| SiC | $430-505 \mathrm{~nm}$ | Blue | $3.6 \mathrm{~V}$ |
| GalnN | $450 \mathrm{~nm}$ | White | $4.0 \mathrm{~V}$ |
(ii) Advantages :
(a) LEDs provide instantaneous turn $\mathrm{ON}$ and have no issues with frequent switching
(b) LEDs consume less power and can operate effectively on low-voltage electrical systems.
(c) LEDs are able to operate at virtually any percentage of their rated power (0 to $100 %$ )
(d) LEDs have good Colour Rendering Index (CRI) $\mathbf{1}$
(iii) Factors :
(a) Wavelength of light emitted by LED depends on forbidden energy gap of semiconductor material which is used for making LED. The wavelength of light emitted by LED is inversely proportional to forbidden energy gap.
(b) Forward current increases as intensity of light increases that reaches to maximum value which on further increase in forward current will lead to decrease in light intensity.
