1. PRACTICAL # 11
Introduction to light emitting diode (LED), its working principle and terminal identification
By:Engr.Irshad Rahim Memon

2. OBJECTIVE
Objective of this practical is learn about LED, how it works and how to identify its terminals

3. Some Concept Regarding LED
Light emitting diode (LED) falls into category of optical devices/diodes.
LED is a two terminal simple PN junction diode with large exposed area on one layer and it is used to emit the light when proper forward bias voltage is applied to it.
Internal working structure and schematic symbol of LED is shown in the figure 01(a) and (b) respectively.

4. Figure 1

5. Working Continued….
When LED is forward biased, conduction electrons from n-type material cross the pn junction to combine with conduction holes in p-type material and finally fall into valence band of p-type material by releasing some energy in the form of heat and light.
This discrete amount of light energy is known as photon. This process of light emission in the form of photon is called electroluminescence

6. Continued….
Doping material determines the wavelength of emitted light and Wavelength determines the color of light and if it is visible or infrared. Different colors are obtained by using different combinations of elements
Typical forward voltage drop across LED ranges from 1.2volt to 3.2volt and reverse voltage drop across LED ranges from 3volt to 10volt. Amount of output light is directly proportional to amount of current flowing through LED as shown in figure 02. Supplied current for an LED should be less than maximum current specified in datasheet.

7. Figure 02

8. Coming Towards Objective now

9. Required Apparatus DC Source Few LEDs Breadboard Multimeter
Connecting Wire etc.

10. Procedure:-
Take different LEDs and identify their terminals with the help (i) Ohm meter (ii) LED casing.
Forward resistance of LED is in the range of kilo ohms and reverse resistance of LED is in the range of mega ohms/out of range or simply forward resistance of LED should be less than reverse resistance.
Terminals of LED can be identified with respect to its casing as given in figure 03.

11. Figure 03

12. Procedure Continued…
Connect LED in forward biased way as shown in figure 02(a) and check voltage drop across its terminals.

13. Figure 02 Circuit on Multism

14. Figure 02 Circuit using Potentiometer on Multism

15. OBSERVATION Terminals of LED are identified
Forward voltage drop across LED terminals is found around 1.2v to 3.2v.
Light out is directly proportional to current through LED

16. ANY QUERY