Introduction to light emitting diode (LED),
its working principle and terminal
identification
By:Engr.Irshad Rahim Memon
 Objective
of this practical is learn about LED,
how it works and how to identify its
terminals
 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.
 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
 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.
 DC
Source
 Few LEDs
 Breadboard
 Multimeter
 Connecting Wire etc.
 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.
 Connect
LED in forward biased way as shown
in figure 02(a) and check voltage drop across
its terminals.
 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