Lesson 1452,
Optoelectronics
Experiment 1, Testing LEDs
with an Ohmmeter
Objectives
1. To show how to use an ohmmeter to
test standard LEDs.
2. To verify the fact that LEDs have many
of the characteristics of ordinary diodes.
3. To point out some precautions to be
taken when using an ohmmeter to test
LEDs.
Introduction
 LED means light-emitting diode
 As the name implies, a LED is a special diode
which gives off light when on
 An ordinary diode is made up of two types
of silicon or germanium that form the PN
junction.
 This type of diode doesn’t give off light
primarily because the case or package
material is opaque. (doesn’t give off light)
 LEDs on the other hand are packaged in a
translucent material that allows the light
generated at the junction to be given off
by the device.
 Gallium arsenide is used to give off red light
 Gallium phosphide gives off green light
 The amount of light produced by these
materials is very small. Consequently, it is
usually necessary to increase the
brightness of the device by using a special
plastic lens within the case itself to
“amplify” the light.
 Many LEDs including the one you will be
using for the experiment, use such an
arrangement.
 LEDs act much like ordinary diodes.
 They pass current when forward biased
 They block current when reversed biased
 Precautions must be taken when testing
LEDs. Like regular diodes, LEDs can only
handle a limited amount of current;
otherwise, they will be permanently
damaged.
 LEDs can be damaged both when testing
for forward biased and reversed biased
conditions.
 LED’s can fail in reverse bias if the voltage is
too high.
 The maximum reverse voltage for many
diodes is only about 3 V.
 It is good practice to check the specification
(spec) sheet to verify the component limits.
 I have shown, on the next slide, two
schematic symbols that are commonly
used. Some publishers use the circle
surrounding the main symbol.
 You may note the symbol is similar to the
diodes schematic symbol, except for the
outward pointed arrows, which point
away from the junction. The direction of
the arrows indicate light emission.
Standard LED Schematic Symbols
Standard LED Schematic Symbols
Typical LED Case / pictorial
Testing LEDs with Ohmmeters
 LEDs act much like a normal diode.
 We can make use of the characteristics
when testing the LED with an ohmmeter.
 As you recall; Diodes can be tested by
making two measurements.
 When the diode is forward-biased, its’
resistance is low
 When a diode is reversed biased the
resistance is high.
 There is a section in this experiment which
states the Analog Multimeter at a x10 or
times ten setting will turn the LED on if it is
forward-biased.
 This is true for the older models of Analog
Multimeters. Very few models now will turn
on the LED.
 Pay attention to the meter readings
discussed on the previous slide.
 The following two slides will show the
schematic diagrams for testing LEDs.
LED Reverse-Biased Schematic
LED Forward-Biased Schematic
Testing with an Ohmmeter
Questions?
Resources
 Rosenow. (2001). Lesson 1452:
Optoelectronics. Cleveland: Cleveland
Institute of Electronics.
The End
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in the CIE Instruction Department.
© Copyright 05/2012
All Rights Reserved / May 2012