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Controls Training and Service Manual
118
Basic Operation of the Opto-Isolator
The purpose of the opto-isolator is to provide isolation of two voltage sources.
This allows a DC voltage source to activate an AC voltage load device or allows
an AC voltage source to provide a DC signal.
We are working with two types of circuits, an input and an output. Therefore, we
require two types of opto-isolators (referred to as optos).
The output opto uses a DC voltage source to activate a Light Emitting Diode (LED)
internal to the IC chip. This causes a triac, also internal to the IC chip, to switch
ON allowing current to flow to the AC load device.
No Current Flow Current Flow
Output Opto-Isolator
The input opto uses an AC voltage source to activate dual LEDs and turn on a
transistor to pass the DC voltage and allow current to flow.
No Current Flow Current Flow
Input Opto-Isolator
Dual LEDs Dual LEDs
AC
Sourc
AC
Sourc
DC
Sourc
DC
Sourc
LED LED
DC
Sourc
DC
Sourc
AC
Sourc
AC
Sourc