Optocoupler:
An
optocoupler is essentially a phototransistor and an LED combined in one package
Operation
When
current flows in LED it triggers the phototransistor.
Operated
as a switch
·
when LED is off transistor is off
·
When a pulse of current flows in LED transistor on.
·
Linear Signal Coupling is also possible
•A
simple circuit with an opto-isolator.
•When
switch S1 is open, LED D1 is off, so Q1 is off and no current flows through R2,
so Vout = Vcc.
•When
switch S1 is closed, LED D1 lights.
•Phototransistor
Q1 is now triggered, so current flows through R2
•Vout
is then pulled down to low state.
Since
the coupling is optical a high degree of electrical isolation is between the
input and output terminals and so it is
also called as optoisolator.
It
isolated low voltage circuits from high voltage circuits.
The
emitted (LED) and detector are contained in transparent insulating material.
•LED
for emitter
•Air as
barrier for isolation
•Phototransistor
for detector
•Transformer
is similar, but only for AC
•Optocoupler
can be used for DC
PARAMETERS:
The most
important parameter for optocoupler is their isolation.
The second
most important parameter is transfer efficiency, measured as the current
transfer ratio or CTR.
CTR is
simply the ratio between a current change in the output transistor and the
current change in the input LED which produced it.
Typical
values for CTR range from 10% to 50% for devices with an output phototransistor
and up to 2000% or so for those with a
Darlington transistor pair in the output.
APPLICATION:
·
dc circuit isolation
·
pulse type coupling application
OTHER OPTOCOUPLER'S
a) Darlington output type
b) SCR output
c) TRIAC output
