Opto
couplers/Opto Isolators and fibre optic IC
·
Opto
couplers or Opt isolators is a combination of light source & light detector
in the same package.
·
They
are used to couple signal from one point to other optically, by providing a
complete electric isolation between them. This kind of isolation is provided
between a low power control circuit & high power output circuit, to protect
the control circuit.
(i)
Current Transfer Ratio:
It
is defined as the ratio of output collector current (Ic) to the input forward
current (If)
CTR
= Ic/If * 100%. Its value depends on the devices used as source & detector.
(ii)
Isolation voltage between input & output:
It
is the maximum voltage which can exist differentially between the input &
output without affecting the electrical isolation voltage is specified in K
Vrms with a relative humidity of 40 to 60%.
(iii)
Response Time:
Response
time indicates how fast an opto coupler can change its output state. Response
time largely depends on the detector transistor, input current & load
resistance.
(iv)
Common mode Rejection:
Even
though the opto couplers are electrically isolated for dc & low frequency
signals, an impulsive input signal (the signal which changes suddenly) can give
rise to a displacement current Ic= Cf*dv/dt. This current can flow between
input & output due to the capacitance Cf existing between input &
output. This allows the noise to appear in the output. Depending on the type of
light source & detector used we can get a variety of opto couplers.
They
are as follows,
·
LED
photodiode shown in figure, here the infrared LED acts as a light source &
photodiode is used as a detector.
·
The
advantage of using the photodiode is its high linearity. When the pulse at the
input goes high, the LED turns ON. It emits light. This light is focused on the
photodiode.
·
In
response to this light the photocurrent will start flowing though the photodiode.
As soon as the input pulse reduces to zero, the LED turns OFF & the
photocurrent through the photodiode reduces to zero. Thus the pulse at the
input is coupled to the output side.
·
The
LED phototransistor opto coupler shown in figure. An infrared LED acts as a
light source and the phototransistor acts as a photo detector.
·
This
is the most popularly used opto coupler, because it does not need any
additional amplification.
·
When
the pulse at the input goes high, the LED turns ON. The light emitted by the
LED is focused on the CB junction of the phototransistor.
·
In
response to this light photocurrent starts flowing which acts as a base current
for the phototransistor.
·
The
collector current of phototransistor starts flowing. As soon as the input pulse
reduces to zero, the LED turns OFF & the collector current of
phototransistor reduces to zero. Thus the pulse at the input is optically
coupled to the output side.
·
The
input & output waveforms are 180º out of phase as the output is taken at
the collector of the phototransistor
·
Control
circuits are well protected due to electrical isolation.
·
Wideband
signal transmission is possible.
·
Due
to unidirectional signal transfer, noise from the output side does not get
coupled to the input side.
·
Interfacing
with logic circuits is easily possible.
·
It
is small size & light weight device.
·
Slow
speed.
·
Possibility
of signal coupling for high power signals.
Opto
couplers are used basically to isolate low power circuits from high power
circuits.
·
At
the same time the control signals are coupled from the control circuits to the
high power circuits.
·
Some
of such applications are,
(i) AC to DC converters used for DC
motor speed control
(ii) High power choppers
(iii) High power inverters
·
One
of the most important applications of an opto coupler is to couple the base
driving signals to a power transistor connected in a DC-DC chopper.
The optocouplers are available in the IC form MCT2E is the standard optocoupler IC which is used popularly in many electronic application.
·
This
input is applied between pin 1& pin 2. An infrared light emitting diode is
connected between these pins.
·
The
infrared radiation from the LED gets focused on the internal phototransistor.
·
The
base of the phototransistor is generally left open. But sometimes a high value
pull down resistance is connected from the Base to ground to improve the
sensitivity.
·
The
block diagram shows the opto-electronic-integrated circuit (OEIC) and the major
components of a fiber-optic communication facility.
·
The
block diagram shows the opto-electronic-integrated circuit (OEIC) and the major
components of a fiber-optic communication facility.
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