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Chapter: Linear Integrated Circuits : Waveform Generators and Special Function ICs

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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.

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.

 

Characteristics of opto coupler:

 

(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,

 

(i) LED – Photodiode opto coupler:


·           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.

 

(ii) LED – Phototransistor Opto coupler:


·           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

 

Advantages of Opto coupler:

 

·           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.

 

Disadvantages:

·           Slow speed.

·           Possibility of signal coupling for high power signals.

 

Applications:

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.

 

Opto coupler IC:



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.

 

Tags : Characteristics, Functional block diagram, Applications, Advantages, Disadvantages Characteristics, Functional block diagram, Applications, Advantages, Disadvantages
Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail


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