Optocoupler or Optoisolator

Optocoupler or Optoisolator

• A package which is a combination of light source like LED and a light detector such as photodiode is called an optocoupler or optoisolator.

1. Construction and Working of Optocoupler

Fig. 6.10.1 shows the basic construction of LED‒photodiode type of optocoupler.

• The LED is forward biased using voltage V₁ and series resistance R₁. The forward current of LED is I₁. Thus LED emits the light.

• The emitted light falls on a photodiode which is reverse biased using a voltage V₂. The reverse current I₂ changes according to the light absorped by the photodiode.

• The output voltage is the difference between the voltage V₂ and the drop across the resistance R₂.

  V_out = V₂‒I₂R₂

• If the input voltage is changed, the amount of light emitted by LED changes. This changes the reverse current of the photodiode. Hence the output voltage changes.

• The output voltage is optically coupled with the input voltage, though electrically isolated. Thus the deviced couples input circuit to output circuit optically hence called optocoupler or optoisolator.

• The various types of optocouplers are,

1. LED - LDR (Light Dependent Resistor) optocoupler

2. LED ‒ photodiode optocoupler

3. LED ‒ phototransistor optocoupler.

2. LED Phototransistor  Optocoupler

• Fig. 6.10.2 shows LED ‒ phototransistor optocoupler.

• The input voltage and a series resistance is used to forward bias the LED. Due to forward current, LED emits the light. The light falls on the phototransistor.

• The phototransistor becomes ON and the resulting current drives the load.

• Thus the input gets optically coupled to the output load providing electric isolation.

• This is very commonly used optocoupler as additional amplification of current is not required.

3. Characteristics of Optocoupler

• The various characteristics of optocoupler are,

1. Current Transfer Ratio (CTR): It is the ratio of output collector current (I_C) to the input forward current (I_F).

2. Isolation voltage between input and output (V_iso): This is important as optocouplers are used to transmit signals from one circuit to another which are having different potentials. It is specified in KV_rms.

3. Response time: It indicates how fast the optocoupler can change its state. It depends on output phototransistor.

4. Common mode rejection: Though optocouplers provide electric isolation, for low frequency signal, a common noise can appear at the output due to stray capacitance. The common mode rejection indicates the ability of the optocoupler to reject such common mode signals. Its value must be high.

4. Advantages of Optocoupler

• The various advantages of optocoupler are,

1. Provides electric isolation between input and output.

2. The response time is very small hence operation is fast.

3. Capable of wideband signal transmission.

4. Due to unidirectional signal transfer, the output does not loop back to the input.

5. Easy interfacing with logic devices.

6. Compact and light weight.

7. Changing load, does not affect the input.

8. The problems such as noise, contact bounce etc. Mat are eliminated.

9. No moving parts hence less maintenance.

5. Applications of Optocoupler

• The various applications of optocouplers are,

1. Mostly used for high voltage applications due to electric isolation.

2. Interfacing the output of computer to external electric circuit.

3. In driving the motors, relays, buzzers, alarms etc.

4. In a.c. to d.c. converters used for d.c. motor speed control.

5. In high power choppers and inverters.

Review Questions

1. Explain the construction and working of optocoupler.

2. Write a note on optocoupler.

3. Write a note on LED - phototransistor optocoupler.

4. State the characteristics of optocoupler.

5. State the advantages of optocoupler.

6. State the applications of optocouplers.