Optical isolators

What are Isolators?

  • Isolator as the name implies is a device which isolates/ opens the circuit or which acts like a switch. Isolators are typically used so that we can isolate a circuit and can do the necessary changes in it.

Optical means use of light.

Thus, optical isolator means the device which isolates a circuit by using light mechanism in it.

Optical isolators are basically used for separating physical contacts between two circuits from each other and still control the circuit.

The two circuits can be high power circuit as well as low power circuit. In some kind of applications, the high power circuit is often required to separate from low power circuit as the high power circuit may damage a low power circuit. Isolation is required for the safety of circuit devices.

In early days, relays were widely used as isolation between the two circuits. The electromagnetic relay is used for this purpose.

Nowadays, relays are replaced by semiconductor devices. These devices act as isolation between the circuits.

This is because of various disadvantages of using relay as an isolator.

The disadvantages are:-

  1. Too much cost of relays.
  2. They are very heavy as compared to other semiconductor devices.
  3. They produce electrical noises due to magnetic fields.
  4. Life span of relay is very less.
  5. Relays contact produce a spark while making and breaking the circuit.

So what is an optoisolator we have seen above.

Now, how this device works. Let’s see.


Optoisolator consists of three components:-

  1. LED to transmit the light
  2. Phototransistor to receive the light
  3. Dielectric medium between them.

It is not necessary that optoisolator contains these items only; other three different combinations can also be used such as IR LED, Photoresistor etc.

  • The control circuit is connected between the circuits 1 & 2 whereas the power circuit is connected between 3 & 4.
  • Whenever we have to control the power circuit, the current is supplied to LED which glows due to which the phototransistor senses the light availability and the power circuit completes its circuit.
  • When the current is not supplied to the LED, the LED doesn’t glow and the photoresistor acts as an open circuit which makes the power circuit incomplete.

So this is all about the optoisolator.
The optoisolator overcomes all the disadvantages of relay.
Thus, optoisolator is widely used rather than relays.


  • Adapted by Power electronics by Khanchandani
  • Images(www.google.com)

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