Optoisolator or Optocoupler Construction and Operating Principle of Optoisolator

An optoisolator or an optocoupler is a device that transfers electrical signals between two electrically isolated circuits through light energy. Opto-isolators prevent the high voltages in one part of the circuit from affecting or destroying other parts of the system.

Basic Construction of Optoisolator

Opto-isolator is a device, which has two diodes: One is a source or emitter of light, usually a light emitting diode (LED) and other is photodiode acting as the photosensor. The LED converts an electrical input signal into light, and the photodiode detects incoming light and based on the incoming light either generates corresponding electric energy. A basic optocoupler is shown below

Operating Principle of Optoisolator

The working principle of optoisolator is simple and interesting. The output signal of one circuit can be controlled by varying input signal in another circuit, where the two circuits are electrically isolated. A powerful light emitting diode (LED) is connected across a variable voltage source. By adjusting the input voltage across the LED, the intensity of the light emitted from the LED can be controlled. The variable source and the LED form the input circuit of the optocoupler or optoisolator.

A photodiode is present in front of the LED so that the light from the LED directly strikes the junction of the photodiode. The photodiode is in reverse biased condition. The reverse biased circuit of the photodiode forms the output circuit of the system. It is also ensured that there is no other light falling on the photodiode junction and the system is protected from any external light, except the light coming from the LED. Initially, no voltage is applied to the LED; hence the LED does not glow. In this condition as no light falls on the photodiode, there would be only dark current flowing through the output circuit. Dark current is the reverse saturation current of the reverse biased photodiode when it entire dark. This is the unavoidable reverse leakage current of the diode. Now, if we increase the voltage across the LED, the LED starts glowing and at same time intensity of the light increases with increasing input voltage across the LED. With increasing light intensity, the reverse current in the photodiode increases, since the reverse current in a photodiode is linearly proportional to the intensity of light falling on the photodiode junction. Also, if we decrease the intensity of light in the input, the output photodiode current will decrease.

Applications of Optoisolator

Optocouplers or of Optoisolators are used in,

  1. Lamp Ballasts
  2. Light Dimmers
  3. Valve or Motor Controllers
  4. Microcontrollers for interfacing with High Voltage Circuits.
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