HOW AN OPTOCOUPLER OR OPTOISOLATOR WORKS?

An optocoupler (or an optoelectronic coupler) is basically an interface between two circuits which operate at different voltage levels. The key advantage of an optocoupler is the electrical isolation between the input and output circuits. With an optocoupler, the only contact between the input and output is a beam of light. Because of this it is possible to have an insulation resistance between the two circuits in high voltage applications where the potentials of two circuits may differ by several thousands of volts.

The most common industrial use of the optocouplers (optically coupled isolators) is as a signal converter between high voltage pilot devices and low voltage solid state logic circuits. Optical isolators can be employed in any situation where a signal must be passed between two circuits which are isolated from each other. Complete electrical isolation between two circuits is often necessary to prevent noise generated in one circuit from being passed to the other circuit. This is especially necessary for the coupling between high voltage information-gathering circuits and low voltage digital logic circuits. The information circuits are almost badly exposed to noise sources and the logic circuits cannot tolerate noise signals.

The ideal isolation scheme should only allow signal flow in one direction, should respond to dc levels, and should offer an extremely large resistance between the input and output circuits. These features are available in a class of optoelectronic devices called optocouplers or optoisolators.

The optical coupling method eliminates the need for a relay controlled contact or an isolating transformer, which are traditional methods of providing electrical isolation between circuits. The optical coupling method is superior in many applications, because it gets rid of some of the less desirable features of relays and transformers.

ADVANTAGES OF OPTOCOUPLER OR OPTOISOLATOR OVER RELAYS AND TRANSFORMERS

·         They are fairly expensive.

·         They are bulkier and heavier than optical devices.

·         They create magnetic fields and switching transients which may be a source of troublesome electrical noise.

·         Relay contacts may develop sparks, which are very undesirable in certain industrial situations.

WORKING PRINCIPLE OF OPTOCOUPLERS

The optocoupler works well on either AC or DC high voltage signals. For this reason, signal converters employing optical coupling  are sometimes referred to the universal signal converters. The optocoupler is a device that contains an infrared LED and a Photo detector combined in one package as shown in figure below:-

It has a LED on the input side and a photodiode on the output side. The left source voltage and the series resistor setup a current through the led. Then the light from the led impinges on the photodiode, and this setup a reverse current in the output circuit this reverse current develops a voltage across the output resistor R. The output voltage then equals the output supply voltage V2 minus the voltage drop across the load resistor R. when the input voltage is varied, the amount of light fluctuates. This means that the output voltage varies in step with the input voltage. The device can couple an input signal to the output circuit.