OPTOCOUPLERS

An optocoupler (or an optoelectronic coupler ) is basically an interface between two circuits which operate at (usually) 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 the output is a beam of light.Because of this,it  is possible to have an insulation resistance between the two circuits in the thousands of megohms. Isolation like this is useful in high voltage application where the potentials of two circuits may differ by several thousand volts.

The most common industrial use of the optocouplers (or optically coupled isolators) is as a signal converter between high voltage pilot devices (limit switches etc.)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(i.e.,the two circuits have no conductors in common)is often necessary to prevent noise generated in one circuit from being passed to the other circuits.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.

In many application SCR and triac power circuits are under the control of sensitive electronic systems.For exmple, it is not unusual to have a microprocessor system programmed to turn motors,lights and heaters on or off.To reduce the possibility  of power-line noise being induced into the control electronics,and to protect it in the events of an SCR or triac failure,it is highly desirable to provide isolation.

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.