Optocoupler or Opto-isolator and its function in circuit

Optocoupler or Optoisolator is a safety component that transfers electrical signals between two isolated circuits by using light signal. A common type of opto-isolator consists of an LED and a phototransistor in the same package. Optocoupler are mainly used in delicate system like between sensor and PLC.

Why optocoupler is used

Optocoupler functions as a galvanic isolation component. That is it maintains the connection between two devices or component without any direct conduction. Components or devices like PLC, SCDA, Sensor etc. Say a sensor is to connected to a PLC, if it is directly connected – any fault in the circuit would damage the PLC.

An opto-isolator connects input and output sides with a beam of light modulated by input current. It transforms useful input signal into light, sends it across the dielectric channel, captures light on the output side and transforms it back into electric signal.

Opto-isolators can pass DC or slow-moving signals and do not require matching impedance between input and output sides. The main function of an Opto-isolator is to block high voltages and voltage transients, so that a surge in one part of the system will not disrupt or destroy the other parts.

Why not use a circuit breaker instead of Optocoupler?

We can use a circuit breaker instead of optocoupler, the circuit breaker will trip the circuit if any fault persists. But this will break the continuity of an important task, say an emergency protection system that need continuous watch. Other than the circuit breaker, optocoupler will simply block the transient due to fault and let pass the signal under normal condition.

Optocoupler principle – function image