An Optocoupler or opto-isolator as it is also called, is a single electronic device that consists of a light emitting diode combined with either a photo-diode, photo-transistor or photo-triac to provide an optical signal path between an input connection and an output connection while maintaining electrical isolation between two circuits.
An opto-isolator consists of a light proof plastic body that has a typical breakdown voltages between the input (photo-diode) and the output (photo-transistor) circuit of up to 5000 volts. This electrical isolation is especially useful where the signal from a low voltage circuit such as a battery powered circuit, computer or microcontroller, is required to operate or control another external circuit operating at a potentially dangerous mains voltage.
The two components used in an opto-isolator, an optical transmitter such as an infra-red emitting Gallium Arsenide LED and an optical receiver such as a photo-transistor are closely optically coupled and use light to send signals and/or information between its input and output. This allows information to be transferred between circuits without an electrical connection or common ground potential.
Opto-isolators are digital or switching devices, so they transfer either “ON-OFF” control signals or digital data. Analogue signals can be transferred by means of frequency or pulse-width modulation.
This circuit shows a 4N26 driving an SCR, which, in turn, is used to control an inductive load. The SCR is a sensitive-gate device ( 1 mA of gate current) and the 4N26 has a minimum-current transfer ratio of 0.2, so the 4N26 input current (IF) must be 5 mA. The 1-kΩ resistor prevents the SCR from triggering with small input changes, and the 1N4005 prevents the SCR from triggering with the self-induced voltage when the SCR turns off.