Optocoupler isolation is the use of optocouplers for isolation, the structure of the optocoupler is equivalent to the light-emitting diode and photosensitive (triode) tube encapsulated together. As the photocoupler shell is sealed, it is not subject to the influence of external light; photocoupler isolation resistance is very large (about 1012 Ω), isolation capacitance is very small (about a few pF) so it can prevent the electromagnetic interference generated by the circuit sexual coupling. Linear mode of operation of the photocoupler is in the photocoupler input control voltage, the output will be proportional to produce a voltage for further control of the next level of the circuit. Linear photocoupler consists of light-emitting diodes and photosensitive transistors, when the light-emitting diode is turned on and emits light, photosensitive tertiary tube conduction, photocoupler is a current-driven type, the need for sufficiently large currents in order to make the light-emitting diode conduction, if the input signal is too small, the light-emitting diode will not be conduction, its output signal will be distorted. In the switching power supply, especially the digital switching power supply, the use of linear optocoupler can constitute the optocoupler feedback circuit, by adjusting the control terminal current to change the duty cycle, to achieve the purpose of precision voltage regulation.
The light-emitting diode converts the input electrical signal into a light signal and passes it to the photosensitive tube to be converted into an electrical signal output, because there is no direct electrical connection, so that both the coupling and transmission of signals, but also isolate the role of interference.