The photoelectric coupler consists of two parts: the semiconductor emitter and the optical receiver assembled in the same closed shell. The emitter mostly can be photodiode. The optical receiver can be photosensitive transistor, or photosensitive field effect tube, photosensitive thyristor and photosensitive integrated circuit etc. The emitter and the optical receiver are separated from each other and by transparent insulation material. The tube foot from emitter is the input end, and the tube foot from optical receiver is the output end. When add electrical signal at input end, photodiode emits light. At the same time, optical receiver produces light current due to photosensitive effect and the current is output from output end, so as to realize electrical signals one-way transmission by the "light" as media. But the input end and an output end of the device is completely and electrically insulated on both ends.
The main features of the photoelectric coupler are: insulation between input end and output end, the insulation resistance generally more than 1010Ω, withstand voltage generally more than 1.5kV, some even above 10kV. Because of the "light" one-way transmission, the signal is transmitted from the emitter to the optical receiver without any feedback, and the output signal will not affect the input end. In addition, the photoelectric coupler can effectively suppress interference and eliminate noise, and its response speed is fast (time constant is usually in microseconds or even nanosecond level), non-contact, long service life, small size, impact resistance, easy to work and logic circuit, which makes it widely used to many fields. For example, it can be as the interface device of signal isolation in computer digital communication and instant control, and the reliability of its work can be greatly increased. In the single-chip switch power supply, the linear photocoupler can be used to form the optical coupler
feedback circuit, which can adjust the control terminal current to change the duty ratio, finally realize precision and stabilized voltage.