Familiar with the electronic element- Optocoupler

In a power supply circuit, Optocoupler always comes with Tl431, their combined function is to control the stability of the primary and secondary feedback loop, ensure the stability of output voltage. Commonly used 817C, 817B, their operating frequency from dozens K to 300K.



The dotted side of the Optocoupler is input, the pin close to the dotted is 1-pin, 1, 2, 3, 4 anticlockwise. Another side without a dot is output.

Characteristics and Principle

  • Transmission of the electric signal by light, generally, the illuminator(Infrared LED) and the light receiver(Photosensitive semiconductor triode) are packaged together.
  • When the input terminal is electrified, the infrared Led shine, and the light receiver receives the light and generates photocurrent between b and c, the photocurrent amplification between c and e, then output.
  • The optical couple has many advantages, small size, long service life, contactless, strong anti-interference ability, one way signal transmission.

The main functions of the optical couple in switching power supply is isolated primary and secondary, provide feedback signal.

  • Optical couple straddles the primary and secondary side, the dotted side in the secondary, another side in the primary.
  • When the output voltage rise, TL431 R-pin voltage rise, and the K-pin loss of voltage, optical couple input current increase, so the optical couple output current increase, FB-pin loss of voltage, inside the IC, PWM reduce, the output voltage reduction.