The function and structure of an optocoupler is shown here. Generally the optocoupler is an optical isolator. IR LEDs are built into the input side of phototransistor optocouplers. A phototransistor is placed on the output side. This optocoupler has two separated circuits which are galvanically isolated. The communication between these two circuits takes place with the invisible infrared LED. The infrared LED sends the light to the phototransistor, and the phototransistor receives the light and transfers the signal into a current. The main task of a phototransistor optocoupler is the separation of low-power control circuits from high-power circuits and the protection against overvoltage.