The compound switch is to connect the high voltage and high current thyristor in parallel with the contact of the magnetic holding relay. Through the microcomputer control, it can automatically find the best input point and the best cutoff point to realize the voltage zero-crossing input and current zero-crossing cutoff. The turn-on and turn-off moment is completed by the thyristor, and the operation current is completed by the magnetic holding relay through the pulse voltage trigger to make the contact close.