PURPOSE: To prevent the breakdown of an element due to the minimum on-time of a GTO, by providing a snapper circuit itself with a minimum on time detecting function.

CONSTITUTION: A transistor TR19 is added to the gate of a GTO15 and while the discharging current of a snapper capacitor 16 has a voltage drop VR across a resistance 17, a base current is flowed to turn on the TR19. Further a GTO arc-extinguishing current generated by the conduction of a TR14 is allowed to by-pass the GTO15 through the TR19, preventing the GTO15 from turning off. Thus, the breakdown of an element due to the minimum on time of the GTO is prevented.