PROTECTING DEVICE OF SELF-EXTINGUISHING SWITCHING ELEMENT

PURPOSE:To reduce the size and to improve the reliability of a protecting device of a self-extinguishing switching element by varying a current controlled by the element in response to the decrease in the voltage of a power source of a gate current generator of the element. CONSTITUTION:The output of an inverter 14 which has a GTO thyristor 6 and a power source 13 is connected with a load 7, and the thyristor 6 is driven from a power source 10 for a gate current generator through gate current generators 11, 12. Current and voltage supplied to the load 7 are detected at 9 by a current detector 8, compared by a comparator 2 with a current command 1, and input to a pulse width modulation controller 3. When an overcurrent is detected by a detector 5, the thyristor 6 is turned OFF. In this case, a voltage detector 15 for detecting the voltage of the power source 10 for the gate current generator is provided, the output is connected with the detector 5 and the comparator 2, and when the voltage drops, the output current and the overcurrent detecting level of the inverter 14 drop.