PURPOSE: To protect a transformer from burning by controlling a FET connected with the transformer based on a DC voltage produced by rectifying an external overvoltage through a full-wave rectifier, thereby cutting off current supply to the transformer.

CONSTITUTION: Initial current Io flows from a DC voltage source 9 through a resistor 10 to a Zener diode 8, thus maintaining Zener voltage. Consequently, N-channel FETs 6, 7 are turned ON to connect the primary windings 12A, 12B of a transformer 12 and to produce pulse voltage in the secondary winding 12C, thus driving a pulse circuit 11. When overvoltage is applied externally onto pulse transmission/reception terminals 2, 4, and the output voltage of a full-wave rectifier 1 exceeds Zener voltage, current flows in reverse direction from the initial current Io, thus turning the FETs 6, 7 OFF. Consequently, the transformer 12 carries no current, thus protecting the transformer from burning.