PURPOSE: To provide a simple and low-cost gate trigger circuit by employing a trigger source comprising a voltage transformer which has an intermediate tap connected with a rectifier including a diode and a capacitor.

CONSTITUTION: A commercial frequency power source 125 is connected to two terminals 114 and 112 on the primary side of a voltage transformer 116. It is also connected to a load 119 such as a blower through a triac 111. On this occasion, an intermediate tap 115 is provided to the winding between the two terminals on the primary side of the transformer 116, to which a rectifying diode and a smoothing capacitor 113 are connected. From the output of this capacitor 113, the intermediate tap 115 is connected to a gate terminal 121 of a triac 111 through a current limiting resistance 123 and a photocoupler 117 on the transistor side. The secondary side of the transformer 116 is connected to a control circuit (control means) 118 to control the output to the abovementioned triac 111 through the photocoupler 117, with the output of which gate trigger current is supplied from the abovementioned intermediate tap 115.