PURPOSE: To prevent the electric disturbance of the load side circuit from affecting the thyristor circuit as well as to avoid the thyristor malfunction by giving the electric insulation or isolation via the photo coupler between the heat-sensitive thyristor circuit and the load side circuit.

CONSTITUTION: An optical connection is secured via photo triac 3 and light emitting diode 13 between load circuit 100 in which AC power source 1, load 2 and photo triac 3 are connected in series and heat-sensitive thyristor circuit 200 comprising AC power source 11, rectifier circuit 12, Zener diode 16, light emitting diode 13, heat-sensitive thyristor 15 connected in parallel to diode 13, etc. Thus, diode 13 emits light when thyristor 15 is turned off, and triac 3 supplies the power to load 2. And diode 13 emits no light when the detected temperature increases to turn thyristor on, and thus the power supply is cut off to load 2. In this case, circuit 200 can be kept at a low potential although circuit 100 features a high potential to ground, thus preventing the electric disturbance effect of circuit 100. Furthermore, the malfunction can be avoided for the thyristor because of a low potential.