pulse signal the potential at the point t is shown by a time chart c in fig3 under such a performance of the circuit if the temperature is lower than the predetermined value the potential at the point r is higher than that at the point q when the current is supplied to the bridge 5 due to the transistor 11 conducting and the comparator 4 then outputs a negative voltage signal which is used to trigger the thyristor 2 on the other hand if the temperature increases up to the predetermined value the comparator output turns to zero even if the temperature increases over the predetermined value so as to invert the above potential difference between the points r and q the comparator output still remains constant at zero because it is supplied only with a negative voltage source therefore only while the temperature is under the predetermined control value of temperature will the comparator 4 continue to output the trigger signal to the thyristor 2 each time the a c voltage crosses the zero level the heater 1 is thus kept energized until the temperature rises up to the predetermined value the a c current through the heater 1 and the time variation of the potential at the output point of the comparator 4 are shown respectively in the time charts e and d in fig3 after the temperature has increased to the predetermined value the potential at the point p remains zero without the generation of the trigger pulses whereas the heater current is.