Abstract: The invention relates to control apparatus for heat actuated devices of the type having a heating resistor and a spring biased motor member movable in response to heat generated by the resistor. A source of rectified AC voltage for the heating resistor is regulated by a controlled rectifier in series with the resistor. There is a comparator circuit for comparing a desired setting with the actual position of the motor member and generating a continuously variable control signal of a value corresponding to the departure value. An ignition circuit for pulsing the controlled rectifier has inputs including the control signal, a half wave AC voltage and a time dependent decreasing signal. The ignition circuit has an input threshold below which pulses are generated and above which pulses are not generated.