Abstract: The apparatus may be used to control heater elements for an over chamber. The heater elements are operated in accordance with the output of a comparator having on one input a voltage representative of the temperature in a chamber. The voltage on the other input is established in accordance with the desired or ultimate temperature in the chamber. As long as the volage on the second-mentioned input is greater than the voltage on the other input, the heater elements will be energized to heat the chamber. The voltage on the second-mentioned input is modulated with a triangular wave so that the heater elements are operated intermittently as the desired temperature of the chamber is approached. The amplitude of the triangular wave varies with the desired temperature so that at higher and higher oven temperatures, the heater elements start to become intermittent at higher and higher temperatures.