Solid state electronic thermostat

TLDR: In this paper, an apparatus for controlling the temperature of a device such as an oven in which a resistance having a positive temperature coefficient of resistance is incorporated in one leg of a bridge and is used to detect the temperature to be controlled.

Abstract: An apparatus for controlling the temperature of a device such as an oven in which a resistance having a positive temperature coefficient of resistance is incorporated in one leg of a bridge and is used to detect the temperature to be controlled. The bridge is energized by an alternating voltage and any unbalance appears across output terminals which are connected respectively to the bases of a first and a second transistor also energized with the same alternating voltage. The emitters of the transistors are connected together and through a common resistance so that the initiation of current flow through one transistor will inhibit current flow through the other. Flow of current through the second transistor triggers a silicon control rectifier which in turn controls a valve which supplies fuel to a burner in the oven. The voltage supplied to the bridge leads the voltage on the control rectifier so that it is always triggered at the beginning of its conducting half cycle. A third transistor is interposed between the second transistor and the silicon control rectifier. The changes in the response of the third transistor to its ambient temperature is compensated for in one of a number of ways. One is to have an ambient temperature sensitive resistance in the leg of the bridge diagonally opposite the first leg in which the first temperature sensitive resistance is placed. A second arrangement is to insert an ambient temperature sensitive diode in a leg adjacent to the first leg. A third arrangement is to insert an ambient temperature sensitive diode across the base and emitter of the third transistor. A delay circuit is provided for charging a condensor to delay the operation of the second transistor, the charge of said condensor being permitted to build up upon the initiation of current through the first transistor.