Overtemperature control system

Claim 1

In a control system to limit the maximuM temperature of an electrically heated bedcover or the like of the type including an electric heater wire and a control wire contained in a single cable distributed through the bedcover with the wires separated by a layer of sensor material having a negative temperature coefficient of electrical impedance such that the layer of sensor material is essentially an insulator at normal operating temperatures and a conductor of significant current at elevated temperatures, the improvement comprising: a. first and second contacts, b. first and second electrical actuators operatively associated with said first and second contacts, respectively, to selectively close and open said first and second contacts, c. a series heating circuit including the heater wire and said first and second contacts, d. a series control circuit including said first and second actuators and the control wire, e. a tie circuit interconnecting said heating and control circuits in parallel with a selected one of said first and said actuators, said tie circuit including the layer of sensor material, f. phase shifting means operative on at least one of said heating and control circuits to provide at all points along the heater and control cable a potential difference between the heater and control wires when both said heating and control circuits are energized from the same a-c power source, whereby substantial current will be shunted away from the selected actuator through said tie circuit when a portion of the layer of sensor material is subjected to an elevated temperature, g. said first and second actuators being operative to close said first and second contacts when said control circuit is energized and there is no appreciable conduction through said tie circuit and the layer of sensor material at normal operating temperatures, to open said first and second contacts when said heating and control circuits are energized and there is appreciable conduction through said tie circuit and the layer of sensor material at elevated temperatures, and to open said first and second contacts when said control circuit is not energized for an appreciable length of time.