Baxter Travenol Laboratories, Inc.

Clarence Wang

Michael B Uffer

The heater shutdown circuit is connected between an electric current source and a heating element in an electric heater and is operable to shut off the flow of current to the heating element when the heating element reaches a predetermined temperature and to maintain the flow of current shut off after the heating element cools to a temperature below the predetermined temperature. The heater shutdown circuit will maintain the electric current shut off until the heater shutdown circuit is disconnected from the current source for a predetermined period of time after which the heater shutdown circuit can be re-energized. The heater shutdown circuit includes a control circuit connected to the current source, a switching circuit coupled between the control circuit and the heating element, and a current sensing circuit coupled between the heating element and the junction between the control circuit and the switching circuit. It is operative in response to a decrease in current caused by an increase in the resistance of the heating element due to the overheating thereof to turn off the switching circuit. The control circuit is operative to turn on the switching circuit upon the initial energization of the shutdown circuit. Thereafter the control circuit reaches a steady-state condition where it is no longer operative to maintain the switching circuit turned on. However, subsequent to the turning on of the switching circuit by the control circuit, the current sensing circuit latches the switching circuit in a turned on condition until there is a decrease in the current through the heating element below a predetermined value.

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Silver Spring, MD, United States of America

Clarence Wang