Temperature responsive control circuit for electric window de-fogger/deicer heater

摘要

A control system for regulating the operation of an electric resistance heating grid for de-icing or de-fogging a window of an automobile in accordance with the temperature of the window includes a manually operable switching relay closed by the driver to start the flow of current through the heating grid upon the accumulation of an unacceptable amount of ice or fog on the window. A holding circuit maintains the switching relay closed until such time as an electric resistance-type temperature sensor in heat exchange relationship to the window surface on which the ice or fog accumulates operates through a transistorized circuit to break the holding circuit when the window reaches a preselected temperature high enough to dissipate any fog or ice accumulated thereon. As a safety precaution and to prevent a too heavy a load on the automobile electrical system the control circuit includes an arrangement for automatically opening the holding circuit in the event that the window does not come up to the preselected temperature within a predetermined maximum time period.