| abstract |
The control may be applied to a domestic hot water system or particularly to a water boiling vessel. An NTC thermistor is coupled in close heat transfer relationship with the heating element and a PTC thermistor is exposed to steam produced when the water boils. A first detector circuit IC1a responds to the rate of change of resistance of the NTC thermistor so as to turn off a triac in response to a rapid resistance change representative of the heating element being powered when there is no water or insufficient water in the vessel, and also is responsive to a predetermined low resistance condition indicative of excessive element temperature. A second detector circuit IC1b turns off the triac, possibly after a delay, in response to a predetermined PTC thermistor resistance representative of the PTC thermistor being heated by steam. In an alternative arrangement, (Fig 1), the NTC and PTC thermistor are series connected to form a potential divider, the junction point of the thermistor being connected to a single detector circuit responsive to rate of change of resistance of either thermistor. The divider may include a current sensor which operates a boil-dry indicator lamp. The triac may be cooled by close thermal contact with water in a pocket in the vessel, (Fig 4A), or the triac may have a chip secured to a large-area copper base plate mounted on the vessel body via an electrically insulating layer so that the triac is cooled by the water that is being heated, (Fig 4c). <IMAGE> |