GB1426399A - Electric apparatus for heating liquids - Google Patents

Abstract

1426399 Automatic control of temperature BELLING & CO Ltd 29 June 1973 [30 June 1972] 31221/73 Heading G3R [Also in Division H2] Apparatus for heating a liquid, e.g. a kettle, comprises a transistor for controlling the supply of current to an electric heating element and two negative temperature coefficient thermistors are sensitive to vapour given off by the boiling liquid and effective to cause the transistor to switch off the current to the heating element when the liquid boils and the other sensitive to the temperature of the heating element and operative to cause the transistor to switch off the current to the heating element if this element overheats, an adjustable resistor being provided to modify the operation of the other thermistor whereby it is made effective to switch off current to the heating element when the liquid reaches a predetermined temperature. The heating element 1 for an electric kettle is energized from an A.C. supply by the manual resetting of a contact.3 of a relay 7. Closure of the contact 3 also supplies a full wave rectifier 4 the output of which energizes the relay 7. In parallel with the relay 7 is a transistor 8 which is normally non-conductive. The control circuit also includes two NTC thermistors 9, 10, the thermistor 9 being located in the kettle in the air space above the water and the thermistor 10 is in thermal contact with the heating element 1. The thermistors 9, 10 are both connected at one end to the base of the transistor 8 and at the other ends to potential dividers 11, 12 respectively. When the water boils the thermistor 9 has a resistance value such that the transistor 8 becomes conductive to short-circuit the relay 7 so opening the contact 3 to interrupt the current flow in the heating element 1. Overheating of the element 1 causes the thermistor 10 to assume a resistance value such that the transistor 8 is made effective to stop the flow of current in the element 1. A resistance 13, which is variable either infinitely or in discrete steps is associated with the potential divider 12, by means of which the thermistor 10 may be made effective to switch off the current supply to the element 1 at a temperature below the boiling point of water. In the embodiment of Fig. 1 the contact 3 is closed manually but in the embodiment of Fig. 2 where the rectifier 4 is connected directly to the A.C. source 2 the system operates automatically such that the contact 3 automatically opens and closes so as to maintain the water at a desired temperature. In an embodiment Fig. 3 (not shown) a second transistor, controlled by the thermistors, is provided to control the transistor 8 and visual and audible indicating devices are incorporated.