Appliances (05-Oct-2009)

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Electrical Appliances and Components

Field of the Invention

The present invention relates to electrical appliances, particularly domestic appliances such as liquid heaters and/or stirrers, and components therefor, particularly, although not exclusively, to heating elements, to a thermal control for controlling the heating element, to a control method for the control and to a computer program product for implementing the control method.

Background to the Invention

Thick film heating elements generally comprise one or more heating tracks that are printed as an ink or paste onto an insulating substrate and fired to form tracks of high electrical resistivity. Connecting tracks may be printed in a separate layer, and fired to form connecting tracks of low resistivity. The insulating substrate may be of an electrically insulating material, such as ceramic, or may be metallic with an insulating surface layer. Such thick film elements are typically used in liquid heating vessels, flow-through heaters, electric irons and other domestic appliances. In liquid heating vessels, the substrate is typically a substantially flat steel plate that forms the bottom of a liquid reservoir, with the tracks deposited on the underside, to form an underfloor heating element.

Heating elements usually require some form of element protection, arranged to disconnect the heating current when the element begins to overheat. In thick film elements, protection may be provided by a mechanical thermostat such as a bimetal in thermal contact with the element. However, in many applications it is preferable to integrate the element protection within the thick film element. An example of this approach is the 'E-fast' (RTM) element protection disclosed for example in WO 2006/083162 A1, in which the thick film element in deposited on a dielectric comprising first and second dielectric layers with an electrically conductive layer in between. When the element overheats, a leakage current is detected between the thick film element and the electrically conductive layer, in response to which the heating current is disconnected. The 'E-fast' element protection has proved successful, but for some applications there is a need for a lower-cost solution.

Patent publication EP-A-286 215 discloses a thick film heater for a hob, including a thick film temperature sensor track of high temperature coefficient. The heating track and the sensor track may be manufactured in the same process. To detect local hot spots, the sensor track may be arranged closely to follow the path of the associated heating track so as to cover a

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large area of the substrate.

Patent publication EP-A-485 211 discloses a thick film heater comprising a conductive heating track formed of pure nickel and a resistive track formed of standard thick film resistor mat...