AU2011236088A1 - Cordless connection between heating vessel and base - Google Patents

Abstract

Abstract Described herein is a heating assembly that comprises a heating vessel (10) and a base (13) for supplying power to the heating vessel (10). The heating vessel (10) comprises a heating chamber (12), a temperature sensor (17), a first cordless power 5 coupling having an electrical connection to the temperature sensor (17) and a heating element (20) for heating contents of the heating chamber (12). The heating element (20) has an electrical connection to the first cordless power coupling. The temperature sensor (17) comprises a positive temperature coefficient (PTC) element and is located so as to be responsive to a temperature in the heating chamber (12). The base (13) 0 comprises a second cordless power coupling that cooperates with the first cordless power coupling such that, in use, electrical power is supplied to the temperature sensor (17) a nd the heating element (20) at substantially the same voltage.

Description

P/001011 Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Cordless connection between heating vessel and base The following statement is a full description of this invention, including the best method of performing it known to us: 1 Cordless connection between heating vessel and base Field of the invention The invention relates to heating vessels and in particular to arrangements in which there is a cordless connection between a heating vessel and a separate base for the vessel. 5 Background of the invention It is often convenient to have a portable heating vessel that can be lifted off a power base. For example, cordless kettles have a main body for holding water that may be detached from a base, making it easy to pour water from the kettle without the potential obstruction of a power cord. In these arrangements there is a need to transfer power 0 from the base to the heating vessel in order to heat the contents. Heating vessels like kettles generally include basic controls such as a boil control and a dry-boil cut-out if the vessel overheats. The dry boil control may use an electromechanical actuator that triggers at a set temperature. These electromechanical actuators are cheap and reliable, but offer limited functionality. 5 In recent years electronic controls have been used to provide more advanced functionality in heating vessels. Where a sufficiently accurate temperature measurement is available, an electronic controller such as a microcontroller or microprocessor may, for example, heat the contents to a user-specified temperature or maintain the temperature at a specified value. An example of a temperature sensing arrangement for 20 electronic kettles is described in WO 2007/131271, "Improved temperature sensor for an electric heating vessel" filed on 11 May 2007, the contents of which are incorporated herein by cross reference. The temperature sensor is generally located close to the contents of the vessel. In arrangements where the electronic controller is positioned in the base, the temperature 25 measurement must be conveyed from the vessel to the base. There is an ongoing need for convenient and reliable methods of transferring this information.

2 Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a 5 person skilled in the art. Summary of the invention According to a first aspect of the invention there is provided a heating assembly comprising: i) a heating vessel comprising: 0 a heating chamber; a temperature sensor having an electrical resistance that varies as a function of temperature, the temperature sensor located so as to be responsive to a temperature in the heating chamber; and a first cordless power coupling having an electrical connection to the 15 temperature sensor; and ii) a base for supplying power to the heating vessel, comprising a second cordless power coupling that cooperates with the first cordless power coupling such that, in use, electrical power is supplied to the temperature sensor. The temperature sensor may comprise a positive temperature coefficient (PTC) element 20 having a first region of operation below a reference temperature in which an electrical resistance of the PTC element decreases with increasing temperature and a second region of operation above the reference temperature in which the electrical resistance of the PTC element increases with increasing temperature.

3 The heating vessel may operate in a range of temperatures below the reference temperature. The first region of operation of the PTC element may comprise a range of temperatures between 2 and 105 *C. 5 The heating vessel may comprise a heating element for heating contents of the heating chamber, the heating element having an electrical connection to the first cordless power coupling such that in use power is supplied to the heating element from the base. The base may comprise a switching circuit that in use switches supplied power to either the heating element or the temperature sensor. 10 The switching circuit may supply power to the heating element or the temperature sensor at substantially the same voltage. The base may supply electrical power intermittently to the temperature sensor via the first and second cordless power couplings. The base may supply power to the temperature sensor at a voltage greater than 42 V 15 AC. The base may comprise an analogue measuring circuit responsive to changes in the electrical resistance of the temperature sensor. The base may comprise an electronic controller that receives an input signal dependent on an output of the analogue measuring circuit. 20 The electronic controller may control operation of the heating element dependent on the input signal. The base may comprise one or more user inputs and a display to indicate information concerning operation of the heating assembly.

4 The heating assembly may comprise a cordless kettle and power base. The first and second cordless power couplings may comprise a 3600 coupling wherein the electrical connection between the heating vessel and the base is independent of a relative angular orientation of the heating vessel and the base. 5 In another aspect of the invention there is provided a heating assembly comprising: i) a heating vessel comprising: a heating chamber; a temperature sensor having an electrical resistance that varies as a function of temperature, the temperature sensor comprising a positive temperature coefficient (PTC) element, the temperature sensor located so as to be responsive to a temperature in the heating chamber; a first cordless 10 power coupling having an electrical connection to the temperature sensor; and a heating element for heating contents of the heating chamber, the heating element having an electrical connection to the first cordless power coupling; ii) a base for supplying power to the heating vessel, comprising a second cordless power coupling that cooperates with the first cordless power coupling such that, in use, electrical power 15 is supplied to the temperature sensor and the heating element at substantially the same voltage. As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps. 20 Brief description of the drawings Embodiments of the invention are described below with reference to the drawings, in which: Figure 1A is a schematic diagram of an electronic kettle with a 5-pole connector to a kettle base, and where the water temperature is measured using a positive temperature 25 coefficient (PTC) element; Figure 1B is a schematic diagram of an alternative arrangement of a kettle and base with a 4-pole connector using neutral sensing; 5 Figure 1C is a schematic diagram of a further alternative arrangement of a kettle and base with a 4-pole connector using active sensing; Figure 2 shows an example of a display and user input that may be provided on the kettle base in the arrangements of Figures 1A-C; 5 Figure 3 is a graph showing the resistance versus temperature characteristic of the PTC element; Figure 4 is a perspective view of a part of a heating assembly with a heating element located on a heat distribution plate and a PTC element thermally isolated from the heating element and heat distribution plate, for use in the kettles of Figure 1A-C; 10 Figure 5 is a cut-away side view of the part of a heating assembly of Figure 4; Figure 6A is an end view of the part of a heating assembly of Figure 4; Figures 6B - 6E show alternative configurations of the heating assembly that provide thermal isolation between the temperature sensor and the heat distribution plate; Figure 7 is a plot of temperature versus time during the boil mode of the kettle; 15 Figure 8 is a flow chart illustrating operation of the kettle in the boil mode; Figure 9 is a plot of temperature versus time during a "keep warm" mode of the kettle; Figure 10 is a flow chart illustrating operation of the kettle in the keep warm mode; Figure 11 is a plot of temperature versus time during a "boil and keep warm" mode of the kettle; 20 Figure 12 is a photograph of a kettle base and the bottom of a heating vessel with a 360 degree connector; and 6 Figure 13 is a photograph of a heat distribution plate and PTC element, showing a void formed in the heat distribution plate to accommodate the PTC element. Detailed description of the embodiments Figure 1A shows a schematic view of a cordless electronic kettle having a heating 5 vessel 10 that may be placed on or removed from a base 13. The vessel 10 has a heating chamber 12, which holds the water to be boiled. The water may be poured into the heating chamber 12 of the kettle through the pouring spout 14 or through the lid of the kettle. A handle 15 is provided for a user to lift the vessel 10 and pour water out of the pouring spout 14. 10 The contents of the heating chamber 12 are heated by a heating element 20 located on the bottom of the vessel 10. Figure 1A shows a sectioned view of an arcuate heating element 20. A temperature sensor using a positive temperature coefficient (PTC) element 17 provides a temperature measurement indicative of a temperature inside the heating chamber 12. Additional temperature sensors (not shown) may be provided to 15 detect overheating of the heating chamber 12. The vessel 10 and base 13 both have a 3600 cordless power coupling, or connector, such that the vessel 10 may be positioned at any angular orientation on the base. An example of such a connector is shown in Figure 12. The connector in the arrangement of Figure 1A has 5 poles including an earth connection 1, active connection 2, neutral 20 connection 3. Two links 4,5 are provided between the PTC element 17 and a control circuit provided on a printed circuit board (PCB) 19 located in the base 13. The connections 2 and 3 may power the heating element 20, for example at a supply voltage of 220-240V. The resistance of the PTC element varies as a function of temperature. This variation in 25 resistance is used to obtain a temperature measurement. A voltage is applied to the PTC element 17 and the resulting current monitored to provide an analogue indication of temperature. The analogue measurement may be converted to a digital signal for input to the control circuit. An advantage of the PTC element 17 is that it may be driven 7 at a relatively high voltage, for example 220-240V. The driving voltage for the PTC element is preferably greater than 42 V AC. The use of relatively high voltages may assist in establishing a reliable connection via connectors 4, 5 when the vessel 10 is placed on the base 13. In contrast, other 5 electronic temperature sensors such as negative temperature coefficient (NTC) thermistors operate at relatively low voltages, for example 5-12 V. At these relatively low voltages a dry connection between vessel and base may be unreliable. The PTC element 17 is self-heating and so power is switched into the PTC element intermittently during heating of the kettle to limit the self-heating effect. The temperature 10 is only measured while power is supplied to the PTC element. However, if the frequency of switching is sufficiently high, an accurate trend of temperatures may be obtained. For example, a kettle with a capacity of 1.7 litres and a heating element of 2200W may take around 5 minutes to boil. The PTC switching may occur at a frequency of 1 to 1000 Hertz. A circuit arrangement using one or more triacs in the base 13 switches power 15 between the heating element 20 and the PTC element 17, ie power is supplied to either the heating element or the PTC element 17, but not to both simultaneously. In one arrangement a sensing current is switched through the PTC element 17 for around 0.04 seconds every 5 seconds. In other arrangements individual switching circuits are provided in the base 13 for each 20 of the heating element 20 and PTC element 17. With a 5-pole connector, power may be supplied independently to the PTC element 17 and heating element 20. Figure 1B shows an alternative arrangement that is similar to Figure 1A except that the 3600 connector has 4 poles. As before, the active 2 and neutral 3 poles transfer power to the heating element. However, there is a single pole 6 between the PTC element and 25 the PCB 19 when the vessel is positioned on the base 13. A short circuit is provided in the vessel 10 between the PTC element 17 and the neutral pole 3, and thus connections 3 and 6 are used when measuring the temperature.

8 Figure 1C show an arrangement similar to that of Figure 1B except that a short is provided between the PTC element 17 and the active pole 2 of the 3600 connector on the vessel 10. Thus connections 2 and 6 are used when measuring the temperature. User interface 5 A user interface 30 may be provided on the base 13. An example is shown in Figure 2, including a display to indicate information concerning operation of the heating assembly such as a liquid crystal display (LCD) 34. In use the LCD 34 displays the real time temperature of the kettle water volume. The LCD 34 may show: " Water temperature in degrees Celcius; 0 0 Time to boil in minutes:seconds (00:00) - count down; * Pre-set water temperature (4 temperature settings); and " Time to expire pre-set water temperature from 20 minutes in minutes:seconds (20:00) - count down 5 The time-to-boil feature is described in more detail in WO 2008/052276 "Electric Heating Appliances with Data Display". The arrangement of Figure 2 also includes a user input, in this case five tactile buttons: . Button 31 to select the Keep Warm mode; . Button 32 to select the Boil/Hold mode; 20 . "<" button 33 for scrolling down through pre-set water temperatures; . ">" button 35 for scrolling up through pre-set water temperatures; and - "Start/cancel" button 36. 25 The buttons may be momentary push buttons or may be a variety of other button types. Other forms of user input, for example, dials may be used. A translucent ring (for example 37) may be provided around one or more of the buttons. These rings are illuminated by LEDs to provide a user with information regarding the kettle's operation. 30 The LEDs are optionally capable of emitting different coloured lights, for example to indicate temperature levels in the kettle. Other types of lights may be used, such as a conventional filament bulb. A "standby" LED may be illuminated when the external power is connected to the kettle base. The buttons are connected to, and provide input 9 to, the controller in the PCB 19 shown in Figure 1. The lights 37 are connected to, and are operated by, the controller. It will be appreciated that a series of buttons may be provided each associated with a particular brewing temperature suitable for different beverages, such as different types 5 of tea. PTC Element The PTC element 17 may be a small ceramic device with a fast heating response time that plateaus once a pre-defined reference temperature or resistance is reached. The shape of these ceramic devices can be designed to be square, rectangular, round, ring 10 or doughnut style, or any other shape. The construction may be solid or perforated. Above the reference temperature, the semiconducting and ferroelectrical properties of the ceramic produce a rise in resistance of several orders of magnitude, and hence provide self-limiting temperature properties. The PTC element 17 may be used as a temperature sensor where the electrical 15 resistance is primarily determined by the temperature of the surroundings of the PTC element. Figure 3 shows PTC resistance on the y-axis and temperature on the x-axis. If a voltage is applied across a PTC element, current will flow and begin to heat the PTC. Since most PTCs are in their NTC region 41 when first energized, heating causes the 20 resistance of the part to drop. The decreasing resistance, in turn, causes more current to flow which heats the element 17 still further. If the voltage is high enough, the element 17 will self-heat via shoulder region 42 until it passes into the PTC region 43 in which resistance rises rapidly with temperature. In Figure 3 a typical resistance/temperature ratio of Rprc as a function of TPTC is shown 25 where RN is the rated PTC resistance (the resistance value at TN), Rmin is the minimum resistance, TRmin is the temperature at Rmin (c becomes positive), RRef is the reference 10 resistance RRef= 2 Rmin and TRef is the reference temperature from where the resistance rises sharply. Unlike an NTC thermistor which has an ability to sense temperature accurately over a wide temperature range, the PTC element is used as a temperature measuring device 5 over a relatively short range of temperatures near the switch temperature. Because the resistance versus temperature characteristic of the PTC thermistor does not lend itself to an equation, most specifications are for the PTC thermistor to be a resistance value at some specific temperature plus or minus some tolerance. When the PTC is being used as a temperature sensor, the amount of current through the PTC must be small so 10 as not to self-heat the thermistor and cause errors. In the kettle application described herein, the PTC element 17 acts as an NTC (heat causes the resistance of the element to drop) and the output is used as temperature feedback for temperature control. However, this temperature feedback is only to be used on the following design conditions: 15 1. the PTC element 17 has a high enough resistance within the temperature sensing range to ensure the element does not self heat. 2. the PTC element 17 is used as a temperature measuring device over a relatively short range of temperatures near the switch temperature, i.e. between around 2.0* and around 105.0 *C. This range is, of course, suitable for boiling water or 20 for keeping water warm at a temperature that falls within this range such as 60*. Different ranges may also be specified for the PTC element depending on the particular application. For example, frypans typically operate at higher temperatures so that the range may be between 2.00 and around 200.0 0 C. 25 The amount of current through the PTC element 17 should be small so as not to cause the element to self-heat. In one arrangement a triac is used to switch the sensing current to flow through the PTC element for a period of 0.04 seconds once every 5 seconds. This has been found to provide a small enough current to ensure sufficiently low self-heating while still providing adequate tracking of changes in the water 30 temperature. An advantage in using a PTC element as a temperature sensor instead of an NTC is that the PTC element can sense using various voltage ranges, for example a 240V supply voltage. This avoids problems caused by impedance/resistive joints between the 11 power base 13 and the vessel 10 when using a thermistor supplied via ELV (Extra Low Voltage) for data transfer. This, in turn makes this application ideal over dry contacts such as those used in the 3600 connector between a kettle base 13 and vessel 10. Heating assembly 5 Figures 4 and 5 show a perspective view and a side view of an assembly 18 that may be provided at the bottom of the vessel 10. Figure 4 shows a bottom view. The base wall of the heating chamber 12 is defined by a contact plate 16. Water stored in the heating chamber 12 is in direct contact with one side of the contact plate 16. The contact plate 16 may be formed from stainless steel. Other materials which are suitable 10 for contacting water and are resistant to high temperatures and oxidation may be used. The contact plate 16 forms part of assembly 18. The assembly 18 is generally located underneath the heating chamber 12 on the opposite side of the contact plate to the heating chamber 12. The assembly 18 is powered from the base 13 via the 3600 connector. 15 The heat used to boil the water is generated by a heating element 20, which is curved and terminates in cold tails 22 carrying electrical connections. Preferably the heating element 20 is powered by electricity. The heating element 20 shown is a resistance element. Other types of heating elements may be used. For a kettle a 2400W sheathed arcuate heating element is often used. 20 The arcuate heating element 20 is bonded to a peripheral region at or near an outer edge of a heat distribution plate or billet 24. The bonding achieves a good thermal coupling between the heating element 20 and the heat distribution plate 24 so that heat generated by the heating element 20 is rapidly and efficiently transferred to the heat distribution plate 24. Many known bonding techniques are suitable, including induction 25 welding, flame or oven welding and impact welding. The heat distribution plate 24 is induction brazed to the contact plate 16 so there is a good thermal coupling between the heat distribution plate 24 and the contact plate 16. Many other known bonding techniques are suitable, including the bonding techniques 12 mentioned above. Alternatively the heat distribution plate 24 may be secured to the contact plate 16 using other known techniques, such as mechanical fasteners. The heat distribution plate or billet 24 may be formed from aluminium, which is a good thermal conductor, and is of sufficient thickness to evenly distribute heat from the 5 heating element 20 across the extent of the heat distribution plate 24. Alternative materials for the heat distribution plate 24 include other metals and metal alloys. The heat distribution plate 24 is generally thicker than the contact plate and formed from a material which is a better thermal conductor than the contact plate. The heat distribution plate 24 defines a void 26. In the arrangement of Figure 4 and 5 10 the void is cylindrical with an outer diameter surrounded by the heat distribution plate 24. The void in this arrangement provides access to the contact plate 16 through the heat distribution plate 24. The void 26 forms a thermally insulating zone. This is because heat which is transmitted from the heating element 20 to the heat distribution plate 24 is not as readily transmitted across the void 26. The region of the contact plate 15 16 located adjacent the void 26 does not conduct significant amounts of heat when compared to the aluminium heat distribution plate 24 because the contact plate 16 is thin and formed from stainless steel, which is not as good a thermal conductor. The PTC element 17 is positioned in the void 26 . The void 26 provides a thermally insulating zone around the PTC element 17. Heat from the heat distribution plate 24 is 20 not readily transmitted to the PTC element 17. As a result, the PTC element 17 is thermally insulated and is not undesirably influenced by the temperature of the heating element 20 and heat distribution plate 24. The thermally insulating zone and the PTC element 17 may be located between the cold tails 22 of the heating element 20. The cold tails do not generate significant 25 amounts of heat, so the electronic temperature sensor 17 is further insulated from the heat generated by the heating element 20. Instead of being empty, the void 26 may be filled, either partially or wholly, with a thermally insulating material, such as silicone or rubber.

13 The PTC element 17 is sandwiched between electrical contact plates 21a, b, enabling the sensing current to flow through the PTC element 17. An electrical insulator 23 may be provided between the electrical contact plate 21a and the contact plate 16 to isolate the heating chamber electrically from the sensing circuit. 5 The temperature sensor assembly that includes the PTC element 17 and electrical contact plates 21a, b and insulator 23 is positioned in close proximity to the contact plate 16. Optionally, the temperature sensor assembly may be touching the contact plate 16. This improves the thermal coupling between the electronic temperature sensor and the contact plate 16. The thermal coupling may be further improved using known 10 techniques, such as applying a heat transfer paste. The temperature sensor assembly may be bonded to the contact plate 16. The PTC element assembly may be attached to the kettle by various means including, for example, a mounting bracket, clips, clamps or screws. The bracket, clips or clamps may be made from materials including aluminium, steel or silicon. 5 It is an advantage that the temperature sensor assembly is in thermal contact with the contact plate. When water contained in the heating chamber 12 of the vessel 10 heats up, the contact plate 16 will heat to a similar temperature. Due to the void 26, the region of the contact plate 16 located within the void is insulated from the heat distribution plate 24 and will more accurately reflect the temperature of the water. Since the temperature 20 sensor assembly is in thermal communication with the contact plate 16, it senses the water temperature with greater accuracy and responsiveness. Figure 6A is a view of the assembly 18, showing the void 26 located between the cold tails 22. Figure 6B illustrates a heating assembly in which the heat distribution plate 24 does not 25 entirely surround the thermally insulating zone provided by inlet 108a. As before, a sheathed arcuate heating element 20 is bonded to a peripheral region of a heat distribution plate 24, which is generally circular. The heat distribution plate 24 acts to distribute heat generated by the heating element 20. The heat distribution plate is 14 bonded to contact plate 16, which forms the base of the kettle's heating chamber. The circumference of the heat distribution plate lies within the bounds of the contact plate 16, which is also generally circular in the illustrated arrangement. A U-shaped inlet 108a is formed at one side of the heat distribution plate 24. The 5 contact plate 16 is accessible through the inlet 108a, which provides a thermally insulating zone. A sensor assembly 106 is positioned in the thermally insulating zone to provide a temperature measurement indicative of the water temperature in the heating chamber. The heat-distribution plate may be cast in the shape having the inlet 108a. Alternatively, 10 the inlet 108a may be removed from a disc-shaped plate, for example by milling the desired inlet shape. In the illustrated arrangement the inlet 108a is positioned between the cold tails 22. This configuration further reduces the effect of the element 20 and heat distribution plate 24 on the temperature measured by the sensor assembly 106. 15 The inlet may have different shapes. A further example is illustrated in Figure 6C, with a smaller inlet 108b. A larger inlet may increase the insulating effect of the thermally insulating zone. However, there may be a design trade-off as the reduced area of the heat-distribution plate 24 may reduce its effectiveness in distributing the heat generated by the arcuate element 20. 20 Figure 6D shows a further alternative in which the area of the heat distribution plate 24 is reduced relative to the area of the contact plate 16. In this arrangement there is sufficient clearance for the sensor assembly 106 to be positioned on the contact plate 16 outside the outer bounds of the heat distribution plate 24. In this configuration the sensor assembly is more remote from the heating effect of the element 20 and heat 25 distribution plate 24 than in the arrangements of Figures 6A-6C. In the arrangement of Figure 6D the sensor assembly 106 is positioned in the vicinity of the cold tails 22. However, the sensor assembly 106 may also be located at other points on the contact plate 16 that lie outside the periphery of the heat distribution plate. An example is shown 15 in Figure 6E in which the sensor assembly 106 is located diametrically opposite the cold tails 22. Figure 13 is a photograph of a PTC element adjacent its mounting position in a void formed in a heat distribution plate. 5 Example of boil mode Operation in the boil mode is described below with reference to the plot of Figure 7 and the flow chart 80 of Figure 8. Referring to Figure 8, in step 82 the vessel 10 is placed on the base 13 and enters a standby mode (area 1 in Figure 7). Step 84 is a check to see whether the boil button 32 is activated. If so the controller (which may, for example, be 10 instructions running on a microprocessor on PCB 19) enters a boil mode, graphically displayed as "Area 2" in Figure 7. When in the boil mode, the controller turns on the heating element 20 for a pre-set period T1, which begins to heat the water in the kettle. The controller may additionally cause an illuminated ring around button 32 to produce, for example, red light, to indicate the controller is in the boil mode and the water is being 15 boiled. The PTC element is inactive during period T1. Step 88 is a check to determine whether period T1 has ended. If so, in step 90 the controller activates the PTC element for preset period T2 (for example 0.04 seconds duration) to sense the water temperature. During period T2 the heating element 20 is inactive. The switching sequence may be programmed into the controller. 20 In step 92 the controller checks whether the PTC element resistance has dropped to a value corresponding to boiling temperature. The resistance of the PTC element may be measured using a shunt. If the resistance has not dropped to the specified value (the NO option of step 92), then in step 94 the controller checks whether period T2 has ended. If so, control flow returns to step 86 to activate the heating element 20. If period 25 T2 has not yet ended, control flow returns to step 90, maintaining the PTC element active. If the PTC element detects a boiling limit (for example 980C), the controller enters a "boiled" mode (indicated by "Area 3" in Figure 7 and step 96 in method 80). In the boiled 16 mode, the controller turns off the heating element 20 and the red light in the illuminated ring. The controller may turn on, for example, a green light in the illuminated ring to indicate that the water is boiled. An audible signal may be emitted to alert the user that the kettle has boiled. 5 In the boiled mode, the temperature sensor continues to sense the temperature of the water. After the heating element 20 is turned off, the water slowly cools. Once the temperature of the water falls to a lower boiling limit, the controller ends the "boiled" mode and returns to "standby" mode (indicated by "Area 4" in Figure 7). At this stage, the controller turns off the green light in the illuminated ring to indicate the water is no 10 longer at or near boiling temperature. A suitable lower boiling limit is 92 0 C, though other limits are also suitable. If the user reactivates the boil button 32 during the boil mode, the boiling operation is cancelled and the kettle returns to standby mode. Keep Warm Mode 15 The keep warm mode is illustrated in the plot of Figure 9 and the flow chart 100 of Figure 10. In step 102 the vessel 10 is placed on the base 13 and enters a standby mode ("Area 1" in Figure 9). Step 104 is a check to see whether the keep warm button 31 is activated. If so the controller (which may, for example, be implemented as instructions running on a microprocessor on PCB 19) enters a keep warm mode, 20 graphically displayed as "Area 2" in Figure 9. In step 106 the controller turns on the heating element 20 for a pre-set period T1, which begins to heat the water in the kettle. The controller may additionally cause an illuminated ring around button 31 to produce, for example, red light, to indicate the controller is in the keep warm mode and the water is being heated. The PTC element is inactive during period T1. Step 108 is a check to 25 determine whether period T1 has ended. If so, in step 110 the controller activates the PTC element for preset period T2 (for example 0.04 seconds duration) to sense the water temperature. During period T2 the heating element 20 is inactive. The switching sequence may be programmed into the controller and effected using a triac.

17 In step 112 the controller checks whether the PTC element resistance has dropped to a value corresponding to a temperature selected by the user, for example using buttons 33 and 35. The resistance of the PTC element may be measured using a shunt. If the resistance has not dropped to the specified value (the NO option of step 112), then in 5 step 114 the controller checks whether period T2 has ended. If so, control flow returns to step 106 to activate the heating element 20. If period T2 has not yet ended, control flow returns to step 110, maintaining the PTC element active. Once the specified temperature is reached (the YES option of step 112), in step 116 the controller turns off the heating element 20 and the red light in the illuminated ring. The 10 controller may turn on, for example, a green light in the illuminated ring to indicate that the water has reached setpoint. An audible signal may be emitted to alert the user. After the specified temperature has been reached, in step 118 the controller checks whether or not a keep-warm time period has expired. This time period may be a set duration (for example, 20 minutes) that is pre-determined for the kettle. Alternatively, the 15 keep-warm time period may be selected by the user, for example via the user interface 30. If the keep-warm period has expired, the kettle returns to the standby mode (step 102). Otherwise (the NO option of step 118), in step 120 the temperature sensor continues to intermittently sense the temperature of the water. The PTC element is active for period T2 and inactive for period T1. 20 After the heating element 20 is turned off, the water cools. The controller monitors the output of the PTC element and, in step 122, checks whether the PTC resistance has increased to a specified level. If so, control flow returns to step 106 to reactivate the heating element. If the temperature is still within range (the NO option of step 122), then control flow returns to step 118. 25 In the example of Figure 9 the keep warm mode maintains the water in the kettle at an average temperature of around 85 degrees. During the keep-warm period, the temperature varies between an upper and lower warm limit, for example +/- 2 degrees or +/- 5 degrees from the average temperature.

18 Boil and Keep Warm Mode Another mode is illustrated in Figure 11. In standby mode (Area 1) the user selects the boil button and the water is boiled (Areas 2 and 3). If the user selects the keep warm button 32, then the temperature is maintained at a specified temperature. In the 5 example of Figure 11 the temperature is 850C and the keep warm operation maintains the temperature at 85*C +/- 2*C (Area 4). The user can select the boil button at any time in areas 1 or 4 to initiate water boiling. The cordless communication between a power unit and a heating unit having a temperature sensor may be used on other applications where the temperature range to 0 be sensed is in the NTC region 41 of the characteristic curve of the PTC device. Examples of other uses include: " Electric frypans and woks; * Rice Cookers; e Slow Cookers; 5 * Hot pots and thermopots; * Electric irons and steam stations; and " Electric blankets. It will be understood that the invention disclosed and defined in this specification 20 extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims (17)

1. A heating assembly comprising: i) a heating vessel comprising: a heating chamber; 5 a temperature sensor having an electrical resistance that varies as a function of temperature, the temperature sensor located so as to be responsive to a temperature in the heating chamber; and a first cordless power coupling having an electrical connection to the temperature sensor; and 10 ii) a base for supplying power to the heating vessel, comprising a second cordless power coupling that cooperates with the first cordless power coupling such that, in use, electrical power is supplied to the temperature sensor.

2. The heating assembly of claim 1 wherein the temperature sensor comprises: a positive temperature coefficient (PTC) element having: 15 a first region of operation below a reference temperature in which an electrical resistance of the PTC element decreases with increasing temperature, and a second region of operation above the reference temperature in which the electrical resistance of the PTC element increases with increasing temperature.

3. The heating assembly of claim 2 wherein the heating vessel operates in a range 20 of temperatures below the reference temperature. 20

4. The heating assembly of claim 2 or claim 3 wherein the first region of operation of the PTC element comprises a range of temperatures between 2 and 105 *C.

5. The heating assembly of any one of the preceding claims wherein the heating vessel comprises: 5 a heating element for heating contents of the heating chamber, the heating element having an electrical connection to the first cordless power coupling such that in use power is supplied to the heating element from the base.

6. The heating assembly of claim 5 wherein the base comprises a switching circuit that in use switches supply power to either the heating element or the temperature 0 sensor.

7. The heating assembly of claim 6 wherein the switching circuit supplies power to the heating element and the temperature sensor at substantially the same voltage.

8. The heating assembly of any one of the preceding claims wherein: the base supplies electrical power intermittently to the temperature sensor via the first 15 and second cordless power couplings.

9. The heating assembly of any one of the preceding claims wherein the base supplies power to the temperature sensor at a voltage greater than 42 V AC.

10. The heating assembly of any one of the preceding claims wherein the base comprises an analogue measuring circuit responsive to changes in the electrical 20 resistance of the temperature sensor.

11. The heating assembly of claim 10 wherein the base comprises an electronic controller that receives an input signal dependent on an output of the analogue measuring circuit. 21

12. The heating assembly of claim 11 wherein the electronic controller controls operation of the heating element dependent on the input signal.

13. The heating assembly of any one of the preceding claims wherein the base comprises: 5 one or more user inputs and a display to indicate information concerning operation of the heating assembly.

14. The heating assembly of any one of the preceding claims wherein the heating assembly comprises a cordless kettle and power base.

15. The heating assembly of any one of the preceding claims wherein the first and 10 second cordless power couplings comprise a 360* coupling wherein the electrical connection between the heating vessel and the base is independent of a relative angular orientation of the heating vessel and the base.

16. A heating assembly comprising: i) a heating vessel comprising: 15 a heating chamber; a temperature sensor having an electrical resistance that varies as a function of temperature, the temperature sensor comprising a positive temperature coefficient (PTC) element, the temperature sensor located so as to be responsive to a temperature in the heating chamber; 20 a first cordless power coupling having an electrical connection to the temperature sensor; and a heating element for heating contents of the heating chamber, the heating element having an electrical connection to the first cordless power coupling; 22 ii) a base for supplying power to the heating vessel, comprising a second cordless power coupling that cooperates with the first cordless power coupling such that, in use, electrical power is supplied to the temperature sensor and the heating element at substantially the same voltage. 5

17. A heating assembly substantially as herein described with reference to any one of the embodiments illustrated in the Figures.