Control unit for a liquid heating appliance

Abstract

A control unit 16 for a liquid heating appliance (e.g., kettle) includes a power supply circuit comprising a trip lever 36 for manually operating a switch. A mount plate 32 of the control unit is moun

Description

The present invention relates to a liquid heating appliance. Liquid heating appliances, such as kettles, are common in many households. Conventional kettles comprise an electrical power supply that is arranged to heat an element at the base of the kettle. The electrical power supply includes a switch, comprising a pair of separable electrical contacts, that can typically be operated by a user of the appliance using a trip lever. In a set position of the trip lever, the electrical contacts are closed and electrical power is supplied to the electrical heating element. In a reset position of the trip lever, the electrical contacts are open and the supply of electrical power to the heating element is interrupted. It is known to provide a kettle with dry-boil protection using a thermally sensitive control comprising one or more thermally sensitive actuators that are mounted in good thermal contact with its heated base. The actuator(s) operate to automatically interrupt the power supply upon detecting an overheat condition, e.g. because the appliance has boiled dry or has been turned on without any water inside. The thermally sensitive actuators in conventional controls are typically snap-action bimetallic elements, also known as “blades”, which normally have a slightly concave shape. The blade is arranged in the appliance to detect the temperature of the heated base. When the temperature of the blade reaches a predetermined operating temperature, the blade “snaps through” so as to adopt a convex, rather than a concave, shape. This mechanical action is used in conventional controls to open a pair of electrical contacts in the control to interrupt the power supply to the heating element. This helps to ensure that the electrical power supply to the heating element is interrupted before the heating element reaches dangerous temperatures. This operation is referred to as Dry Switch Off (DSO). It is important that, as part of this operation, the trip lever is moved from the set position to the reset position. Typically, the movement of the blade as it snaps through is also used to move the trip lever to the reset position. However, the Applicant has appreciated that, in some situations, the blade does not deliver the force and travel required to move the trip lever to the reset position. Such situations can arise, for example, when the appliance is operated at a low voltage (e.g. 50V rather than 240V). The reduction in the heating gradient that results from the lower power causes the blade to snap through with a reduced force. If the weight of the trip lever is too high, this reduced force can be insufficient to move the trip lever to the reset position. This can result in the trip lever failing to be moved at all, or the trip lever operating after a delay (referred to as DSO Delay). In the latter case, after the blade has snapped through without operating the trip lever, the thermal inertia in the heating element continues to transfer heat energy into the blade. This means that the blade gradually becomes increasingly convex until the travel of the blade, and the force exerted on the trip lever, is sufficient to move the trip lever to the reset position. The present invention aims to provide a control unit that addresses the problems outlined above. When viewed from a first aspect, the invention provides a control unit for a liquid heating appliance, the control unit comprising: an electrical power supply circuit for supplying electrical power to an electrical heater of a liquid heating appliance, the circuit comprising a switch; a trip lever for manually operating the switch, wherein the trip lever is pivotally moveable between a set position, in which the switch is closed, and a reset position, in which the switch is open; a mount plate for mounting to an underside of a heat diffuser plate of the liquid heating appliance, the mount plate comprising: a strap, comprising an inner aperture; and a column, extending out of the plane of the mount plate from an upper surface of the mount plate and defining a shelf; a snap-action bimetallic element arranged to operate at a predetermined temperature, the bimetallic element comprising: a hoop defining an inner cut-out, the hoop comprising a supported portion and an actuator portion that are separated by the inner cut-out; and a central tongue, extending from the supported portion, into the inner cut-out of the hoop, towards the actuator portion; wherein the actuator portion is arranged to move relative to the central tongue when the bimetallic element operates at the predetermined temperature to exert a force on the trip lever for moving the trip lever from the set position to the reset position; wherein the snap-action bimetallic element is mounted on the upper surface of the mount plate such that: the central tongue is arranged within the inner aperture of the strap of the mount plate so as to prevent movement of the central tongue away from the mount plate when the bimetallic element operates; and the supported portion of the hoop rests on the shelf of the column so as to prevent movement of the supported portion towards the mount plate when the bimetallic element operates. When viewed from a second aspect, the invention provides a liquid heating appliance comprising: a liquid heating vessel; an electrical heater comprising a heat diffuser plate and arranged to heat liquid contained within the liquid heating vessel; and a control unit mounted to an underside of the heat diffuser plate, the control unit comprising: an electrical power supply circuit for supplying electrical power to the electrical heater, the circuit comprising a switch; a trip lever for manually operating the switch, wherein the trip lever is pivotally moveable between a set position, in which the switch is closed, and a reset position, in which the switch is open; a mount plate mounted to the underside of the heat diffuser plate, the mount plate comprising: a strap, comprising an inner aperture; and a column, extending out of the plane of the mount plate from an upper surface of the mount plate and defining a shelf; a snap-action bimetallic element arranged to operate at a predetermined temperature, the bimetallic element comprising: a hoop defining an inner cut-out, the hoop comprising a supported portion and an actuator portion that are separated by the inner cut-out; and a central tongue, extending from the supported portion, into the inner cut-out of the hoop, towards the actuator portion; wherein the actuator portion is arranged to move relative to the central tongue when the bimetallic element operates at the predetermined temperature to exert a force on the trip lever for moving the trip lever from the set position to the reset position; wherein the snap-action bimetallic element is mounted on the upper surface of the mount plate such that: the central tongue is arranged within the inner aperture of the strap of the mount plate so as to prevent movement of the central tongue away from the mount plate when the bimetallic element operates; and the supported portion of the hoop rests on the shelf of the column so as to prevent movement of the supported portion towards the mount plate when the bimetallic element operates. Thus the present invention provides a control unit for mounting to the underside of a heat diffuser plate of a liquid heating appliance and comprising a bimetallic element for actuating a trip lever that can be manually operated to operate a switch in the electrical power supply circuit of the control unit. The strap and the column of the mount plate allow the central tongue and the supported portion of the hoop of the bimetallic element respectively to be held in a relatively fixed position, while the moveable actuating portion of the hoop of the bimetallic element is free to move to exert a force on the trip lever. By restraining the movement of the central tongue and the supported portion of the hoop, the mechanical advantage can be improved, as all of the movement in the bimetallic element when the bimetallic element “snaps through” occurs in the actuating portion of the hoop, meaning that the force exerted on the trip lever is greater. This helps to reduce the risk of the control unit failing to move the trip lever to the reset position, and to reduce the risk of a delay in the operation of the trip lever. Preferably the predetermined temperature at which the bimetallic element operates corresponds to a temperature of the heat diffuser plate during an overheat (e.g. “dry-boil”) scenario. For example, the predetermined temperature may be set at a value in the range of 120 °C to 220 °C, e.g. between 120 °C and 160 °C (e.g. for magnesium oxide heaters) or, e.g., between 200 °C and 220 °C (e.g. for printed heaters). The bimetallic element is preferably configured to move from a concave configuration to a convex configuration when the bimetallic element operates at the predetermined temperature. It will be appreciated that, in both configurations, the bimetallic element comprises a concave surface and a convex surface. Preferably the bimetallic element is mounted on the mount plate such that, in the concave configuration, the convex surface of the bimetallic element faces the mount plate. The bimetallic element may have a substantially circular profile. The hoop of the bimetallic element may be substantially annular, for example the actuator portion may be annular. However, in preferred embodiments, the hoop of the bimetallic element comprises two substantially parallel straight sides, connected at a first end by the supported portion and connected at the second end by the actuator portion. The actuator portion may be substantially annular. The central tongue extends into the inner cut-out between the two parallel sides of the hoop. The width of the central tongue preferably reduces along its length as it extends into the inner cut-out. Preferably the inner cut-out is U-shaped, being defined around the central tongue and, for example, along the two substantially parallel sides. Preferably the inner cut-out is fully defined by the hoop and the central tongue of the bimetallic element. The Applicant has identified that, owing to varying tolerances in the manufacturing process for control units and / or variations in how a control unit is mounted to the underside of a heat diffuser plate in an appliance, there is a problem with existing control units that the thermal communication between the bimetallic element and the heat diffuser plate can vary between appliances. This thermal communication depends on the orientation of the bimetallic element, e.g. the angle between the bimetallic element and the underside of the heat diffuser plate, and the extent to which the bimetallic element is compressed against the underside of the heat diffuser plate. Variations in either of these parameters can result in the operating temperature of the bimetallic element changing when the control unit, comprising the bimetallic element, is mounted to the heat diffuser plate. This can potentially reduce the reliability of the control unit, which may consequently not actuate the trip lever at the desired DSO temperature. Embodiments of the present invention address this problem with existing control units. Thus, preferably the mount plate comprises a bridge portion from which the strap extends. The bridge portion is preferably arranged to support the central tongue of the bimetallic element. Preferably the bridge portion abuts the entire length of the central tongue. This helps to ensure that the bimetallic element is mounted on the mount plate in the required orientation in order to reliably actuate the trip lever at the intended temperature. Preferably, the mount plate comprises a recessed portion defined around the bridge portion. Preferably at least the actuator portion of the hoop of the bimetallic element is arranged directly above at least part of the recessed portion. Preferably the recessed portion provides a gap between the underside of the actuator portion of the hoop of the bimetallic element and the upper surface of the mount plate. This allows the actuator portion of the hoop of the bimetallic element to move relative to the mount plate. Preferably the recessed portion is arranged to receive at least the actuator portion of the hoop of the bimetallic element when the bimetallic element operates at the predetermined temperature. Preferably the mount plate further comprises a separator protrusion that extends vertically out of the plane of the mount plate from the upper surface of the mount plate. In some embodiments the column comprises the separator protrusion. Preferably, in use, the separator protrusion is arranged to abut the underside of the heat diffuser plate of the liquid heating appliance when the mount plate is mounted to the underside of the heat diffuser plate. Preferably the separator protrusion extends out of the plane of the mount plate to a greater distance than the shelf of the column, e.g. to a height substantially coincidental with the snap-action bimetallic element that rests on the shelf. This means that, when the control unit is mounted to the underside of the heat diffuser plate, the separator protrusion, rather than the shelf, abuts the underside of the heat diffuser plate. The height of the separator protrusion may be selected to ensure that the snap-action bimetallic element also abuts the underside of the heat diffuser plate but without the bimetallic element being unduly compressed. It will be appreciated that providing such a separator protrusion can reduce the variation in the extent of the compression of the bimetallic element by the underside of the heat diffuser plate, as the separator protrusion can define a minimum separation between the heat diffuser plate and the rest of the mount plate. The shelf is preferably arranged to support an outer edge of the supported portion of the bimetallic element. Preferably the shelf is arranged to support the outer edge of the bimetallic element directly opposite the actuator portion of the bimetallic element. Preferably the separator protrusion is arranged adjacent the edge of the supported portion of the bimetallic element. This means that the minimum separation between the heat diffuser plate and the rest of the mount plate can be provided at least in the area of the bimetallic element, where it can be most advantageous to ensure a suitable separation. Preferably, the vertical distance between the upper surface of the shelf and the upper surface of the separator protrusion is greater than the thickness of the supported portion of the bimetallic element. This can help to prevent the bimetallic element from being pinched between the diffuser plate and the shelf when the control unit is mounted to the heat diffuser plate. This pinching of the bimetallic element can affect the operating temperature of the bimetallic element, meaning that the bimetallic element may not operate at the desired “dry switch off’ (DSO) temperature. Thus, embodiments of the present invention can improve the reliability of the control unit. Preferably the column comprises the separator protrusion, as well as the shelf. Preferably the column is a single monolithic structure comprising the shelf and the separator protrusion. This can reduce the risk of the shelf and the separator protrusion being subject to different manufacturing tolerances, meaning that the distance between the heat diffuser plate and the bimetallic element can be more reliably set. In preferred embodiments, the mount plate is formed of a metallic material and the column is bent out of the plane of the mount plate. Preferably the strap comprises two upright members, extending out of the plane of the mount plate, connected by a connecting member, wherein the connecting member is substantially parallel to the plane of the mount plate. In preferred embodiments, the mount plate is formed of a metallic material and the strap is bent out of the plane of the mount plate. Preferably the central tongue is arranged between the two upright members such that respective edges of the central tongue abut respective upright members of the strap. Preferably the upper surface of the central tongue is arranged to abut the underside of the connecting member. This helps to provide a tight fit between the strap and the central tongue. Preferably the area of the inner aperture of the strap is substantially equal to the cross-sectional area, in the plane perpendicular to the plane in which the central tongue extends, of the portion of the central tongue that is received within the inner aperture. This means that the central tongue can be received tightly within the inner aperture, thereby helping to reduce movement of the bimetallic element relative to the mount plate in a direction parallel to the plane of the mount plate. In some embodiments, the mount plate further comprises a tongue stop extending out of the plane of the mount plate from the upper surface of the mount plate. In preferred embodiments, the mount plate is formed of a metallic material and the tongue stop is bent out of the plane of the mount plate. Preferably the tongue stop is arranged to abut a distal end of the central tongue of the bimetallic element. This further helps to ensure that the bimetallic element is mounted in the correct orientation on the mount plate, thereby helping to ensure that the trip lever is operated reliably. Preferably the tongue stop is arranged below the inner cut-out of the bimetallic element. This means that the tongue stop does not interfere with the movement of the actuator portion of the bimetallic element when the bimetallic element operates at the predetermined temperature. Preferably the mount plate is a single monolithic component. Preferably the column is formed as a partial cut-out of the mount plate that is bent out of the plane of the mount plate. This can help to simplify manufacturing of the mount plate. The liquid heating vessel of the liquid heating appliance may be any suitable or desired shape, defining a volume for receiving liquid to be heated. Preferably the liquid heating vessel is able to safely contain liquid, especially water, when heated to boiling. The liquid heating vessel may be made from any suitable or desired material. Preferably the liquid heating vessel is made from stainless steel, e.g. foodsafe stainless steel. The electrical heater preferably comprises an electrical heating element. The electrical heating element may be a thick film heating element. The electrical heating element is preferably a sheathed electrical heating element. The heat diffuser plate is preferably arranged beneath the base of the liquid heating vessel. The electrical heating element may be mounted to an underside of the heat diffuser plate. Preferably the control unit further comprises a push rod that is operated on by the actuator portion of the bimetallic element to move the push rod so as to apply a force to the trip lever. The bimetallic element may be arranged in alignment with (e.g. above) the push rod. The push rod may extend downwardly from the bimetallic element towards the trip lever. The trip lever may comprise a force transfer pad that is arranged to be acted on by the push rod. The force provided by the actuator portion of the bimetallic element may therefore be transferred by the push rod to the trip lever, e.g. via the force transfer pad. Preferably the switch comprises a fixed electrical contact and a moveable electrical contact, wherein the moveable electrical contact is arranged to be moved by movement of the trip lever between the set position and the reset position. The control unit preferably further comprises a leaf spring, wherein the moveable electrical contact is mounted on the leaf spring. Preferably the force applied by the actuator portion is exerted on the leaf spring to separate the moveable electrical contact from the fixed electrical contact. The Applicant has identified that the present invention can help to more reliably separate the fixed and moveable electrical contacts, as the force applied by the bimetallic element to separate these contacts can be increased. In some embodiments, the force is exerted on the leaf spring by the push rod. In some embodiments, the force exerted on the leaf spring (e.g. by the push rod) is transferred to (e.g. the force transfer pad of) the trip lever. In some embodiments, the force exerted by the actuator portion of the bimetallic element is transferred to the trip lever via the push rod and, in turn, the leaf spring. In some embodiments, the control unit comprises a secondary snap-action bimetallic element arranged to operate at a further predetermined temperature. Thus, the snap-action bimetallic element described above may comprise a primary snap-action bimetallic element and a secondary snap-action bimetallic element. The secondary snap-action bimetallic element may be arranged to operate as a backup or “fail-safe”, such that the secondary bimetallic element is arranged to operate in the event that the (i.e. primary) bimetallic element is faulty. The further predetermined temperature may be greater than the predetermined temperature of the (primary) bimetallic element. It will be appreciated that any or all of the features of the control unit described herein in relation to the (primary) bimetallic element can, and preferably do, apply additionally to the secondary bimetallic element. For example, the mount plate may comprise a further strap and a further column for supporting the secondary bimetallic element. In some embodiments, the control unit comprises an electronic boiling detection means arranged to detect that liquid within the liquid heating vessel has reached boiling point and to operate the switch or the trip lever to open the switch upon said detection. The electronic boiling detection means may comprise an electronic temperature sensor (e.g. a thermistor), for example arranged to measure (directly or indirectly) the temperature of liquid within the liquid heating vessel. The electronic temperature sensor may be mounted on the control unit (e.g. protruding from the mount plate) to contact the heat diffuser plate or to extend through the heat diffuser plate. In such embodiments, the control unit may further comprise a processor or control circuit and preferably the electronic temperature sensor is configured to provide a measurement signal representative of the temperature of liquid within the liquid heating vessel to the processor or control circuit. The control unit may also comprise a relay or triac configured to exert a force acting to open the switch or acting on the trip lever for moving the trip lever from the set position to the reset position. In some embodiments, the control unit comprises a further snap-action bimetallic element arranged to detect that liquid within the liquid heating vessel has been converted into steam and to operate the switch or the trip lever to open the switch upon said detection. For example, the further snap-action bimetallic element may be arranged to operate at a predetermined temperature of 70-95 °C. Preferably the further snap-action bimetallic element is arranged to exert a force on the trip lever for moving the trip lever from the set position to the reset position upon detecting that liquid within the liquid heating appliance has been converted into steam. The further snap-action bimetallic element may have a substantially circular profile. Unlike the bimetallic element described above, the further snap-action bimetallic element may comprise a central tongue arranged to move to exert a force on the trip lever for moving the trip lever from the set position to the reset position when the further snap-action bimetallic element operates at the predetermined temperature. When viewed from a further aspect, there is provided a control unit for a liquid heating appliance, the control unit comprising: an electrical power supply circuit for supplying electrical power to an electrical heater of a liquid heating appliance, the circuit comprising a switch; a trip lever for manually operating the switch, wherein the trip lever is pivotally moveable between a set position, in which the switch is closed, and a reset position, in which the switch is open; a mount plate for mounting to an underside of a heat diffuser plate of the liquid heating appliance, the mount plate comprising: a strap comprising two upright members, extending out of the plane of the mount plate, connected by a connecting member, wherein the connecting member is substantially parallel to the plane of the mount plate, and the two upright members and the connecting member together define an inner aperture; and a column, extending out of the plane of the mount plate from the upper surface of the mount plate and defining a shelf; a snap-action bimetallic element arranged to operate at a predetermined temperature; and a push rod, positioned below the bimetallic element and extending downwardly towards the trip lever; wherein the bimetallic element comprises: a hoop defining an inner cut-out, the hoop comprising a supported portion and an actuator portion that are separated by the inner-cut out; and a central tongue, extending from the supported portion, into the inner cut-out of the hoop, towards the actuator portion; wherein the actuator portion is arranged to move relative to the central tongue when the bimetallic element operates at the predetermined temperature to exert a force on the push rod so as to apply a force to the trip lever for moving the trip lever from the set position to the reset position; wherein the bimetallic element is mounted on the upper surface of the mount plate such that: the central tongue is arranged within the inner aperture of the strap of the mount plate so as to prevent movement of the central tongue away from the mount plate when the bimetallic element operates; and the supported portion of the hoop is supported by the shelf of the column so as to prevent movement of the supported portion towards the mount plate when the bimetallic element operates. It will be appreciated that terms such as “below”, “lower” and “above” used herein are relative to the intended orientation of the liquid heating appliance when it contains liquid to be heated, and thus to the direction in which gravity acts on the liquid. It will be appreciated by those skilled in the art that many variations and modifications to the embodiments described above may be made within the scope of the various aspects and embodiments of the invention set out herein. Any aspect of the invention described herein may (and preferably does) include one or more (e.g. all) of the optional and preferred features outlined herein. Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows a perspective view of a liquid heating appliance comprising a control unit in accordance with an embodiment of the present invention; Fig. 2 shows an exploded view of the heating arrangement of the appliance of Fig. 1; Fig. 3 shows a perspective underside view of the control unit of the liquid heating appliance of Fig. 1; Fig. 4 shows a perspective topside view of the control unit of the liquid heating appliance of Fig. 1; Fig. 5 shows a cross-sectional side view of the control unit of the liquid heating appliance of Fig. 1; Fig. 6a and 6b show perspective topside views of the mount plate of the control unit of the liquid heating appliance of Fig. 1; and Fig. 7a and 7b show cross-sectional side views of the operation of a bimetallic actuator of the control unit of the liquid heating appliance of Fig. 1. Figure 1 shows a perspective view of a liquid heating appliance 2, hereinafter the appliance 2, in accordance with an embodiment of the present invention. The appliance 2 comprises a liquid heating vessel 4, a spout 6 and a handle 8. The top of the appliance 2 is closed with a lid 10. The appliance 2 is arranged to rest on a power base stand 12 having a 360-degree base electrical connector part 14 provided in the centre for supplying the appliance with electricity. The appliance 2 further comprises an electrical heater (not shown in Figure 1) for heating a volume of liquid contained within the liquid heating vessel 4. The heater comprises an electrical heating element mounted to the underside of a heat diffuser plate. The heating element is arranged in conductive thermal communication with the base 23 of the liquid heating vessel 4 via the heat diffuser plate when electrical energy is provided to the heater, thereby heating the contents of the liquid heating vessel 4. A control unit (not shown in Figure 1) is also mounted to the underside of the heat diffuser plate, and controls the provision of electrical power from the power base stand 12 to the electrical heater, as described in more detail below. Figure 2 shows an exploded view of the heating arrangement of the appliance 2 of Figure 1, which includes the electrical heater 18, the control unit 16 and the base 23 of the liquid heating vessel 4 of the appliance 2. The other components of the appliance 2 have been removed for clarity. The control unit 16 and the heater 18 are both mounted below the base 23 of the liquid heating vessel 4. The control unit 16 is arranged to receive the base electrical connector part 14 of the power base 12 and to control the provision of electrical power from the power base stand 12 to the electrical heater 18. The heater 18 comprises a sheathed electrical heating element 18a and a circular heat diffuser plate 18b. The heating element 18a is of a conventional ‘horseshoe’ construction with an electrical termination 19 at either end. The heating element 18a extends substantially around the circumference of the underside of the diffuser plate 18b. The control unit 16 is arranged to be mounted centrally on the underside of the diffuser plate 18b. Figure 3 shows a perspective underside view of the control unit 16 shown in Figure 2. The control unit 16 comprises a moulded plastics body 26 in which there is formed a cordless electrical adapter part 28 on its bottom side. The cordless electrical adapter part 28 is a 3-pole conductor part comprising an earth pin 28a, a live ring 28b, and a neutral ring 28c. The live ring 28b and the neutral ring 28c are concentrically arranged around the central earth pin 28a. The base electrical connector (not shown in Figure 3) comprises a central aperture to receive the earth pin 28a and a coaxial annular aperture to receive both the live and neutral rings 28b, 28c Electrical contacts housed in the central and coaxial apertures contact the live ring 28b and neutral ring 28c respectively to connect the live and neutral poles of the power supply circuit when the connector / adapter parts 14, 28 are brought together. The control unit 16 comprises a pair of electrical tabs 29a, 29b, which are electrically connected to the cordless electrical adapter part 28. The neutral tab 29a is connected to the neutral ring 28c, and the live tab 29b is connected to the live ring 28b. Both tabs 29a, 29b are connected to the respective contacts of the cordless electrical adapter part 28 via a switch comprising a fixed electrical contact and a movable electrical contact (not shown in Figure 3), the moveable electrical contact being moveable to be brought into and out of electrical contact with the fixed electrical contact so as to respectively close and open the switch. Each of the electrical tabs 29a, 29b is further connected via a flying lead (not shown) to a respective electrical termination 19 of the heating element 18a. Thus, operation of the switch controls the supply of electrical power to the heating element 18a. The control unit 16 further comprises a further snap-action bimetallic element 31, referred to herein as a steam blade, supported by the moulded plastics control body 26. The steam blade 31 is a snap-action bimetallic element set to operate at a predetermined temperature. In use, when the control unit 16 is mounted within the appliance 2, the steam blade 31 is arranged to detect the temperature of steam generated by water boiling within the liquid heating vessel 4, e.g. via a steam channel extending from the top of the vessel 4 to the control unit 16 via a steam delivery tube, as is known in the art. The control unit 16 comprises a trip lever 36 that is pivotally mounted on the control body 26. The trip lever 36 is pivotally movable between a set position and a reset position. A distal end of the trip lever 36 is arranged to contact the steam blade 31, while a proximal end of the trip lever 36 is arranged to operate the switch (not shown in Figure 3) that connects the pair of electrical tabs 29a, 29b with the cordless electrical adapter part 28. The trip lever 36 is arranged to operate the switch to separate the fixed electrical contact from the moveable electrical contact of the switch when in the reset position, and to bring the electrical contacts together when in the set position. The steam blade 31 is arranged such that its operation at its predetermined temperature, e.g. 85 °C, causes the trip lever 36 to pivot so as to open the switch (i.e. to move to the reset position), thereby disrupting the supply of electrical energy to the pair of electrical tabs 29a, 29b and, consequently, to the heating element 18a. This allows the heater 18 to be switched off when a temperature is detected by the steam blade 31 that is representative of water within the liquid heating vessel 4 reaching boiling. The trip lever 36 is also manually operable to open and close the switch. Figure 4 shows a perspective topside view of the control unit 16 shown in Figure 2. The control unit 16 comprises a primary snap-action bimetallic element 30a and a secondary snap-action bimetallic element 30b, referred to herein as a first and a second dry-boil blade 30a, 30b respectively. The first and second dry-boil blades 30a, 30b are supported by a metal mount plate 32 fixed on the top side of the control body 26. The dry-boil blades 30a, 30b are each set to operate independently at respective predetermined temperatures, e.g. 130 °C and 145 °C. The dry-boil blades 30a, 30b are positioned horizontally on the top surface of the control unit 16 such that, when the control unit 16 is mounted to the underside of the heat diffuser plate 18b, the dry-boil blades 30a, 30b are in thermally conductive communication with the heat diffuser plate 18b. This means that the dry-boil blades 30a, 30b are arranged to detect the temperature of the heat diffuser plate 18b. The dry-boil blades 30a, 30b are arranged such that their operation at the predetermined temperature(s) causes a switch within the control unit 16 to be opened, thereby interrupting the supply of electrical energy to the heater 18. As is known in the art, this allows the heater to be switched off in the event of a “dry-boil” scenario, in which no liquid is present within the liquid heating vessel 4. The mount plate 32 of the control unit 16 comprises two bosses 32b extending from the upper surface of the mount plate 32. The bosses 32b help to provide a minimum separation between the underside of the heat diffuser plate 18b and the control unit 16 when the control unit 16 is mounted to the heat diffuser plate 18b. Figure 5 shows a cross-sectional side view of the control unit 16 shown in Figure 2. The control unit 16 comprises a first push rod 40a that is arranged directly below the first dry-boil blade 30a. As will be described in more detail below, the first push rod 40a is arranged directly below an actuator portion of the first dry-boil blade 30a. The first push rod 40a is arranged so as to be moved vertically downwards upon operation of the first dry-boil blade 30a. The control unit 16 further comprises a leaf spring 42, comprising a fixed end (not shown in Figure 5) that is mounted to, and electrically connected to, the live ring 28b. The leaf spring 42 also comprises a moveable portion 42b, which extends from the point of deflection of the leaf spring 42 to the end of the leaf spring 42 that is distal to the fixed end. The first push rod 40a is arranged to contact the distal end of the moveable portion 42b of the leaf spring 42. The moveable electrical contact of the switch that controls the supply of power to the heater 18 is provided on the moveable portion 42b of the leaf spring 42. Downwards movement of the leaf spring 42, caused by downwards movement of the first push rod 40a, causes the moveable electrical contact on the leaf spring 42 to separate from the fixed electrical contact of the switch. Separation of these contacts results in an opening of the power supply circuit, thereby interrupting the supply of electrical power to the heater 18. The trip lever 36 comprises a force transfer pad 48a arranged below the movable portion of the leaf spring 42 for transferring the downward movement of the first push rod 40a, via the moveable portion of the leaf spring 42b, to the trip lever 36. This causes the trip lever 36 to pivot in an anticlockwise direction (when viewed as shown in Figure 5) so as to move into the reset position, in which the electrical contacts of the switch are separated. Figures 6a and 6b show perspective views of the mounting arrangement for the first dry-boil blade 30a on the control unit 16 of Figure 2. In Figure 6b, the dry-boil blade 30a itself has been removed for clarity. The dry-boil blade 30a is substantially planar and comprises a hoop 37, defining an inner cut-out 31a. The hoop 37 comprises an actuator portion 37a and a supported portion 37b, wherein the actuator portion 37a and the supported portion 37b are separated by the inner cut-out 31a. The dry-boil blade 30a further comprises a central tongue 31b, extending into the cut-out 31a from the supported portion 37b, towards the actuator portion 37a. The actuator portion 37a of the hoop 37 is arranged directly above the push rod 40a such that, when the dry-boil blade 30a operates at its predetermined operating temperature, the actuator portion 37a moves downwards relative to the central tongue 31b and contacts the push rod 40a, thereby applying a downwards force to the push rod 40a. The mount plate 32 of the control unit 16 comprises a bridge portion 39a that supports the central tongue 31b of the dry-boil blade 30a. A recess 39b is defined in the mount plate 32 around the bridge portion 39a such that the recess 39b is directly below the hoop 37 of the dry-boil blade 30a. This means that, when the dryboil blade 30a operates at the predetermined temperature, the hoop 37 of the blade 30a moves downwards into the recess 39b (as shown in Figures 7a and 7b). The mount plate 32 further comprises a strap 33 extending upwards from the bridge portion 39a and returning to the bridge portion 39a so as to define an aperture between the bridge portion 39a and the strap 33. The central tongue 31b of the dry-boil blade is received and held tightly within the aperture 33a of the strap 33, between the bridge portion 39a and the strap 33. This helps to ensure that, when the blade 30a snaps at its predetermined operating temperature, all of the movement in the blade relative to the mount plate 32 is in the hoop 37. This helps to increase the mechanical advantage of the blade 30a, meaning that the actuator portion 37a of the blade 30a pushes on the push rod 40a with a greater force. The mount plate 32 further comprises a column 35 extending perpendicularly out of the plane of the mount plate 32. The column 35 comprises a shelf 35a (shown in Figure 6b) for supporting the edge of the supported portion 37b of the hoop 37 of the dry-boil blade 30a. The shelf 35a helps to ensure that, when the dry-boil blade 30a snaps at its operating temperature, the supported portion 37b of the hoop 37 cannot move closer to the mount plate 32. This helps to further increase the mechanical advantage of the blade 30a, as all of the movement in the blade relative to the mount plate 32 is in the actuator portion 37a of the hoop 37, i.e. the portion of the blade 30a on the opposite side of the cut-out 31a to the supported portion 37b. Thus, the actuator portion 37a of the blade 30a pushes on the push rod 40a with a greater force. The combination of the blade-retaining strap 33 and the supporting shelf 35a means that a substantial proportion of the operating force of the blade 30a is transferred to the push rod 40a. This means that a greater force is transferred from the blade 30a to the trip lever 36 via the push rod 40a, the leaf spring 42b, and the force transfer pad 48a, thereby reducing the risk of the trip lever 36 failing to operate. The column 35 further comprises a separator protrusion 35b that extends a greater distance from the upper surface of the mount plate 32 than the shelf 35a. The column 35 is a single monolithic component that comprises both the shelf 35a and the separator protrusion 35b. The upper surface of the separator protrusion 35b is arranged to contact the underside of the heat diffuser plate 18b when the mount plate 32 is mounted to the heat diffuser plate 18b. Thus, the separator protrusion 35b is arranged to maintain a particular separation between the upper surface of the mount plate 32 and the underside of the heat diffuser plate 18b. More particularly, the separator protrusion 35b helps to define the vertical separation between the blade 30a and the underside of the heat diffuser plate 18b. As discussed above, the mount plate 32 also comprises bosses 32b for helping to maintain a separation between the heat diffuser plate 18b and the control unit 16. However, owing to manufacturing tolerances, the separation provided by the bosses 32b may not be reliable, meaning that the first and second dry-boil blades 30a, 30b may be compressed against the underside of the heat diffuser plate 18b to a different extent than is intended. This means that the respective operating temperatures of the blades 30a, 30b may not be the desired temperatures, resulting in unreliable operation of the DSO mechanism. By providing the shelf 35a and the separator protrusion 35b on the same component (the column 35), rather than as separate components which may be subject to varying manufacturing tolerances, the desired separation between the heat diffuser plate 28b and the control unit 16 can be more reliably obtained. The vertical distance between the upper surface of the shelf 35a and the upper surface of the separator protrusion 35b is greater than the thickness of the blade 30a. This means that, when the control unit 16 is mounted to the underside of the diffuser plate 18b, the blade 30a is not pinched between the mount plate 32 and the diffuser plate 18b, which could cause the blade 30a to operate at an undesired temperature. The mount plate 32 further comprises a tongue stop 32a protruding out of the plane of the mount plate 32 from the upper surface of the mount plate 32 adjacent the distal end of the central tongue 31b. The tongue stop 32a, together with the strap 33, helps to prevent movement of the blade 30a parallel to the plane of the mount plate 32. The central tongue 31b of the blade 30a defines an aperture 31c that is shaped to accommodate a stud 39c that protrudes from the upper surface of the bridge 39a of the mount plate 32. The engagement of the stud 39c within the aperture 31c further helps to prevent movement of the blade 30a parallel to the plane of the mount plate 32. Figures 7a and 7b shows cross-sectional side views of the blade mounting arrangement of the control unit 16 of Figure 2. Figure 7a shows the dry-boil blade 30a at a temperature below its predetermined operating temperature, and Figure 7b shows the dry-boil blade 30a at a temperature above its predetermined operating temperature, after the blade 30a has snapped through. Figure 7b shows that, after the blade 30a has snapped through, the actuator portion 37a of the hoop 37 of the blade 30a is arranged within the recess 39b of the mount plate 32, around the bridge portion 39a of the mount plate 32. However, as the central tongue 31b is held between the strap 33 and the bridge portion 39a of the mount plate 32, and as the supported portion 37b is held by the shelf 35a of the column 35, the respective positions of the central tongue 31b and the supported portion 37b of the blade 30a do not change when the blade 30a snaps through. As discussed above, this means that all of the movement in the blade 30a is in the actuator portion 37a, meaning that the push rod 40a is moved downwards towards the force transfer pad 48a of the trip lever 36 with a greater force. Consequently, the risk of the control unit 16 failing to move the trip lever 36 to the reset position (as shown in Figure 7b) is reduced. Once the push rod 40a is pushed down against the force transfer pad 48a, the trip lever 36 pivots to move from the set position to the reset position.

Claims

1. A control unit for a liquid heating appliance, the control unit comprising:an electrical power supply circuit for supplying electrical power to an electrical heater of a liquid heating appliance, the circuit comprising a switch;a trip lever for manually operating the switch, wherein the trip lever is pivotally moveable between a set position, in which the switch is closed, and a reset position, in which the switch is open;a mount plate for mounting to an underside of a heat diffuser plate of the liquid heating appliance, the mount plate comprising:a strap comprising an inner aperture; anda column, extending out of the plane of the mount plate from an upper surface of the mount plate and defining a shelf;a snap-action bimetallic element arranged to operate at a predetermined temperature, the bimetallic element comprising:a hoop defining an inner cut-out, the hoop comprising a supported portion and an actuator portion that are separated by the inner-cut out; anda central tongue, extending from the supported portion, into the inner cut-out of the hoop, towards the actuator portion;wherein the actuator portion is arranged to move relative to the central tongue when the bimetallic element operates at the predetermined temperature to exert a force on the trip lever for moving the trip lever from the set position to the reset position;wherein the bimetallic element is mounted on the upper surface of the mount plate such that:the central tongue is arranged within the inner aperture of the strap of the mount plate so as to prevent movement of the central tongue away from the mount plate when the bimetallic element operates; andthe supported portion of the hoop is supported by the shelf of the column so as to prevent movement of the supported portion towards the mount plate when the bimetallic element operates.

2. The control unit of claim 1, wherein the mount plate comprises:a bridge portion from which the strap extends, wherein the bridge portion is arranged to support the central tongue of the bimetallic element; anda recessed portion defined around the bridge portion for receiving at least the actuator portion of the hoop of the bimetallic element when the bimetallic element operates at the predetermined temperature.

3. The control unit of claim 1 or 2, wherein the mount plate further comprises a separator protrusion that extends vertically out of the plane of the mount plate from the upper surface of the mount plate to a greater distance than the shelf of the column.

4. The control unit of claim 3, wherein the vertical distance between the upper surface of the shelf and the upper surface of the separator protrusion is greater than the thickness of the supported portion of the bimetallic element.

5. The control unit of claim 3 or 4, wherein the column comprises the separator protrusion.

6. The control unit of any of claims 3 to 5, wherein, in use, the separator protrusion is arranged to abut the underside of the heat diffuser plate of the liquid heating appliance when the mount plate is mounted to the underside of the heat diffuser plate.

7. The control unit of any preceding claim, wherein the area of the inner aperture of the strap is substantially equal to the cross-sectional area of the central tongue in the plane perpendicular to the direction in which the central tongue extends.

8. The control unit of any preceding claim, further comprising a push rod, wherein the actuator portion of the bimetallic element is arranged to act on the push rod to move the push rod so as to apply the force to the trip lever.

9. The control unit of any preceding claim, wherein the switch comprises a fixed electrical contact and a moveable electrical contact, wherein the moveableelectrical contact is arranged to be moved by movement of the trip lever between the set position and the reset position.

10. The control unit of claim 9, further comprising a leaf spring, wherein the moveable electrical contact is mounted on the leaf spring.

11. The control unit of claim 10, wherein the force applied by the actuator portion is exerted on the leaf spring to separate the moveable electrical contact from the fixed electrical contact.

12. The control unit of any preceding claim, wherein the snap-action bimetallic element comprises a primary snap-action bimetallic element and a secondary snapaction bimetallic element.

13. The control unit of any preceding claim, comprising a further snap-action bimetallic element arranged to detect that liquid within the liquid heating appliance has been converted into steam and to open the switch upon said detection.

14. The control unit of claim 13, wherein the further snap-action bimetallic element is arranged to exert a force on the trip lever for moving the trip lever from the set position to the reset position upon detecting that liquid within the liquid heating appliance has been converted into steam.

15. The control unit of any preceding claim, comprising an electronic boiling detection means arranged to detect that liquid within the liquid heating vessel has reached boiling point and to operate the switch or the trip lever to open the switch upon said detection.

16. A liquid heating appliance comprising:a liquid heating vessel;an electrical heater comprising a heat diffuser plate and arranged to heat liquid contained within the liquid heating vessel;a control unit as claimed in any preceding claim, mounted to an underside of the heat diffuser plate.

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