GB2083328A - Boil-dry switch mechanism for kettle - Google Patents

Abstract

An automatic cut-out mechanism for an electric kettle includes a sensor (25) of shape memory effect material normally in a heat conducting path from the kettle element (17). The sensor (25) is coupled to a snap-action switch mechanism (35). When the snap-action mechanism is operated by the sensor (25) being subject to elevated temperature, the former removes the latter from the heat-conducting path so that the sensor is not subjected to high temperature for prolonged periods. <IMAGE>

Description

SPECIFICATION Kettle This invention relates to a boil-dry switch mechanism for an electric liquid-heating con trainer, referred to hereinafter as an electric kettle. Such a mechanism detects excessive temperature, usually of the heating element, arising through a "boil-dry" condition, as when the heating element is energised when inadequately covered by the liquid to be heated.

A boil-dry switch mechanism for an electric kettle usually consists of a temperature sensor which is arranged in a heat conducting path such that it is subject to a temperature depen dent on the temperature of the heating ele ment and opens an electric power circuit for the element on the occurrence of excessive element temperature; the sensor has been in the past a bi-metallic element and has been designed to operate the contacts with a snap action.

In the present invention, a sensor made of SME material is employed. By an "SME mate rial" is meant a material, usually an alloy, having a transition temperature range, in which the modulus of elasticity varies progres sively and significantly with change of temper ature in a reversible manner. If an element of SME material, which is usually a heat-treated alloy, is deformed at a temperature below the transition temperature range, it will restore progressively towards the undeformed condi tion as the temperature is raised through the range. The phenomenon is reversible, in that a subsequent lowering of the temperature produces a return towards the deformed con dition. The preferred SME material of the present invention is a copper-zinc-aluminium manganese alloy.

There is the danger that if an SME sensor is subject for prolonged periods to excessive temperatures relative to the transition temper ature range, it may lose its properites. To avoid that danger, the heat conducting path to the SME sensor in the present invention is broken on operation of the switch mechanism to open the power circuit, so as to minimise the period for which the sensor is subject to high temperature.

The invention thus provides, in one embodi ment, an electric kettle having a liquid-heating electric element; and a snap-action switch mechanism which includes contacts in a power circuit for the element, a temperature sensor which is made of SME material and which is arranged to open the contacts when the temperature of the sensor exceeds a given value, a heat conducting path to the sensor whereby the sensor temperature is dependent on the element temperature, and means for breaking the heat conducting path on opera tion of the mechanism to open the contacts.

The invention will be more readily understood by way of example from the following description of an electric kettle with a boil-dry switch mechanism in accordance therewith, reference being made to the accompanying drawings, in which Figure 1 illustrates the kettle in vertical section, and Figures 2, 3, 4 and 5 show the heating element of four boil-dry switch mechanisms.

While the electric kettle may have other configurations, that shown in Fig. 1 has its electric heating element suspended from the fixed lid or roof of the kettle. Thus the kettle is shown as having a pan-like container 1 2 to which is secured an integral lid 1 3 and handle 14 with a spout 15 formed in the lid 13. The lid 1 3 contains a socket for an electric supply plug 1 6 and switches (not shown) for the control of the current to a heating element 17, which is suspended from the lid 1 3 by a rigid tube 1 8 carrying supply leads 1 9 (Fig.

2); tube 1 8 does not contain heating elements.

A steam outlet passage 20 formed in the lid 1 3 leads from the kettle interior to the surrounding air, the steam passing over the thermal sensor 21 which operates one of the control switches through plunger 22. When the kettle boils, the switch is operated to open the circuit to the heating element 1 7.

The boil-dry sensor is illustrated in Fig. 2 in the form of an element 25 of an SME alloy located in a second metal tube 26. Tube 26 is mounted in the lid by a bulkhead fitting, which also mounts the tube 18, and extends downwardly parallel to tube 1 8. At its lower end, the sensor tube 26 is sealed by a metal cap 28 which is attached to the heating element 1 7 in good heat conducting fashion.

Within the tube 26, the temperature sensor 25 is normally in heat-conducting contact with the cap 25 but is not secured to it; in the form shown in Fig. 2, the sensor 25 is in the form of a helical spring engaged in compression against the cap 28, but the sensor may take other forms.

The sensor 25 operates a switch (not shown) in the hollow lid 1 3 through a remote control link, illustrated in Fig. 2 as a push rod 32 the lower end of which engages against, and is secured to, the upper end of sensor 25 and which extends upwards into the lid. A snap-action mechanism 35 is associated with the push-rod 32. The snap-action mechanism can take any known form, being illustrated as a pivoted lever 36 engaged by a compression spring 37 biasing the lever towards either of two over-centre positions. The lever 36 is slotted on the rod 32. In the non-actuated condition shown in full line, spring 37 biases the lever anti-clockwise thus maintaining sensor 25 in compression against the cap 26.On detection of an excessive temperature, the sensor 25 forces the lever 36 over-centre, when the spring 37 drives the rod upwards with a snap action, operating the switch to the OFF position and lifting the sensor 25 out of engagement with the cap 28.

The SME alloy forming the sensor 25 has a transition temperature range over which the modulus of elasticity increases progressively with temperature; temperature changes below To at the lower end of that range have little effect on the sensor and particularly do not affect the effective length of the spring form of the sensor shown in Fig. 2; Tz is the temperature at which an SME alloy starts to generate a stress or a strain. The composition of the alloy and the heat path from the element 1 7 to the sensor 25 are so chosen that, during normal operation of the kettle with the element 1 7 properly immersed in liquid, the sensor 25 is never subject to a temperature exceeding Tz and degradation of the sensor is avoided.Furthermore, because the sensor is lifted away from the cap 25 once the snap action mechanism has operated, the sensor 25 is subject to temperatures exceeding Tz only for so long as is necessary to cause actuation of the switch, and does not experience excessively high temperatures likely to cause degradation. The boil dry switch can be reset manually once cooling of the element has taken place in order to return the sensor to its active position as shown.

The arrangement shown in Fig. 3 is generally similar in principle to that of Fig. 2, but differs from the latter in two particulars.

Firstly, a snap-action switch 40 is employed to actuate contacts 41 in series with the heating element 17, and a separate snapaction mechanism such as that shown at 35 in Fig. 2 is dispensed with. The upper end of push rod 32 is consequently held captive on the contact blade 42 of switch 40 and is lifted by that blade, when the switch snaps into open position.

Secondly, the rod 32 includes a lower, offset part 43 terminating in an integral disc 44 through which a stem 45 passes freely.

Stem 45 is made of copper and has at its lower end a shoe 46, the SME sensor 25 being located between shoe 46 and disc 44.

In the normal position shown, shoe 46 is in engagement with, and in good heat-contacting with the bottom of tube 26, which is made of copper with an integral pip or strap 26A. Strap 26A is welded or otherwise attached to element 1 7 to provide a heat conducting path from the element to the tube 26.

While the element 1 7 is properly immersed in water, the sensor 25 is never subject to a temperature much in excess of 100 C. However, if a boil-dry condition arises by the water level being below the element, heat is transmitted quickly from element 1 7 to sensor 25 which when its temperature exceeds Tz applies thrust to link 32 and thence to the snapaction switch 40. Movement of the link 32 without lifting shoe 46 off the bottom of tube 26 is permitted by the lost motion device constituted by the stem 45 movable in disc 44.However, when switch 40 is operated and snaps into the open position, the upper face of disc 44 engages an abutment 47 on the stem and removes the shoe 46 out of heat transmitting contact with the bottom of tube 26, so that the heat path from element 1 7 to sensor 25 is broken.

The arrangement of Fig. 3 has the minor disadvantage that the element 1 7 cannot be removed without disturbing the boil-dry switch 40 which is normally fixed inside the kettle assembly. That disadvantage is avoided in the modification of Fig. 4, in which the thrust link between the sensor 25 and the switch 40 is in two separable parts.

The upper part of the link is constituted by a pin 50 which is attached to blade 42 of switch 40, as before. The lower part consists of a rod 51 which is located within the tube 26 and is generally similar to link 32 of Fig. 3 at its lower end, but at its upper end has a larger diameter and is formed with a shoulder 52; pin 50 is a press fit in a bore in the upper end of link part 51. At the upper end, tube 26 is enlarged to form a shoulder 53 and a compression spring 54 is engaged between shoulders 52 and 53.

The operation of the boil-dry mechanism of Fig. 4 is generally similar to that of Fig. 3, except that spring 54 lifts link part 51, sensor 25 and shoe 46, when permitted to do so by the upward movement of pin 50.

Fig. 5 shows a further variant in which the element 1 7 is normally connected to pin 60 through a leaf spring contact blade 61 biased against a fixed contact 62 electrically connected to the element. Sensor 25 is located in a tube 63 attached to element 17, and is engaged between a copper shoe 64 on the lower end of rod 65 passing through the sensor 25 and a housing 66. The end of a rod 65 can engage the free extremity of blade 61, and further carries a detent 67 overlying.

a slider 68 which emerges from the kettle casing as a manual reset button.

Slider 68 is guided by a tongue 70 which is received in a slot 71 in the slider and is biased to the right by a spring 72. The upper surface of the slider is formed with a shoulder 73 from which extends a wedge-like cam face 74. The upper end of sensor 25 engages a ring 75 which can slide on rod 65 and which carries a second ring 76 so that a collar 77 fast on rod 65 is located between rings 75 and 76. A spring 78 biases the rings downwardly and maintains the shoe in contact with the bottom of tube 63.

Fig. 5 shows the condition of the mechanism after the reset button has been pressed; the slider 68 is held against the bias of spring 72 by the engagement of shoulder 73 with detent 67. If a boil-dry condition arises. sen sor 25 forces ring 75 upwards until it en gages collar 77; thereafter rod 65 is raised progressively, but blade 61 is not engaged unless detent 67 clears shoulder 73.

If the temperature of sensor 25 rises suffici ently, detent 67 clears shoulder 73, allowing spring 72 to drive slider 68 to the right. Cam face 74 rides past detent 67 and lifts the detent by cam action, causing blade 61 to be lifted off contact 62 abruptly. At the same time the upward movement of rod 65 lifts the shoe 64, the sensor 25 and the rings 75, 76 upwardly so that the thermal path from ele ment 1 7 and sensor 25 is broken, as before.

The composition of the SME alloy of sensor 25 is preferably: Copper 81% Zinc 7% Aluminium 10% Manganese 2%

Claims (14)

1. An electric kettle having a liquid-heat ing electric element and a switch mechanism including a temperature sensor which is made of an SME material and which is arranged in a heat conducting path such that it is subject to a temperature dependent on the tempera ture of the element and opens an electrical power circuit for the element on the occur rence of excessive element temperature, in which kettle the heat conducting path is broken on operation of the mechanism to open the power circuit, so as to minimise the period for which the sensor is subject to high temperature.

2. An electric kettle having a liquid-heat ing electric element; and a snap-action switch mechanism which includes contacts in a power circuit for the element, a temperature sensor which is made of an SME material and which is arranged to open the contacts when the temperature of the sensor exceeds a given value, a heat conducting path to the sensor whereby the sensor temperature is dependent on the element temperature, and means for breaking the heat conducting path on opera tion of the mechanism to open the contacts.

3. An electric kettle according to claim 2, in which the sensor is located in a closed tube which extends within the housing of the ket tle.

4. An electric kettle according to claim 3, in which the tube is connected to the element, the sensor is movable between operative and inoperative positions in which it is, and is not, in a heat-transmitting path from the tube, respectively, and the sensor is so linked to a snap action device of the mechanism that the sensor is moved from the operative position to the inoperative position when the device is actuated by the sensor to open the contacts.

5. An electric kettle according to claim 4, in which the snap action device is a snap action switch which includes the contacts and to which the sensor is coupled by a push rod.

6. An electric kettle according to claim 4, in which the sensor is connected to a push rod extending through the tube and arranged to open the contacts when moved sufficiently by the sensor, and the snap action device is coupled to the rod.

7. An electric kettle according to claim 6, in which the snap action device comprises a spring urged slider engaged by a detent on the rod, the slider being normally held by the detent, but driving the rod into contact-opening position when released by the detent on movement of the rod under the action of the sensor.

8. An electric kettle according to any one of claims 5 to 7, in which the sensor is coupled to the rod by a lost-motion device permitting limited actuating movement of the rod before the sensor is moved towards its inoperative position.

9. An electric kettle according to claim 8, in which the sensor is located between an end of the rod and a shoe which is carried from the rod by the lost motion device, and which is in good heat conducting contact with tube, until actuation of the mechanism.

1 0. An electric kettle according to any one of the preceding claims, in which the sensor is in the form of a helical compression spring of an SME alloy.

11. An electric kettle according to any one of the preceding claims, in which the SME material is an alloy of copper, zinc, aluminium, and manganese.

1 2. An electric kettle substantially as herein described with reference to the accompanying drawings.

1 3. A boil-dry cut-out switch mechanism for an electric kettle according to any one of the preceding claims, comprising a snap-action switch device, a temperature-sensor of SME material connected to the switch device by a link such that the switch device is operated by the sensor but when so operated moves the sensor from a heat-receiving operative position to a heat-isolated inoperative position.

14. A boil-dry cut out switch mechanism for an electric kettle substantially as herein described with reference to the accompanying drawings.