WO2001006334A1

WO2001006334A1 - Improvements relating to water heating vessels

Info

Publication number: WO2001006334A1
Application number: PCT/GB2000/002814
Authority: WO
Inventors: Robert Andrew O'neill; Robin Keith Moore
Original assignee: Otter Controls Limited
Priority date: 1999-07-21
Filing date: 2000-07-20
Publication date: 2001-01-25
Also published as: CN100373280C; AU6005700A; EP1198741A1; CN1373866A

Abstract

A water boiling vessel has a thick film heating element (3) provided with a bimetallic overtemperature protection control for switching off the heating element if it overheats, for example because the vessel is switched on empty and, to enable the vessel selectively to be operated to produce hot water at a selected temperature lower than boiling, an auxiliary heater (11) is provided on the thick film heating element in the region thereof to which the bimetallic control is responsive and a resistive track is connected in series with the auxiliary heater and is contactable selectively along its length so as to vary its effective resistance and thereby vary the current through the auxiliary heater. The auxiliary heater increases the heating of the bimetallic control above that normally received in operation of the vessel so that it operates at a lower temperature than it normally would, thereby switching off the vessel prematurely before it had time to boil. Also disclosed is an integrated 360° cordless appliance inlet connector and dry boil control which includes a means to connect selectively to the resistive track.

Description

IMPROVEMENTS RELATING TO WATER HEATING VESSELS

Field of the Invention:

This invention concerns improvements relating to electrically powered

water heating vessels such as domestic kettles and hot water jugs for example,

and, more particularly, concerns arrangements whereby such vessels may be

switched off or their power supply reduced at a water temperature below

boiling.

Background of the Invention:

Steam controls which operate to switch off or reduce the power supply

to the electric heating element of a water boiling vessel when water boils in

the vessel are well known, and commonly operate by responding to the

generation of steam when water boils in the vessel, the stem venting from the

vessel interior into a region of the vessel whereat a thermally-sensitive switch

actuator, a bimetal or shape memory effect device for example, is located.

Controls are also well known which are intended to protect the heating

element of a water boiling vessel from overheating, for example as a result of

the vessel being switched on empty or being allowed to boil dry, and

commonly comprise a bimetallic switch actuator which is held in close

thermal contact with the heating element and is calibrated to operate at a

certain overtemperature.

It has also been proposed to provide a water heating vessel with means

whereby hot water can be provided at a temperature lower than boiling, for example for use m making coffee Electronic control arrangements have been

proposed which enable the temperature of the hot water produced by a water

heating appliance to be selected by the user, and bimetallic arrangements are

known which operate similarly to an adjustable thermostat, in that the

operating temperature of the bimetal is controllable by the user, or which

operate by varying the heat transfer relationship between the heating element

and the bimetal, for example by moving the bimetal relative to the heating

element

The bimetals that are employed m controls for water heating vessels

are nowadays commonly of a stressed, snap-acting type m view of the fact that

such bimetals have well defined characteπstics which are stable over a peπod

of time So-called creep acting bimetals were previously employed but are not

nowadays widely used, mamly on account of their relative lack of stable

operating characteπstics Creep acting bimetals, which exhibit a progressive

deflection or deformation as a function of increasing temperature, as opposed

to the snap-acting type which exhibits a change from one stable state to

another at a predetermined temperature, are however useful in applications

where the operating temperature of a control is required to be adjustable It is

however, disadvantageous to use a creep-acting bimetal to operate switch

contacts directly since the relatively slow movement of the bimetal (as

compared to a snap-actmg bimetal) will give πse to arcmg at the switch

contacts with disadvantageous consequences and for this reason, where creep-acting bimetals are proposed to be used in present day controls, it would

be good design practice to isolate the bimetal from the switch contacts of the

control by means of a snap-acting mechanism, an overcentre lever for

example, which translates the slow movement of the bimetal into a

snap-action at the switch contacts. For obtaining temperature adjustability,

the mechanical relationship between the creep bimetal and the snap-acting

mechanism can be adjustable.

One such arrangement as abovedescribed is proposed in Strix

Limited's GB-A-2 329 523. A standard U18 control as presently available

from Strix has a creep bimetal riveted to the metal carrier plate of the control

and the creep bimetal is arranged to interact with the trip lever of the control

by way of a manually operable selector which provides for adjustability of the

mechanical relationship between the creep bimetal and the trip lever. The

arrangement of GB-A-2 329 523 provides the U18 control with the additional

facility of an adjustable means for switching off the power supply to a heating

element at a temperature below boiling which is selectable, in dependence

upon the calibration and adjustment of the creep bimetal, by operation of the

manually operable selector.

For the reasons abovementioned, creep bimetals are, in our considered

opinion, an undesirable component for inclusion in a control for a water

boiling appliance. It is in the nature of creep acting bimetals that some form

of initial temperature calibration is required during manufacture, and it is notable that the embodiment described in GB-A-2 329 523 includes set screws

which have to be adjusted to determine the action of the creep bimetal. This

adds to the cost and renders the control susceptible to variation of its operating

temperature as the calibration and adjustment changes as the control ages.

Objects and Summary of the Invention:

It is thus the principal object of the present invention to provide for

adjustability of the water temperature at which a control will operate to switch

off or reduce the power supply of the heating element of a water boiling vessel

without evoking the disadvantages that are inherent in the arrangement that is

described in GB-A-2 329 523 abovementioned.

Whilst the arrangement of GB-A-2 329 523 is not restricted in its

application to any particular kind of electric heating element, its greatest

application at the present time is to electric kettles and hot water jugs utilizing

so-called thick film heating elements. A thick film heating element comprises

a generally planar substrate, commonly but not essentially formed of metal,

such as stainless steel for example, which carries a printed or otherwise

formed heating track or layer on at least one side thereof, electrically

insulating layers being provided as necessary between the heating track or

layer and the substrate and overlying the heating track or layer. In accordance

with the present invention, in one of its aspects, we propose to make use of

the properties inherent in thick film heating elements to achieve adjustability

of the operating temperature of a thermally sensitive control associated with a thick film heating element. More particularly, we propose to enable the

thermal output of a predetermined part of a thick film heating element which

is to have a thermal sensor associated therewith to be selectively adjusted,

whereby the performance characteristics of the sensor in relation to the

volume of water being heated will change correspondingly.

According to the present invention, in one of its aspects, therefore, an

electrically powered water heating vessel has a thick film heating element, a

thermally-responsive control is located in thermal contact with a

predetermined part of the thick film heating element, and means are provided

for enabling the thermal output of that part of the thick film heating element to

be selectively adjusted.

Thick film heating elements are commonly manufactured by printing

with conductive ink onto the surface of a substrate formed of stainless steel

and provided with one or more insulating layers, commonly of glass. On such

a printed element the track temperature is related to the water temperature,

normally exceeding it by an amount which depends on the power density of

the track and on the thermal conductivity of the substrate and dielectric. In the

case of our X4 control described in our GB-A-2 339 088 on a 2.5 to 3kW

element the track temperature will be about 160°C, that is 60°C above the

boiling water temperature. In order to avoid nuisance tripping of the control

during boiling the control bimetal is normally set at about 180°C. If, on the

track portion which heats the bimetal, the heat output is increased, then the bimetal can be made to nuisance trip at a water temperature below boiling

point. If the amount of additional power applied to that part of the track were

arranged to be controlled, then the temperature at which the control "nuisance

tripped" could be controlled to give water at a controlled temperature below

boiling. In application to a cordless vessel, because of the latch out function

of the X44 (latched) variant, the control would remain off until the associated

cordless vessel was lifted off its base and reset. On the X46 (auto cycling) or

X48 (auto cycling with thermal fuse) variants, the control would cycle to

maintain the water at the preselected temperature. Of course, the present

invention is not limited in its application to our X4 series of controls.

The means by which the heat output of the track beneath the bimetal

may be adjusted could take a number of forms, but the preferred means is to

add an auxiliary track beside the main heater track at the location where the

bimetal senses the heater temperature. The current through the auxiliary track

could be controlled by connecting it via a variable resistance to the power

supply. It is anticipated that a power of around 20W would be necessary to

give a range of about 20°C depression of switch off point. This could be done

by provision of a variable resistor separate from the heating element and

having a control knob accessible to the user of the appliance, but such a

resistor would be costly and would dissipate a considerable amount of heat,

making it inefficient. The control could be electronic, since the control of a

power of 20W is well within the capability of low cost components. However the preferred option is to form the vaπable resistor on the thick film heating

element m the form of a resistive track pπnted on the element, and to provide

an arm carrying a brush or sliding contact arranged to make electπcal

connection to the resistive track at any selected position along the track or to

any selected one of a plurality of contacts connected to predetermmed track

locations which preferably are spaced apart from each other unequally to

provide a linear change of power to the auxihary track as the brush or sliding

contact is moved, the arrangement thus increasing or decreasing the amount of

resistive track included in circuit with the auxiliary heater There could also

be a "boil" position where the sliding contact made no contact with the

resistive track, providing a setting where the auxiliary heater track has no

effect and the vessel boils water normally Such an arrangement would

however mean that with the sliding contact in its "boil" position, no heat

would be generated by the auxiliary track and thus no contπbution to the

overall element power would be available from that area of the heater This is

inefficient use of the available space on the heating element and would lead to

an unnecessaπly large and costly heater, and to avoid this it is preferred in

accordance with the present invention to arrange that the "boil" position

places the entire resistive track in circuit and that the power density in the

auxiliary track is the same as in the adjacent main power track so that both

operate at the same temperature The techniques of pπnting tracks for

vaπable resistors with sliding contacts are well known and would be directly applicable to this construction. The resistive track could be an arc preferably

concentπc with the element substrate disc (and/or the appliance), and the

adjustment arm could project from a slot in the wall of the vessel body and be

arranged to slide around the body. Alternatively the arc could be formed as a

small diameter arc of more than 180°, as would be normal in a conventional

potentiometer, in which case the sliding arm could be pivoted from the centre

of the arc and a lever could be connected to the sliding arm and project from

the side of the appliance

In the above embodiment which contemplates the use of an X4 control

in the practice of the invention, the bimetal actuator of the control would also

continue to function as a dry boil protector. However if a more flexible lower

power solution is desired, then a thermal sensor separate from those provided

on the X4 control could be provided. This sensor should have a low thermal

capacity to reduce the power required from the auxiliary heater track It could

take the form of a PTC sensor, with an electronic control system, or preferably

it could be a small contactstat. Such a contactstat would require a smaller (m

area) heater track to make it function and, since the temperature of the heater

depends on its power density (not the absolute power), then a small track will

only require a small amount of power. The temperature setting of the

contactstat would be approximately 105°C to avoid nuisance tripping duπng

boiling, and the auxiliary heater track could be arranged to be adjustable to

provide between 0° and 65°C temperature πse to provide water selectively at temperatures from 40°C for heating babies bottles up to boiling. A further

advantage of this approach is that the auxiliary heater track would normally be

run at very low or no power, which would limit the amount of scale build up.

Scale build up over the main bimetal heater area results in a rise in running

temperature, which could affect the switch off point of a temperature control

based on the main bimetal. This is, after all, the reason why the bimetal is set

higher than the running temperature to avoid nuisance tripping.

A further aspect of the invention, useful to maintain water at a preset

temperature, for example simmering, or for maintaining a water bath at a set

low temperature, makes use of the principle disclosed in our GB-A-2 248 144

which describes how a temperature may be controlled by the reset temperature

of an automatically resetting bimetal control by ensuring that the bimetal

differential is less than the self heating caused by the current flow through the

bimetal. In the case of the present invention, the heating is proposed to be

supplied not by through current but by the auxiliary heater track, which raises

the bimetal through its differential until it switches off. The track then rapidly

falls to the water temperature and, if this is below the remake temperature of

the bimetal, then the control switches the heater back on and repeats the cycle.

This cycling action continues, with the water being heated intermittently, until

the water reaches the bimetal remake temperature, whereafter the bimetal will

cycle slowly to maintain the heater and the water at its remake temperature

just as in GB-A-2 248 144. To take best advantage of this effect it would be desirable to have a variable remake bimetal, that is to say an adjustable

bimetal, with a fixed auxiliary track power, but a variable (adjustable) remake

bimetal could be used with an adjustable auxiliary track power, in which case

one of these adjustments, possibly the power of the auxiliary heating power,

might be factory set and the other enabled to be determined by the user of the

appliance by provision of an appropriate control on the appliance.

Adjustability of the remake temperature of a bimetal, particularly a dished,

snap-acting bimetal as best contemplated for use in the practice of the present

invention, is readily provided for example by provision of an adjustable

back-stop for the bimetal and/or by provision of adjustability of the stressing

of the bimetal.

In the practice of this aspect of the invention, the controlled

temperature would be unaffected by any scale build up, since at the controlled

temperature there is no temperature drop across the dielectric/substrate/scale

sandwich. It should be noted in connection with the previously mentioned

example of the cycling X46, that if the normal remake temperature (which is

above 100°C) of such a control is used in the practice of this further aspect of

the invention, then the water temperature would tend to rise steadily until it

boiled. For this reason the alternative contactstat version with its lower

setting and remake below 100°C, is to be preferred where a controlled

maintained water temperature is desired. In accordance with this further aspect of the invention, therefore, the

thick film heating element of an electrically powered water heating vessel has

a predetermined part thereof provided with an auxiliary heater and a

snap-acting bimetallic actuator is arranged in thermal contact with such part of

the thick film heating element so that, in use, its operation is dependent upon

the heat output of said auxiliary heater, said snap-acting bimetallic actuator

being adjustable as regards it remake temperature.

Switching off the appliance on boil can be done by any of the known

ways, such as by use of our Jl or Z5 controls described in GB-A-2 212 664

and GB-A-2 331 848 respectively, or by means of the concept described in

GB-A-2 265 070, or by sensing the rate of rise of the water temperature as in

our electronic kettle control described in GB-A-2 228 634.

According to yet another optional feature of the invention a thick film

heating element provided with an auxiliary heating track at a location whereat

a thermal control, for example a bimetallic control, is to be juxtaposed with

the heating element and with means enabling the heat output of the auxiliary

track to be adjusted, all as aforesaid, additionally has a keep warm track and

means enabling the heat output of the keep warm track to be adjusted. The

means enabling the heat outputs of the auxiliary and keep warm tracks are

preferably commoned so that one and the same manual operation can perform

both functions and preferably comprise respective variable resistance elements

juxtaposed with each other and arranged to be adjusted by means of one and the same slider control, and the slider control preferably has an off position

where no contact is made with either variable resistance element so that

neither the auxiliary heater nor the keep warm heater is operative.

A water heating vessel provided with such a heating element and with

an X4 element protector control monitoring the condition of the heating

element, with one of its bimetallic actuators located to be responsive to the

heat developed by the main heating element track and by the auxiliary track

and the other located to be responsive to the heat developed by the main

heating element track and by the keep warm track, can have an on/off control

constituted by the reset switch of a boil sensing control such as our

aforementioned Jl or Z5 steam controls and a further control constituted by

the abovementioned slider or, more preferably, can have the two such controls

commoned together. Such a water heating vessel can, with the tracks and

bimetals appropriately configured, demonstrate different modes of operation,

namely a boil mode wherein the slider control is in its off position so that

neither the auxiliary heater track nor the keep warm track is operative and the

vessel is controlled by operation of the boil sensing control, a boil and keep

warm mode wherein the slider control is positioned so that the keep warm

track provides its maximum power output and the auxiliary track its minimum

whereby the vessel contents are initially heated to boiling by operation of the

main heater track which is then switched off by the Z5 steam control

whereafter the keep warm track keeps the vessel contacts close to boiling, and a heat and keep warm mode wherein the slider control is positioned at an

intermediate position so that the auxiliary heater operates to switch off the

power supply to the main heating element track when the vessel contents are

at some selected sub-boiling temperature and the keep warm track then

maintains the vessel contents at that selected temperature. By selection of the

appropriate mode, the user of such a water heating vessel can obtain boiling

water, or water that has boiled and which has been maintained close to

boiling, or water that has just been heated to some selected intermediate

temperature, and the mode selection is simple and uncomplicated.

The above and further features of the present invention are set forth in

the appended claims and will be further explained by reference to the

following detailed description of exemplary embodiments which are

illustrated in the accompanying drawings.

Description of the Drawings:

Figure 1 illustrates in plan view an exemplary track layout of a thick

film heating element for an electrically powered kettle or hot water jug

embodying the present invention.

Figure 2 is a schematic circuit diagram showing a thick film heating

element, for example as in Figure 1, in a water heating vessel provided with

dual dry-boil protection and with a steam control;

Figure 3 is a plan view of an alternative thick film heating element

embodying the present invention; Figures 4A, 4B, 4C and 4D show how a modified form of our X4

control interfaces with the heating element of Figure 3, Figure 4A illustrating

the areas of the heating element that are required to register with specific parts

of the control, Figure 4B showing a plan view of the modified X4 control

assembled with the heating element, Figure 4C showing an enlarged

perspective view illustrating the modification of the X4 control and Figure 4D

being a scrap cross-sectional view of the modified part of the control,

Figures 5 and 6 are plan views of yet further alternative thick film

heating elements embodying the present invention,

Figure 7 is a plan view of yet another thick film heating element

embodying the present invention which additionally includes a controllable

keep warm track, and

Figure 8 is a circuit diagram illustrating the use of a thick film heating

element according to Figure 7 with a control substantially as shown in Figures

4 A, 4B, 4C and 4D

Detailed Descnption of the Embodiments

Referring to Figure 1 , a circular thick film heating element is shown in

plan view as compπsing a stainless steel disk 1 upon which there is formed an

electπcally insulating layer 2 upon which a conductive ink pπnted heating

element proper 3 is provided The heating element proper 3 is made up of

heater track sections 4 which are joined by conductive bπdges 5 at their

extremities to avoid current crowding where the track changes direction The heating element shown is adapted for use with an X4 control having two

generally similar bimetallic switch actuators which, when the control is

affixed to the heating element by spot welding of metal mounting feet of the

control to the stainless steel disk 1 at the three locations 6, register with the

heating element m good heat transfer relationship therewith generally in the

regions 7 and 8 thereof The main power supply terminals to the heating

element compπse conductive pads 9 and 10 which are arranged to be

contacted by spπng terminals of the X4 control

In accordance with the teachings of the present invention, an auxiliary

heater track 11 is provided in the region 7 of the heating element which

co-operates with one of the bimetals of the X4 control and, coupled thereto by

means of a conductive section 12, is a resistive track section 13 which, in use,

will serve to enable the thermal output of auxiliary heater track 11 to be

adjusted The auxiliary heater track 11 is coupled at one end to mam heater

terminal 9 and resistive track section 13 has associated therewith a

schematically illustrated wiper 14 which is connected to that pole of the power

supply which supplies terminal 10 of the main heating element 3 In use, the

seπes-connected resistive track section 13 and auxiliary heater track 1 1 are

connected in parallel with the mam heating element track 3 and the position of

the wiper 14 along the resistive track section determines the heat output of the

auxiliary heater 1 1 and thus, as hereinbefore described, determines the operation of that bimetallic actuator of the X4 control that registers with

region 7 of the thick film heating element

The position of wiper 14 on the resistive track section 13 is made

controllable by the user of the vessel and preferably, to enable the effect of the

auxiliary heater 11 to be switched off when the vessel is to be used for boiling

water, has a control position whereat it makes no contact with the resistive

track section 13 The mechanical arrangement of the wiper 14 and/or its slider

knob is preferably such that contact between the resistive track section 14 and

the wiper 14 is made and broken with a snap action so as to minimize arcing,

and the respective part of the resistive track section, which will preferably be

provided at the high resistance end thereof, may be configured to withstand

such arcing as might occur

Figure 2 shows the thick film heating element of Figure 1 in an

electrically heated water boiling vessel provided with a dual-protection dry

boil control such as our aforementioned X4 control and with a steam control

such as our aforementioned Jl or Z5 controls In the figure, the two dry boil

controls are designated 21 and 22 and are designed to register with the regions

7 and 8 of the thick film heating element, and the steam control is designated

23 The control position of wiper 14 where it does not make contact with

slider track 13 is designated 24, this position of the wiper serving to isolate

auxiliary heater track 1 1 so that the vessel operates as a water boiling vessel Figure 3 shows an alternative thick film track layout designed

specifically for use with a modified form of our X4 control which is described

hereinafter. Employing the same reference numerals as were used in relation

to Figure 1, the thick film heating element of Figure 3 has regions 7 and 8

designed to register with the bimetals of the X4 control and the region 7 has

associated therewith an auxiliary heater track 11 which is connected to a track

section 13 which is designed to co-operate with a control slider 14 to

constitute a variable resistance enabling the thermal output of the auxiliary

track 11 to be controlled by the vessel user.

Figures 4A to 4D illustrate the attachment of a modified X4 control to

a thick film heating element as shown in Figure 3. Figure 4A merely shows

the regions of the heating element which are required to register with specific

parts of the control, namely the regions 6 whereat the feet of the metal chassis

of the control are affixed to the metal substrate 1 of the heating element, the

regions 7 and 8 whereat the bimetals of the X4 control thermally contact the

heating element track, the terminal regions 9 and 10 whereat the spring

terminals of the X4 control make electrical contact with the heating element

track, and the variable resistance track section 13 adapted to be contacted by a

control wiper of the X4 control.

Referring to Figure 4B, this shows a modified X4 control 40 affixed to

the heating element. As is known, and as described in GB-A-2 339 088

aforementioned, the X4 control comprises a cordless vessel inlet connector 41 of the 360° type pioneered by us which enables a cordless vessel to be set

down upon its base in any relative rotational orientation and, integrated

therewith, first and second heating element protector controls generally

designated 42 and 43 which can be bimetallic or a combination of one

bimetallic and one fusible material as described in our abovementioned

application. Provision is made at location 44 of the control for the attachment

of a Z5 steam control to the X4 control.

Figures 4B, 4C and 4D show the modification to the X4 control. The

metal chassis of the control has an extended part 51, which has a central pivot

hole 52 at the centre of the arc of an arc shaped section 53 with end stops 54.

A plastics moulding 55 has a pivot peg 56 and an overlapping arm 57 and is

designed to snap into place as shown, so that the pivot peg 56 locates in the

pivot hole 52 of the chassis extension. The overlapping arm 57 fits under the

chassis extension to prevent the moulding coming off its pivot and acts as a

guide as the moulding is rotated. At the outer end of the moulding is a box

section 58 with a snap fitting lid 59. A carbon brush assembly 60 fits into the

box section, with the brush projecting towards the element surface. The braid

pigtail of the brush assembly has a quick connect receptacle attached which is

fitted to the tab 61 of the X4. This tab is connected to the neutral supply point

of the element via the neutral side dry boil thermal control, as shown in the

circuit diagram of Figure 2. As is also seen from the circuit diagram of Figure 2, the boil control,

which may be either directly plugged into the X4 or remotely mounted (as is

the case in the Figures, plugging into the receptacles marked 62 and 63 on the

isometπc view), will act to switch off the power to the element, independently

of the connection of the brush assembly This will ensure that in the event of

a failure of the lower temperature feature, the boil control will always prevent

the water boiling continuously

When the X4 control is mounted to the heating element, the brush

makes contact with the arc shaped slider track 13 The angle of rotation of the

moulding is greater than the angle subtended by the slider track so that at one

extreme of travel the brush is not in contact with the slider track In this

position the auxiliary heating track 12 is not energised and the appliance w l

behave as a normal automatic kettle This is the "Boil Position" in Figure 2

As the brush engages the slider track, current w l flow m the auxiliary track,

causing the live side dry boil blade to operate at a temperature depending on

the power in the auxiliary track, as already descπbed, and varying the position

of the brush on its slider track will vary the power in the auxiliary track, and

hence the water temperature at which the control will switch off

Figures 5 and 6 show yet further alternative thick film track layouts

designed for use with the aforementioned modified X4 control Employing

the same reference numerals as were used in relation to Figures 1 and 3, the

thick film heating elements of Figures 5 and 6 each have regions 7 and 8 designed to register with the bimetals of the X4 control and the region 7 has

associated therewith an auxiliary heater track 11 which is connected to a track

section 13 designed to cooperate with a control slider of the X4 control to

constitute a vaπable resistance enabling the thermal output of the auxiliary

track 11 to be controlled by the user of the vessel

In both of the heating element layouts of Figures 5 and 6 the mam

power track 4 is shown reasonably wide, whereas the temperature control

auxiliary track 1 1 is somewhat narrower, since it is of lower power and thus

higher resistance The arc shaped control track 13 is moved away from the

stroke of the sliding brush contact and has a seπes of silver hnks 14' pπnted

over (or under) it The silver links lead to a seπes of closely spaced silver

pads 15 which he in an arc beneath the path of the brush contact The spacing

of the pads ensures that the brush is always in contact and may bπdge two

pads with little loss of contact area

The silver pads 15 have two purposes Firstly, it is known that making

contact to resistive areas of a thick film pπnted heater can cause problems

from localised overheating, which may burn out the track or the contacting

component Although the current through the track m this case is relatively

low, the mateπal of the track has a much higher resistivity than the mam track

and may cause problems Thus it has been our practice always to provide

silver pads when making spπng contact to a thick film heater The problem

may be worse with the proposed sliding contact when the point of contact is moving and any arcmg could damage the track surface Secondly, it will be

noticed that the segments of the arc shaped track 13 between the silver links

14' are not of equal length This allows a linear change of power to the

auxiliary track as the brush is rotated, rather than the hyperbolic relationship

that would occur with equal length segments or with direct contact with the

arc shaped track This will allow a linear temperature scale on the appliance

A further feature of the design of Figures 5 and 6 is that there is no

"boil" position where the brush is isolated from the arc shaped track Such a

position would mean that no heat is generated in the auxiliary track, and thus

no contπbution to the overall element power would be available from that

area of the heater This is inefficient use of the available space and would

lead to an unnecessaπly large and costly heater In the present layout, m the

"boil" position, which is with the whole arc included, the power density m the

auxiliary track is arranged to be the same as in the adjacent main power track

Thus the auxiliary track will run at the same temperature as the mam boil

track, not causing nuisance tπppmg and contπbuting both to the normal power

output of the element and to providing the necessary dry boil response from

the bimetal blade at that position As the brush is moved to the minimum

included arc length, so the power density in the auxiliary heater track is

increased leading to the bimetal being actuated at progressively low er

temperatures, according to the invention Referring now to Figure 7, shown therein is yet another embodiment

of a thick film heating element according to the present invention which, in

addition to incorporating an auxiliary track and associated arc shaped slider

track enabling the heat output of the auxiliary track to be controlled, also

incorporates a keep warm track and an associated control enabling the heat

output of the keep warm track to be controlled. The control for the keep

warm track is actually commoned with the control for the auxiliary track by

virtue of comprising an arc shaped slider track parallel to and concentric with

the arc shaped slider track that is associated with the auxiliary track and by

virtue of employing the same control to determine the position of respective

carbon brushes associated with the respective arcuate slider tracks. As will be

explained more fully hereafter, this arrangement enables a kettle or hot water

jug, for example, selectively to provide boiling water, or water which has been

boiled and then has its temperature maintained close to boiling, or water

which is heated to a selected temperature lower than boiling and then is

maintained substantially at that temperature. Furthermore the arrangement

hereinafter described enables this to be achieved by use of just one manual

control.

The same reference numerals are used in Figure 7 as were used to

denote like parts in previous embodiments. As shown, the thick film heating

element 1 has arcuate main heater track sections 4 connected at their ends by

conductors 5 so as to avoid cuπent crowding, the main heater track extending between neutral and live terminals 9 and 10 respectively. An auxiliary track

section 1 1 is connected between a tapping point 20 of the main heater track,

selected to apply the required sub-mains voltage to the auxiliary track section,

and an adjustable resistance defined by the track section 13 with its associated

contact points 14' and tapping points 15 to be contacted selectively by a

sliding brush contact. As shown, an end tapping point 15' is provided upon

which the brush contact can be parked out of electrical contact with the track

section 13; when the brush contact is in this position, the auxiliary track 11 is

not powered.

An additional track section 21 is provided in the centre of the heating

element and serves as a keep warm track. As shown, the keep warm track 21

is connected at one end to an additional power supply terminal 22 provided on

the heating element by means of a conductor 23 and, at its other end, is

connected by means of a conductor 24 to a further adjustable resistance 25

configured similarly to and physically in parallel with the adjustable resistance

that is associated with the auxiliary track section 11. The provision of the two

adjustable resistances physically in parallel and closely proximate to each

other enables both to be operated by means of one and the same control slider.

The control that is used with the heating element of Figure 7 is a

variant of the X4 control described hereinbefore with reference to Figures 4A

to 4D. According to this variant, the sliding brush mechanism of the Figure 4

proposal is provided with two separate brushes (or a single broader one) attached to a flexible lead or braid, which in turn is connected to one terminal

of the boil control Z5 by means of the standard X4 tabs and receptacles

provided for the purpose. The variant X4 has a third spring contact 22, in

addition to the standard live and neutral element contacts, which is located

beside the auxiliary track 1 1. Figure 8 shows how the control connects to and

interacts with the heating element.

The control is fitted with a high temperature bimetal blade

(180/190°C) on its neutral side to protect the main and auxiliary tracks. A low

temperature blade (110/120°C) is fitted to the live side to act as a keep warm

thermostat and provide secondary protection. The potentiometer slider has

either one or two contacts (depending on the design, to ensure that both

resistance tracks have good contact pressure), which contact the two variable

tracks, so that as one resistance increases, the other is reduced. At one end of

the travel there is an isolated position, to act as an "Off. The auxiliary track

1 1 is shown being fed from a tapping, part of the way along the main track, to

give a lower voltage which is better suited to the available resistive materials

used to print its heater track. This is simply for convenience, and the auxiliary

track could be fed from the full supply voltage if desired.

The heating element of Figure 7 when combined with the

abovedescribed control enables the following selectable modes of operation,

namely: . Boil: With the slider in the Off position, the kettle may be

switched on by means of the Z5 control, which functions as

usual, and switches the kettle off when the water boils. Since

the slider contacts are not connected to anything, they have no

effect and the auxiliary and keep warm tracks are inoperative.

2. Boil and Keep Warm: With the slider position in the max

position, when the Z5 is switched on the water will come to the

boil and switch off the Z5. The slider contact of the keep warm

track 21 provides the maximum power to that track which is

designed to keep the water close to boiling. The slider contact

of the auxiliary track 11 gives a very low heat to its associated

heater track, which is insufficient to cause the bimetal

(thermostat) associated with it to operate. The water may be

re-boiled by operating the Z5 at any time.

3. Heat and Keep Warm: With the slider positioned between max

and min, and when the Z5 is switched on, the main track will

heat the water until the thermostat bimetal is tripped by the

auxiliary track 1 1. After this the keep warm track 21 will

maintain the water temperature. The two variable resistors are

arranged so that the "keep warm" setting follows the

"Vari-Temp" setting determined by auxiliary track 11 as the

slider is moved, so that the maintained temperature is close to the Van-Temp temperature There wfll clearly be some

vaπation in the temperature given by the keep warm track,

depending on ambient conditions and the amount of water in

the kettle, so the track is preferably targeted always to give a

slightly lower temperature than the Van-Temp track This

means that the thermostat bimetal will cycle occasionally, as

necessary, to switch on the mam heater and return the water

temperature to the required value This avoids the possibility

of the water temperature slowly πsmg m an uncontrolled

manner, and at the same time limits the cycling rate of the

thermostat to increase its fespan

Dry boil protection of the system is provided by the dry boil bimetal of

the X4 which locates m the region 8 of the heating element This will switch

off the main tracks, but leave the keep warm track 21 energised if the slider is

not at the off position The keep warm track 21 is positioned centrally to

ensure that its heat does not cause damage to seals or the vessel wall In the

event of failure of the dry boil bimetal, the other bimetal associated with

auxiliary track 11 will operate, albeit somewhat more slowly because of the

lower track power density Even if the slider is set to off, there will be

sufficient thermal transfer from the main tracks to ensure that this low set

bimetal will operate Should the keep warm track remain energised, it is

probable that it will act to maintain the Van-Temp bimetal in an operated condition, giving a form of manual latch, to be reset by disconnecting the

kettle from the supply. However this action is dependent on the slider

position and is not relied on in the design of the system.

The design of the "user interface" is a matter of choice. Two separate

controls could be provided, one for the boil control, and one for the slider.

However this would still leave the dilemma of having two "Off positions,

and a simple user may not realise that both controls must be off to completely

switch off the appliance. A better scheme is to have a single, sliding, control,

which has three states: Off, where both the Z5 and the sliders are in their

"Off positions; a Boil position, where the Z5 is switched on, and can return

the control of the "Off state, the sliders remaining in their "Off position; and

a Simmer range of positions, where the Z5 is on, and can have no effect on the

sliding control, and the sliders are between Max and Min. The status of the

appliance would be indicated by indicating lamps. Such a mechanism would

be within the capability of a reasonably skilled mechanism designer, and

might make use of lost motion and Geneva wheel type functions.

The invention having been described herein by reference to specific

arrangements and an exemplary embodiment, it is to be understood that

modifications and variations thereto would be possible without departure from

the scope of the invention. For example, in accordance with the teachings of

GB-A-2 340 367, the main track portions 3 of the thick film heating element 1

could be formed to exhibit a PTC (positive temperature coefficient of resistance) characteristic whereas the auxiliary heater 11 could be formed with

an NTC characteristic for the purpose of compensating for supply voltage

variations. Furthermore, whereas in the foregoing the auxiliary heater is

formed as an integral part of a thick film heating element, the invention is

applicable to the control of other kinds of heating elements by means of

bimetallic actuators. For example, an underfloor heating element of the metal

sheathed, mineral insulated, resistance wire type could be controlled in

accordance with the teachings of the present invention by means of a

bimetallic control by provision of a separate auxiliary heater, for example a

small thick film heater, in close thermal contact with the bimetal and by

provision of a variable resistance to control the auxiliary heater output. The

metal mounting plate of an X4 control (item 27 in Fig ID of GB-A-2 339 088

for example) could even be provided with a small thick film heater on a part

thereof extending under one of the bimetals for this purpose.

Claims

Claims:

1. In or for an electrically powered water heating vessel, a thick film

heating element having a predetermined part thereof adapted to be

juxtaposed with the bimetallic actuator of a thermally-responsive

control, and means enabling the thermal output of that part of the thick

film heating element to be selectively adjusted.

2. A heating element as claimed in claim 1 wherein said part of the

heating element has an auxiliary heater track formed thereon, and said

adjustment means is such as to enable the current through the auxiliary

heater track to be selectively adjusted.

3. A heating element as claimed in claim 2 wherein said adjustment

means comprises an adjustable resistance including a resistive track

section formed on the thick film heater.

4. A heating element as claimed in claim 3 wherein said resistive track

section has an associated plurality of contact portions coupled thereto

at spaced apart locations.

A heating element as claimed m claim 4 wherein said locations are

unequally spaced apart so as to enable the output of the auxiliary

heater track to be linearly adjusted

An electπcally powered water heating vessel including a thick film

heating element, a thermally responsive control for protecting the

heating element against overheating, said control including a

bimetallic actuator juxtaposed with the thick film heating element, an

auxiliary heater associated with said bimetallic actuator, and means

enabling the heat output of said auxiliary heater to be adjusted

A water heating vessel as claimed in claim 6 wherein the bimetalhc

actuator is juxtaposed with a predetermined part of the thick film

heating element which is provided with an auxiliary heater track

constituting said auxiliary heater

A water heating vessel as claimed in claim 7 wherein said adjustment

means compnses an adjustable resistor connected in seπes with said

auxiliary heater track, said adjustable resistor compπsing a resistive

track section formed on the thick film heating element and means

enabling the operatι\ e length of said resistive track section to be

adjusted

9. A water heating vessel as claimed in claim 8 wherein a movable wiper

is associated with the resistive track section for enabling the operative

length thereof to be adjusted.

10. A water heating vessel as claimed in claim 9 wherein the wiper

cooperates with a plurality of contacts coupled to respective locations

of said resistive track section.

11. A water heating vessel as claimed in claim 10 wherein said locations

are unequally spaced apart whereby adjustment of the wiper linearly

adjusts the output of the auxiliary heater track.

12. A water heating vessel as claimed in claim 9 or 10 or 1 1 wherein the

wiper is additionally selectively movable to a position whereat it

makes no contact with the resistive track thereby disconnecting said

auxiliary heater.

13. A water heating vessel as claimed in claim 9 or 10 or 1 1 wherein the

power output of the auxiliary heater track matches that of the main

heater track when the maximum length of resistive track section is

selected.

14. An electrically powered water heating vessel including a heating

element, a thermally-responsive control associated with the heating

element for protecting the same against overheating, an auxiliary

heater associated with said control, and means enabling the heat output

of said auxiliary heater to be selectively adjusted thereby to adjust the

vessel temperature at which said control will operate.

15. A water heating vessel as claimed in claim 14 wherein said control

comprises a snap-acting bimetallic element and means are provided

enabling the operating characteristics of said bimetallic element to be

adjusted.

16. A water heating vessel as claimed in claim 15 wherein the heating

element is a thick film heating element, the auxiliary heater comprises

a track formed on said thick film heating element, and for adjusting the

heat output of said auxiliary heater, a resistive track is provided on the

thick film heating element in series with the auxiliary heater and a

wiper is associated with said resistive track for adjusting the resistance

thereof by adjusting the effective length thereof.

17. A water heating vessel as claimed in claim 16 wherein the wiper

cooperates with a plurality of contacts connected to respective

locations of said resistive track section.

18. A water heating vessel as claimed in claim 17 wherein said locations

are unequally spaced apart whereby linear adjustment of the wiper

linearly adjusts the output of the auxiliary heater.

19. A water heating vessel as claimed in claim 16 or 17 or 18 wherein the

power output of the auxiliary heater track matches that of the main

heater track when the maximum length of resistive track section is

selected.

20. An electrically powered water heating vessel including a thick film

heating element having associated therewith a temperature-sensitive

control comprising an automatically resetting snap-acting bimetallic

actuator in thermal contact with the heating element, an auxiliary

heater being associated with the bimetallic actuator for causing the

same to snap from its cold to its hot configuration in a relatively short

time period, and the remake temperature setting of the bimetallic

actuator being adjustable, the aπangement being such as to enable water to be heated in the vessel to the remake temperature of the

bimetallic actuator and maintained at that temperature.

21. A water heating vessel as claimed in claim 20 wherein the auxiliary

heater comprises a track formed on the thick film heating element.

22. A water heating vessel as claimed in claim 20 or 21 wherein the heat

output of the auxiliary heater is aπanged to be adjustable.

23. A water heating vessel as claimed in claim 22 as dependent upon

claim 22 wherein an adjustable resistance is provided on the thick film

heating element in series with the auxiliary heater.

24. A water heating vessel as claimed in claim 23 wherein said adjustable

resistance is aπanged so that adjustment thereof enables the output of

the auxiliary heater to be controlled linearly.

25. A water heating vessel as claimed in claim 23 or 24 wherein the heat

output of the auxiliary heater at one adjustment of the adjustable

resistance matches that of the main heater track of the thick film

heating element.

26. A thick film heating element as claimed in any of claims 2 to 5, or a

water heating vessel incorporating such a thick film heating element,

or a water heating vessel as claimed in any of claims 6 to 13, 16 to 19

and 20 to 25, wherein the major part of the thick film heating element

has a PTC characteristic and the auxiliary heater has an NTC

characteristic.

27. A thick film heating element as claimed in any of claims 2 to 5 and 26,

or a water heating vessel incorporating such a thick film heating

element, or a water heating vessel as claimed in any of claims 6 to 13,

16 to 19 and 20 to 26, wherein the heating element further comprises a

"keep warm" track portion.

28. A thick film heating element as claimed in claim 27 wherein means

are provided for enabling the power output of said "keep warm" track

portion to be adjusted.

29. A thick film heating element as claimed in claim 28 wherein said

adjustment means comprises an adjustable resistance including a

resistive track section formed on the thick film heater.

30. A thick film heating element as claimed in claim 29 wherein said

resistive track section has an associated plurality of contact portions

coupled thereto at spaced apart locations.

31. A thick film heating element as claimed in claim 28 or 29 or 30

wherein the adjustment means of the "keep warm" track portion is

adjacent to a corresponding adjustment means for the auxiliary heater

track whereby both adjustment means may be adjusted together.

32. A thick film heating element as claimed in claim 31 wherein

adjustment of the power output of the auxiliary heater track such as to

increase the same is effected commonly with adjustment of the power

output of the "keep warm" track portion, and vice versa.

33. A thick film heating element comprising a main heater track portion,

an auxiliary heater track portion located at a position whereat a

thermal control is to be juxtaposed with the heating element, a "keep

warm" track portion, a first adjustable track portion connected to the

auxiliary heater track and configured to enable its resistance to be

selectively adjusted for selectively adjusting the power output of the

auxiliary heater track, and a second adjustable track portion connected

to the "keep warm" track portion and configured to enable its resistance to be selectively adjusted for selectively adjusting the power

output of the "keep warm" track.

34. A thick film heating element as claimed in claim 33 wherein said first

and second adjustable track portions extend alongside one another so

as to be adjustable simultaneously by one and the same manual

control.

35. A thick film heating element as claimed in claim 34 wherein the first

and second track sections are simultaneously adjustable in opposite

senses so that as the resistance of one increases, so the resistance of

the other reduces.

36. A thick film heating element as claimed in any of claims 33 to 35 in

combination with a thermally-responsive control comprising a first

bimetallic actuator juxtaposed with the heating element at said

position thereof, a second bimetallic actuator juxtaposed with the

heating element so as to be responsive to the temperature of said "keep

warm" track portion, and adjustment means enabling said first and

second adjustable track portions to be manually adjusted.

37. A liquid heating vessel incorporating a heating element and

thermally-responsive control as claimed in claim 36.

38. A liquid heating vessel as claimed in claim 37 further comprising a

boil control responsive to the boiling of liquid in the vessel to

switch-off the power supply to the main heater track portion.

39. A thermally responsive control for a water heating vessel, said control

including a bimetallic actuator arranged so as in use of the control to

be responsive to the temperature of a heating element so as to protect

the same against overheating, and wherein the control further

comprises a selectively operable member enabling the heating of said

bimetallic actuator to be controlled for adjusting the heating element

temperature at which the control will operate.

40. A thermally responsive control as claimed in claim 39 which is

adapted for use with a thick film heating element having an auxiliary

heater located to interact with said bimetallic actuator and wherein

said selectively operable member comprises part at least of an

adjustable resistor for determining the current in said auxiliary heater.

A thermally responsive control as claimed in claim 40 which is

adapted for use with a thick film heating element further having a

resistive track coupled in senes with the auxiliary heater and wherein

said selectively operable member compnses a slider mechanism for

co-operation with said resistive track to vary the effective resistance

thereof

A thermally responsive control as claimed m claim 39 or 40 or 41

wherein the control compnses first and second bimetallic actuators and

only one of said actuators is arranged to be effected by said selectively

operable member

A thermally responsive control as claimed in claim 39 or 40 or 41

further compπsing a fusible component actuator

A thermally responsive control as claimed in any of claims 39 to 43

which is integrated with an appliance mlet connector

A thermally responsive control as claimed in claim 44 wherein the

appliance inlet connector is for a cordless appliance and enables the

appliance proper to be set onto its base with any relative rotational

oπentation

46. A thick film heating element substantially as herein described with

reference to any of Figures 1, 3, 5, 6 and 7 of the accompanying

drawings.

47. A thermally responsive control substantially as herein described with

reference to Figures 4B, 4C and 4D of the accompanying drawings.

48. A thermally responsive control as claimed in claim 47 in combination

with a thick film heating element as claimed in claim 46, in or for a

water heating vessel.

49. An electrically powered water heating vessel wherein the electrical

circuit of the vessel heating element and its associated controls is

substantially as herein described with reference to Figure 2 of the

accompanying drawings.

50. An electrically powered water heating vessel wherein the electrical

circuit of the vessel heating element and its associated controls is

substantially as herein described with reference to Figure 8 of the

accompanying drawings.