CN1135060C - Electric heaters - Google Patents

Abstract

The desired power rating of a thick film heater is obtained by making electrical connections to the heater track at positions which define the appropriate resistance therebetween.

Description

Heater

This invention relates to electric heaters, particularly of the plate element type for use in appliances such as water heaters, water heaters and irons, among other applications.

A flat panel heater consists of an insulating substrate on which is mounted a resistive heating track. Typically, a glass, ceramic or glass-ceramic insulation is mounted on a metal base, such as a metal plate, on which rails are laid. The above can be achieved by various methods, such as bonding a thin layer of resistive material on the substrate, or using the more common printed circuit technology, laying a layer of resistive ink on the substrate. Spraying is also one of the commonly used methods. Using any of these techniques, the result is a heater in the form of a planar heater which may conveniently be formed as the base of the heating vessel, or be formed on the base, or be bonded to the base.

Resistive heating tracks can also be laid on non-planar substrates, such as tubes or other three-dimensional objects.

EP-A-0574310 discloses an example of a water heater having a plastic body and a printed circuit type panel heater as a base. Electrical contacts are made at the end of each spiral track, connected to the power supply by wires attached to the track.

However, it is very difficult to produce printed circuit heaters with the same resistance value and thus the same power output. The difference in the thickness of the printed track, the difference in the nominal resistivity of the ink, and the change in time and temperature during the process will result in a difference in the resistance of the final track, so the power of the heater is also different. In practice, manufacturers are doing well to keep power within ±15% of nominal. However, international electrical standards dictate that power be maintained between +5% and -10% of nominal. Obviously, if you just suppress the part that is out of tolerance, as some manufacturers do, then the reject rate will be high, maybe 25%, which is very wasteful and expensive.

Various attempts have been made to overcome these problems. Manufacturing tolerances can be improved through the use of precious materials and house cleaning equipment. But doing so is too expensive. Additionally, a laser can be used to "cut" the track to increase its impedance. However, it is difficult and expensive to use this method for tracks that require 2kW thermal effects.

The present invention attempts to solve these problems. According to a first aspect of the present invention there is provided a heater assembly comprising an electric heater comprising a resistive heating track on an insulating substrate, the electric heater having at least one electrical contact location, the The location is properly separated from the heater track by a section of material, the resistance value of which is much lower than that of the heating track material, and the heater device also includes an elastic contact in contact with the contact position, and the elastic contact Made of or mounted on a leaf spring.

According to a second aspect of the present invention there is provided a liquid heating vessel comprising a heater assembly comprising an electric heater comprising a resistive heating track on an insulating substrate , the electric heater has at least one electrical contact position, which is properly separated from the heater track by a section of material whose resistance value is much lower than that of the heating track material, and the heater device also includes a contact position with the contact position An elastic contact that is in contact with each other, the elastic contact is made of or mounted on a spring leaf.

And, viewed from the first aspect, the invention provides a method of manufacturing an electric heater device comprising a heater which in turn comprises a resistive heating track laid on an insulating substrate, and Corresponding to the required heater power, the heater has a predetermined resistance value, and the method provided by the invention includes:

a) A resistance heating track is laid on the substrate, the resistance value of the track is greater than its predetermined value;

b) determining a position on the track where the contacts are brought out such that the resistance therebetween reaches a predetermined value; and

c) Make an electrical contact at this position on the track.

Thus, according to the present invention, the resistance value of the resistance heating track is deliberately made higher than the required value (5% higher under the worst tolerance condition), and the electrical contacts are drawn out at selected positions so that the track resistance between the electrical contacts reaches the required value .

Using this method, scrap is substantially eliminated while achieving the desired track resistance without expensive fabrication equipment and materials.

For the heater, its predetermined value does not need to give a "nominal" rating, but has a kind of selectivity, for example, it can be any point within the tolerance range required by the target. Another example is that the heater energy generated by the selected resistance is preferably within the -10% region of the tolerance range, which will reduce the power density of the heater and thus prolong its life.

In the actual production process, the track is made longer than the required length to achieve the predetermined nominal resistance value, and at least an electrical contact is made inward from one end of the track to obtain the required track resistance value.

Thus, viewed from a second, broader perspective, the present invention provides an electric heater assembly comprising a heater having a resistive heating track laid on an insulating One end is provided with an electrical contact for connection to a power supply.

There are several ways to locate electrical contacts on a track. In the first method, the total resistance of the track can be measured first, so that, after knowing the resistivity of the track material, it can be calculated as the amount that the track should be shortened to achieve the required resistance. It is also possible to obtain a series of measured resistance values from a table or similar form, and use this to estimate the amount that the track should be shortened. In addition, the resistance value between two selected points on the track can be directly measured until the requirement is met.

For each pair of joints, the position of one of them can be fixed, such as the end of a track. In addition, one of the joints can also be made on the track before the required position of the other is determined.

There are a certain number of separate contact locations on the track, such as one or more contact pads, which can be made into electrical contacts.

Once the required contact locations are determined, electrical contacts can be made at these locations. It may happen that the heater itself does not have contacts when it is produced, but when the heating vessel is installed later, the contacts of the heater need to be added. In this case, the heater may be installed with indicating points at which electrical contacts are to be made later. For example, markings can be made at relevant locations on the track.

Therefore, viewed from another broader point of view, the present invention also provides a method of producing an electric heater comprising a resistive heating track laid on an insulating layer and heating, corresponding to the power required by the heater device has a corresponding predetermined resistance value, this method includes:

a) laying a resistance heating track on the substrate, the resistance value of the track is greater than a predetermined value;

b) determining positions on the track at which electrical contacts are made such that the electrical resistance therebetween reaches a predetermined value; and

c) Mark the heaters to indicate where on the track the electrical contacts should be made later.

The heating track may be of the type that requires a protective glaze. If so, it is permissible to apply the glaze either before or after the required electrical contact locations have been established. In the first method, the position of the contact point can be established first, and then the glaze layer is added, and one or more windows are made at the appropriate position of the glaze layer so as to connect to the bottom of the track. In the second method, a glaze layer with one or more windows can be added first. The purpose of the window is to connect the bottom of the track, and then select the contact position on the track under appropriate allowable conditions to make the resistance between them reach the required value. .

However, the track is preferably of a material that does not require a protective glaze. This reduces manufacturing costs by removing the glaze layer and makes making the tracks easier.

Several contacts can be made on the track to become electrical contacts. If so, several contact pads can be made appropriately where the track requires, for example in the window of the protective glaze layer. However, the rails are preferably made with electrical contacts directly. This avoids the need to install separate contacts on the rails and reduces the corresponding costs.

Therefore, the tracks are more preferably made by spraying or printing with printable and sprayable inks, without the need for a glaze layer containing pure nickel or a high nickel alloy.

Thus, in a preferred embodiment, the track is substantially exposed without glaze, so that the track can be made into a contact without having to install separate contacts.

The rail joint can be soldered or brass welded, but preferably a contact is used to abut against the rail joint to form an electrical contact. Therefore, the contact is preferably a spring contact. Such contacts are conveniently mounted on a contact holder which forms a control portion of the heater, as will be discussed further below. The contact is preferably knife-edged so that there is a good line of contact between the contact and the track.

The shape of the track can be selected to suit a specific application. But generally, heaters made from printed or painted circuits are very sensitive to sharp bends because the increased current density at the bends will cause potential hot spots that can damage the heater. Therefore, sharp bends should generally be avoided, so a helical track, such as a circular helical track, is usually preferred. A simple spiral has the advantage that the voltage drop between adjacent tracks is relatively slow, so that the spiral tracks can be packed close together without the risk of arcing between the tracks.

In practical application, as mentioned above, where the contacts of the bracket are used to make the electrical contacts of the heater track, the relative positions of the contacts will be fixed for the convenience of production. Therefore, the preferred track should meet the requirement that there are a certain number of different positions to install the contacts, and the interval between the positions is constant. Because the track spacing is parallel, the above content can be easily realized, and each electrical contact can be made on the track respectively.

Thus, in the embodiments described above, the inner and outer rails have parallel arcuate segments, most preferably annular segments, at least partially and preferably substantially circumferentially. Regardless of the configuration between the tracks, the use of ring segments is especially preferred because the heater and contacts can be placed in a desired rotational orientation to achieve the desired resistance of the heater. It will be appreciated that the inner section corresponds to the outer section of the track in the form of a disc or point on its axis.

When the heater is installed, for example, at the bottom of a water heater, the position of the track joint of the heater relative to the container may be fixed. Therefore, the installation position of the heater in the container must meet: when the container is installed, the track joint must be in the correct position to achieve the required power of the heater. In order to facilitate the realization of the above content, it is not necessary to indicate the position of the electrical contact on the heater, but to make an indication on the heater to align with the corresponding reference point on the container, so as to ensure the position of the heater relative to the contact during installation for correct.

Viewed more broadly, therefore, the present invention provides a method of producing a liquid heating vessel comprising, among other things, an electric heater comprising a resistive heating track laid on an insulating substrate , corresponding to the required power of the heater, the heater has a certain predetermined resistance, and a set of electrical contacts used to lead out the contacts, and these contacts are arranged at predetermined positions related to the container. The method provided by the invention includes:

a) laying a resistance heating track on the substrate, the track resistance value is greater than a predetermined value;

b) Determine the position where the heater should be installed relative to the container, so that when installed, the container contact forms an electrical contact with the track, so that the track reaches a predetermined resistance value;

c) Make an indicator mark on the heater to indicate the position, which is used to align the reference point on the container, so that when the heater and the contact are installed on the container, the electrical contact can be made at the required position of the track;

d) a heater is installed on the vessel with an indicator symbol for alignment with the reference point; and

e) Install the electrical contacts at this location on the heater

Determination of the calibration position can be achieved by various methods. For example, the total resistance of the track can be measured, and the position used to indicate can be found from the table, which shows the different measured resistance values for the track, a certain angle or a straight line from the end of the track to the indicated position displacement. Knowing the nominal resistivity per unit length of the track material, these figures can be calculated in advance, or better yet, derived from actual measurements.

As mentioned above, the contacts are preferably mounted in a control unit of the container which is mounted on the base of the container. For example, the control unit can be provided with a function to protect the components from overheating. For another example, the control unit can include a reset switch. When the control unit is installed in place, the reset switch is set on the thermal contact of the heater. But this control also can be a kind of electric control mode, and this has just become another preferred feature of the present invention.

In the familiar electrical control of liquid heating vessels, the electrical components require a drive voltage lower than the mains voltage, which is typically 50V when the supply voltage is 240V. So far, this drive voltage has been properly derived from the mains supply. However, it is a preferred feature of the invention that the drive voltage can be obtained from the contacts of the heater track itself. Accordingly, it is preferred that the heater track has at least one contact location between the respective power supply contact locations to "pick up" an intermediate voltage from the track.

The concept of providing a driving voltage for a flat-plate heater is novel in itself, so from another broad point of view, the electric heater provided by the present invention, taken together, includes a resistor laid on an insulating substrate Heated tracks, and a set of electrical controls. Among them, electrical contacts are made at selected positions between the track and the control unit to provide the driving voltage required for control.

From another point of view, the present invention provides an electric heater, which includes a resistance heating track laid on an insulating substrate. There is a contact position between the ends of the track to obtain a certain voltage from the track. The voltage is lower than the supply voltage of the rail.

The location of this junction can be determined empirically. For example, to obtain 50V from a 240V power supply, the contact should occupy approximately 1/5 of the length of the heater track.

In the above-mentioned heater track device with some parallel track sections connected to the power supply, if the position of the middle contact on the track is fixed, then when the contact point connected to the power supply changes, the voltage obtained by different heaters from this point can still be different, since the relative lengths of the orbitals on either side of the midpoint change. Therefore, the track can have another contact section parallel to the power supply contact section. For example, a helical track may have an additional arcuate track segment between the original arcuate end positions of the track. The use of such parallel rail arrangements is particularly advantageous when the spatial relationship between the power contacts and the intermediate voltage contacts is predetermined, for example mounted on a common support such as a control support. In this device, because the relative position between the track and the power supply contact is changed in order to achieve the required track resistance value, the position of the middle contact is also changed. To a certain extent, this offsets the resistance between the track and the power supply contact. Changes in the ratio of track lengths on both sides.

Obviously, the invention also extends broadly to the invention of suitable heaters. Therefore, viewed from another broad perspective, the present invention provides a heater comprising a resistive heating track laid on an insulating substrate, the track having electrical contacts spaced from at least one end of the track. open, this terminal can make a power contact.

This contact is preferably a part of the track itself, although it is equally possible to mount or connect a separate electrical contact to the track.

Preferably the track has a first exposed section for connection to a first power supply contact of the track, and a second exposed section parallel to the first section for connection to a second power supply contact on the track.

The first segment and the second segment preferably adopt coaxial circular arcs. For example, the inner segment may be located near the center of the heater track. But in another embodiment the track is not a single helix but can be formed by folding itself such that the inner track is positioned adjacent to the outer track segment and works in the opposite direction to the outer track segment. Such an arrangement has the advantage that, since both contact sections occupy a relatively large ratio, the arrangement can, if necessary, realize a wide range of resistance variations. A possible disadvantage of this arrangement, however, is that because of the potentially greater voltage between adjacent curves of the track, a greater distance must be maintained between the curves.

If necessary, a plurality of parallel track sections can be mounted at one end of the track to achieve a wider range of resistance variation, selectively making one or the other of the track sections an electrical contact.

Alternatively, the track may include a third exposed section, parallel to and located between the first and second sections, which may make a further electrical contact to take a voltage from the track which is lower than the supply voltage. Thus in a preferred embodiment the track may comprise three coaxial circular arc segments.

The heater substrate is preferably in the form of a flat plate, most preferably an insulating metal plate.

The resistive track and its different contact locations can be made of the same resistive material and/or have the same resistivity. However, the contact locations or contact areas may be made of a different material and/or resistivity and/or width than the rest of the track, for example, by a multiplex printing process in which different resistive materials or different thickness of resistive material was successfully laid. Specifically, the material of the contact position or the contact region may be a material that is easy to make a contact, for example, a material that is not easily oxidized.

Also, while the heater track of the present invention is preferably disposed in substantially the same plane, it may also be disposed in multiple planes. For example, it may have a first track section covered with a glaze or the like, and a second section mounted on top of the glaze with contact locations, with electrical contacts between the two sections of the track.

It will be appreciated that the concept of having an intermediate contact location on the heater track has broader application than is limited to electrical controls only. Therefore, the middle position can be used for more than just picking up a section of voltage from the rail.

For example, control could be applied through one or both heater track segments to selectively energize the track. So different parts of the track could have different resistance values or different thermal effects, for example it could be used in hollow tanks or similar to provide the effect of boiling or simmering a liquid.

Thus, in a very broad sense, the invention also extends to an electric heater comprising a resistively heated track on an insulating substrate, the track having an electrical contact location between the ends of the track. between.

The invention also extends to a water heater incorporating a heater or heater unit according to the invention, and to a method of manufacturing such a vessel incorporating a method according to the invention Steps for producing the heater unit.

It was mentioned above that the region of the track adjacent to the contact point may be made of a different material than the rest of the resistive heating track. It can be seen from another aspect of the present invention that it is better to use a low-resistance material such as silver between an electrical contact position section and the heating track. The advantage of the above approach is that this part of the track will not generate a lot of heat, which will reduce the heat transfer to the contact location, which also reduces the heat transfer to the contact made from this area. Specifically, under overheating conditions, the repeated heating of the contacts at this place will lead to the toughening of the spring sheet, resulting in a reduction in the contact pressure, which in turn leads to an increase in resistance heat generation at the contact position. The benefits of the above approach are reflected.

Therefore, from another point of view, the present invention provides an electric heater, which includes a resistance heating track laid on an insulating substrate, the heater has at least one electrical contact position, and the contact position is connected to the heating track. The space is properly separated by a section of low-resistance material, preferably, the resistance value of this material is much lower than that of the heating track material.

The low-resistance material is preferably silver or a material with a high silver content. This will provide a good electrical connection to the normal heating track with little thermal effect.

The joint location is preferably made as a contact pad, eg with silver, which provides for a good electrical contact with the heater.

The low resistance material may partially or completely cover the surface of the heating track or underlie the heating track. However, it preferably overlaps the end section of the heating track. This length of low resistance material will be defined within a box to provide adequate thermal insulation for the contacts through the bottom of the box. However, the length of the low resistance material is preferably at least 8 mm.

In a preferred embodiment, the low resistance material is applied using conventional track application techniques, such as serigraphy. Therefore, a silver or high silver ink can be used with this material.

This aspect of the invention also extends to a heater assembly having a resilient contact in contact with a contact location, preferably using a leaf spring to form the contact or having the contact mounted on the leaf spring. As stated above, the possible problems of reduced contact pressure, as well as actual heat transfer to other parts of the appliance, are alleviated by the present invention.

Some embodiments of the present invention will be described herein by way of example only and with reference to the accompanying drawings, in which:

Accompanying drawing 1 is the perspective view of a heater according to the present invention;

Accompanying drawing 2 has shown the heater rail side of accompanying drawing 1;

Accompanying drawing 3 shows that the heater of Fig. 1 is positioned on the base of a liquid heating vessel;

Accompanying drawing 4 has shown a control mechanism for heater, and it is positioned on heater;

Accompanying drawing 5 shows that the control mechanism of accompanying drawing 4 is installed on the heater;

Accompanying drawing 6 is a plan view of another form of heater track suitable for the present invention;

Accompanying drawing 7 is another embodiment of the present invention; And

Figure 8 shows yet another embodiment of the present invention.

With reference to accompanying drawing 1 and accompanying drawing 2 have illustrated a thickness film flat panel heater 2, and heater 2 is used to do in the base of a water heater or above, or make base. It consists of a stainless steel plate with a diameter of 81mm and a thickness of about 0.5mm. There is an electrically insulating ceramic layer 6 on the surface of the steel plate. A thick film resistance heating track 8 is printed on the ceramic layer 6 in a familiar way. The track 8 is pure nickel or high nickel alloy, it is exposed on its length, without protective glaze layer.

Rail 8 is about 2.4mm wide, and it comprises an outer annular end section 10 and an inner annular end section 12, and end section 10 has a constant average radius R ₁ of about 34.5mm, and the periphery of flat plate 4 that it extends substantially completely, The end section 12 has a constant mean radius _R2 of about 5.25mm and spreads out about 300° about the axis of the plate, the end section 12 being coaxial with the outer end section 10. The track 8 also has a central coaxial annular segment 14 having a constant mean radius _R3 of approximately 16.75 mm.

Ring segments 10, 12, 14 are connected to inner and outer helical segments, respectively. The transition between the individual helical segments and the annular segments is a smooth curve, which avoids current build-up and resulting hot spots at excessive bends. The distance between adjacent curves of the spiral track section is about 0.8 mm to 1.0 mm. A mounting stud is installed in the center of the plate 4 .

The plate 4 will be mounted on or in the base 24 of the liquid heating vessel 26, or as the base 24, as schematically shown in FIG. 3 . And, through the projection on the fuselage 36 of an electrical control unit of heater by each elastically installed electrical contact piece 30,32,34, electrical contact is done in three positions 9,11,13 of track 8, these three Each of the two positions is one on each annular segment 10, 12, 14 of the track, (as shown in Figure 3). As shown in FIG. 4 , the control body 36 is mounted above the stud 20 .

A control fuselage 36 is mounted on the vessel base in a predetermined angular orientation with respect to a reference point 38 illustrated in FIG. 3 . The contacts 30 and 32 are connected to the wires installed on the control body 36 and the neutral wire terminal of the power supply to supply power to the track 8, while the contacts 34 are connected to the electrical components of the control mechanism for receiving power from the track 8. Obtain a drive voltage to drive these electrical components.

The track 8 is manufactured with a total resistance, measured between its ends, that is greater than that required to achieve the desired power of the heater. Therefore, the power supply connection must be led to the appropriate position of the inner and outer track segments 12, 10 by the contacts 32, 30 of the control body 36, like this, the track length therebetween is limited so that its resistance corresponds to the required power of the heater. reach the predetermined value.

Because the installation position of the control body 36 is fixed with respect to the container base, when the container is assembled, the heater 2 must be installed on the container base with a special angle orientation to ensure that the control body is installed on the container base 24. At 36, the contacts 30, 32 should be connected to the track 8 at a suitable position. To facilitate the foregoing, the heater is marked with an indicator symbol 42 for alignment with the fiducial 38 within the container.

One way to establish the position of the indicator symbol is to measure the total resistance of the track 8, and then calculate the position based on it. After the length of the touch track, the resistivity of the track material and the position relative to the reference point 42, the electrical contact can be made in the container. on the inner track. For example, in order to obtain the required resistance value of the track, the amount by which the track should be shortened can be calculated from the given data, so that when the heater is installed, the position of the indicator symbol is displaced from an end joint by a predetermined amount, which takes into account the joint The position is relative to the position of the container. These data can all be pre-calculated and tabulated so that in final production and inspection, the proper position can be obtained simply by reading the table, or calculated for each component.

Another method of determining the position of the indicator mark on the heater is to measure the resistance between a pair of probes that are positioned in the same relative position as that between the contacts 30,32. The two probes are moved integrally around the axis of the track 8 until the measured resistance value meets the requirement. Once this position is reached, the indicator symbol 42 can be made on the heater 2 in the same orientation relative to the determined contact position as the contacts 30 , 32 relative to the container reference point 38 . For example, if the contacts 30 are positioned at a 10° offset from the datum point 38 , then the markings 42 on the heater are positioned at a 10° offset from the corresponding contact locations on the outer track 10 .

Then, the heater is installed on the container base 26 with the indicator 42 aligned with the reference point 38, and the control mechanism 36 is installed on the container base subsequently. Electrical contacts should be made between the correct locations.

The intermediate track section 16 also makes an electrical contact by the contacts 34 . This contact makes the voltage taken from the rail lower than the supply voltage to drive the electrical components in the control mechanism.

Because the middle and end track sections are concentric, a satisfactory joint can be made on the middle track section 14 substantially without regard to the relative angular position between the control body 36 and the heater track 8.

The purpose of setting the intermediate contact 34 is to make the obtained voltage be the required value at the position mentioned above.

In the illustrated embodiment, the voltage drawn from the intermediate junction will be approximately 1/5 of the supply voltage, ie approximately 50V for a 240V supply voltage. Therefore, the intermediate junctions made on the track should take about 1/5 of the track length. The precise voltage drawn by contacts 34 varies slightly with the relative angular position between control body 36 and rail 8 because of the variation in relative track length between the intermediate and end contacts. But this change will not be so large that it will adversely affect the control elements.

In the embodiment described above, it is possible to achieve a change in resistance approximately equal to the resistance of the outer circular track section 10 . In practice, the approximate maximum and minimum values of the track area (the same is true for resistance) can be limited between contacts in the canyon, which are about 2500mm ² and 2100m ² respectively, and the total possible area change reaches 400mm ² , which accounts for about 10% of the minimum range of resistance. 19%.

Greater variation can be achieved using the heater track configuration illustrated in FIG. 6 . In this embodiment, the track 50 is a double helical structure, the outermost of which is a circular end section 52, and another substantially circular end section 54 is disposed immediately inside. As in the previous embodiments, each end section 52, 54 makes a separate electrical contact. Because the length of the inner annular track end section 54 is substantially larger than that of the previous embodiment, and because the individual end sections 52, 54 work in opposite directions relative to each other, this device can have the ability to achieve a greater range of adjustment. advantage. Thus, the change in resistance possible with this track shape is equal to the sum of the lengths of the track segments 52,54.

If pneumatic controls such as those described above are used for the track, an individual annular track section can be fitted in place.

In this embodiment, the spacing between adjacent curves of the tracks must be kept sufficiently large to prevent electrical breakdown, for example because the tracks 52 and 54 will be at a potentially sufficient mains voltage relative to each other. So the usual pitch is 2mm.

Another embodiment of the invention is depicted in Figure 7 of the accompanying drawings. In this embodiment, a heating track 60 includes an inner segment 62 having a lower resistance of about 14Ω and an outer segment 64 having a higher resistance of about 190Ω. The track is laid, for example printed, on an insulating substrate which is covered, for example printed, with an outer contact ring 66 and an inner contact ring 68 connected to the outer section of the track, The contact ring 68 is connected to the inner section of the track, and a number of contact pads 70 are provided on an intermediate section 72 of the track 60 . These contact rings and pads are made of a material that conducts electricity well on the track. They do not require thermal effects. A protective glaze layer is located on the heating track 60, but this glaze layer is installed with a window to record the contact pad 70, and does not extend inwardly to the inner contact ring 68 or outwardly to the external contact. Head circle 66. Thus, electrical contacts are made on the rails via the exposed inner and outer races 66, 68 and contact pads 70, for whatever purpose.

This heater will be used in hollow pots or similar where different thermal effects are required, initially bringing the liquid in the container to a boil and then simmering.

Figure 8 shows a portion of a heater assembly according to another embodiment of the present invention. Through a length of 4.0mm, a width of 0.2mm silver-plated beryllium-copper leaf spring contact 78, a control mechanism 74 of the heater device is electrically connected to the electric heater 76.

During operation of the heater unit, the control mechanism supplies power to a palladium-silver alloy heating track 80 of the electric heater 76 . Each end of the heating track 80 terminates at a junction 82 with a silver "tail". The silver tail 84 is a film made by using silver or printing ink containing a large amount of silver in a pot printing method. Silver tails 84 may be applied to the heated substrate either before or after the resistive heating tracks 80 are applied to the heated substrate during manufacture. In both cases, heating track 80 and silver tail 84 overlap at junction 82 so that good electrical conduction occurs between them. The silver tails are covered with a layer of glass, glass-ceramic, but at the end of the tail furthest from the joint 82, the covering exposes a silver contact pad 86 with a diameter of 8mm. The spring piece 78 is in contact with the contact pad 86 with a contact pressure of about 150 gm (not less than 100 gm), thereby conducting electrical conduction with the heating track 80 through the silver tail 84 .

It will be seen that the resistive heating track 80 is separated from the leaf spring contact 78 by a silver tail 84 having a length of 100 mm from the edge of the contact pad 86 to the junction 82 . Thus there is no direct heating of the heater track 80 on the contacts 78 . The low resistance of the silver tails prevents resistive heating in the area of the contact pads, thus reducing the temperature in this area, especially under overheating conditions.

Using the device shown in Figure 8, a 2.2kW dry switch within a period of 7 seconds, the highest temperature reached by the leaf spring 78 is 150°C. After such a period of 50 hours as required in the stress test, the contact pressure on the spring leaf 78 drops by about 15% due to thermal toughening. The contact resistance between the leaf spring 78 and the contact pad 86 thus rises to a value not greater than 10 mΩ, which is acceptable compared to the initial value of 7 mΩ. In this case, for a 2.2kW, 9A heater, the power loss at the contacts will be less than 1W.

The embodiments described above may be varied within the scope of the invention. For example, the track material 80 may extend to the contact pads 86 under or over the silver tails, but it is more effective to short them with the contact pads 86 .

Although in the embodiments described above the substrate is shown as a flat plate, it may have grooves around its perimeter for the purpose of mounting it in or on the base of the container. In this arrangement, a plastic container side wall or base may have a downwardly supported portion for receiving and mounting the heater circuitry. The above device is described in our PCT application WO96/18331.

The present invention is not limited to flat-plate heaters, but is equally applicable to heaters whose substrates are partially tubular, polygonal or three-dimensional.

Also, although the particular embodiment described generally is a circular spiral, other track shapes may be used, such as triangular or rectangular or other polygonal spirals, all having a series of alternating turns of decreasing size. For example, if the track uses a rectangular slab, the track curve can mostly use a rectangle to fill the edge of the slab.

The meaning of the word "luoshi" can then be interpreted broadly, rather than being limited to a curved spiral.

Furthermore, the invention in its broadest sense is not limited to the use of curved heating tracks, but any track shape, even a straight track. It is important to be able to make the electrical contacts of the tracks at the required locations to obtain a predetermined track resistance. It will thus be seen that, in a very broad sense, the invention presents a method of obtaining an electric heater with the required power output comprising a resistive heating track supplemented on an insulating substrate, also in the The electrical contacts of the power supply are made at a certain position on the track, so that the resistance between them is limited to an appropriate value.

Finally, although in the preferred embodiment the invention is disclosed for use in a water heater, it can also be used for example in other water heaters, again for example in dishwashers or flood heaters, or in appliances of the same type as irons.

Claims (5)

1. heater assembly, it comprises an electric heater, described electric heater comprises a resistance heating trace that is layered on one deck dielectric substrate, this electric heater has an electric terminal position at least, this position separates with heater tracks with one section material, the resistance value of this section material is significantly less than the heating rail material, and described heater assembly comprises that also described spring contact is made or is installed on the spring leaf by a spring leaf with a spring contact of described connecting point position contact.

2. according to the heater assembly of claim 1, wherein, described low electrical resistant material is for silver or contain a large amount of silver.

3. according to the heater assembly of claim 1 or 2, wherein, the length of described low electrical resistant material is at least 8mm.

4. according to claim 1,2 or 3 heater assembly, wherein, described contact is a power contact head.

5. heating vessel, it comprises a heater assembly, described heater assembly comprises an electric heater, described electric heater comprises a resistance heating trace that is layered on one deck dielectric substrate, this electric heater has an electric terminal position at least, this location-appropriate ground separates with heater tracks with one section material, the resistance value of this section material is significantly less than the heating rail material, described heater assembly comprises that also described spring contact is made or is installed on the spring leaf by a spring leaf with the contacted spring contact of described connecting point position.

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