GB2061680A - Electrically heated panels - Google Patents

Abstract

In order to modify the heat output distribution from an electrically heatable panel 17 bearing deposits of electrically conductive material which form bus strips 15, 16 extending along opposed side margins of the panel and electrical resistance heating elements 1 to 14 extending between the bus strips 15, 16 at intervals therealong in a generally ladder-like array which has a centre defined by the space 24 between the two central elements 7, 8 (or by the central element if there is an odd number of elements) the total conductance of the heating elements 1 to 7 above the centre 24 of the array is less than that of the heating elements 8 to 14 below the centre 24, this conductance differential being at least partially attributable to the fact that at least one of the upper elements 1 follows a course of which at least a portion lies adjacent and substantially parallel to a portion 22, 23 of a bus strip 15, 16 so that the element 1 is longer than the distance between its points of connection to the bus strips. In order to economise on conductive material, the bus strips 15, 16 decrease in width in a direction towards an end remote from the location of current input terminals 20, 21. Such panels are useful for forming heated vehicle windows. <IMAGE>

Description

SPECIFICATION Electrically heatable panels This invention relates to an electrically heatable panel comprising a substrate bearing deposits of electrically conductive material which form bus strips extending along opposed side margins of the panel and electrical resistance heating elements extending between said bus strips at intervals therealong in a generally ladder-like array. Such a ladder-like heater arrangement is a very practical one for heating a quadrilateral panel area. It finds favour for example with various vehicle manufacturers for electrically heatable windows of their vehicles. The arrangement of the heating elements and bus strips affords advantages in the panel manufacture and combines an effective heating function with acceptable effects on the optical properties of the window.In such windows the heating elements are usually disposed generally horizontally and the bus strips, which are themselves formed by electrically conductive coatings on the glass substrate, are often disposed so that each of them runs substantially parallel with the corresponding adjacent side boundary of the window.

In the manufacture of such electrically heatable panels attempts have been made to make the electrical resistances of the various heating elements substantially equal by carefully controlling the deposition of electrically conductive material so that the deposited elements have the same widths and thicknesses, within narrow tolerances. This accuracy in the formation of the electrically conductive coatings is not at all easy to achieve in mass production. Consequently it has been proposed to compensate for inequalities in the deposited conductors by removing deposited electrically conductive material from one or more selected regions and/or by adding electrically conductive material to one or more selected regions to modify the electrically conductive paths of one or more of the heating elements.

Certain performance specifications are sometimes laid down which cannot satisfactorily be reconciled with the provision of heating elements with equal heat outputs. The heating effects of a panel are influenced by secondary factors dependent on environmental factors and particularly by convection currents. Such convection currents have an important influence on the stabilised heat distribution over the panel during periods of operation of the heater if the panel is installed in a vertical position or at some other inclination to the horizontal. Electrically heated vehicle windows clearly fall in this category.

Thus, in some circumstances, it is desirable to modify the heat output distribution as between those heating elements which lie above the centre of the array and those heating elements which lie below the centre of the array. For the purpose of this specification, the centre of the array is defined as the central heating element of the array where there is an odd number of heating elements in the array, or as the space between the two central elements of the array where there is an even number of elements in the array.

It is known to adopt different coating specifications for forming different heating elements and/or to form individual elements of varying thickness in order to achieve a modified heat output distribution from the heater. These methods of modifying the heat distribution pattern are useful but they are difficult to control under cost effective volume production conditions.

It is an object of the invention to enable advantages of a ladder-like heater arrangement to be combined with a modified heat output distribution pattern without dependence on variations in the composition or in the thicknesses of the coating deposits forming the heating elements.

According to the present invention, there is provided an electrically heatable panel comprising a substrate bearing deposits of electrically conductive material which form bus strips extending along opposed side margins of the panel and electrical resistance heating elements extending between said bus strips at intervals therealong in a generally ladder-like array which has a centre defined by the central element if there is an odd number of elements or by the space between the two central elements if there is an even number of elements, characterised in that the total conductance of the heating elements lying on one side of the centre of the array (which side will be referred to as "the upper side" or "above the centre of the array") is less than the total conductance of the heating elements lying on the other (or lower) side of the centre of the array and in that this conductance differential is at least partially attributable to the fact that one or more said elements lying above the centre of the array follows a course of which at least one portion lies adjacent and substantially parallel to a portion of a said bus strip whereby the course of such element is longer than the distance between its points of connection to the bus strips measured along the surface of the substrate.

The invention makes it possible without complicated manufacturing steps to make panels having a heating energy distribution which varies over the panel area in a manner which will wholly or partially compensate for the differential heating effects of convection currents when the panel is used in a vertical or upwardly inclined position. This compensating effect can be achieved even if the heating elements are identical in respect of their composition and their widths and thicknesses.

In a panel according to the invention the conductance of, and thus the heat output from the heater array is greater at one side of its centre than it is at the opposite side thereof. It is intended that a panel according to the invention shall be installed with the higher heat output part of the heater below its other part. The lower heat output from the upper part of the heater will compensate or partly compensate for the heating effect of rising convection currents on the upper part of the panel.

The adoption of heating element characteristics according to the invention affords the further advantage that it can favourably influence the current distribution within the bus strips.

Preferably the outermost heating element at the upper side of the array of said elements, or each of two or more adjacent heating elements including that outermost one is deposited to follow a course of which at least one portion lies substantially parallel to a neighbouring portion of a said bus strip whereby the course of such element is longer than the distance between its points of connection to the bus strips measured along the surface of the substrate.

This distribution of heating elements following a said longer course is helpful for achieving the most favourable heating effects when the panel is installed as intended.

As has been stated, heating panels in accordance with the invention may be constituted as heatable windows for vehicles, and especially as heatable rear windows. Such rear window panels may be curved or flat. In known panels, it is usual for all the heating elements to be parallel to each other, or at least to give the appearance of being parallel. Thus for example, all the heating elements may be deposited in such a way that when the panel is viewed in a particular way, for example via the rear view mirror of a vehicle in which it is installed, the elements all appear straight and parallel, notwithstanding any curvature of the panel.

It is envisaged that in most embodiments of the present invention, most and preferably all of the heating elements which do not follow a said longer course are substantially parallel with one another. Such parallelism tends to give the panel an appearance which is not visually distracting, and this is of course very important for road safety where the panel is used as a vehicle window. Whether or not this feature is present, it is preferred, and for the same reason, that the or each element following a said longer course lies, except at a region or regions near the bus strips or one of them, substantially parallel with the element adjacent the centre of the array and on the opposite side thereof to the respective element following a said longer course.

Advantageously, at least one said bus strip is shaped so that its inner boundary has at least one recess or rebate, and in that that portion of the course of at least one heating element which lies adjacent and substantially parallel with a portion of a said bus strip also lies within a said recess or rebate. The adoption of this feature also tends to give the panel an appearance which is not visually distracting.

Said bus strips preferably have end portions projecting into a third side margin of the panel, and there is preferably at least one said heating element following a said longer course which is connected at or towards a said end of a said bus strip and which has an end portion which lies substantially parallel to a said bus strip end portion.

Preferably there is a plurality of said heating elements each following a said longer course and such elements all lie above the centre of the array of elements.

The invention can be applied using substrates of any composition. A panel which is intended to fulfil only a heating function can be opaque.

In the most important embodiments of the invention the substrate is transparent and the panel forms or is adapted to serve as a window. Of particular importance are embodiments wherein the panel is an electrically heatable glass panel for use as a vehicle window.

The invention includes a heatable transparent panel constituting or suitable for constituting a vehicle window, such panel being as hereinbefore defined and having bus strips which over at least the greater part of their length have inner boundaries which converge so that their spacing is less above the centre of the array than it is below the centre of the array. A heater with converging bus strips may occupy a substantially trapezoidal area and is particularly useful in a panel for forming a vehicle rear window of generally trapezoidal shape, the panel being orientated so that the said inner boundaries of the bus strips converge upwardly. The lengthening of the courses followed by heating elements in the upper part of the panel can be and preferably is more than sufficient to compensate for the narrower spacing of the bus strips in that part of the panel.

The heating elements of a panel according to the invention can be of the same composition and of the same width and thickness, or different elements can differ inter se in respect of any one or more of these factors for achieving given effects. Furthermore, any of the elements can have a composition, width or thickness which different from one part of the element length to another, for achieving required effects. For example the elements or some of them can have portions of increased width at or near their ends in order to modify the temperature gradient across the panel, for example to promote rapid heating of the central zone of the panel.

A panel according to the invention may have leading wires connected to the bus strips or terminal elements may be connected to such bus strips ready for receiving lead-in wires. In some embodiments of the invention the points (hereafter called "terminal points") at which such wires or elements are connected to the bus strips are located at or near the ends of the bus strips which are on said one side of said central axis.

Advantageously the outermost heating element on the said upper side of the centre of the array, or each of two or more adjacent elements including that outermost one, follows a said longer course and the terminal points are located at positions along the bus strips which are between that or those elements and the other heating elements of the panel. In such a heater there is a division of current in opposite directions along the bus strips. The arrangement is beneficial for reducing the amount of electrically conductive material which has to be deposited in order to form bus strips capable of a given electric current distribution to the heating elements without overheating of such strips.The said positioning of the terminal points also has the consequence that the electric current flow along the said outermost heating element or along a group of elements including that outermost one is less than it would be if the terminal points were located at the ends of the bus strips, adjacent the ends of said outermost heating element.

That consequence can also be beneficial e.g. for reducing the minimum length of the course which heating elements of a said group must follow in order to keep the heating currents therethrough below a certain value.

The invention includes panels which are as hereinbefore defined and wherein the bus strips each decrease in width in a direction towards an end which is remote from a position where a lead-in wire or terminal element is connected to such a strip. It has been found that by providing bus strips with this characteristic, an important saving in electrically conductive coating material can be achieved without offsetting disadvantages in terms of the heater performance.

Preferably, each bus strip tapers progressively in width over at least the greater part of the distance between the said position where the lead-in wire or terminal element is located and a said remote end of the bus strip.

The provision of bus strips exhibiting a said decrease in width is of advantage for reducing manufacturing costs, regardless of any geometrical pattern of the heating elements themselves.

The invention accordingly includes any electrically heatable panel comprising a substrate bearing electrically conductive material which forms bus strips extending along at least the greater part of the length of opposed margins of the panel, electrical resistance heating elements extending between such bus strips at intervals therealong and lead-in wires or lead-in wire terminals connected to said bus strips, characterised in that each said bus strip tapers progressively, decreasing in width over at least the greater part of the distance between the location of said wire or terminal and an end of the bus strip which is remote from the location of said wire or terminal. The said wires or terminals may be connected to the bus strips at one end thereof or at positions between heating elements.

Particularly important heated glass panels according to the invention, suitable for forming the rear windows of vehicles, are of elongate form and have bus strips and heating elements arranged so that a power supply of 1 50 W to the heater will maintain, at the side of the panel carrying the heater, surface temperatures of at least the following minimum values above ambient temperature measured at the specified zones: Median axis of panel along which temperatures Surface temperature in "C above ambient: are measured Each end of axis Middle of axis

Longitudinal 12 (horizontal) axis 22 Vertical axis 20 The heaters of a panel according to the invention can e.g. have an overall electrical conductance of between 1 and 0.3 reciprocal ohms.

The heaters can be formed by conductive compositions and by deposition techniques as known per se. For example a conductive coating composition can be deposited in the required pattern by a serigraphic process, for example a process using a photographically produced screen image of the required heater pattern. The deposited composition may be in the form of a paste comprising particles of an electrical conductor, e.g. silver, and of a vitreous or vitrifiable binder dispersed in a liquid vehicle. Following deposition of such a composition the panel is heated to cause evaporation of the liquid vehicle and melting and/or vitrification of the binder.

The resistances of the heating elements can be adjusted following deposition of a said electrically conductive paste, by electro-deposition of a metal or metals thereon, e.g. by electrodeposition of successive coatings of copper and nickel.

Certain embodiments of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of a panel according to the invention; and Figures 2 to 4 are plan views of parts of other panels according to the invention.

The panel shown in Fig. 1 is of trapezoidal shape and is intended to form the rear window of an automobile. The panel comprises a glass substrate bearing an electrical resistance heater which has been formed in situ on the panel by deposition of an appropriate coating composition as known per se.

The heater comprises an array of fourteen heating elements indicated at 1, 2, 3,... 7, 8. . 13, 14 numbered from the top of the figure, each having a width between 0.4 and 0.8 mm and a thickness of 40 jtm. The heating elements 1 to 14 extend between bus strips 15, 1 6 which extend along the two opposed side margins of the panel 1 7.

End portions 1 8, 1 9 of the bus strips 15, 1 6 respectively project into and indeed extend part way along the top margin of the panel 17, and terminal element 20, 21 are provided at the ends of these end portions. Furthermore upper portions of the bus strips are shaped to provide rebates 22, 23, and lower portions of the bus strips progressively decrease in width towards their lower ends.

Since the array of heating elements contains an even number of elements, fourteen, the centre of the array is defined by the space 24 between the two central elements numbered 7 and 8.

Heating elements 2 to 14 are conventionally arranged. They are substantially parallel to one another and they are shown as following substantially straight-line courses between their respective points of connection to the two bus strips 15, 16.

The uppermost heating element 1, however, follows a longer course, though it is, over the greater part of its length and except at its end regions near the bus strips, parallel with the other heating elements 2 to 14 and in particular parallel with the heating element 8 which is adjacent the centre 24 of the array and on the opposite side thereof to the heating element 1. The course followed by the heating element 1 is longer than a straight line course between its points of connection to the bus strips 15, 1 6 because end portions of the heating element 1 lie adjacent and substantially parallel to the portions of the bus strips 15, 16 which define the rebates 22, 23. In fact, the embodiment illustrated adopts two further optional features relating to the heating element 1.Firstly, those end portions of the heating element each lie within a respective bus strip rebate 22 or 23, and secondly, the extreme end portions of the first heating element lie parallel to and are connected to the ends of the respective bus strip end portions 18, 1 9.

Because the heating element 1 follows a said longer course, its conductance will be less than that of an otherwise similar element which followed a straight line course, and it is this reduction in conductance of that heating element which at least in part contributes to the fact that the total conductance of the heating elements 1 to 7 above the centre 24 of the array is less than the total conductance of the heating elements 8 to 14 below the centre of the array.

This conductance differential will in use give rise to a corresponding heat output differential in the two parts of the heating array, so that the upper part of the panel when installed as a vehicle window will not become unnecessarily overheated. The lower heat output from the upper part of the panel will at least in part compensate for the fact that that upper part is not only heated electrically, but also by convection currents of warm air which have been heated by contact with lower parts of the panel.

It will be appreciated that the drawings are not to scale. In particular, the bus strips 15, 16 may be narrower and closer to the margins of the panel 1 7 than illustrated, so that when the panel is framed, the bus strips are wholly or mainly within the mounting frame. Indeed the end portions of the uppermost heating element 1 which are out of line with its major central portion may also be accommodated within the frame so that all that is exposed to view is a series of straight parallel horizontal lines.

The bus strips 15, 1 6 decrease in width in the downward direction. The strips may for example have a width of 25 mm at their upper ends, where the terminal elements 20, 21 are located, a width of from 30 to 35 mm at the level of heating element 2, and a width of from 5 to 1 5 mm at their lower ends. For a given electrical potential between the terminal elements, e.g. a potential of 14 volts, the voltages across at least those heating elements which are in the lower half of the panel will be lower than they would be if the outer boundaries of the bus strips were to run parallel with the inner boundaries thereof, but the differences are quite acceptable.

On the other hand the tapering of the bus strips involves an important economy in the use of the electrically conductive coating material.

In a further embodiment of the invention, bus strips are used which have the form of strips 1 5 and 16 except that the inner boundaries of the bus strips above the level of heating element 2 are straight continuations of the convergent lower portions of those boundaries, or run vertically, and heating element 1 follows an entirely straight course between the bus strips, parallel with the other elements. This embodiment is an example of the present invention in its second aspect in which the benefits of employing at least one heating element which follows a said longer course are not obtained but which affords the advantage of an important economy in the consumption of electrically conductive coating material.

Fig. 2 shows part of another panel according to the invention, which is also intended for use as the rear light of an automobile.

In Fig. 2, a panel 25 has an array of thirteen heater elements 101, 102, 103, 104,... 107, 108,... 11 2, 11 3 extending between a pair of bus strips deposited on opposite side margins of one face of the glass substrate forming the panel. One of these bus strips is shown at 26, and this bus strip is shaped to provide a rebate 27 towards its upper end. The bus strip 26 has an end portion 28 (in which part of the rebate 27 is formed) which extends horizontally along part of the upper margin of the panel 25. The lower ten heating elements 104 to 11 3 follow substantially straight line courses between the two bus strips, and they are substantially parallel to one another. The upper three heating elements 101 to 103 follow longer courses.An end portion of the third heating element 103 follows a course which is substantially parallel and is adjacent to the boundary of the rebate 27 in the bus strip and is connected to the bus strip at a position within the rebate and towards the end of the end portion 28 of the bus strip 25. The first two heating elements 101, 102 are also connected to the bus strip within the rebate at positions towards the end of the bus strip end portion 28, and these heating element end portions run parallel to and are enfolded by the end portion of the third heating element 103.

Thus these three adjacent heating elements have end portions which lie substantially parallel with the neighbouring portion of the bus strip which also defines the boundary of the rebate 27.

Since there is an odd number of elements in the array, the centre of the array is defined by the central element, in this case the element 107. The major part of the lengths of the uppermost three heating elements runs substantially parallel to the element below the centre of the array, that is, to heating element 108.

By virtue of the longer course followed by the three uppermost heating elements 101 to 103, the six heating elements which lie above the centre of the array (coincident with heating element 107) have a greater total length than the six heating elements 108 to 11 3 which lie below the centre. Since the various heating elements are formed of the same material and have the same uniform nominal cross section, this entails that the upper six elements have a lower total conductance than the lower six elements. Terminal elements or lead-in wires can be connected to the bus strips at or near their upper ends, for example close to the corners of the panel.

Fig. 3 shows part of a panel in which there are heating elements which are connected to the bus strips in pairs, the elements of each pair being electrically connected in parallel and having common points of junction with the bus strips. Thus, heating elements 29, 30 join together at point 31 and a corresponding point at the other side of the panel. One end of elements 29, 30 join the bus strip 32 at point 33 and the other end of that element joins the opposite bus strip at a corresponding point. Heating elements 34, 35 are of similar form and join together in a similar way, to join the bus strip 32 at point 36. The inner margins of the bus strips have recesses such as 37 which accommodate the common end portions of heating elements 29-30 and 34-35 which run parallel with those strips.The other heating elements of the panel, which include elements 38 and 39 follow a straight line course. Terminal elements such as 40 for connection to lead-in wires are soldered to the upper ends of the bus strips.

Fig. 4 shows part of a panel in which each of heating elements 41 and 42 follow a to and fro course along the panel between the bus strips 43, 44. Each of those elements has three parallel reaches which run along the panel perpendicularly to the bus strips and the intervening portions of the element run parallel with the bus strips. The inner margins of the bus strips have recesses such as 45 which accommodate those intervening portions, such as 46, 47, of the elements.

Said intervening portions are given a larger cross-section than the other portions of the elements in order to decrease the heating effect near the panel sides. The elements 41 and 42 follow a course which is very much longer than the shortest distance between their points of connection to the bus strips. The other heating elements such as 48 of the panel follow a straight line course. Lead-in wires can be soldered to the bus strip at or near the upper ends of the bus strips.

In referring to the drawings, the course followed by various heating elements or portions of them has been referred to as a straight line course. Strictly this implies that the various panels are planar. It will be appreciated that in practice many heating panels, especially those destined for use as automobile rear lights, will be curved. In these circumstances, it would be appropriate for the so-called "straight line course" to be modified to an approximately geodesic course, or to a course of such form that it would give the appearance of a straight line when viewed via the rear view mirror of a vehicle in which the panel was installed.

Claims (11)

1. An electrically heatable panel comprising a substrate bearing deposits of electrically conductive material which form bus strips extending along opposed side margins of the panel and electrical resistance heating elements extending between said bus strips at intervals therealong in a generally ladder-like array which has a centre defined by the central element if there is an odd number of elements or by the space between the two central elements if there is an even number of elements, characterised in that the total conductance of the heating elements lying on one side of the centre of the array (which side will be referred to as "the upper side" or "above the centre of the array") is less than the total conductance of the heating elements lying on the other (or lower) side of the centre of the array and in that this conductance differential is at least partially attributable to the fact that one or more said elements lying above the centre of the array follows a course of which at least one portion lies adjacent and substantially parallel to a portion of a said bus strip whereby the course of such element is longer than the distance between its points of connection to the bus strips measured along the surface of the substrate.

2. A panel according to Claim 1, characterised in that the outermost heating element at the upper side of the array of said elements, or each of two or more adjacent heating elements including that outermost one is deposited to follow a course of which at least one portion lies substantially parallel to a neighbouring portion of a said bus strip whereby the course of such element is longer than the distance between its points of connection to the bus strips measured along the surface of the substrate.

3. A panel according to Claim 1 or 2, characterised in that the or each element following a said longer course lies, except at a region or regions near the bus strips or one of them, substantially parallel with the element adjacent the centre of the array and on the opposite side thereof to the respective element following a said longer course.

4. A panel according to any preceding claim, characterised in that at least one said bus strip is shaped so that its inner boundary has at least one recess or rebate, and in that that portion of the course of at least one heating element which lies adjacent and substantially parallel with a portion of a said bus strip also lies within a said recess or rebate.

5. A panel according to any preceding claim, characterised in that said bus strips have end portions projecting into a third side margin of the panel and in that there is at least one said heating element following a said longer course which is connected at or towards a said end of a said bus strip and which has an end portion which lies substantially parallel to a said bus strip end portion.

6. A panel according to any preceding claim, characterised in that there is a plurality of said heating elements each following a said longer course and in that such elements all lie above the centre of the array of elements.

7. A panel according to any preceding claim, characterised in that the substrate forms or is adapted to serve as a vehicle window.

8. A panel according to any preceding claim, characterised in that over at least the greater part of their length, the inner boundaries of the bus strips converge so that their spacing is less above the centre of the array than it is below the centre of the array.

9. A panel according to any preceding claim, characterised in that the bus strips each decrease in width in a direction towards an end which is remote from a position where a lead-in wire or terminal element is connected to such a strip.

10. A panel according to Claim 9, characterised in that each bus strip tapers progressively in width over at least the greater part of the distance between the said position where the lead-in wire or terminal element is located and a said remote end of the bus strip.

11. An electrically heatable panel comprising a substrate bearing electrically conductive material which forms bus strips extending along at least the greater part of the length of opposed margins of the panel, electrical resistance heating elements extending between such bus strips at intervals therealong and lead-in wires or lead-in wire terminals connected to said bus strips, characterised in that each said bus strip tapers progressively, decreasing in width over at least the greater part of the distance between the location of said wire or terminal and an end of the strip which is remote from the location of said wire or terminal.

1 2. An electrically heatable panel, substantially as described with reference to any of the figures of the accompanying drawings.