[001] This invention relates to flexible printed circuit boards (PCBs), and in particular to elongated PCBs adapted to carry components at different locations. The invention also relates to an arrangement for connecting sensors at different locations. The invention also relates to an improved arrangement for applying sensors to water heater tanks. Background of the invention

[002] In the water heater control field, an indication of the temperature of the water in the tank is usually required.

[003] Where the water is heated from the bottom, a single temperature sensor at the top of the tank has been used, because convection causes the hot water to rise to the top of the tank. However, in the quest for greater efficiency and convenience, other methods of heating water are being implemented. These other methods can include the use of processor controlled arrangements to control the amount of energy used to heat the water. The use of processors has opened up the possibility of using multiple temperature sensors to more precisely control the temperature. This has application, for example, in "top down" water heating applications, where the hot water enters at the top of the tank. Such arrangements usually include a water circulation path so that the cooler water is displaced from the bottom of the tank into a heat exchanger or other source of heat, from whence the heated water enters the top of the tank. This mode of operation can result in a fairly clear temperature stratification, with a layer of hot water above a layer of cooler water. In some applications, it may be desirable to use temperature sensors to locate the thermal transition layer. This can be done using a plurality of temperature sensors located at different heights in relation to the tank. The temperature sensors can be associated with a controller which can be used to set the location of the thermal transition, so that the effective capacity of the tank can be adjusted to maximize efficiency for the expected average load. For example, the temperature sensors may be located to detect when the thermal transition is at a selected zone, such as one quarter capacity, half capacity, three quarters capacity and full capacity, so that only the required amount of water is heated to the top of the predetermined operating temperature range, at which point the heat input can be turned off until the temperature of the water in the selected zone falls below the set minimum operating point. [004] In the field of printed circuit boards (PCBs), components are traditionally mounted on hard PCBs and flexible PCBs are used to connect between circuits in different positions within a piece of equipment.

[005] Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates, at the priority date of this application. Summary of the invention

[006] An embodiment of the invention provides an ambient condition sensor arrangement including one or more elongated flexible printed circuit strips to which one or more ambient condition sensing components are attached at predetermined locations.

[007] The or each circuit strip can have conductor tracks on a first side of the strip, and the or each sensor can be located on said first side and connected to two or more corresponding tracks.

[008] The conductor tracks can be formed on the flexible PCB using screen printing.

[009] The conductor tracks can be formed using a silver screen printing process.

[010] The sensors can be located at different distances along the strip.

[011] The sensors can be temperature sensors applied to a water heater tank

[012] The PCB can be a flexible ribbon cable.

[013] The conductor strip can be in the form of a flexible printed circuit board.

[014] The conductor strip can carry sets of conductor tracks for each sensor.

[015] The conductor strip may have a common conductor to which one terminal of two or more sensors is connected, and an individual signal line can be provided for each sensor.

[016] Alternatively, a two line bus using an appropriate communication protocol, such as I ² C, can be utilized where the electronic components include the appropriate interfaces. •

[017] The printed circuit strip is adapted to be attached to the external surface of a water heater storage tank with a second side of the printed circuit strip opposite to the first side contacting the external surface of the tank.

[018] A first temperature sensor can be located to obtain a measurement of the temperature in the proximity of the top of the tank.

[019] Second and third sensors can be located to measure the temperature of at second and third predetermined locations.

[020] Reinforcing patches can be mounted on the strip to reduce stress on the components.

[021] Vias or plated through holes can be provided to connect a conductor on one side of the strip with a conductor on the other side of the strip.

[022] The sensors can be surface mounted components.

[023] The strip can include end-connection pads at least one end adapted for connection to a second flexible printed circuit strip.

[024] One strip can include first end connection pads at a first end and second end connection pads at a second end.

[025] The first end connection pads can be accessible for connection through the front face, and the second end connection pads are accessible for connection through the second face.

[026] End connector pads of adjoining strips can be connected.

[027] Preferably, the strip-joining contact of a first strip is on the opposite face to the mating strip-joining contacts of a second strip.

[028] The connector arrangements can be in the form of one or more solder pads.

[029] The connectors can be zero insertion force connectors.

[030] In a further embodiment, an intermediate connector is used to join adjacent strips.

[031] Preferably, such a connector comprises a connector strip having strip-joining connection contacts at either end which are, in a "face down" orientation, adapted to match the strip-joining contacts of the strips being joined.

[032] The pads of the first connector can be on one side of the strip and the pads of the second connector can be on the other side of the strip.

[033] According to a further embodiment of the invention there is provided a temperature sensor arrangement for a water heater storage tank, wherein the strip has a first face in contact with the tank and a second face opposite the first face on which one or more sensors are mounted.

[034] According to a further embodiment of the invention there is provided a water heater temperature control system including a processor control system having an input to which a flexible printed strip is connected by two or more end connector pads.

[035] In alternative embodiment, a first strip is arranged with its component side facing away from the water heater tank, and a second strip is arranged with its component side facing the water tank, so that the strip-joining contacts face-to- face and can be joined, for example, by soldering. In such an arrangement, an electrical insulating layer can be inserted between the strip and the water tank.

[036] The sensors can be thermally insulated from the ambient atmosphere by a layer of insulation used to insulate the tank.

[037] The thermal insulation may be an insulation blanket, or a foam injected between the outside of the tank and the housing of the system.

[038] A further embodiment of the invention provides a control system including a processor control system having an input to which a flexible printed strip is connected by two or more end connector pads. Brief description of the drawings

[039] An embodiment or embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[040] Figure 1 shows a water heater tank to which a connector strip is attached;

[041] Figure 2 shows the elements of a strip utilized in forming a connector strip;

[042] Figure 3 illustrates a connector strip;

[043] Figure 4 illustrates a component suitable for attachment with the strip of Figure

3;

[044] Figure 5 A is a plan view of a connector strip;

[045] Figure 5B is a sectional view of the strip of Figure 5 A along line A-A;

[046] Figure 5C is a sectional view of the strip of Figure 5A along line B-B;

[047] Figure 5D shows an alternative embodiment of a view similar to Figure 5C;

[048] Figure 6 is a plan view of a strip-joining connector;

[049] Figure 7 is a schematic representation of a strip with thermistors attached;

[050] Figure 8 is a representation of a further embodiment of the invention having one common conductor;

[051] Figure 9 shows a pair of strips adapted to be connected in series; and

[052] Figure 10 shows diagrammatically details of a cross section through an assembly of components.

Detailed description of the embodiments

[053] The invention is described in relation to the use of two or more ambient condition sensors in the control of a water heater system, but is of more general application. In the field of water heater systems, individual temperature sensors have been inserted into the tank and connected to a relay driver circuit or the like to switch a heating element on or off. [054] A particular problem which has been encountered in implementing this invention is that the length of conductor strip which is available can be less than the distance over which the sensor readings are to be taken. Accordingly, strips have been designed with strip-joining contacts or end-connection pads to enable an end of one strip to join an end of another strip or to a connector. However, the invention is also applicable to a single strip. [055] Generally, flexible PCBs are used as connectors rather than for mounting components. In applying this invention to water heater tanks, advantage can be taken of the fact

that, where a water heater tank is made of a thermally conducting material, a temperature sensor may be affixed to the outside of the tank and in contact with the tank. The use of a thin flexible PCB means that temperature sensors can be mounted on one side of the PCB and the other side can be in contact with the outside of the tank. The thinness of the flexible PCB permits the heat to transfer through to the temperature sensor on the other side. Preferably, the side in contact with the tank does not carry any conductive tracks or connection points, although conductive tracks can be applied to the side in contact with the tank if they are adequately insulated. [056] Where a number of such sensors are to be used in conjunction with a controller, they can be connected by individual wires to the controller. However, this requires a number of individual wire/component connections to be soldered.

[057] This invention proposes the use of a conductor strip on which two or more sensors are mounted. The conductor strip can be mounted on the outside of a water heater tank or other environment where it is required. The conductor strip forms a flexible substrate to carry the electronic components.

[058] Previous flexible PCBs have been comparatively short.

[059] Figure 1 shows a water heater tank 100 to which a connector strip 102 has been affixed. The strip can be affixed to the tank by adhesive, adhesive tape or other suitable means. The connector strip is connected at one end to a controller 106.

[060] Electrical components 104 may be affixed to the connector strip at intervals.

The components in this embodiment are temperature sensors, for example, thermistors, which are two terminal devices whose resistance changes with changes in temperature. Thus the sensors may only require two terminals to connect them back to the controller [061] When the tank is in its complete form, it includes an outer jacket and the space between the outer jacket and the tank is packed with insulating material (not shown). The insulating material may be packed sufficiently tightly so that it holds the connector strip 102 in place. Thus the strip only needs a sufficient attachment to hold it in place while the outside jacket and insulation are installed.

[062] The strip can be made of a base of a flexible insulation material which can be sufficiently thin so that the heat transfers from the tank through the base material to the sensor. The flexibility of the strip ensures adequate contact between the strip and the tank surface for the required thermal transfer. Thus the sensors do not need to be mounted in direct contact with the tank wall.

[063] Where the strips have the contact pads on one side only, alternative strips may be mounted so that the sensors of one strip are on the opposite side to the tank wall, and on the

second strip, the sensors may be in direct contact with the tank wall. This enables the contact pads of one strip to connect directly to the connector pads of the other strip.

[064] In those embodiments in which the conductive tracks are facing the tank, an additional electrical insulator can be interposed between the flexible PCB and the tank.

[065] Figure 2 shows the basic construction of the connector strip. A base insulator

202 has conductive tracks 204 printed thereon. An insulating cover 306, shown in exploded view, is applied over the tracks and lower insulator.

[066] As shown in Figure 3, a plurality of conductive tracks can be formed on the insulating base 302. The tracks may be formed in pairs 304 where the components to be affixed are two terminal devices. The pairs of tracks may terminate at predetermined distances along the strip, usually with connection pads 306. Thus pair 304 terminates before pair 314, and pair 308 continues to the end of the strip.

[067] Components such as thermistors can be soldered to the contact pads which are located at the ends of pairs of conductors 306 and 314.

[068] The pair of conductors 308 can serve as strip-joining contacts for connection to another strip.

[069] The "upper" insulation layer 316 includes "window" cut-outs, such as 310 which correspond to pads 306 when the layer 316 is superimposed on the layer 302. This provides access to permit a sensor component to be soldered onto the pads. While individual windows are shown corresponding with each connector pad, a "double " window can be used to expose pairs of connector pads as shown at 1004 in Figure 10.

[070] Figure 4 shows a surface mount component 402 having solder pads 404, 406.

These pads are suitable for connection to the pads such as 306 in Figure 3. The pads 404, 406, and/or the pads 306 can be pre-soldered to facilitate the soldering of the component to the strip.

The component 402 can thus be soldered to the strip in a surface mounting operation. Surface mounting has the advantage compared with "through hole" component connections that the component connections are on the same side as the copper rack and the component. Thus the surface mounted component connections can be arranged on the opposite side of the PCB from the water tank, while through hole connections would be exposed to short circuiting against the tank. [071 ] Figure 5 A shows a plan view of a strip adapted at the right hand end to connect to either a device such as a controller or further strip via connector pads 508, and to connect to components via intermediate pads such as 506, and to connect to a further strip via pads 509.

[072] Figure 5B shows a sectional view of the strip of figure 5 A along line A-A. the base member 502 carries conductor 518. Insulation cover 514 overlies the strip 502. Holes 510, 512 are cut out of insulation cover 514 to expose the underlying pads 508, 506. These pads may be pre-soldered, as mentioned above, to facilitate the attachment of surface mounted components such as 402 in Figure 4. The insulation cover 514 is thin and flexible and conforms to the surface around the conductors and the surface of support base 502 as shown at 516. The base 502 and the conductor 518 are also thin and flexible. The thickness and other dimensions of these elements are not shown in proportion for the sake of clarity. The area of the pad is large in comparison with the thickness of the insulation cover 514 and permits surface mounting of components.

[073] Figure 5C shows a sectional view of the strip of Figure 5 A along line B-B. A conductor 520 runs the full length of the strip and ends in pads 508, 509 which are exposed by cut-outs in the insulation cover 514.

[074] Figure 5D shows an alternative arrangement to that of figure 5C, in which the strip-joining pads 508 at a first end of the strip are exposed through cut-outs in the insulation cover 514 to form "upper pads", and the pads 509 are exposed through cut-outs in the support base 502 at the other end of the strip to form "lower pads". This arrangement allows the ends of adjoining strips to be directly connected by overlaying the upper pads of one strip with the mating lower pads of the second strip. These connections can be made using surface mounting techniques.

[075] Where the 'lower pads' are not provided, the connector member of figure 6 can be used. This connector member can be jnade of a similar structure to the strips and can have mating connector pads 602, 604 at either end connected by conductors 606 and adapted to connect to the "upper pads" of a first and a second strip. The number of pads and strips can be chosen as required.

[076] Figure 7 schematically represents a strip 700 with a pair of thermistors, 714,

716, connected.

[077] Thermistor 714 is connected via conductors 704 to pads 702.

[078] Thermistor 716 is connected via conductors" 708 to pads 706.

[079] A pair of through-conductors 712 connect a first pair of pads 710 to a second pair of pads 718.

[080] The pads 702, 706, 710 may be connected to a connector of a controller or to a further strip. The pads 718 may connect to a further thermistor on a second strip.

[081] While the embodiments have shown three sets of conductors, the invention is suitable for connecting several components via printed conductors.

[082] The strip shown in Figure 7 is not to scale, as, in practice, the length would normally be many times the width.

[083] While the components are shown as having two terminals, the strips can be used to connect components having three or more terminals to an equal number of conductors.

[084] Figure 8 illustrates an embodiment of the invention in which one common conductor is used to connect to one of the terminals of the sensors.

[085] Figure 8 shows a sensor strip 800 having a plurality of conductors 804, 806, 810,

812, 818 on a flexible PCB 802.

[086] Conductor 804 is a common conductor to which two or more components may be connected. Conductor 806 terminates in a connection pad 814 and common conductor 804 carries a matching pad to provide for the connection of a first component.

[087] Conductor 816 bends towards common conductor 804 after conductor 806 has terminated. Conductor 816 terminates in pad 820 which is spaced from a second pad on common conductor at a distance to facilitate the connection of a second component.

[088] Conductor 818 then bends towards common conductor 804. This may occur on the strip 802 or on a second strip 9 not shown) to which strip 802 is connected via connection pads 822.

[089] The further conductors 810, 812, also can be connected through to a second strip via connector pads 822. These conductors may also be bent into proximity with common conductor 804 to connect to pads at locations on the second strip where further sensors are to be attached.

[090] Strip 800 can be connected to the control circuitry via connection pads 824.

[091] In Figure 8, one of the conductors, 804, carries a common reference potential.

In this case, two or more sensors may connect to this single conductor 804. This can be achieved by printing the tracks to bend towards the common track after each previous track ends, as shown by tracks 814, 818. This provides the same distance between surface mount pads such as

81. Thus, a system may be used where, for two terminal devices, only one track per device, plus the common track are required. Of course, the same result can be obtained by bending the common track after each intervening track ends. Other track configurations can be used. All that is required is that the space between the pads at the point where the sensors are to be attached is the appropriate distance for attaching the sensors.

[092] Figure 9 shows a pair of strips which are adapted to be connected in series. A first strip 900 has a plurality of conductors which terminate at a first end in an end connector 902 made up of a plurality of end connector pads. A common conductor 904 is formed to make a component connector 906, 908 with the adjacent conductors, each adjacent conductor terminating at its respective component connector and the common conductor 904 being stepped across to the correct component spacing at each succeeding component connector. The end connector 902 can be connected to a SMD connector attached to a flexible cable connected to a control board.

[093] The continuation conductors (those which have not terminated in a component connector) and the common conductor 904 form a strip-joining connector 910 at the second end of the strip.

[094] The second strip 901 has a strip-joining connector 903 at a first end, which is adapted to be connected to strip-joining connector of strip 900. As described above, this may be done in a number of ways, including having connector 910 on the upper surface and connector

903 on the under-surface, providing a joining piece having appropriate strip-joining connectors at either end, or facing strip 901 in the opposite direction to strip 900 so that solderable surface of the pads of connector 903 overlie solderable surface of the pads of connector 902. As can be seen, this arrangement permits strips to be joined to form a single strip.

[095] We have found that the FR4 laminate is suitable as the flexible PCB. Single sided 0.2 mm FR4 laminate provides a suitable degree of strength and flexibility. The conductor tracks can be covered with a layer of insulating photoresist or other polymer coating which can have apertures accurately located to expose the connection pads. An alternative material for the

PCB is polyamide.

[096] If a double sided PCB laminate is used, vias can be provided to connect connector pads on one side to conductor tracks on the other side of the laminate.

[097] In a further embodiment, additional layers of laminate can be attached opposite each component attached to the PCB to provide additional rigidity in the region of the components.

[098] Figure 10 shows a component 1002 connected to two conductors 1010 and 1020 through an aperture 1004 in the insulation 1022. The component has two solder pads 1006 and

1008.

[099] Also shown in Figure 10 is the addition of two "reinforcing patches" 1012 and

1014 of laminate or other suitable material mounted on the opposite side of the laminate 1000.

The first layer 1012 extends beyond the connector pads of component 1002. The second patch

1.014 can be somewhat snorter than the first patch, but can still extend beyond the ends of the component 1002. This arrangement thus provides a progressive stiffening of the strip in the region of the component. However, a single stiffening patch, or more than two stiffening patches can also be used. A patch having a tapered trapezium cross-section can also be used.

[0100] In an alternative, the reinforcing patch may be an hollow device, rectangular, annular or other suitable shape, with sufficient stiffness and adapted to encompass the component. This device can be mounted on the same side as the component.

[0101] Also shown in Figure 10 is a via or through-hole 1016 which connects the conductor 1010 on the top side of the strip to the connector pad 1018 on the lower side of the strip.

[0102] In the case where an I ² C bus is used, each sensor is provided with an interface chip an a protocol control chip is connected between the controller and the bus to enable each individual sensor's interface chip to communicate with the controller.

[0103] Where ever it is used, the word "comprising". is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of. A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear.

[0104] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text. AU of these different combinations constitute various alternative aspects of the invention.

[0105] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein.