ELECTRICAL HEATING
Field of the Invention This invention relates to thermal fuses and, more particularly, to thermal fuses used in connection with electrical sheet heaters.
Background of the Invention
U. S. Patents Nos. 4,485,297 issued November 27, 1984, 4,523,085 issued June 11, 1985, 4,542,285 issued September 17, 1985, 4,626,664 issued December 2, 1986, 4,633,068 issued December 30, 1986, 4,656,339 issued April 7, 1987, 4,690,347 issued September 1, 1987, 4,749,844 issued June 7, 1988, 4,752,672 issued June 21, 1988, 4,774,397 issued September 27, 1988, 4,849,255 issued July 18, 1989, 4,888,089 issued December 19, 1989, 4,892,998 issued January 9, 1990, 5,019,797 issued May 28, 1991, and 5,038,018 issued August 6, 1991, all of which are hereby incorporated by reference, disclose electrical heating devices in which a semi-conductor pattern is printed on an insulating substrate.
Generally, the semi-conductor pattern includes a parallel of parallel longitudinally-extending stripes and a heating region (e.g., a plurality of bars) extending between the stripes. A pair of longitudinally-extending metallic conductors are juxtaposed with the longitudinally-extending stripes, and the metallic conductors are held in tight engagement with the stripes by a sealing layer that overlies the metallic conductors and is sealed to the substrate on opposite sides of the stripes. In operation, the metallic conductors are connected to a power source, and heat is generated by the resulting current flowing through the semi-conductor pattern.
One use of such heating devices is as a wall or floor heater. In uses of these types, and in other uses in which the heating device is located in a closed area, there is a chance of localized over-heating when, for example, a portion of the heater is closely surrounded or covered by a thermal insulator. In such a situation, it is important that heat not build up to a point where there is a risk of heating some portion of the building or heater to its flame or ignition temperature. Such overheating can be prevented by "turning off the heater or some portion of it before too-high temperatures are reached,
and it would be desirable to accomplish this by providing a thermal fuse that is responsive to local temperature conditions to break the electrical circuit that provides power to the heater or heater portion.
Summary of the Invention
In one of its aspects, the present invention is directed an electrical sheet heater having a heat generating semiconductor pattern on an electrically insulating substrate and a thermal fuse (e.g., a fuse that is responsive to temperature rather than current) attached to the heater in close proximity to the semiconductor pattern and connected in series with the semiconductor pattern. In a second aspect, a thermal fuse useful with such heaters includes a low temperature melting point conductor sandwiched between a pair of insulating sheets and overlying at least one recess in one of the sheets such that the conductor will sag or flow into the recess and break when the temperature of the wire exceeds the solidus temperature of the wire. In preferred embodiments in which the low melt conductor is either a wire or foil, the heater includes a plurality of heating panels, each panel has its own associated thermal fuse, and the fuses and circuitry are such that the flow of power to individual panels may be terminated without affecting power to other panels.
Brief Description of the Drawing
Figure 1 is a top plan view of a multi-panel electrical heater embodying the present invention.
Figure 2 is a top plan view of a panel of the heater of Figure 1.
Figure 3 is a perspective view, partially in section, of a portion of the panel of the heater of Figure 1.
Figure 4 is an exploded perspective view of the panel of Figures 2 and 3. Figure 5 is a plan view, partially in section, of the thermal fuse used in the electrical heater of Figures 1 through 4.
Detailed Description
Referring more particularly to Figures 1 through 4, an electrical heater generally designated 10 includes a plurality of heater panels, each of which is generally designated 12, connected to each other end-to-end. The overall length of a heater depends, of course, on the number of panels it contains. As shown, each panel is about a foot long, and the heater may be cut to a desired length by separating adjacent panels along cut lines 11. Heater comprises a plastic, typically polyester ("Mylar"), substrate 14 about 0.004 to 0.007 inches thick on which a semi-conductor pattern 16 of colloidal graphite is printed, typically by silk-screen printing. The semiconductor pattern includes a pair of parallel longitudinal stripes 18a, 18b spaced about 12 inches apart, and a central or "heating" portion 20 in the area between stripes 18a, 18b. The heating portion 20 of each panel 12 comprises a plurality of substantially identical, parallel semi-conductor bars 22, each about 0.5 inch wide and spaced about 0.125 inches apart, extending perpendicularly between stripes 18a, 18b.
As shown most clearly in Figure 2, stripe 18a is about 1/8 inch wide and extends continuously along one end of all of the bars 22 of a panel. Stripe 18a does not extend between adjacent panels 12; rather the stripe 18a of each panel 12 only connects the bars 22 of the particular panel and terminates a short distance, e.g., about 1/16 inch, beyond the respective bars 22 at the opposite ends of the panel. Stripe 18b, on the other hand, extends the full length of heater 10 and is about 1/4 inch wide. Unlike stripe 18a, the portion of stripe 18b in each panel is not continuous. Rather, stripe 18b has narrow breaks or interruptions (not shown) between adjacent bars 22 of each panel and also between adjacent panels. It will be noted that, in each panel, the distance from the bar 22 at each end of the panel to the end of the panel is about 0.625 inches; and that there thus is an about 1.25 inch space between the heating portions 20 of adjacent panels 12.
A pair of metal conductors or bus-bars 24a, 24b, typically thin, flat tinned copper strips about 0.125 inches wide, extend longitudinally of the heater. Conductor 24a is parallel to, and spaced about 1/4 inch outwardly from, longitudinal stripe 18a. Conductor 24b is juxtaposed over longitudinal stripe 18b. A thin plastic (polyester/polyethylene hot
melt) cover sheet 26 having a total thickness of about 0.005 to 0.007 inches overlies substrate 14, semi-conductor pattern 16 and conductors 24. Although conductors 24 are not themselves bonded to the underlying stripes or substrate, the polyethylene hot melt layer of cover sheet 26 bonds tightly to the substrate 14 on either side of the conductors and holds them tightly in place. Electric leads 26 connect heater 10 to a source of electric power (not shown). As shown, each lead 26 includes a crimp-on connector 28 having pins which pierce the plastic sheets 14, 26 and engage one of conductors 24a, 24b. In addition to the relatively resistive semi-conductor pattern 16, a number of highly conductive electrodes are printed on substrate 14 using a highly conductive ink (e.g., a silver ink comprising a mixture of silver particles in a binder as described in aforementioned patent no. 4,849,255). One silver electrode, desigated 30, is printed over semi -conductive stripe 18b and extends continuously (sandwiched between stripe 18b and bus-bar 24b) the length of heater 10. Additionally, each panel 12 of the heater 10 includes two silver electrodes, designated 32 and 34, respectively, one at each end of the respective panel. Each silver electrode 32 is generally "L" shaped. One arm of the "L"is printed over the semi-conductive stripe 18a of the respective panel; the other arm of the "L" extends inwardly slightly more than half the panel width, spaced slightly from and parallel to the bar 22 at the adjacent end of the panel. The other silver electrode 34, at the other end of each panel, is generally "T" shaped. The cross-bar of the "T" underlies busbar 24a, and the upright of the "T" similarly extends inwardly slightly more than one half the width of the panel. Each of the printed silver ink electrodes 30, 32 and 34 is about 3/16 inch wide.
Each panel 12 also includes a thermal fuse, generally designated 40. Each fuse 40 extends longitudinally of the panel, substantially midway the width of the panel, is positioned so that overlies semi-conductor pattern 16, and is sandwiched between the semiconductor pattern 16 on substrate 14 and cover sheet 26. The opposite ends of the fuse extend beyond the bars 22 at the opposite ends of the panel end to, and are connected, to electrodes 32, 34. It will thus be seen that, when electrical power is applied to heater 10 through leads 26, the current path in each panel 12 extends from bus-bar
conductor 24a through electrode 34 to fuse 40, through fuse 40 to electrode 32, through electrode 32 (including the portion of electrode 32 that is juxtaposed with stripe 18a) to bars 22, and then through bars 22 to juxtaposed stripe 18b, electrode 30 and conductor 24b. As discussed in, e.g., aforementioned patent no. 4,485,297, the watt density produced by the panels 12 of heater 10 is a function of the resistance of the portion of the pattern forming bars 22, the number of bars per unit length of the heater, the bar width, and bar length, and the applied voltage, and can be varied as desired for the particular intended application.
Referring more particularly to Figures 4 and 5, thermal fuse 40 includes a low melting point wire 42 sandwiched between a pair of polyester ("Mylar") expansion layers 44, 46, each of which is about 0.007 inches thick. Each expansion layer includes a series of aligned oblong (0.25 inches long and 0.13 inches wide) or circular (0.25 inches in diameter) holes 48, spaced apart on 0.38 inch centers so that the distance between the adjacent edges of adjacent holes is about 0.13 inches. As shown, wire 42 overlies, and is aligned with the centers of, holes 48. A conductive silver electrode 50 is printed transversely across the inner side of each end of expansion layer 44, so that one of the electrodes 50 will engage each end of wire 42. The inner side of expansion layer 46, i.e., the side facing wire 42 and expansion layer 44, is coated with a pressure sensitive adhesive so that the two expansion layers will bond tightly together and hold the wire 42 in position between them.
It will be noted that expansion layer 46 (1.5 inches wide) is substantially wider than expansion layer 44 (0.5 inches wide), and that four holes 52 are provided at each end of each expansion layer, on opposite sides of electrode 50 and closely adjacent the opposite side edges of the expansion layer. As shown most clearly in Figures 2 and 3, since expansion layer 46 is wider than expansion layer 44, the printed silver electrodes 50 at the opposite ends of expansion layer 46 extend beyond the side edges of expansion layer 44 and, on the opposite sides of expansion layer 44, engage a respective one of electrodes 32, 34. Top cover sheet 26 bonds to substrate 14 through the holes 52 in expansion layer 46, and holds the juxtaposed silver ink electrodes at the opposite ends of
each panel 12 tightly together, thus insuring good current flow between electrodes 32, 34 and wire 42.
In addition to the wire 42 and expansion layers 44, 46, the thermal fuse 40 of the preferred embodiment also includes a top assembly layer 52 which overlies expansion layer 44 and thus is positioned between expansion layer 44 and the top sheet 26 of heater 10, and a dielectric layer 54 between expansion layer 46 and the bars 22 of semiconductive pattern 16. As will be seen, the dielectric layer 46 insures that, despite holes 48, wire 42 is electrically isolated from the bars 33 of the semiconductor pattern. Assembly layer 52 insures that the hot melt adhesive on the inner side of cover sheet 26 does not bond to wire 42 through holes 48 in expansion layer 36, which could happen without the assembly layer.
It will be appreciated that, in the manufacture of a heater 10, the structure comprising thermal fuses 40 typically extends the full length of the heater. To electrically isolate the fuse 40 in one panel from the fuses in other panels, and thus insure that the individual panels are individually connected in parallel between conductors 24, 24a, a hole 60 is punched through a portion of the fuse including wire 42, severing the wire, at each end of each panel.
The current passing through wire 42, and the rest of heater 10, is relatively small; a typical applied power is 17 watts per panel at 240 volts. Accordingly, wire 42 itself carries relatively little current and may be quite small in cross-section. In the illustrated embodiment, wire 42 is circular in cross-section; in other embodiments a thin tape of rectangular cross section, may be employed. The diameter of a wire, or thickness and width of a tape, will of course vary depending on the required current carrying capacity. In the illustrated embodiment, wire 42 has a diameter of about 0.013 inches; in similar heaters, thin tapes or foils about 1/8 inch wide and about 0.003 inches thick may be employed. In either event, the wire or tape is made of a low melting point metal alloy, e.g., a 50% Bi, 25% Pb, 12.5% Sn, 12.5% Cd alloy having a liquidus temperature of 73 deg. C (165 deg. F) and a solidus temperature of 70 deg. C (158 deg. F). The wire used in the preferred embodiment is sold by Belmont Metals, Inc. of Brooklyn, NY, which
also sells a wide range of other low melting alloy wires and tapes. Similar low melting alloys are commercially available from other sources.
It will be apparent from Figure 1 that the individual panels 12 of heater 10 are electrically connected in parallel between conductors 24a and 24b. Thus, if the thermal fuse 40 of any particular panel breaks, electrical power will no longer flow through that particular panel, but will continue to flow (and to cause heat to continue to be generated by) the other panels 12 of the heater.
In operation, heaters such as heater 10 are typically attached to the joists in a home or similar building structure and are mounted horizontally. When the heater is mounted in such a way that heat is freely conducted away from it, the relatively low power flow through the heater results in the temperature of the heating portions 20 of the heater panels, and of the regions adjacent them, to remain quite low, typically in the range of about 120 degrees F to 130 degrees F, and only rarely as high as about 150 degrees F. However, there are circumstances in which heat flow in the region around the heater is impeded and the temperature may build up in or near some panel 12 of the heater. The wire 42 in the fuse 40 of each panel is sensitive to the temperature in the region adjacent to the wire and to that of adjacent portions of the particular panel 12. If the temperature in the region adjacent the heater, or that of the heater panel 12 of which the fuse 40 is a part, should rise to above the solidus temperature of the alloy from which wire 42 is made, the wire 42 will soften. If the temperature exceeds the solidus and, typically before the temperature reaches the liquidus temperature (at which the wire will melt), portions of the wire overlying holes 48 in expansion layers 44, 46 will sag or flow under the force of gravity into one or more of holes 48, breaking the wire and terminating flow of power through the associated panel. The wire 42 typically breaks when a portion of it sags into a hole 48, at temperatures only slightly above the solidus temperature, even though the depth of the hole 48 is only about half the wire diameter. In any event, the wire will break when its temperature reaches the liquidus temperature and the portion of the wire overlying a hole melts and flows into the hole.
As should be evident, the desired solidus/liquidus temperatures of the material used in a thermal fuse such as fuse 40 depends on the temperature at which it is desired or important for the fuse wire to break, e.g., to disconnect a heating panel of which the fuse is a part from the heating circuit; and the particular alloy used in the fuse wire is selected depending on the intended use of the fuse. For example, the ignition or flame temperature of paper is typically about 450 deg. F, and that of other building materials typically is higher. As indicated above, it is desirable to insure that the wire 42 of a thermal fuse 40 will break before the temperature in the region adjacent a heater panel and potentially flamable building material reaches the ignition/flame temperature of the material. Accordingly, according to the present invention, the alloy from which wire 42 is made is chosen so that the wire will soften/melt, and break the electrical circuit heating the associated heater panel, at a temperature well below the ignition/flame temperature of the heater or adjacent materials wire 42. In heaters such as those discussed above, the tape or wire is made of an electrically conductive metal alloy that has a solidus temperature no more than about 300 degrees F, and in most circumstances not more than about 200 degrees F (about 110°C). In the particular embodiment disclosed herein, the solidus temperature is about 150°F (a little more than 70°C).
It will be evident that, according to the present invention, thermal fuses may be used with devices other than electrical heaters of the type discussed above; and that when used in a sheet heater such as those discussed need not be positioned overlying the heating region.
Thus, while the invention has been described in connection with what is presently considered to be a practical and preferred embodiment, it is to be understood that the invention is not to be limited by the disclosed embodiment. On the contrary, the invention includes and covers any of the various modifications and arrangements that are within the spirit and scope of the following claims, including but not limited to those that may be equivalent to or obvious in view of the embodiments specifically disclosed herein.
What is claimed is: