Abstract:
An electrical resistance heating element includes a pair of spaced apart electrical conductors, and a strip of resistive material disposed between the conductors and extending along their lengths. A strip of heat-sensitive, electrically conductive material is also disposed between the conductors and also extends along their lengths. The strips of resistive material and heat-sensitive, electrically conductive material together provide an electrical path between the conductors widthwise along their lengths. The heat-sensitive strip interrupts an electrical path between the conductors and widthwise through the resistive material strip only over portions of the resistive material strip which attain a predetermined temperature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical resistance elements, and more particularly relates to an electrical resistance element having a heat-sensitive disconnect capability. 
     2. Description of the Prior Art 
     Conventional electrically energized, elongated heating elements currently on the market and commonly referred to as heating cords generally utilize high resistance metallic conductors, such as nichrome wires, embedded within a plastic substrate, whereby the conductors are coupled in series in order to form a complete electrical path through the element and thus generate heat. A common use for such elongated heating elements is to wrap the element around a water pipe to prevent the pipe from freezing. 
     One of the major problems with such commercially available heating elements is that they may produce &#34;hot spots&#34;, for example, if they are double wrapped such that a portion of the element is wrapped over another portion. The hot spots occur at the double wrapped portions of the elongated heating element. The element may overheat and cause a fire. Usually, such resistance elements are sold with warning labels advising the consumer not to double wrap the element. 
     Another problem with many conventional elongated heating elements is that they have a fixed length. Those conventional elements that may be cut to a desired length require that the two nichrome wires be reconnected at the ends that are cut to form a closed circuit. This requirement is not only an annoyance but also could be dangerous when 115 volts is employed in energizing the element. 
     U.S. Pat. Nos. 4,758,815 and 4,823,106, each of which issued to the present inventor, Walter Lovell, the disclosures of which are incorporated herein by reference, disclose a heat generating, electrical resistance tape element. The resistance tape element basically includes a pair of flat, parallel, spaced apart ribbon conductors and an elongated strip of commercially available magnetic recording tape which at least slightly overlaps and contacts the ribbon conductors. The electrical conductors and magnetic recording tape are laminated between top and bottom plastic covering sheets. The laminated heating tape disclosed in the Lovell U.S. patents mentioned above has many advantages over conventional heating elements including its ability to be rolled up tightly for shipment, its capability of being manufactured inexpensively and its ability to be cut to a desired length without the need to connect the terminal portions together prior to use. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an electrical resistance element with a heat-sensitive disconnect capability. 
     It is a further object of the present invention to provide an electrical resistance element in the form of a heating tape which is continuous and may be cut to any desired length without requiring an electrical connection at the cut portion. 
     It is another object of the present invention to provide an electrical resistance element in the form of a continuous tape, which element will continue to provide heat over its entire length except for specific portions where a hot spot occurs. 
     It is another object of the present invention to provide an area heater having a heat-sensitive disconnect capability. 
     It is yet another object of the present invention to provide an electrical resistance element which overcomes the disadvantages of known electrical resistance elements. 
     In one form of the present invention, an electrical resistance heating element includes a pair of spaced apart electrical conductors, and a strip of resistive material, which becomes warm when current passes through it, disposed between the conductors and extending along their lengths. Another strip of a heat-sensitive, electrically conductive material is also disposed between the conductors and extends along their lengths. The strips of resistive material and heat-sensitive, electrically conductive material together provide an electrical path transversely between the conductors along their lengths. The heat-sensitive strip will interrupt an electrical path between the conductors only over portions of the resistive material strip which attain a predetermined temperature. 
     In one preferred form of the invention, the strip of resistive material contacts one conductor along its length, and the strip of heat-sensitive conductive material contacts the strip of resistive material and the other conductor along their lengths to provide an electrical path between the conductors and through the resistive material. The heat-sensitive, electrically conductive material, in one form of the invention, may shrink in width wherever a &#34;hot spot&#34; occurs along the length of the resistive material. At those portions of the resistive material strip where the heat-sensitive material has shrunk in width, the heat-sensitive material no longer contacts either the resistive heating material or one of the conductors so that no current will flow transversely through the resistive heating material over those over-temperature portions. 
     In another form of the invention, the heat-sensitive material retains its original width at elevated temperatures but &#34;bubbles&#34; or forms discontinuous droplets at the hot spot areas to interrupt the flow of current through the resistive material at those areas. 
     Preferred forms of the electrical resistance element, as well as other embodiments, objects, features and advantages of this invention, will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a top perspective view of a portion of an electrical resistance heating element formed in accordance with one form of the present invention. 
     FIG. 2 is a top perspective view of a second embodiment of the electrical resistance element of the present invention. 
     FIG. 3 is a top perspective view of a portion of a third embodiment of the electrical resistance element of the present invention. 
     FIG. 4 is a top perspective view of a fourth embodiment of the electrical resistance element of the present invention. 
     FIG. 5a is a perspective view of one form of an area heater formed in accordance with the present invention. 
     FIG. 5b is a partial perspective view of another form of an area heater formed in accordance with the present invention. 
     FIG. 6 is a top perspective view of a sixth embodiment of the electrical resistance element of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1 of the drawings, it is seen that an electrical resistance element 1 for use in heating pipes, for example, and constructed in accordance with a first embodiment of the present invention basically includes first and second electrical conductors 2, 4, in spaced relationship from each other. Preferably, the conductors 2, 4 are flat, parallel, elongated ribbon conductors. The resistance element further includes first and second elongated strips of resistive material 6,8 which became warm when sufficient current is passed through them. In one preferred mode, and as disclosed in U.S. Pat. Nos. 4,758,815 and 4,823,106 mentioned previously, the disclosures of which are incorporated herein by reference, the first and second resistive material strips 6,8 may be formed from magnetic recording tape and will be referred to hereinafter as first and second magnetic recording tape portions. The electrical resistance heating element also includes an elongated strip of heat-sensitive, electrically conductive material 10. 
     The first electrical conductor 2 preferably includes inner and outer edges 2a, 2b. Similarly, the second electrical conductor 4 includes inner and outer edges 4a,  4b. The first and second electrical conductors 2, 4 are preferably in spaced relationship from each other and define a specified conductor gap width between their inner edges 2a, 4a. 
     The first magnetic recording tape portion 6 also preferably includes inner and outer edges 6a, 6b. The inner edge 2a of the first electrical conductor 2 electrically communicates with the outer edge 6b of the first magnetic recording tape portion 6. 
     The second magnetic recording tape portion 8 preferably also includes inner and outer edges 8a, 8b. The first and second magnetic recording tape portions 6, 8 are preferably in spaced relationship from each other and define a specified tape gap width between their inner edges 6a, 8a. 
     The heat-sensitive strip 10 preferably includes opposite first and second edges 10a, 10b, defining the outer edges of the heat-sensitive strip. Further, the heat-sensitive portion has at least the specified tape gap width. The first edge 10a of the heat-sensitive portion 10 is in electrical communication with the inner edge 6a of the first magnetic tape portion 6. The second edge 10b of the heat-sensitive portion 10 is in electrical communication with the inner edge 8a of the second magnetic recording tape 8. 
     The inner edge 4a of the second conductor 4 is in electrical communication with the outer edge 8b of the second magnetic recording tape portion 8. The first and second conductors 2, 4, the first and second magnetic tape portions 6, 8, and the heat-sensitive portion 10 are in electrical communication with each other, thereby allowing electrical current to flow through them widthwise along the entire length of the resistance heating element. 
     The current flow through the heating element 1 raises the temperature of the magnetic recording tape portions 6, 8, thereby generating the desired heat. The heat-sensitive portion 10 substantially retains its width 11 at the normal operating temperature of the resistance heating element so that an electrical path is provided transversely through the magnetic recording tape portions 6, 8 along the entire length of the resistance heating element. However, the heat-sensitive portion 10 reduces its width 11 at a predetermined higher temperature so that, wherever a &#34;hot spot&#34; occurs over the length of the resistance heating element, the heat-sensitive strip will shrink in width at those areas and no longer make contact with at least one of the first and second tape portions 6, 8 only at those hot spots in the element where the heat-sensitive strip has shrunk. Thus, current no longer passes through those areas where the magnetic recording tape has overheated, although current continues to pass through the rest of the magnetic recording tape portions 6, 8 which has not overheated and is operating at a normal temperature. 
     The electrical resistance element 1 of a first embodiment as shown in FIG. 1 can be wrapped around a pipe to heat the pipe to a desired temperature. When wrapping the element 1 around a pipe, for example, it is possible that some sections of the resistance element will overlap each other, resulting in a &#34;hot spot&#34;. The heat-sensitive strip will cause a discontinuity in the electrical path between the conductors and through the magnetic recording tape portions only at the &#34;hot spot&#34; in the element, thereby reducing the temperature over the portion of the resistance element which overheated. 
     A second embodiment of the electrical resistance element 1 previously described is shown in FIG. 2. The electrical resistance element 1 includes first and second conductors 2, 4, first and second magnetic tape portions 6, 8, and a heat-sensitive portion 10, as in the previous embodiment shown in FIG. 1. However, in the embodiment shown in FIG. 2, the first and second magnetic tape portions 6, 8 have a partially overlapping relationship with the first and second conductors 2, 4. More specifically, the outer edges 6b, 8b of the first and second magnetic recording tape portions 6, 8, and the inner edges 2a, 4a of the first and second conductors 2, 4 have portions in overlapping relationship in a specified overlap area. 
     It is desirable to at least partially overlap the magnetic recording tape portions 6, 8 and the conductors 2, 4 for a number of reasons. First, overlapping allows the electrical components of the resistance element to be laminated together in a protective covering, as will be described in greater detail. Second, the heat output (and current passing widthwise through the magnetic recording tape portions 6, 8) of the resistance element can be controlled. The edge portions of the magnetic recording tape strips which are overlapped with the conductors 2, 4 are effectively shorted out, and current passes through substantially only the non-overlapped width of the magnetic recording tape strips 6, 8, and only the non-overlapped portions will generate heat. The disconnect capability of the resistance heating element will still be present as long as the heat-sensitive strip can shrink to a width such that at least one of its edges is no longer in electrical contact with the first or second magnetic recording tape portions 6, 8. 
     As mentioned previously, the electrical resistance element may include a protective covering. As shown in the embodiment of FIG. 2, a first non-conductive cover 14 and a second non-conductive cover 16 are provided for this purpose. The first and second conductors 2, 4, the first and second magnetic recording tape portions 6, 8, and the heat-sensitive portion 10 are sandwiched between the first and second covers 14, 16. 
     The outer edges 2b, 4b of the first and second conductors 2, 4 are positioned away from the outer edges 14a, 14b, 16a, 16b of the first and second covers 14, 16 to form margin portions enabling the first and second covers 14, 16 to be tightly laminated to each other within the margin portions. The first and second magnetic recording tape portions 6, 8, the first and second conductors 2, 4, and the heat-sensitive portion 10 are pressed firmly and thus laminated together between the first and second cover sheets 14, 16, in a similar manner to that described in the previously mentioned Lovell patents. Thus, the conductors 2, 4, the magnetic recording tape portions 6, 8, and the heat-sensitive portion 10 continually maintain firm contact with each other as the tape is wrapped or curled without the need for electrically conductive adhesives between the contacting portions of the resistance heating element. 
     A third embodiment of the electrical resistance element 1 previously described is shown in FIG. 3. The electrical resistance element 1 includes first and second conductors 2, 4, a heat-sensitive portion 10, and a magnetic tape portion 6, as in the previous embodiment shown in FIG. 1. However, in the embodiment shown in FIG. 3, only one magnetic tape portion 6 is included in the electrical resistance element 1. The inner edge 6a of the magnetic tape portion 6 is in electrical communication with the first edge 10a of the heat-sensitive portion 10. The first edge 10a of the heat-sensitive portion 10 electrically communicates with the inner edge 6a of the first magnetic recording tape portion 6. The second edge 10b of the heat-sensitive portion 10 electrically communicates with the inner edge 4a of the second electrical conductor 4. Thus, the heating element 1 is provided with one magnetic tape portion 6 for heating but operates in the same manner as the embodiments previously described. 
     A fourth embodiment of the electrical resistance element 1 previously described is shown in FIG. 4. The electrical resistance element 1 includes first and second conductors 2, 4, a heat-sensitive portion 10, and a magnetic tape portion 6, as in the previous embodiment shown in FIG. 1. Also, the electrical resistance element 1 includes a first non-conductive cover 14 and a second non-conductive cover 16, as in the previous embodiment shown in FIG. 2. The outer edges 2b, 4b of the first and second conductors 2, 4 are positioned away from the outer edges 14a, 14b, 16a, 16b of the first and second covers 14, 16 to form margin portions. The margin portions enable the first and second cover sheets 14, 16 to be tightly laminated to each other within the margin portions, as in the previous embodiment shown in FIG. 2. The first magnetic recording tape portion 6, the first and second conductors 2, 4, and the heat-sensitive portion, 10 are pressed firmly together between the first and second cover sheets 14, 16. 
     The embodiment shown in FIG. 4 includes only the first magnetic recording tape portion 6, and the second magnetic recording tape portion 8 is eliminated. The first magnetic tape portion 6 is in partially overlapping relationship with the first conductor 2. More specifically, the outer edge 6b of the first magnetic recording tape portion 6, and the inner edge 2a of the first conductor 2 have portions that are in overlapping relationship in a specified overlap area. Thus, one magnetic recording tape portion is used for heating, and the conductors 2, 4, the magnetic recording tape portion 6, and the heat-sensitive portion 10 are pressed between the covers 14, 16 to continuously maintain contact with each other during normal operation. 
     An area heater 30, 32 formed in accordance with the present invention, is shown in FIGS. 5a and 5b. The area heater is composed of a parallel arrangement of electrical resistance elements 1, each having the structure of one of the embodiments described previously and shown in FIGS. 1-4 or FIG. 6 which will be described. The first and second conductors 2, 4 of each resistance element of the arrangement may be respectively interconnected, and connected to a source of power. Thus, an area heater will have a planar, heat radiating surface 34 which will radiate heat evenly to a room, for example. If a &#34;hot spot&#34; develops on the planar, heat-radiating surface, the heat-sensitive strip in the resistance element situated at the hot spot will automatically stop the current flow transversely through the magnetic recording tape (or other resistive heating material) only at the hot spot. The area heater 30 may be incorporated into a wall or floor 36 of a room, as illustrated by FIG. 5b, or may be designed to be a free-standing room heater 32 with a heat controllable thermostat 38, as shown in FIG. 5a. 
     The heat-sensitive electrically conductive strip 10 has been described so far as being of the type that shrinks in width at a predetermined temperature. A suitable shrinkable film which may be used for this purpose is Part No. 142GA Fleetmark H. S. Silver, manufactured by Xebec Company of North Haven, Conn. This particular film is designed to shrink at approximately 158° F. 
     It is, however, envisioned to alternatively use a strip of conductive heat-sensitive material which retains its original width but forms bubbles or electrically discontinuous droplets 40 at elevated temperatures, thus interrupting the flow of current widthwise wherever a hot spot occurs on the resistance heating element. Such a heat-sensitive material may be used in any of the embodiments of the invention previously described, and is shown in FIG. 6 in one such embodiment. 
     As shown in FIG. 6, the resistance heating element 1 includes first and second conductors 2, 4, a heat-sensitive portion of the droplet or bubble type 10&#39; and first and second resistance material strips such as magnetic tape portions 6, 8, arranged in the manner described previously with respect to FIG. 1. The heat-sensitive portion 10&#39; provides an electrical path through the first and second magnetic tape portions along the entire length of the electrical resistance element 1 at normal operating temperatures. 
     Whenever a hot spot occurs, the electrically conductive film of the heat-sensitive portion 10&#39; will form into discontinuous bubbles or droplets 40 at that hot spot, and disrupt the flow of current widthwise through the heat-sensitive strip as well as widthwise through the first and second magnetic recording tape portions 6, 8 only at that hot spot. The overheated portion of the electrical resistance element will, accordingly, cool, thus lessening the risk of fire. 
     Indium Corporation of America, located in Utica, N.Y., manufactures a suitable ribbon which may be used as heat-sensitive strip 10&#39;. One ribbon it manufactures is composed of 55.5% bismuth and 44.5% lead. This ribbon will form discontinuities at 255° F. Another ribbon manufactured by this company is composed of 10% cadmium, 50% bismuth, 26.7% lead and 13.3% tin. This ribbon will form discontinuities at 158° F. Accordingly, a heat-sensitive strip may be selected such that non-conduction occurs at a desired temperature. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modification may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.