Positive temperature coefficient thermistor heating pad

Abstract: A heating pad using positive temperature coefficient thermistors as a primary heat source with the thermistors being electrically and mechanically connected to electrically conductive sheets, preferably woven, with dielectric material preferably separating the conductive sheets. A covering of dielectric material preferably is used to electrically separate the heating pad from the environment. The heating pad construction uses the conductive sheets as the primary heat transfer means from the thermistors, resulting in a relatively even heat distribution over the heating pad. The heating pad is preferably made of materials that are flexible so as to enable the pad to flex to adapt to items of different shapes to be heated by the pad.

Description: Background of the Invention 1. Field of the Invention The present invention relates to electrical heating devices that use positive temperature coefficient thermistors as self-regulating heaters. 2. Description of the Prior Art As exemplified in U.S. Patent No. 4,072,848, electrical heating cables have been used commercially for some time to provide heat to pipes and tanks in cold environments. Heating cables as disclosed in U.S. Patent No. 4,072,848 based their temperature control on the use of variable resistance heating materials which provide a self-regulating feature. The heating materials are generally formed into chips made of barium titanate or solid solutions of barium and strontium titanate which are made semiconductive by the inclusion of various dopants. These chips are referred to as positive temperature coefficient thermistors and have a relatively low temperature coefficient of resistance at low temperatures. As the temperature of the thermistor rises, a sharp rise in the resistance occurs at a point termed the "Curie point". The transition from low resistivity to high resistivity occurs at a relatively sharp point as shown in U.S. Patent No. 4,072,848. As these chips are well known to those skilled in the art, no further discussion of their construction is necessary. As a voltage is applied to the thermistor, the thermistor generates heat due to resistance effects. This heat is then transferred to the environment, such as the pipe to which the cable is attached. As the temperature of the thermistor and the surrounding environment increases, the thermistor temperature reaches the Curie point, the heat producing capability of the thermistor is reduced and the thermistor cools down. Thus the thermistor temperature settles on or near the Curie point, with the temperature of the surrounding environment being based on the thermal conductivities of the various materials in contact with the thermistor. Prior art thermistor-based devices were cables and other similar devices which covered only small lateral areas, even though they could be extended for long distances. While the prior art cables could be shaped in serpentine patterns to cover larger lateral areas, this often resulted in uneven temperature distributions over the surface area and was hard to manufacture. U.S. Patent No. 4,330,703 shows several examples of prior art cables utilizing heat generating layers of materials and having electrical conductors formed of metal sheets, grid of meshes. The heat generating materials are located over the entire area of the cable, not in discrete and separated areas as is the practice in thermistor-based cables. Additionally, the electrical conductors are thin, utilized only to supply electrical current to the heat generating materials and not utilized to conduct appreciable amounts of heat. It is already known an electrical heating device with the features of the first part of the claim 1 (FR-A1-2 496 382). In the known device the electrical heating chips are arranged in a single line, so that the device has the form of a heating cable. It is further known a planar heat generating device (US-A-4 401 885) in which between two and three heating chips are arranged. They are arranged between rigid metallic plates. The plates are held fixed to one another by an insulating cover layer of synthetic resin. The cover extends through holes in the device and exerts a prestress on the heating chips. The invention proposes an electrical heating pad with the features of claim 1. Further developments are subject matter of the sub-claims. Summary of the Invention The heating pad of the present invention has substantially flat, preferably woven, electrical conductors disposed in overlying parallel relationship and having a plurality of spaced thermistors electrically connected thereto, wherein the electrical conductors serve as the primary heat transfer means by dissipating heat produced by the thermistors away from them. The thermistors are spaced in a grid or substantially uniform pattern over the area of the heating pad. Such construction results in an efficient heat transfer between the conductors and the thermistors, thus allowing heat to be removed from the thermistors. Also such construction enables the thermistor to produce high power levels with a given applied voltage before the thermistor reaches the self-limiting temperature or Curie point. Such heat transfer using the electrical conductors improves the temperature distribution over the surface of the pad because the heat is transferred in all directions along the electrical conductors, which are good thermal conductors, and away from the thermistors, limiting the amount of local heat and improving the heat balance of the pad. The construction of a heating pad of the present invention allows ease of manufacture because complex serpentine paths are not required. The use of the woven electrical conductors significantly decreases the thermal or mechanical stresses which occur at the connections between the conductors and thermistors because of the dispersed multidirectional forces which are exerted because of the small size and great number of wire strands in the material. Brief Description of the Drawings Fig. 1 is a perspective view in partial cross-section of a heating pad constructed according to the present invention. Fig. 2 is a graph illustrating the unit power produced at given temperatures and given voltages for the heating pad of Fig. 1. Fig. 3 is a graph representing the unit power produced at given temperatures and given voltages for a heating pad according to Fig. 1. Description of the Preferred Embodiment Referring to the drawings, the letter P generally designates a heating pad according to the present invention. Fig. 1 illustrates the preferred embodiment of a heating pad P constructed according to the present invention. A plurality of thermistors 10 are inserted into a separating dielectric insulator 12. The separating dielectric 12 contains a series of holes or cavities 14 in which the thermistors 10 are installed. The spacing between the holes 14 is varied depending upon the specific size of the thermistors 10 and the number of thermistors 10 required for a given desired thermal output of the heating pad P. Preferably the holes 14 are slightly smaller than the size of the thermistors 10 so that the thermistors 10 are positively retained in the separating dielectric 12. The thermistors 10 are shown as being circular in cross-section, but any desired shape can be used, with the holes 14 have corresponding shapes. The dielectric material may be rubber, thermoplastic resins such as polyethylene or polytetrafluoroethylene, asbestos fiber, or any satisfactory material which is an electrical insulating material and is capable of withstanding the temperatures of the thermistors 10, while conducting sufficient heat as desired and being flexible to allow the heating pad P to be flexed as desired. Conductive sheets 16, 18 are installed parallel to each other and on opposite sides of the separating dielectric 12 to provide the source of electrical energy converted by the thermistors 10 to heat. The conductive sheets 16, 18 are attached to the thermistors 10 by soldering, brazing, welding or otherwise electrically and mechanically connecting the conductive sheets 16, 18 to the surfaces of the thermistors 10. Conductors 17, 19 are attached to the conductive sheets 16, 18 and to the voltage source (not shown) used to supply electrical energy to the heating pad P. After the conductive sheets 16, 18 have been connected to the thermistors 10, an insulating layer 20 is provided to protect the heating pad P from the environment. In this way, short circuit and potential shock conditions are prevented. If further mechanical or corrosion resistant protection is desired or where a more rigid surface is desired, a metallic sheath 22 can be formed over the insulating layer 20 of the heating pad P. The metallic sheath 22 may be aluminum, stainless steel, copper or any satisfactory metal or metal alloy that can be formed about the pad. Such construction, using conductive sheets 16, 18 of adequate heat transfer capability, results in the conductive sheets 16, 18 becoming the primary heat transfer means. The use of the conductive sheets 16, 18 as the primary heat transfer means results in increased heat removal from the thermistors 10 and a more even temperature distribution over the surface of the heating pad P. Thus, by reason of this invention, heat is removed from the thermistors 10 and the heat is evenly distributed over the area of the heating pad P. The conductive sheets 16, 18 are preferably formed of copper wire cloth approximately the same size and shape as the heating pad P. The conductive sheets 16, 18 can alternately be formed of wire cloth made of aluminum, stainless steel or other metallic conductors. Alternatively, carbon or graphite fibers, conductively coated fiberglass yarn or other similar materials of known construction as are commonly used in automotive ignition cables and as disclosed in U.S. Patent No. 4,369,423 may be used. The fibers can be electroplated with nickel to further improve the conductivity of the fibers. Sufficient numbers of the fibers are woven to provide a conductive sheet which is capable of carrying the necessary electrical and thermal loads. In yet another alternative, the conductive sheets could be solid metallic sheets of materials such as copper, aluminum or other suitable materials. An exemplary copper cloth is comprised of 0.011 inch diameter copper wire formed into a mesh having 16 wires per inch in either direction. The individual copper strands may be coated with a tin, silver, aluminum or nickel plated finish. The conductive sheet construction according to the present invention is preferably formed with a large number of smaller wires which are woven into sheets. The increased number of contacts of smaller wire and the mesh or woven pattern developed by the woven conductors decreases the thermal and mechanical stresses which occur at the connection between the conductive sheets 16, 18 and the thermistor 10. The thermal stresses arise due to differing expansion rates and other reasons and the mechanical stresses occur due to the flexible nature of the heating pad P. Because the woven wires are small and are arranged in several different directions, the forces exerted on each strand or wire are low, thereby increasing the reliability of the heating pad P. A heating pad P according to the present invention can be cut or formed into almost any desired shape. The exemplary embodiment shown in Fig. 1 is formed into a square, but the heating pad P can be formed into circular shapes irregular shapes or regular or irregular polygons as desired. Because the thermistors 10 are relatively small, and the other materials used in the present invention are preferably flexible, the heating pad P is adapted to be flexed so as to substantially conform to an item such as a vessel or pipe to be heated. Example A heating pad P was constructed of copper wire cloth according to Fig. 1 with Curie temperature 124-128° C. thermistors 10. A one foot square separating dielectric layer 12 of coated fiberglass having a thickness of 0.07 inches was used. Twelve thermistors 10 were placed in openings 14 distributed evenly over the area of the separating dielectric layer 12. Copper wire cloth having a 16 by 16 mesh and formed of 0.011 inch diameter wires was formed into sheets one foot square which were then soldered to pre-tinned thermistors 10 with a silver bearing, high temperature solder alloy. This heating pad P was then insulated with high temperature RTV silicone to form the insulating layer 20. The completed heating pad P thus formed had a resistance of 90 ohms at room temperature of approximately 77°F. This heating pad P was then placed in an environmental chamber, and tested as equilibrium temperatures of -35°F, 0°F., 50°F., 100°F., and 200°F. and energized at voltages ranging from 0 to 300 volts. The power consumption at the various voltages and temperatures was recorded and the results are shown in Figs. 2 and 3. It can thus be seen that the present invention provides a construction which produces high power levels with a given applied voltage before the thermistors reach the self-limiting temperature. In another test, the same heating pad P was energized by approximately 120 volts while the heating pad P was suspended in a free air environment having a temperature of 76°F. Temperature measurements were taken at a series of locations on the surface of the heating pad P. The maximum and minimum temperatures at positions directly over the thermistors 10 were 199°F and 178°F. The average temperature directly over the termistors was approximately 183°F. The outer edges of the heating pad P had temperatures of 111°F, 116°F, 112 °F and 102°F. The average temperature on the surface area at locations between the thermistors 10 was approximately 121°F, with a maximum of 134°F and a minimum of 108°F. Such results indicate the efficient heat transfer from the thermistors 10 to the conductive sheets 16, 18 and the good thermal conduction of the conductive sheets 16, 18. It will be understood that because the heat is generated initially at the thermistors, the pad may be selectively formed or cut into any desired shape while still retaining approximately the same watts per square foot capability for the selected area, assuming an equal area of remaining heating pad per thermistor. The foregoing disclosure and description of the invention are illustrative and exemplanatory thereof, and various changes in the size, shape and materials as well as in the details of the illustrated construction may be made.

Claims: An electrical heating pad, comprising: first and second conductor means extending parallel to each other and spaced from each other for conveying electrical current and for conducting heat; each of said conductor means comprising a substantially flat sheet (16, 18) of electrically and thermally conductive material; heating means formed of variable resistance heating material electrically connected between said first and second conductor means for producing heat when current flows therethrough, said variable resistance heating material substantially increasing in resistance when a temperature limit is reached to reduce the current flowing through said heating means so as to control the heat output of the heating pad, said heating means comprising a plurality of chips (10) of said variable resistance heating material; electrical insulating means (12) disposed between said conductor means for preventing contact between said first and second conductor means; and electrical insulating material (20) disposed externally of said conductor means for preventing short circuit or shock, characterized in that said plurality of chips (10) is arranged in a two-dimensional array (grid). The heating pad of claim 1, wherein said electrical insulating means (12) comprises an insulating material having openings (14) at spaced intervals in which said variable resistance chips (10) are disposed. The heating pad of claim 2, wherein said openings (14) are uniformly spaced from each other for locating said chips (10) uniformly over the area of the heating pad (P). The heating pad according to one of the preceding claims, wherein each of said conductor means comprises woven wire cloth. The heating pad of claim 4, wherein said wire cloth is copper. The heating pad according to one of the preceding claims, further comprising a metallic enclosure (22) formed over said electrical insulating material (20). The heating pad of claim 6, wherein said metallic enclosure (22) is formed of stainless steel. The heating pad according to one of the preceding claims, wherein said conductor means comprises a plurality of electrically conductive fibers woven into cloth. The heating pad according to one of the preceding claims, wherein said conductor means (16,18), said electrical insulating means (12) and said electrical insulating material (20) are all flexible to enable the heating pad (P) to be flexed for conforming to various items to he heated by the heating pad (P).

Inventors: GOSS DAVID C, YAGNIK CHANDRAKANT M, GOSS, DAVID C., YAGNIK, CHANDRAKANT M.
Applicants: THERMON MFG CO, THERMON MANUFACTURING COMPANY
Application year: 1988
Main classification: H05B3
IPC codes: H05B3