FLEXIBLE ELECTRIC SHEET HEATER

Abstract: A flexible electric heater (10) which eliminates the need for long copper conductors, simplifies electrical connections and provides for heating closely along the side edges of the sheet heater includes a thin elongated polyester (Mylar) substrate (12) having a semiconductor pattern (14) printed on the surface and having a pair of spaced connector portions (26) and a pair of U-shaped heating portions extending in opposite directions from and electrically connected to the connector portions (26). The U-shaped heating portions each include a pair of leg portions (18) lying close to the side edges of the substrate and joined by a U-shaped end portion (22). The connector portions (26) have a higher electrical conductivity than the heating portions and a metallic conductor (30) is disposed in face - to - face engagement with each of the connector portions (26) and has an area that is less than that of the respective connector portion (26) and an area: perimeter ratio not greater than D/2, wherein D is the major dimension of the conductor (30). A thin polyester cover sheet overlies the substrate (12), semiconductor pattern (14) and conductors (30) and is bonded to the substrate to form a hermetically sealed unit. Connector wires (40) are connected to the conductors (30) by metallic staples (44) penetrating the entire thickness of the heating device (10) including the respective conductors (30).

Description: Description FLEXIBLE ELECTRIC SHEET HEATER This invention relates to electrical heating devices. Background of Invention United States Patent No. 4,485,297, here incorporated by reference, discloses flexible sheet heaters including a pair of longitudinally-extending (typically copper) conductors, and a semi-conductor pattern extending between and electrically con¬ nected to the conductors. The heaters there disclosed provide generally superior performance and substantially even heat dis¬ tribution, and are useful in a wide range of applications. There are circumstances, however, when it is desirable to provide heating closer to the side edges of the heater than can be accomplished with the heater design shown in the aforemen- tioned applications, or over a long, thin area. It also may be desirable to provide a heater construction which, because it eliminates the need for long copper conductors, is less expen¬ sive than the heater design of the aforementioned applications. Summary of Invention The present invention provides an electrical sheet heater which, while retaining important advantages of the structure of the heaters described in the aforementioned applications, eliminates the need for long copper conductors, simplifies connection, and provides for heating of long thin areas and to closely adjacent the heater side edges. In general, I have discovered that an electrical heating device comprising a substrate having an upper insulating sur¬ face, a semi-conductor pattern carried on the insulating sur¬ face, and a pair of metallic conductors mounted in face-to-face engagement with said semi-conductor pattern may have the afore¬ mentioned advantages if the semi-conductor pattern includes generally U-shaped heating portions extending in opposite directions from and electrically-connected to a pair of metal- lie conductors. Each of the conductors is in f ce-to-face engagement with a respective connector portion of the semi¬ conductor pattern; and each of the heating portions includes areas of different resistivities (ohms/square) connected electrically in series. In preferred embodiments in which an insulating layer overlies the substrate, semi-conductor pattern and metallic conductors, the two heating portions have approximately the same watt density. Also, each of the heating portions forms an approximately 180° loop extending between the connector por- tions and includes a pair of parallel, spaced-apart leg por¬ tions of a first conductivity and an intermediate (base) por¬ tion of a second conductivity; and each of the metallic con¬ ductors has an area:perimeter ratio in the range of about —-- to ——wherein d is the major dimension (height, width, or diam- eter) of the conductor. In most preferred embodiments, the connector portions are more conductive and wider than the abutting ends of the heating portions and the total length of the edges of the metallic conductors that overlie and engage the connector portions is more than twice the width of the abutting heating portion ends. Drawings Figure 1 is a perspective, exploded view, partially in section of a heater embodying the present invention. Figure 2 is a sectional view, taken at 2-2 of Figure 1 but not exploded, of the heater of Figure 1. Detailed Description Referring to the drawings, there is shown an electrical sheet heater, generally designated 10, designed for use at the mullion between the freezer and refrigeration compartments of a refrigerator. Heater 10 comprises a plastic substrate 12 on which is printed a semi-conductor pattern 14 of colloidal graphite. As shown, heater 10 is 2 inches (5 cm) wide and 30 inches (76 cm) long. Substrate 12 is 0.004 inch (0.010 cm) thick, essentially transparent, polyester ("Mylar"). The semi-conductor pattern 14 is centered on substrate 12, so as to provide an uncoated area 16 along the side edges and at each end. Along the edges, uncoated area 16 is approxi¬ mately 3/16 inch (0.5 cm) wide; it has a minimum width of about 3/8 inch (1 cm) at each end. As shown, pattern 14 includes a pair of parallel portions 18, each 27.00 inches (68.6 cm) long and 0.750 inches (1.9 cm) wide, with an uncoated space 20 (0.125 inch, or 0.318 cm) wide throughout most of its length) therebetween. Each parallel portion 18 extends substantially the full length of substrate 12. At the opposite ends of the substrate, the adjacent ends of portions 18 are connected by generally U-shaped end portions 22, each 1.125 inch (2.86 cm) long and 1.625 inch (4.13 cm) wide. As shown, the base of each end portion 22 is essentially semicircular, and the two legs of each "ϋ" abut, and have the same width as, a respective one of parallel portions 18. A circular, 0.500 inch (1.27 cm) in diameter, uncoated space 24 is provided in each end portion 22, concentric with the semi¬ circular portion thereof. Near the center of the heater 10, and on opposite sides of a transverse center line, a rectangular (and nearly square so that its area:perimeter ratio is not greater than about d/2 (wherein d is the major dimension, e.g., height or width. *_f>i_?_ thereof) and, preferably, is not significantly greater than the d/4 ratio of a square or circle) connector portion 26 is cen¬ tered on each parallel portion. Each connector portion 26 is about 0.750 inch (1.9 cm) long and 0.875 inch (2.2 cm) wide and, since it is centered on a respective parallel portion 18, extends about 0.0625 inch (0.16 cm) beyond each side thereof. As will be evident, the inner longitudinally-extending edges of the two connector portions 26 are aligned, but the adjacent transverse edges of the two connector portions 26 are spaced about 0.250 inch (0.64 cm) from each other. Because the connector portions are compact and do not extend any significant distance longitudinally or transversely of the heater 10, the heating portions of the semi-conductor pattern (e.g., the parallel portions 18 and end portions 26) may extend to very closely adjacent the heater side edges. Connector portions 26 have a greater conductivity than do either parallel portions 18 or end portions 22 and, in the illustrated embodiment, parallel portions 18 are more conductive (less resistive) than the end portions. In the illustrated embodiment in which the pattern 14 is printed onto substrate 12 in three sequential printing steps, end portions 22 are printed one layer (e.g., about 1/2 to 1 mil, or 0.0013 to 0.0025 cm) thick, parallel portions 18 are printed two layers thick, and connector portions 26 are printed three layers thick. All three portions are printed with a conductive ink having a resistance of about 100 ohms per square when printed at 1/2 mil (0.0013 cm) thickness. It will be noted that the semi-conductor pattern 14 of the illustrated embodiment provides two essentially identical current paths between connector portions, and that the two paths have the same watt density. A tin-plated copper disc 30, 0.002 inch (0.005 cm) thick and 0.500 inch (1.27 cm) in diameter (D), is placed in the center of each connection portion 26. The perimeter: area ratio of disc 30 is D/4. The perimeter of disc 30 is greater than twice the width of parallel portions 18 of pattemly. A thin plastic cover sheet 32, comprising an essentially 5 transparent co-lamination of an 0.005 cm. (0.002 in.) thick polyester ("Mylar") and an 0.007 cm (0.003 in.) thick adhesive binder, e.g., polyethylene, overlies substrate 12, semi¬ conductor pattern 14 and discs 30. As indicated, a 3/8 inch (1 cm) diameter hole 25 is punched through the heater at each end 10 thereof, concentric with the circular unprinted area 24 in each end portion 24, to receive a screw or the like when the heater 10 is installed. Since the diameter of hole 25 is less than that of unprinted area 24, an unprinted annular area remains surrounding each hole. - | c Typically, discs 30 are not themselves bonded to the underlying semi-conductor material, and the cover ssheet 32 bonds poorly to the semi-conductor pattern. However, the polyethylene forming the bottom layer of cover sheet 32 bonds well to substrate 12. Thus, when the cover sheet and substrate 20 are laminated together (as taught in aforesaid Patent No. 4,485,292), the polyethylene bottom layer of cover sheet 32 bonds the cover sheet tightly to the longitudinally-extending, uncoated (with semi-conductor material) areas 16 of substrate 12 between the outer periphery of the semi-conductor pattern 14 and 25 the adjacent outside edge of the substrate 12, and to the uncoated (with semi-conductor material) area 20 between parallel semi-conductor pattern portions 18, and to the uncoated annular area surrounding each hole 25. Because the substrate 12 and cover sheet 32 are sealed tightly to each other in the areas 16 30 between the outside edge of pattern 14 and the outer edges of the heater, the unit is essentially heremetically sealed. SUBSTITUTE SHEET - 5a - For connecting the heater to a source of power (not shown and for the heater of the illustrated embodiment a conventional 120 volt a.c. source), a wire 40 is connected to each of discs 30. As shown most clearly in Figure 2, the wire connection is made by centering the stripped end 42 of each wire over a respective disc, and then driving a copper staple 44 over the stripped end 42 of each wire through the entire thickness of SUBSTITUTE SHEET -6- the heater, including the respective disc 30. The staples 44 thus provide good electrical connection between discs 30 and wires 30, and also assist in holding the discs 30 tightly against the underlying semi-conductor pattern connection por- tions 26. In the preferred practice of the invention, the stapling is accomplished using an Autosplice brand connecting system, provided by the Autosplice division of General Staple Co., Woodside, New York. In some circumstances it may be desirable to drive the staples in the opposite direction from that shown, crimping the ends of the staples over the stripped end 42 of wire 40. Other Embodiments In other embodiments, connection portions 22 and discs 30 may be circular, oblong, square or rectangular, so long as the area:perimeter ratio of the discs (and generally of the connec¬ tion portions also) is not greater than about d/2 or D/2, respectively. Similarly, the shape of the heating (e.g., parallel and end) portions of the semi-conductor pattern may vary, as may the number of heating portions that extend'between the connection portions. Generally, the watt density' of .each heating portion will be the same. These and still other embodiments will be within the scope of the following claims. What is claimed is:

Claims: -7- Clai s 1. In an electrical heating device comprising a generally flat substrate having an upper insulating surface, a semi-conductor pattern carried by said upper surface of said substrate, and a pair of metallic conductors mounted in face-to-face engagement with said semi-conductor pattern, that improvement wherein: said semi-conductor pattern includes generally U-shaped heating portions extending in opposite directions from and electrically connected to each of said conductors; and each of said heating portions includes areas of different resistivities (ohms per square) connected electrically in series. 2. The heating device of claim 1 wherein each of heating portions includes a pair of parallel, spaced apart leg portions of a first conductivity and an intermediate portion of a second conductivity different from said first conductivity. 3. The heating device of claim 2 wherein said semi- conductor pattern includes a pair of spaced connector portions, each of said leg portions has one end thereof connected to a respective one of said connector portions, and the other end thereof connected to a said intermediate portion. 4. The heating device of claim 1 wherein said heating portions are substantially identical. 5. The heating device of claim 3 wherein said connecting portions are adjacent each other, and a said heating portion extends from each of said connecting portions in a direction forming an angle of not less than about 90° with a line extending directly from one of said connecting portions to the other of said connecting portions. 6. The heating device of claim 1 wherein said semi¬ conductor pattern includes a pair of spaced connector portions, -8- each of said heating portions extending between and being elec¬ trically connected to each of said connector portions, and said connector portions have an electrical conductivity greater than that of said heating portion. 7. The heating device of claim 6 wherein each of said metallic conductors is in face-to-face engagement with a respective one of said connector portions, and has an area that is less than that of said respective one of said connector portions and an area:perimeter ratio not greater than about D/2 wherein D is the major dimension (length, width, or diameter) of the respective one of said conductor. 8. The heating device of claim 1 wherein an insulating layer overlies said semi-conductor pattern and said conductors and sealingly engages portions of said substrate that are free from said semi-conductor pattern. 9. _ The heating device of claim 7 wherein said connector portions have an area:perimeter .ratio not greater than about d/2 where d is- the major dimension of the respective connector portion. 10. The heating device of claim 8 wherein said area:perimeter ratios are about d/4 and D/4, respectively. 11. The heating device of claim 1 including a wire electrically connected to each of said conductors, each said electrical connection being made by a connector engaging said wire and extending through a respective said conductor disc. 12. The heating device of claim 1 wherein the minimum distance from said heating portion to a side edge of said heater is less than the width of either of said heater portions. 13. The heating device of claim 1 wherein each said heating portion extends from one of said connection portions through a loop of at least 180° to the other of said connectio portions. Ω Φ P ft TJ O i rt μ- O P CO O Hi 01 P μ- a tr Φ P rt μ- P ι TJ O ti rt μ « O P CO

Inventors: GRISE FREDERICK G J, GRISE, FREDERICK, G., J.
Applicants: FLEXWATT CORP, FLEXWATT CORPORATION
Application year: 1985
Main classification: H05B3
IPC codes: H05B3