US3218594A - Electrical resistor - Google Patents

Description

Nov. 16, 1965 c J c 3,218,594

ELECTRICAL RESISTOR Filed July 27, 1962 United States Parent O 3,218,594 ELECTRICAL RESISTOR Charles J. Ganci, Bellerose Manor, N.Y., assignor to Ward Leonard Electric Co., Mount Vernon, N.Y., a corporation of New York Filed July 27, 1962, Ser. No. 212338 1 Claim. (Cl. 338-293) This invention relates to electrical resistors having a vitreous enanel coating on an insulating base with a resistive element mounted on the base and embedded in the coating. The invention is directed particularly to the insulating base and its relationship to the resistive element, terminals and the coating.

In the resistors of this type the insulating base provides a strong rigid support for the resistive element to firmly hold the element in the given fixed relation to prevent shorting of the resistive element or physical damage to it and maintain the turns of wire wound element in proper heat radiating relation. The base may have a number of different shapes-tubular, elliptical, disc, rectangular, or other suitable form. The resistive element is mounted on an exterior surface of the base for good heat radiation ,and Secured to the base by attachment to electrical terminals mounted on the base and by interlocking with projections on the base or encircling the base. The essential properties of the base are that it is rigid to maintain the proper relation of the parts of the resistive element and that it is tough or strong to withstand the internal strains and physical shock. In view of the high temperatures to which the base is subjected, it should have a high specific heat and a high thermal conductivity to absorb the heat and rapidly conduct the heat to produce a cooling of the base after the removal of the high heats of the manufacturing process or during actual operation of the resistor. The coefiicient of expansion should also be relatively low in order to withstand rapid changes in temperatrue, thus having a high ability to withstand thermal Shock.

The resistive element may be a resistance wire or a resistance ribbon wound around a tubular base or mounted on the side of a disc-shaped or rectangular-shaped base. Terminals embedded in the base Or encircling the base may be provided for receiving the ends of the wire or ribbon for connection to an external circuit. The wire or ribbon may be made from a number of different metal alloys, such as nickel, chrome and iron. Other alloys may be used. The element may have a resistance from a few tenths of an ohm to more than a million ohms and from low to high wattages.

The resistive element is covered by a coating of vitreous enamel cement or the like which adheres to the element and to the base to provide a tough, Shock resistant covering electrically isolating the resistive element. In these coatings diflicultes are encountered in securing the proper interrelation of the base, resistive element and coating to secure a rugged, durable resistor of an efficient size and inexpensive to manufacture. These members are subjected to high temperatures in the manufacturing operation and in subsequent use.

The flow of heat in and out of the resistor creates stresses in the members resultingin the cracking of the coating, breaking of the resistive element and weakening of the supporting base so that it is more readily subject to breakage under Shock. One of the main reasons for these difficulties is the rigidity of the base and coating and the diference in the coefficients of expansion of the members. The coating should have a coefficient of expansion equal to that of the base or lower than that of the base. It is desirable to have the coefficients of expansion of the base and coating nearly equal. Heretofore it has not been possible to provide a Patentecl Nov. 16, 1965 base that had a coeflicient of expansion near that of the coating and still secure a base at a moderate cost with the other desirable features, such as a high specific heat, high thermal conductivity and high strength and density. Also, the metal of the terminals should have an average coeflicient of expansion only slightly higher or slightly lower than the base. Preferably the coefficent of expansion should be nearly equal to that of the base to provdie a tight and secure mounting of the terminals on the base.

An object of the invention is to provide a resistor with a coated insulating base having a coefiicient of expansion nearly equal to or slightly greater than the coefiicient of expansion of the coating, that is moderate in cost and has the other necessary properties tor a satisfactory base.

Another object of the invention is to provide a resistor with an insulating base having a coefcent of expansion equal to or substantially equal to the coefficient of expansion of the terminals on the base.

Other and further Objects will be apparent from the following description taken in connection with the drawings in which:

FIG. 1 is a sectional view of a tubular resistor;

FIG. 2 is a fragmentary perspective view of a resistor having an elliptical-shaped cross section;

FIG. 3 is a side view of a rectangular resistor;

FIG. 4 is a sectional view of FIG. 3 along lines 4-4;

and v FIG. 5 is a sectional View of the disc-shaped resistor.

In FIG. 1 the tubular supporting base 10 has end terminals 11 and 12 encircling the base and a resistance wire 13 helically wound around the base and welded to the terminals 11 and 12. The vitreous enamel coating 14 covers the resistance wire 13, the terminals 11 and 12 and the outer surface of the insulating base. The tabs 15 and 16 formed as part of the terminals extend through the coating for attaching the resistor to an electrical circuit.

A resistor having a base 20 with an elliptical cross section, having a long axis in one direction and a short axis in the other direction to form a flat type of base, is illustrated in FIG. 2. As in the embodiment of FIG. 1, terminals 21 and 22 are provided at each end and a resistance wire 23 is wound around the base in a conventional manner and Secured to the terminals by the ends of the wire overlapping the terminals and being welded or soldered thereto. The terminals have tabs 25 and 26 for connecting the resistor to an external circuit. Vitreous enamel coating is provided on the outer surface of the base to cover the resistance wire and the portions of the terminals encircling the base.

In FIGS. 3 and 4 a resistor having a generally rectangular base 30 with a flat surface 31 is illustrated. Posts 32 and 33 extend normal to the surface 31 and a resistance wire 34 is looped around the posts 32 and 33 to form a serpentine resistance element extending in a plane. Terminals 35 and 36 have portions extending through openings in the base for attachment of the resistance wire 34 thereto. In FIG. 3 the portion 35a of the terminal 35 extends through the opening 37 in the base 30. The terminal 36 correspondingly extends through an opening in the base. A recess 38 is formed above the surface 31 by the side walls 39 extending around the surface 31 and projecting normal thereto. The walls 39 have a height greater than the height of the posts 32 and 33 and the diameter of the wire 34. The recess 38 is filled with a vitreous enamel coating 40 to embed and cover the resiistive element 34. Mounting brackets 41 and 42 with slots 43 and 44 are formed as part of the base for securing the resistor to a chassis or panel or otherwise mounting the resistor.

In FIG. 5 another fiat type of resistor is illustrated in a circular form with a base 50 having projections 51 molded in the base to form generally radially extending grooves 52. The resistance wire 53 is wound in a serpentine fashion around the projections 51 fitting in the grooves 52. Terminals 54 and 55 are provided projecting through holes 56 and 57 for attachment to the ends of the resistance wire 53 in any suitable manner. The base is provided with a central opening 58 mounting the resistor in a conventional manner. Circular walls 59 and 60 extend normal to the surface 61 on which the resistance wire 53 is mounted. The walls 59 and 60 form a recess which is filled with a vitreous enamel coating 62 to embed and cover the resistance wire.

The vitreous enamel coating having an average coeflicient of expansion of approximately 6 10- per degree centrigrade is formed from the mixture of material set forth in the following Table 1.

Table 1 Constituent: Percent BaO 3.29 SiO 46.70 MgO .66 ZrO 1.98 B O 11.16 CaO 1.65 CaF 3.29 L O 1.98 Al O 7.68 ZnO 11.50

CI'203 NiO .66 Ti 1.65 PbO 6.25

This mixture is fritted and milled to proper mesh. A slurry or slip is then applied to the base and wire winding and terminals of the resistor shown in FIG. 1 by dipping the resistor in the slurry or by spraying or by other means` The resistor is then fired with a heating cycle of approximately 10 to 15 minutes at 1650 F. to 1750 F. As previously stated, the resistance wire wound on the base may be a nickel-chrome-iron alloy or other resistance alloys.

The metal of the terminals may be alloys of nickel and iron, nickel, iron and cobalt and the like. These alloys have average coefficients of expansion of 5 to SXIO- inches per degree centigrade depending on their composi- 'tion. For example, an alloy of 29% nickel, 17% cobalt and the balance iron has an average coefficent of expansion of 6 10- an alloy of 42% nickel and the balance iron has an average coefiicient of 5.3 to 5.5 1O- and an alloy of 46% nickel and the balance iron has an average coeflicient of 8 10- The coeflicient of expansion is over the range of 20 to 500 C. A preferred alloy is 45% nickel and the balance iron, having an average coeflicient of expansion of 7 10- over the foregoing temperature range.

The base 10 is made chiefly from A1 O The aluminum oxide powder is mixed with other desired ingredients to form the compositions as set forth in the following table.

Table 2.-Hgh alum'na ceram'c bod'es The mixture is finely ground, then molded or extruded under pressure into a tubular form. The tubular .form is then fired at a temperature of approximately 3000 F. to 3300 F. to form a rigid, hard, non-porous ceramic core having a coefficient of expension in the range of 6.5 to

8)( 10* inches per degree centigrade in the range of 25 to 800 C. The preferred range of aluminum oxide is to 99% depending on the mixture, with %-96% aluminum oxide being the favored composition because of cost, 99% being more costly. The sintering of the finely ground aluminum oxide forms a recrystallization and a bonding of the crystals to form a high strength body. The kaolin contributes the plasticity to the composition for molding or extruding into the desired shape. The flint and calcium oxide and magnesium oxide act as fluxes to reduce the melting point of the composition. The aluminum oxide is the alpha-corundum type. The CaO and MgO may be introduced in the form of dolomite or of chemically pure composition. The aluminum oxide forms a high density, high strength base with a high specific heat and thermal conductivty. The coefficient of expansion is greater than that of the vitreous enamel composition and interrelates the two coeflicients of expansion so that the thicker base has a slightly higher expansion to compensate for the slower cooling due to its inner position in relation to the outer thinner coating. The base has good insulating and dielectric properties and a high dielectric and mechanical strength.

In FIGS. 3 to 5 a different relationship of the base and coating is illustrated. In these latter embodiments, the base and coating are planar with the thick base supporting the resstive element and the enamel coating. The enamel coating is confined by the walls of the recess.

It is thus seen that both the base and coating have a low coeflicient of expansion, with the coating having a coefl icient of expansion less than that of the base. Thus the resistor may withstand severe thermal shock without fracture. With the base having a coeflicient of expansion equal to or greater than the coating, the coating is placed in compression on cooling and does not form cracks or crazes which correspondingly cause cracks to occur in the surface of the base thereby weakenng the structural strength of the base. This increases the reliability and life of the resistor and increases the Operating conditions to which the resistor may be subjected. The high specific heat and high termal conductivity of the base also contribute to the superior qualities of the resistor.

As described, the base has a coefiicient of expansion equal to or slightly greater or slightly less than the coefficient of expansion of the terminal material. The coefficient of expansion of the base ranges from 7.2 to 7.8 1O- inches per degree centigrade, while the terminal material coefiici-ent of expansion ranges from 5 to 8 10* inches per degree centigrade.

A single composition of the vitreous enamel coating is set forth. However, it is understood that other vitreous enamel coatings may be used which have the desired characteristics and have a coefiicient of expansion equal to or less than that of the base. In the description of the embodiments of the resistor emphasis has been placed on the combination of the vitreous enamel type coating with the base in View of the stringent requirements to be met. However, it is understood that other types of coatings may be used with the bases, such as the various types of cements. Of particular note is an alumina and zirconium silicate cement comprising 75% alumina and 25% zirconium silicate. Such a cement has an average coeflicient of expansion of approximately 7.25 10 inches per degree centigrade. Other cements such as a phosphate and sodium silicate compositions may be used. Other coating materials such as varnishes, silicones or pla-stics may be used where the properties of these materials are desired. However, as previously noted the vitreous enamel coating is the most difi'icult coating to satisfactorily combine with a base.

I claim:

An electrical resistor comprising a rigid sintered alumina base with a coeflicient of expansion in the range of 6.5 to 8 10- inches per degree centigrade in the temperature range of 20 to 800 degrees centigrade, wherein said base in planar and has a flat surface with projections extending therefrom and side walls extending therearound to recess said surface, a Wire resistive element laced around said projections, spaced terminals connected to said resistive element for passing current through said resistive element and having a coeficent of expansion substantially equal to the coeficent of expansion of said base in the range of 5 to 8 10* inches per degree centigrade, and a vitreous enamel coating firmly adhered to said base covering said Wire resistve element within said side walls, said coating having a coefficient of expansion in a range from less than to equal to the coefficient of eX- pansion of said base and covering said terminals, said Wire resistive element and said base thereby formng a high resistance to thermal and hysical Shock.

576,202 2,o58,253 2,084,819 2,405,449 2,425,o32 2,647,192 2,691,088 2,803,729 2332375 2,866,o66

References Cited by the Examiner UNITED STATES PATENTS Carpenter 338-293 X Leonard 338-293 X Parsons 338-293 X Morgan et al. 338-253 Robinson et al. 338-309 X Deyrup 1.06-49 Berkelhamer 338-302 Ungewiss 338-302 Kohring 338-268 X Norton 338-264 X Neely 219-536 RICHARD M. WOOD, Pr'mary Examiner.

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