US3295090A - Electrical resistor having a core element with high heat dissipating properties - Google Patents

Description

Dec. 27, 1966 B F. HAY 3,295,090

ELECTRICAL RESISTOR HAVING A CORE ELEMENT WITH HIGH HEAT DISSIPATING PROPERTIES Original Filed Feb. 26, 1962 VIIIIIIIIIA YIIIIII II) //V V6 7' 01? BER/V141? A #4 Y United States Patent Nebraska Original application Feb. 26, 1962, Ser. No. 175,654. 1963, Ser. No.

Divided and this application Sept. 3, 306,242

1 Claim. (Cl. 338-302) This invention relates to a base core means for resistors and the like. This is a divisional application of the co-pending application Serial No. 175,654 filed February 26, 1962.

The use of ceramic core shafts in resistors is old. In general, such cores are produced by firing certain clays. While such ceramic cores for resistors are in universal usage, they do have many shortcomings. Firstly, they do not provide sufficient thermal conductivity. Secondly, they are low in electrical resistivity at high temperatures. Thirdly, their useful life is limited.

Therefore, one of the principal objects of this invention is to provide an improved core for resistors and like that will remain substantially cooler when in use than present type ceramic resistor cores.

A further object of this invention is to provide a resistor that has excellent electrical resistivity at high temperatures.

A still further object of this invention is to provide a resistor that permits reduction in size Without materially affecting its capacity.

A still further object of this invention is to provide an insulative core for resistors or the like that will not conduct electricity but will effectively dissipate heat created by the resistance element thereon.

Still further objects of this invention are to provide an improved core for resistors and the like that is economical in manufacture, and durable in use.

These and other objects will be apparent to those skilled in the art.

This invention consists in the construction, arrangements, and combination, of the various parts of the device, whereby the objects contemplated are attained as hereinafter more fully set forth, specifically pointed out in the claims, and illustrated in the accompanying drawing, in which:

FIGURE 1 is a side view of an ordinary wire wound resistor using the core element herein contemplated,

FIGURE 2 is a longitudinal sectional view of the resistor shown in FIGURE 1, and more fully illustrates its construction; and

FIGURE 3 is a sectional view of the resistor taken on line 3-3 of FIGURE 1.

While the core shown is used in conjunction with a wire wound resistor, it can also be successfully used as a core for ribbon type resistors, film type resistors, deposited type resistors, and the like.

In the drawings, the numeral generally designates the base core shaft. -On each end of this shaft are two metallic caps 11 and 12, respectively. The numeral 13 designates the terminal rod of the cap 11, and the numeral 15 designates the terminal rod of the cap 12. The numeral 16 designates the metallic wire wound around the core 10, having one end electrically contacting the cap 11, and its other end electrically contacting the cap 12. As herebefore indicated, the invention herein contemplated resides in the core 10, per se. Instead of using substantially all clay in the production of this core, this invention contemplates the use of beryllium oxide formed into a core unit. Such a core is preferably completely 3,295,090 Patented Dec. 27, 1966 ice comprised of beryllium oxide, but impurities in the oxide may prevent the core from being one-hundred percent beryllium oxide. Impurities in the form of additives such as earth clays, water or the like can be added to the beryllium oxide to affect its consistency during the molding of the core. But regardless of whether these impurities are inherent in the beryllium oxide used, or whether they occur through the use of additives in core forming process, the core should never be comprised of beryllium oxide that is less than ninety percent by both weight and volume, and purity in excess of ninety-five percent is desired. Thus if the core is comprised only of beryllium oxide, and the oxide is ninety-five percent pure, ninety-five percent of the resulting core will be beryllium oxide, which comes within the prescribed limitations. This high purity of the beryllium oxide is required because at lower values, the thermal coefficient of the beryllium oxide is so affected that it is not suited for use as a resistor core. Eighty-five percent is an absolute minimum purity value. The raw material comprised of beryllium oxide, its impurities and/or additives are fired or heat baked in the same manner as other ceramic cores. These purity values of berylium oxide prevent the core from conducting electricity, but provides enough beryllium to permit the heat dissipation phenomenon to take place. The mechanism of heat dissipation begins with the heat generated by the wire 16 as power is applied thereto. The beryllium oxide particles provide both a thermally conductive path because of their physical juxtaposition, and a radiator effect in that they radiate thermal energy in all directions (perpendicular to the particle surface) and thus, the heat is spread evenly throughout. After mixing and shaping, the product is then fired or heat baked in the usual manner. The result is a beryllium oxide ceramic base core of superior quality and operation.

Temperature rise tests on a beryllium oxide ceramic core designed within the above limits in comparison to ordinary standard ceramic cores have been conducted. With 3 watts applied to the resistance wires of both test specimens, the beryllium oxide cores measured 25 C. cooler than standard types. With 5 watts of power applied to the resistors using beryllium oxide cores, the temperature rise did not exceed 275 C. Due to the possible error of thermocouple measurements on this type of resistor, a load life test was conducted to determine the stability of the beryllium oxide core resistors when operated at 5 watts. Past test history has shown that so long as the hot spot temperature does not exceed 275 C., a drift of less than 1 percent in 1,000 hours of load can be expected.

In comparative load life tests on beryllium oxide cores vs. standard resistors of identical size, wound from the same spool of wire, and processed in an exact manner at the same time, the resistance deviations were noted as follows:

Max. Min.

1. Standard core, percent 1. 21 .7 2. Beryllium oxide core, percent After 280 hours of load life, deviations were again noted with the following results:

Max. Min.

1. Standard core, percent 1. 76 2. Beryllium oxide core, percent .49

- portion of self-generated temperature in the resistor is dissipated by conduction through the lead material. The portion of heat being dispensed in this manner will vary in accordance with the length and diameter of the resistor :body. There is no doubt that beryllium oxide ceramic is a more eflicient materal for use as a substrate for power resistors.

This invention is therefore directed to an improvement in wattage dissipation of a resistor core fabricated from beryllium oxide ceramic material as compared to those made from conventional ceramics of identical size.

As an example, a core having dimensions of A; inch outside diameter b-y inch long used as a substrate for a resistor will result in a power rating of 3 Watts based on a hot spot temperature of 275 C. when made from conventional ceramics. The same size core in beryllium oxide ceramics will result in a resistor having a power rating of 5 watts based on a 275 C. hot spot. The application of 3 watts to a core having the above dimensions is equal to approximately 15 watts per square inch of core surface, and the application of 5 watts to a core of similar dimensions is equal to approximately 25 watts per square inch of core surface. This approximate same advantage should be evident on any resistor of identical size when comparing the performance of beryllium oxide ceramics to that of conventional ceramics.

Some changes may be made in the construction and arrangement of my base core means for resistors and like without departing from the real spirit and purpose of my invention, and it is my intention to cover by my claims, any modified forms of structure or use of mechanical equivalents which may be reasonably included within their scope.

I claim:

In an electrical resistor having a core element with high heat dissipating properties,

an elongated straight cylindrical insulative core of solid and homogeneous construction, and being com prised substantially of beryllium oxide,

said beryllium oxide comprising at least 90 percent by weight of said insulative core,

an electrical resistance element in engagement with and substantially continuously covering the outer.

surface of said insulat-ive core,

and terminal elements on the ends of said core and in electrical contact with said resistance element,

said terminal elements being in firm operative engagement with a suificient portion of the surface of said insulative core whereby heat in the center portion of said core can be substantially uniformly distributed over the length of said core and can be quickly dissipated from said resistor through the beryllium oxide in said core and thence through said terminal elements;

the operative engagement between said terminal elements and the surface of said insulative core being such that when suflicient power is applied to said resistor to cause said resistor to reach its maximum operating temperature and said power is applied for 1000 hours, the temperature rise at the warmest point on said resistor will not exceed 275 degrees centigrade, and the resistance value of said resistance element will deviate no more than one percent from its initial resistance.

References Cited by the Examiner UNITED STATES PATENTS 1,536,321 5/ 1925 Amsler 338282 X 1,942,080 1/ 1934 Young 338264 X 2,075,876 4/1'937 Van Wedel 106-55 X 2,645,701 7/ 1953 Kerridge et al. 338-302 2,734,344 2/1956 Lindenblad 1364 X 2,802,896 8/1957 Tierman et al. 338274 X 2,944,910 7/1960 Schurecht 10646 3,048,914 8/1962 Kohrin-g 378-273 3,067,048 12/19 6 2 Gion 106-55 X 3,136,878 6/1964 Staller 219239 3,165,417 1/1965 Turner 106-55 X RICHARD M. WOOD, Primary Examiner. V. Y. MAYEWSKY, Assistant Examiner.

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