US2333476A - Electrical resistor - Google Patents

Description

Nov. 2,1943. DUSTQN 2,333,476

ELECTRICAL RESISTOR Filed June 21,1941 4 I INVENTOR Zr/e 2115011 Jim; 1. ATTORNEY Patented Nov. 2, 1943 ELECTRICAL RESISTOR Merle Duston; Detroit, Micln, assignor to United States Rubber Company, New York, N. Y., a corporation of New Jersey Application June 21, 1941, Serial No. 399,012

3 Claims.

This invention relates to resistors designed for use in an electronic circuit such as radio or amplifler apparatus. More particularly the invention relates to a rigid or flexible resistance in which a conducting rubber composition i applied to fabric.

Heretofore, many types of resistors have been made from rods r tubes ofcarbon or graphite compositions. Also paper or similar materials have been treated with conductive compositions. Other conventional types used carbon, graphite or metalized material applied over nonconducting supports such as glass or ceramic materials. In the manufacture of conventional types of resistors, difllculty has been experienced in maintaining definite resistance values. In many cases it is necessary to obtain a number of conventional resistors and actually test each resistor in order to find one which is within the tolerance desired. There is available on the commercial market various types of flexible resistors which employ metal wire as the resistance element. Although accuracy is obtainable in such a type of resistor, it is necessary to provide 'a resistor which is comparatively large in size in order to obtain values greater than 2,000 ohms. Besides the bull; of such resistors, they are also expensive to manufacture. By the very nature of wire wound, nonflexible type resistors, they are also limited to low resistance values, 50,000 ohms representing a high value in a convenientsize resistor.

Among the objects of my invention are to provide a non-metallic fixed resistor element, as distinguished from a variable resistor, in which the specific resistanc and current carrying capacity can be accurately predetermined over wide resistance ranges, either by changing the area of the resistance fabric or by changing the composition of the conducting materials constituting the resistance element; to provide a flexible resistor of high resistance value; to provide a small fixed resistor with comparatively high current carrying capacity and low temperature rise; to provide an improved terminal connection joining with the conductive film; to provide a treated fabric fixed resistor having excellent high frequency characteristics; to provide a resistor in which fully in the following detailed description when considered in connection with the accompanying drawing, in which:

Fig. 1 is a longitudinal view, partly in section, of a resistor forming an embodiment of my invention; I

Fig. 2 is a longitudinal view, partly in section, of a resistor 01' my invention embedded within moisture protective means;

Fig. 3 is a longitudinal view, partly in section, of a resistor shown embedded in a modified form of moisture resisting composition;

Fig. 4 is a front view, partly in section, of a resistor in which the resistance element is positioned' in spiral convolutions and embedded in a moisture resisting composition;

Fig. 5 is a perspective diagrammatic view of the resistance element employed in the resistor illustrated in Fig. 4; and,

Fig. 6 is a longitudinal view, partly in section,

of a resistor illustrating a method of connecting terminals with the conducting element.

With reference to the drawing and in particular to Fig. 1, I show a flexible fixed resistor embodying a flexible base tube I. This base tube is in the form of a conventional varnished, cambric sleeve generally referred to in the radio field as spaghetti. It is also to be understood that the base tube may be formed of a rigid rod or tube. Over the base tube l is helically wrapped a ribbon of fabric 2 having its surface coated with a film 30f a conducting rubber composition. Preferably the fabric ribbon 2 is formed of quare woven cotton fabric or an equivalent material. The square woven fabric is desirable because it forms a good flexible base to which a high degree of adhesion with the rubber composition film 3 may be obtained. The coated ribbon of fabric 2 is wrapped around the base tube l in a helical manner with the margins of the ribbon lying in spaced relation so that the path of electrical conductivity must follow the helical path of the conducting ribbon 2.

A composition suitable for forming the conducting film is a rubber composition containing acetylene carbon black. It is preferable that the rubber film be applied to the fabric in the form of a cement. A representative film composition which has been found satisfactory is as follows:

Parts by weight Vulcanizing agent .75

The coating composition 3 is preferably made by stirring a finely divided electrically conducting black into rubber cement containing rubber dissolved in an organic solvent such as benzene, gasoline, etc. Good results are obtained by adding conducting black in the amount of 25% to 50% based on the weight of the dried composition. More particularl the invention contemplates the use of a carbon black which per so has an electrical resistivity of not more than about 0.25 ohm-centimeter when compressed at 2500 pounds per square inch. The cement may be uncured or curable, and the composition may contain any desired vulcanizing and accelerating ingradients. The purpose of the rubber is to furnish a flexible binder for the conducting black. Cements made by the preferred method have resistivities less than 1 ohm-centimeter and preferably between 0.1 and 1.0 ohm-centimeter.

In some cases it may be desired to replace the rubber in whole or in part by other flexible binders; for example, artificial rubbers such as Buna, neoprene, Thiokol, and various natural or artificial resins or plastics such as shellac, cellulose acetate, viscose, nylon, Vinylite, casein, etc. Various of these material (having the black processed in them) may be used in the form of varnishes and paints. It has been found that thermal acetylene black has the desired electrical resistivity, although other carbon blacks may be treated so as to furnish the desired electrical resistivity. For example, it is possible to make regular carbon black more conducting by heating it in an inert atmosphere at 2000 degrees Fahrenheit.

The black may also be mixed with the binder on a mill or other mixing device or in the case of latices such as rubber latex (naturally or artificially prepared), and black may be stirred into the latex, and the latex composition applied to the support, and the coating thereafter applied to furnish a dry residue. In other cases the milled mixture may be calendered on the support.

In order to secure the ends of the coated fabric 2, a terminal member 4 is providedat each end of the base tube I and functions to clamp the base tube l and the coated ribbon 2 together. Extending from each cap member 4 is an extension 5 to which a terminal wire 6 may be soldered.

Referring to Fig. 2. I show a modified form of the invention in which the resistor is similar in all respects to that described in Fig. 1 with the exception that the resistance element is in the form of a fabric tube or sleeve I coated with a conducting rubber composition 8 in the same manner as the coating 3 on the ribbon 2. The fabric sleeve '1 is formedover the base tube i, this may be a rigid tube or rod. and the cap member 4 joins the base tube with the coated sleeve 1. The res stor as thus assembled is placed within a tube 9 formed of a material such as a wax impregnated cardboard. The interior of the tube 9 is then filled with a moisture proofing substance H) which embeds the resistor assembly rigidly within the tube 9 leaving the terminal wire 8 projecting therefrom. This moisture proofing material may be in the form of any insulating substance such as wax, tar, cements, or plastics.

With reference to Fig. 3, I show a resistor which is similar in all respects to the resistor shown in Fig. 2 except that it is permanently embedded within a rigid moisture proofing substance such as a vulcanized hard rubber composition l I.

In Figs. 4 and 5, I show a modified form of resistor in which the fabric ribbon l2, coated with a conducting rubber composition, is wrapped spirally with a strip l3 of an insulating material lying between the convolutions of the conducting strip I2. Wire leads I 4 and ii are attached to each end of the strip I! by being bent thereagainst and cemented by use of conductive adhesive to form a suitable electrical contact. The wire I may be bent as shown in Fig. 4 and the entire assembl is embedded within a moisture proofing material such as hard rubber vulcanized composition l6. By arranging the strip of conducting fabric in this manner, it is possible to obtain a resistor with a high resistance capacity at the same time resulting in a relatively small article.

Another method of forming a good mechanical and electrical connection between the wire leads and the resistor is illustrated in Fig 6. This view shows a conducting rubber sleeve assembly similar to the sleeve assembly as illustrated in Fig. 2. Lead wires l1 are joined to the coating 8 on the resistor sleeve by positioning the wire I! against the surface of the coating 8 near the ends of the resistor sleeve and electrodepositing a layer of metal l8, such as copper, at the ends of the assembly. Due to the electrical conductivity characteristics of the coating 8, the electrodeposited copper l8 readily adheres to the coating 8. In the same operation the wire I1 is mechanically embedded in the copper deposit I8 with the result that a good electrically conducting terminal is provided at each end of the resistor and the wires I! are strongly embedded in the copper deposit. The complete assembly is thereafter coated with a moisture resisting film such as lacquer or varnish I9 which in addition to its moisture proofing effect functions as an insulating medium over the resistor assembly, and allows the resistor to be bent, thus adding flexibility to the function of the resistor.

Fabric materials such as the sleeve 1 and the ribbon l2 upon which the conducting rubber is applied functions with greater efficiency than other bases upon which conducting materials may be applied such as paper or like material due to the fact that effective surface area on which the conductive material is deposited, is greater for a given area. This feature will allow greater current carrying capacity and better heat radiation than is possible with non-porous substances. It is to be understood that the fabric referred to herein can be made from threads or yarns of glass, plastics, cotton, wool, silk, or like materials.

Although conductive rubber fabric resistances will not stand the high temperatures of carbon and wire resistances the improved radiation properties allow better resistance stability for a given size. As an example a treated fabric sleeve 2%" long and T g" in diameter having a. resistance of 800 ohms changes in resistance under a load of approximately 2 watts, less than 5%, with a rise in temperature from 75 F. to a. maximum of 230 F., and will remain constant for a number of hours and through numerous cycles. In another example a treated fabric sleeve mounted on a glass tube and having a no-load resistance of 7000 ohms at 75 F. changed in resistance under a load of 1 watt, less than 5%, with a rise in temperature from 75 F. to a maximum of 187 F., and less than 2% from a load .1 watt to 1 watt.

The above tests were made after curing resistors under excessive load for 1 cycle of 30 minutes after which a stable condition was obtained. Through subsequent cycles of whatever length, the resistors maintained reasonably stable temperatures and resistance characteristics.

It is to be understood that the resistance values can be controlled by the area of the conductive fabric or by changing the conductive compound thus permitting the resistance values to cover a wide range from a few ohms to several meg. ohms.

As thus shown and described, it is believed apparent that I have provided a novel improvement in resistors in which the resistance values may be controlled with a high degree of accuracy while at the same time permitting economical manufacture of such resistors; and while I have shown and described preferred embodiments of my invention, it is to be understood that it is susceptible of those modifications which appear within the spirit of the invention and the scope rubber cement containing a sufficient amount of acetylene carbon black to impart substantialrcurrent conducting properties to the film, and metal terminals engaging opposite ends of the conducting film.

2. A small fixed electrical resistor of relatively high current carrying capacity and low temperature rise, comprising a length of core of insulating material, a flexible fabric support surrounding said core and provided with a current conducting coating firmly adhered to the fabric and which consists of a fllm of a dried residue of rubber cement containing a sufiicient amount of conducting carbon black to impart substantial current conducting properties to the film, and

metal terminals engaging opposite ends of the conducting film.

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