US2333477A - Electrical variable resistor - Google Patents

Description

, Nov. 2, 1943. M DUSTON ELECTRICAL VARIABLE RESISTOR Fil-ed June 21, 194i /5 /4 16 g /iv i INVENTOR Patented Nov. v2, '1943 ELECTRICAL VARIABLE RESISTOB Merle Duston, Detroit, Mich., assigner to United States Rubber Compa ny, New York, N. Y., a

corporation of NewvJersey Application June 21, 1941, Serial No. 399,073

Claims.

This invention relates to resistors designed for use in an electronic circuit such as radio or amplier apparatus. More particularly the in Vention relates to a fabric and conductive rubber composition in combination with means for forming contact with the resistance element at vvariable points.

It is recognized that resistance elements have been used in the form of strips, discs or irregular shaped units made from paperI plastics, or non-conductive materials treated with carbon or graphite and mounted in a suitable housing with arrangements for making variable contacts between the resistance element and `fixed terminals. Variable resistors have also been made usin'g resistance wire, but size limitations ordinarily confine these resistors to values under 10,000 ohms. In most applications this limitation is. not high enough to provide a suitable volume control unit. From the start of the radio industry up to the present time, variable resistor controls have been a continual source of trouble in radio and other sound amplifying devices. The high resistance composition volume controls become noisy after extended use due to the current carrying limitations, particularly since a small portion of the resistance element is subject to most of the load. Also the friction between the mowng metal contact and the composition element has a deteriorating effect upon the resistance element.

In the practice of my invention I provide in combination with a variable contacting means a resistance element which preferably is formed of a square woven fabric treated with a conducting rubber composition, and in some cases including a graduated electrodeposited layer of metal, that is a layer of metal which is electrically deposited so that it gradually increases in thickness; over a portion of the conducting rubber treated fabric. 1t has been found that a high degree of adhesion is obtained between a fabric base and a conducting rubber composition. Thus under Yordinary conditions the conducting rubber film will not flake away or otherwise separate from the fabric base. Fabric materials on which conducting rubber is applied function with greater efliciency than other bases such as paper or like material due to the fact that effective surface area, on which the conductive material is depositedI is greater for a given area, This feature will allow greater current carrying capacity and better heat radiation than is possible with substantially non-porous substances. Also, experiments have shown that fabric resistance elements are much more stable than solid rubber strips.

In addition to coating the fabric base with a conducting rubber composition, it is also possible to coat the variable means for contacting the resistance element with a thin layer of conducting rubber composition. Thus both contacting surfaces may be formed of a conducting rubber composition, and a high degree of fidelity and quiet operation is obtained. Due to the flexible characteristics of the conducting rubber compovide a variable resistor in combination with a,

resistor element in which the speciflc resistance 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 variable resistance element of small size and of relatively large carrying capacity; to provide a conducting rubber fabric resistance element in combination with a graduated metallic film deposited on a portion of the element; to provide a variable resistor in which both contacting points are formedof a conducting rubber'composition thus being conducive to quiet operation and resulting in better distribution of the current carrying load; and, to provide a variable resistor which may be manufactured elciently and economically. These and other objects and advantages will rappear more fully in the following detailed description when. considered 'in connection with the accompanying drawing in which:

Fig. l is a plan view of avariableresistor as embodied in my invention; l

Fig. 2 is a transverse view thereof, in section, taken along lines II-II of Fig. 1;

Fig. 3 is a perspective view of the resistance element employed in the construction as shown inv Figs. 1 and 2;

Fig. 4 is a plan view of a variable resistor illustrating a modified form of my invention; and.'

, Fig. 5 is a transverse view thereof, in section, taken along lines V-V of Fig. 4.

With reference to the drawing and in particular to Figs. 1 and 2, I show a variable resistor which includes a cup shaped housing I, a central bushing 2, attached to the housing, and an operating shaft 3 rotatable within the bushing 2. Lying within the cylindrical surface of the cup shaped sistivity. For example, it is member I is a strip of fabric 4 having at least its" outer surface coated with a layer of conducting rubber composition. An insulated ribbon 5, interposed between the conducting strip 4 and the inner cylindrical surface of the cup shaped member I, serves to insulate, electrically, the conducting strip 4 from the metal Wall of the housing I. The conducting strip 4 and the insulating strip 5 are held in place by terminal screws 5 and 'I attached to the housing I but insulated electrically therefrom. The strip of fabric 4 is preferably in. the form of a square woven cotton fabric although it is to be understood that other forms of fabric may be used such as glass, plastics, wool, silk or like materials. Applied to the surface of the fabric 4 is a layer or film of conducting rubber composition. Preferably this lm is applied to the fabric in the form of a cement. A representative iilm composition which has been found satisfactory is as follows:

lDarts by weight Rubber Conducting carbon black 85. Zinc oxide 15. vAntioxidant .'75 Accelerator v 2.00 Deodorant .l0 Softener 7.00 Vulcanizing agent .'15

The coating composition 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 to 50% based on the weight `of the dried composition. More particularly the invention contemplates the use of a carbon block which per se has an electrical resistivity of not more than about 0.25 ohmcentimeter 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 ingredients. The purpose of the rubber is to furnish a ilexible 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, Thokol, and various natural or articial resins or plastics such as shellac, cellulose acetate, viscose, nylon, Vinylite, casein, etc. Various of these materials (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 reregular carbon black more conducting by heating it in an inert atmosphere at 2000 degrees Fahrenheit.

The black may a mill or other mixing device 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. l

Attached to the control shaft 3 is an arm4 8 fastened thereto by means of a set screw' 9. The outer end of the arm 8 is slotted for the reception possible to make also be mixed with a binder on j or in the case of' of a lever III plvotally pinned to the arm 8 at II. The lever III is provided with a slot i2 which engages with a reduced portion I3 of a roller I4. A tension spring I5 normally urges the roller I4 into contact engagement with the coated fabric strip 4. By this arrangement rotary movement of the shaft 3 moves the roller I4 along the circumferential length of the resistance element i so as to provide a variable effect in the length of the resistance. A spring 3| (Fig. 2) serves to space the arm 8 and its associated connecting parte in proper position Within the housing i, and also serves to form a better electrical connection b.,- tween the housing and therotatabie'parts within the housing. A projecting portion 3?! or the lever I0 functions as a stop by contacting with the side of the arm 8 to prevent the roller from moving past the heads of either of the binding posts 6 or l.

The outer surface of the roller it may be covered with a layer it of a conducting rubber composition. This layer is formed oi material substantially the same as that employed in coating -the fabric strip il. This layer however is vuicanized and is bonded directly to the roller it.

.By providing the roller IQ with an outer layer of -effective area. A binding post Il secured to the housing I provides a terminal for forming an electrical connection leading to the roller through the metalcasing l, shaft 3, arm and lever ill.

In addition to coating the fabric strip fi with a conducting rubber composition, the coated fabric strip may be treated in part with an additionalbonducting material in the form of an electrolytically deposited metal 29. The ilrn of co.-- ducting rubber applied to the strip i being conductive permits the deposit of metal 2t to be formed directly thereon. The length oi the deposit in relation to the length of the strip t should be equal to approximately 1/3 of the total length of the strip 4. Preferably the deposit 2Q is in the form of electrolytically deposited copper and in addition the deposit is graduated, that is, it is heaviest nearer the terminal end of the strip i and thinnest as it reaches'approximately 1/3 of the length of the strip d. The largest area of the deposited metal-29 is essentially thin so that it retains some resistance properties. Modern ldesigns of the electronic apparatus require various resistance tapers and many times failures occur at the lower resistance end o the resistance element. This is due to excessive current dissipation over a comparatively small portion of the resistance element. The electroplating of a portion of the coated strip 4 overcomes this condition while materially increasing the current carrying capacity and still retaining certain low resistance properties.

In Figs. 4 and 5, I show a modified form of the invention in which the resistance element is flat and horseshoe-shaped. Essentially this modiiication includes a cup shaped member I8, a bushing I9 attached centrally ofl the cup shaped member and an operating shaft 20 extending through the bushing I9. A resistance element 2I of horseshoe-shape and a similarly 'shaped insulating layer 22 are attached to the base of the cup shaped member by terminal screws 23 and 24. These screws are insulated from the metal wire of the cup shaped member I8. Slidably keyed to the shaft 20 is an arm 2| which rotatably supports a contact roller 25 engageable with the resistance element 2l. A nut 26 secured tothe shaft- 20 and a spring 21 function to place the roller 25 firmly against the resistance element 2|. Similar to the treatment of the fabric strip 4, the resistance element may be treated over a portion of its length by containing a graduated coating 28 of an electrodeposited metal. A binding post 30 secured to the housing I8 provides a terminal for forming an electrical connection leading to the roller 25.

By f providing resistance elements as herein described, ithas been demonstrated that good resistance ranges can be obtained up to 500,000

ohms. It has also been found that the conducting rubber composition'which-is bonded to the fabric, provides a surface which will stand excessive wear and usage without danger of the conducting film loosening from the fabric or other- Wise deteriorating. i

While I have thus shown and described a novel construction of a variable resistor, it is to be understood that the invention is susceptible of those modifications which appear within the spirit of the lnvention and the scope of the appended claims.

I claim:

1. A variable electrical resistor comprising a housing, a resistance element secured to thehousing, said resistance element comprising a fabric support 'and a film of electrically conducting rubber composition containing a substantial amount of conducting acetylene black adhered tp the fabric support, a centralshaft rotatably 'mounted with the housing, and contact means attached to the shaft and resiliently engageable with the resistance element for movement over its surface.

2.v A variable electrical resistor comprising a housing, a resistance element secured to the housing, said resistance element comprising a fabric support, a film of a dried residue of an electrically conducting rubber cement containing a large 4amount of conducting carbon black adhered to the fabric support, a central shaft rotatably mounted with the housing, a contact roller engageable with the resistance element, and means attached to the shaft for resiliently supporting the roller.

3. A variable electrical resistor comprising a housing, a resistance element secured to the housing, said resistance element comprising afabric support, a film of a dried residue of lan electrically conducting rubber cement adhered to the fabric support, a central shaft rotatably mounted with the housing, a contact roller engageable with the resistance element, means attached to the shaft for resiliently supportingA the roller, and a thin layer of an electrically conducting rubber composition bonded to the outer surfaceof the contact roller.

4. A variable electrical resistor comprising a housing, a resistance element secured to the housing, said vresistance element comprising a fabric support, a lm of a dried residue of an electrically conducting rubber cement adhered and contact means attached to the shaft and resiliently engageable with the resistance element.

5. A variable electrical resistor comprising a, housing, a resistance element secured to the housing, said resistance element comprising a fabric support, a film of a dried residue of an electrically conducting rubber cement adhered to the fabric support, a thin layer of electrodeposited metal formed on a portion of the resistance element in a. graduated thickness, the thicker portion lying at the end of the resistance element, a central shaft rotatably mounted with the housing, a contact roller engageable with the resistance element. contact means attached to the shaft and resiliently engageable with the resistance element, and a thin layer of an electrically conducting rubber composition bonded to the outer surface of the contact roller.

6. A variable electrical resistor comprising a cup-shaped housing, an arcuate sheet resistance element secured to the inner bottom surface of the cup-shaped member, said resistance element comprising a fabric support, a film of a dried residue of an electrically conducting rubber cement adhered to the sheet element, an electrically insulating layer interposed between the resistance element and the housing, a central shaft rotatably mounted with the housing', a contact roller engageable with the resistance element, and means attached to the shaft for resiliently supporting the roller and advancing it over said arcuate fabric support.

'7. A variable electrical resistor comprising a cup-shaped housing, a ribbon resistance element secured to the inner circumferential wall of the cup-shaped housing, said resistance element com- 'prising a fabric support, a film of a dried residue of an electrically conducting rubber cement adhered to the fabric support, an electrically insulating ribbon interposed between the resistance element and the housing, a central shaft rotatably mounted with the housing, a contact roller engageable with the resistance element, a thin layer of an electrically conducting rubber composition bonded to the outer surface of the contact roller, and means attached to the shaft for l resiliently supporting the roller.

8, A variable electrical resistor comprising a cup-shaped housing, a ribbon resistance element secured to the inner circumferentiall wall of the cup-shaped housing, said resistance element comprising a fabric support, a film of a dried residue of an electrically conducting rubber cement adhered to the fabric support. a thin layer of metal electrically deposited on part of the film-covered portion of the resistance element in a graduated thickness, the thicker portion lying at the end of the resistance element, an electrically insulating ribbon interposed between the resistance element and the housing, a central shaft rotatably mounted with the housing, a contact roller engageable with the resistance element, and means attached to the shaft for resiliently supporting the roller.

9. A variable electrical resistor comprising a cup-shaped housing, a ribbon resistance element secured-to the inner circumferential wall of the cup-shaped housing, said resistance element comprising a fabric support, a film of a dried residue of an electrically conducting'rubber cement adhered to the fabric support, a thin layer of electrodeposited metal formed on a portion of theV resistance element in a graduatedthickness, the thicker portion lying at the end of the resistance 4 assamv secured to said surface and comprising a fabric support and a film of electrically conducting rubber composition adhered to the fabric support a contact roller engageable with the resistance element, a carrier movable relative to the resistance element and connected to said roller to advance it along said element, and a thin layer of an electrically conducting rubber composition bonded to the outersurface of the con 10 tact roller. y

IJERLE DUSTON.

Images