US2916717A - Variable resistor - Google Patents

Description

Dec. 8, 1959 c, w, HARTMAN ErAL 2,916,717

VARIABLE RESISTOR 3 Sheets-Sheet 1 Filed April 21, 1958 Vi/syn; AGE/51 5321:

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VARIABLE RESISTOR 3 Sheets-Sheet 2 Filed April 21, 1958 METALIZED FILM SILVER E'Zmian WHariman ATM-m1" MIaZZy A. i a!" den.

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VARIABLE RESISTOR Filed April 21, 1958 3 Sheets-Sheet 3 h J mdwv pw E'Zmi an WHarZ-m'an Ari Zzur M Dally my'qa A..Barian United States Patent VARIABLE RESISTOR Clinton W. Hartman, Elkhart, Ind., Arthur M. Daily, Edwardsburg, Mich., and Wayne A. Bar-den, Eikhart, Ind., assignors to Chicago Telephone Supply Corpo= ration, Elkhart, Ind., a corporation of Indiana Application April 21, 1958, Serial No. 729,741 13 Claims. (Cl. 338-154) This invention relates to variable resistors and potentiometers and has as its purpose to provide a control of this type which is extremely small in its overall dimensions, but nevertheless is capable of reliable, stable operation at temperatures as high as 250 centigrade, Though these objectives have exceptional size reduction has already been achieved, so that control manufacturers are already producing what has been termed a miniature control and even a subminiature control, none of the presently available variable resistors combines both the desired small size and the capability of operating satisfactorily at elevated temperatures such as those here contemplated.

This invention does achieve this object, and to better illustrate the point, the overall diameter of the casing in which the control of this invention is housed is only one-half inch in diameter, its depth, or axial dimension is less than three-eighths of an inch, and the unit is stable at 250 centigrade.

To a large degree, the unprecedented results obtained by this invention are due to the nature of the resistance path and the insulated base upon which it is mounted. The base is a ceramic which, of course, is capable of withstanding high temperatures and, in addition, has good dimensional stability and is unaffected by moisture; and the resistance path is a very thin film of metal, often referred to as a metalized film bonded directly to the ceramic base and, as such, greatly excels the performance capable with carbonaceous type resistance elements or others heretofore used in variable resistors.

The exceptional insulating qualities of the ceramic base, of course, provide the necessary electrical clearance be tween those portions of the control which are at different voltage potentials, as for instance the terminals, despite the fact that they are, of necessity, relatively close to one another. However, the use of ceramic for the base, due toits inherent brittleness, presented problems in the mounting of the terminals and in the connection of the end terminals to the resistance path; and, accordingly, one of the objects of this invention is to provide an improved manner of connecting end terminals to the ends of the resistance path which lends itself to the present situation wherein the base is formed of ceramic and, hence, incapable of withstanding the application of localized pressure or impact.

In this connection, it is another object of this invention to provide a base assembly which, in addition to the ceramic base per se upon which the resistance element is mounted, includes a protective outer base or cover formed of insulating material which is capable of withstanding the application of localized pressure and which provides a buffer for the ceramic base to receive the pressure needed to effect tight mounting of the terminals, and which also mounts the base assembly in the casing or shell of the unit so that the ceramic base is relieved of even those strains which result from clinching over the hold-down tangs or cars by is secured to the base assembly.

been sought before, and

which the casing or shell 2,916,717 Patented Dec. 8, 1959 With a view toward assuring against abrasive wear on the surface of the resistance path which wouldresult from having a contactor slide thereacross, the variable resistor of this invention utilizes aflexible contactor disc, or diaphragm mounted upon the base with its peripheral portion overlying the resistance path, with a localized portion thereof pressed into engagement with the IE sistance path by means of a pressure finger carried by an arm mounted to rotate about the axis of the control. Although this general construction is old in the art, being illustrated for instance inthe early Stoekle PatentNo. 1,653,745, the present invention adapts this old expedient to a unit of the small size. here contemplated, through a novel way of mounting the resilient contactor disc or diaphragm on the base assembly wherein the actual securement of the disc also electrically connects thetdisc to the center terminal of the control.

Still another object of this invention is to provide positive assurance against having the contactor disc bridge the ends or hop off points of the the disc is in either extreme position of adjustment.

With the above and other objects in view which will appear as the description. proceeds, this invention resides in the novel construction, combination and arrangement of parts substantially as hereinafter described, and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of the herein disclosed invention may be made as come within the scope of-the claims.

The accompanying drawings illustrate one complete example of the physical embodiment ofthe invention,-constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:

Figure l is a longitudinal sectional view at a greatly enlarged scale through a variable resistor or potentiometer constructed in accordance with this invention;

Figure la is a side view of the control drawn to actual scale, which is one-tenththat of Figure 1;

Figure 2' is a perspective view of the base assembly of the control, viewing the same from its inside, and having part of the contactor disc or diaphragm broken away to uncover the structure therebeneath; Y t

Figure 3 is an exploded perspective view of the casing or housing of the control and of the shaft with the contactor actuating arm thereon;

Figure 4 is a plan view of the base assembly, viewing the same from the inside, as in Figure 2, and also having part of the contactor disc broken away; v

Figure 5 is a detail sectional view through Figure 4 on the plane of the line 5-5 to illustrate the manner in which the end terminals of the control are mounted and connected to the ends of the resistance path;

Figure 6 is a detail perspective view of the pressure applying portion of the arm to illustrate a slightly modified form of pressure applying finger;

Figure 7 is a fragmentary detail view similar to a portion of Figure l, to illustrate a slight modification of one of the elements of the variable resistor; and

Figure 8 is an enlarged sectional view substantially; on the'plane of the line 8-3 in FigureS to illustrate the manner in which the ends of the resistance path are electrically connected with their respective terminals.

Referring now more particularly to the accompanying drawings, the numeral 5 designates generally the base assembly of the variable resistor of this invention which, as shown, closes the open end of a cup-shaped metal casing 6, within which the instrumentalities of the control are contained. i

The bottom or end wall 7 of the cup-shaped casing 6 has a mounting thimble 8 and a stop-plate 9, fixed thereto, the latter being clamped between the outer face of resistance element when the wall 7 and a flange 10 on the thimble, and the thimble being secured to the casing in any suitable way as by having the inner end portion of the thimble passed through the casing end wall 7 and a shouldered portion thereof staked or swedged over the inner face of the wall 7.

The thimble is externally threaded to receive a clamping nut (not shown) and its flange 10 is preferably hexagonal with two diametrically opposite corners thereof engaged in sockets formed by out-struck portions of the stop plate 9 to secure the thimble against rotation with respect to the casing.

The thimble 8 not only provides the means by which the entire unit may be mounted on a supporting panel in the conventional way. but also provides a bearing for the operating shaft 11 of the resis or. At its inner end. the shaft 11 has two arms '12 and 13 solidly fixed thereto. The arm 12 is stam ed from thin spring stock and. in the assembled condition of the parts, is biased rearwardly to rovide contact pressure, but the arm 13 is formed of stiff heavier gage metal since its purpose is merely to provide a movable stop abutment coonerable with a fixed stop 14 projecting into the casing from the stop plate 9, to define the limits of rotation of the shaft.

The rotating structure comprising the shaft with the two arms 12 and 13 thereon. is restrained against forward displacement by having the front face of its stiff arm 13 bearing against the inner end of the thimble, and is held assembled with the thimble by a conventional C washer 15 mounted on the shaft at the front end of the thimble. Since the engagement of the arm 13 with the inner end of the thimble serves as a thrust bearing for the rotating structure. and since it is essential to smooth adjustment of the resistor that rotation of its operating shaft be as free as possible, the inner end of the thimble and also the surface of the arm 13 which bears thereon should be smooth.

The arm 12 has a flat central portion 16 by which it is secured to the control shaft, and an outer U-shaped portion 17 embracing the central portion and connected thereto only at the ends of its legs. This outer U-shaped portion 17 is sprung rearwardly toward the base assembly and at its rearmost portion which is formed by the bight of the U. it carries a pressure applying finger or shoe 18. In the preferred embodiment of the invention this pressure applying finger or shoe is simply a small cylindrical piec of Teflon, or other similar material, seated in a socket 19 formed in the bight of the U-shaped portion 17.

However, if desired, the pressure applying finger or shoe may take the form of a spool, as shown at 20 in Figure 6. In this case. the bight of the U-shaped portion 17 has a rearwardly directed flange 21 which is bifurcated to interengage with the spool-shaped configuration of the finger or shoe 20.

In either case, however, the pressure applying finger or shoe constitutes the rearmost extremity of the rotor and bears upon the peripheral portion of a resilient contactor disc 22 to hold a localized part thereof deflected out of its normal flat state and firmly pressed against the surface of a resistance path 23.

The contactor disc 22 and the resistance path 23 form part of the base assembly 5 which also includes a round, essentially flat disc or body 24 of ceramic material upon which the resistance path is formed, and an outer base or cover disc 25 of fiber glass or other similar and suitable good insulating material. The two discs 24 and 25 are held together by a rivet 26 which passes centrally through the discs and also through a center terminal 27 to clamp these parts together between a shoulder 28 on the rivet which bears against the inner face of the ceramic disc and the rearmost portion of the rivet which is rolled over the terminal, as at 29. The resilient disc 22 is fixed on the rivet 26 by being clamped between the rolled over hollow end of the rivet and a forwardly facing shoulder 30 thereon. Preferably the hole in the disc 22 through which the rivet passes, is serrated as shown in Figure 4, since this assures a more solid connection between the disc and the rivet.

The center terminal 27 should be secured against rotation, and to this end the terminal has a tongue 31 projecting forwardly therefrom into aligned holes in the bases 24 and 25; and its actual terminal portion, of course, projects perpendicularly from the base 24, as shown.

Attention is directed to the fact that the diameter of the ceramic base 24 is such that it fits easily into the metal casing 6 but the diameter of the outer or protective covering disc 25 is large enough to project over and seat upon the edge of the casing, in which position it is held by a plurality of tongues 32 extending from the edge of the casing side wall through notches 33 in the outer base or disc 25, and struck over the rear face thereof.

The inner face of the ceramic base 24 has an arcuate or substantially annular ridge 34 rising therefrom, concentric to the axis of the control. The top of this ridge slopes radially, or in other words is frustoconical, and this frustoconical surface of the ridge has the resistance path 23 bonded directly thereto. Preferably the ceramic base is glazed to assure the smoothest possible surface for the resistance path. The resistance path is a thin metal film, often referred to as a metalized film, deposited directly on the base in any suitable manner, as by vaporization of the metal in a vacuum. The thickness of the film, of course, depends upon the resistance intended for the unit, and may range from to 2,000 angstrom units.

The metal of which the film is formed preferably is Nichrome which, as is well known, is an alloy of nickel and chromium. As best shown in Figure 4, the arcuate resistance path has ends circumferentially spaced apart a short distance, and located in the space therebetween is a protuberance 34 which rises from the frustoconical top of the ridge. At opposite sides of this protuberance, the ridge is radially interrupted as at 35 to define the ends of the resistance path; and each interruption has a pocket 36 opening axially into the bottom thereof. These pockets have flat bottoms and outwardly sloping sides, as shown in the detail sectional view, Figure 5, and as will be shown, play an important part in connecting the resistance path with its terminals 37.

The terminals 37 are stamped and formed of suitable metal and have a substantially T-shaped formation. The stem of the T passes through aligned slots 38-38 in the bases 24 and 25, and the head of the T bears against the inner face of the ceramic base 24. The terminals are secured in this position by having their portions directly adjacent to the mouths of the slots 38 in the outer base or cover 25 swedged over the base, as at 39. The terminals 37 thus coact with the rivet 26 in holding the two bases tightly together.

One arm 40 of the head of each terminal is longer than the other and is bent to have its outer portion extend substantially radially into the adjacent interruption 35. A tongue 41 on the underside of this outer portion of the arm 40 extends into the pocket 36 which opens to the interruption 35, and is electrically connected with the adjacent end of the resistance path.

The manner in which the connection is made between the terminals and the ends of the resistance path is an important feature of this invention. It comprises a conductor in the form of a coating of metal of good conductivity, preferably silver, painted or otherwise applied to the bottom and sides of the interruptions 35 and of the pockets 36, and against which the tongues 41 of the terminals are pressed by the securement of the terminal to the base. To further assure good electrical conneetion at this point, the tongues 41 are actually soldered to the silver conductor, as at 42.

Attention is directed to the fact that although the silver conductor coating of each interruption 35 is extended onto the adjacent top surface of the plateau or ridge, it does not come in contact with the resistance path. Direct physical connection between the silver and the metalized film resistance path is definitely not desirable. Experience has shown that the silver migrates into the metalized film and, in so doing, actually disrupts the connection between the resistance path and the terminal.

It has been found, however, that if the silver conductor coating is electrically connected with the metalized film resistance path through a bridging strip or band of gold, and the silver is kept from contacting the metalized film, a stable and wholly reliable connection is obtained. Accordingly, a strip 43 of gold is applied or suitably bonded to the frustoconical top surface of the ridge 34 near each of the interruptions 35 before the silver conductor coating and the metalized film are applied, and, as best seen in Figure 8, the silver conductor coating overlaps one edge portion of the gold strip or band and the end of the metalized film resistance path overlaps the opposite edge portion thereof but without coming close to the silver.

Besides gold, platinum, palladium, iridium, and alloys thereof may be used with good results as the bridging strip between the silver and the metalized film. These metals, which may be considered noble metals have good resistance to oxidation and will not migrate into the metalized film.

As clearly illustrated in Figure 1, the mounting of the resilient contactor disc or diaphragm 22 is such that the peripheral portion of the disc or diaphragm overlies the resistance path, but in spaced relation thereto, except Where the disc is deflected and pressed against the resistance path by the pressure finger or shoe 18. The angle of the slope or conical surface of the resistance path and the height of the disc above the base are such that Where the disc is pressed against the resistance path the engagement therebetween extends across the full Width of the resistance path, theoretically along a radial line, but actually along a narrow radially extending strip.

Because of the sloping or conical disposition of the top surface of the plateau or ridge 34 on which the resistance path is located, the deflection or deformation of the disc or diaphragm needed to make good contact with the resistance path is not as severe as it would be if the resistance path were on a flat surface parallel to the base. Moreover, by having the surface of the resistance path frustoconical, rather than flat, the contact between the disc and the resistance path approaches more nearly the desirable line contact obtained with a cylindrical resistance path, without losing much of the advantage of the more readily utilized fiat type of resistance path.

Obviously, the surface onto which the resistance path is applied need not be an absolutely true frustrum of a cone. A somewhat curved or rounded surface, such as shown at 34a in Figure 7, is also satisfactory. In fact, in the molding of the ceramic base, a slight roundness or curvature for the top of the ridge 34 is practically inevitable.

Upon rotation of the control shaft, the pressure finger or shoe 18 slides across or along the peripheral portion of the resilient disc to move the area or line of contact between the disc and the resistance path along the path with a rolling action, as distinguished from a sliding action and, hence, adjustment of the control does not subject the resistance path to objectionable abrasion.

An important feature of the invention is performed by the small protuberance 34'. This protuberance extends above the frustoconical surface of the resistance path so that the resilient contactor disc cannot bridge the two ends or hop ofi points of the arcuate resistance path. As the pressure finger is brought to one end of its range of travel, the contactor disc rolls against the protuberance and is thereby held off of the end of the resistance path at the opposite side of the protuberance.

From the foregoing description taken in connection with the accompanying drawings, it will be readily apparent to those skilled in this art that this invention provides a variable resistor especially well. adapted for use in situations where space is at a tremendous premium and where the operating conditions are such that temperatures as high as 250 C. may be encountered.

What is claimed as our invention is:

1. A variable resistor comprising: a ceramic base having a substantially annular ridge projecting from one face thereof, the top surface of said ridge being substantially frustoconical; an arcuate resistance path on the substantially frustoconical top surface of said ridge having ends which are located in substantially close circumferentially spaced relation; a thin resilient metal disc secured at its center to the base so as to be coaxial with said ridge and the resistance path thereon, the marginal portion of the disc being unbroken and overlying the top of the annular ridge in spaced relation to the arcuate resistance path thereon; an arm mounted to rotate about an axis which is fixed with respect to the base and is perpendicular to said face thereof, said axis passing through the center of the disc; a pressure finger on the arm pressing down onto the disc near its periphery to hold a localized peripheral portion of the disc deflected and firmly pressed against the resistance path; means to rotate the arm; and a protuberance on said ridge projecting above the substantially frustoconical top surface thereof at a point between the ends of the resistance path, said protuberance lying beneath the adjacent marginal portion of the disc and being engageable thereby upon such deflection of the disc which causes it to contact one of the end portions of the resistance path, so that said protuberance precludes contact of the disc simultaneously with both end portions of the resistance path.

2. In a variable resistor: a base of insulating material,

said base having a substantially annular ridge projecting from the top thereof; an arcuate resistance path on the top of said ridge having ends which are located in substantially close circumferentially spaced relation; said ridge having spaced substantially radial interruptions therethrough at the ends of the resistance path; a terminal for each end of the resistance path, said terminals being T-shaped stampings, the heads of which bear against the top of the base inside the annular ridge with one end portion thereof projecting into one of said interruptions in the ridge and electrically connected with the adjacent end of the resistance path, the heads of the T- shaped terminals having a height no greater than that of the annular ridge and the stems of the T-shaped terminals passing downwardly through the base; and means fixed with respect to the base intermediate the ends of the resistance path and projecting above the top of the annular ridge to be engageable by a contactor tracking on the resistance path and thereby prevent shorting of the terminals by such contactor.

3. in a variable resistor, the structure set forth in claim 2 further characterized by the fact that said last named means comprises a protuberance integral with the annular ridge and projecting from the top thereof between substantially radial interruptions.

4. A variable resistor having a base of insulating material, a resistance path on the base and a contactor to traverse the resistance path, said resistor being characterized by the fact that: the resistance path is formed by a film of metal on the base; and by low resistance end portions for the resistance path, each of which comprises an area of silver on the base near but definitely spaced from the adjacent end of the resistance path, and an area of nobel metal on the base in intimate contact with the area of silver and the adjacent end of the resistance path to electrically connect the same without danger of having the silver migrate into the metal forming the resistance path.

5. The variable resistor set forth in claim 1, wherein said ridge has radial interruptions therethrough at opposite sides of its protuberance to define the ends of the resistance path; and further characterized by the provision of a pair of terminals mounted on the base and each having a portion received in one of said interruptions and electrically connected with the adjacent ends of the resistance path.

6. The variable resistor of claim wherein the electrical connection between each terminal and the adjacent end of the resistance path comprises an area of a noble metal in intimate contact with the end portion of the resistance path and with a conductor of good conductivity which conductor is in intimate contact with the adjacent terminal, but has no physical connection with the resistance path.

7. The variable resistor of claim 6, wherein said conductor covers the surfaces of said interruptions, and wherein said terminal portions in the interruptions have parts thereof soldered to the conductor.

8. The variable resistor of claim 7, wherein said parts of the terminal portions which are soldered to the conductor, are tongues which project from the terminals into pockets in the base, the surfaces of which are covered by said conductor.

9. A stator assembly for a variable resistor comprising: an outer mounting disc of insulating material; a slightly smaller inner disc of ceramic material; means coaxially securing said discs flatwise together, comprising a pair of end resistor terminals extending through both discs at locations spaced from their common axis, shoulder portions on said end terminals engaged with the exposed faces of the connected discs and between which the discs are clamped, and a rivet passing centrally through both discs; a center terminal on the exposed face of the outer disc secured thereto by said rivet; said ceramic disc having a substantially annular ridge projecting from its exposed face, concentric with said rivet, and the top of said ridge being frustoconical; an arcuate resistance path on the frustoconical top of said ridge; extensions on said end terminals overlying the exposed face of said ceramic disc and electrically connected to the ends of the resistance path; a thin resilient metal disc secured at its center to said rivet so as to be coaxial with said ridge and the resistance path thereon and to have a marginal portion of one face of the disc spaced from but in opposing relation to said resistance path, whereby a localized marginal portion of the disc may be deflected and pressed into electrical engagement with the resistance path; and means fixed with respect to the disc at a location intermediate the ends of the resistance path and thus engageable by a deflected marginal portion of the disc for inhibiting simultaneous engagement thereof with both ends of the resistance path.

10. A variable resistor comprising: a ceramic base; a cover of insulating material over one face of the base; a

thin resilient metal contactor disc overlying the other face of the base; a terminal for the contactor disc at the exposed side of the cover with a part thereof fiatwise engaging the center of the cover; a rivet passing through the center of the contactor disc, the ceramic base, the cover and said terminal portion securing said parts together with the peripheral portion of the contactor disc spaced from the underlying portion of the ceramic base, and electrically connecting the contactor disc with its terminal; an arcuate resistance path on the ceramic base under and facing the peripheral portion of the contactor disc; a terminal for each end of the arcuate resistance path, said terminals passing through the ceramic base and said insulating cover and being electrically connected with the ends of the resistance path; an arm mounted to rotate about an axis perpendicular to and fixed with respect to the base and passing through the center of the contactor disc, the arcuate resistance path being concentric to said axis; a pressure finger on said arm pressing against the contactor disc to hold a localized portion thereof against the resistance path; and means for rotating said arm.

11. A resistor having a resistance path and a metal terminal electrically connected with the resistance path, characterized by the fact that the connection between the terminal and the resistance path comprises: a conductor of metal having good conductivity directly physically connected to the terminal and extending from the terminal toward the resistance path but terminating at a distance therefrom; and a bridging strip of a noble metal other than silver directly physically joined to the resistance path and to said conductor.

12. The resistor of claim 11 wherein the bridging strip is gold and the conductor is silver.

13. The resistor of claim 11, wherein the resistance path is a thin film of metal applied directly to one surface of a ceramic body; wherein the terminal is mounted on said ceramic body remote from the nearest part of the resistance path; wherein the bridging strip is gold bonded to said surface of the ceramic body with an edge portion of the strip underlying the adjacent portion of the metal film forming the resistance path; and wherein said conductor is a coating of silver on the ceramic body and on an edge portion of the gold bridging strip remote from the edge portion thereof underlying the resistance path.

References Cited in the file of this patent UNITED STATES PATENTS 1,851,933 Schellenger Mar. 29, 1932 2,000,177 Kennet May 7, 1935 2,644,064 Jack et al. June 30, 1953 2,748,234 Clarke et a1. May 29, 1956 2,789,191 Arisman et al. Apr. 16, 1957

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