US2041212A - Enclosed terminal construction for resistance elements - Google Patents

Description

May 19, 1936.

N. C. SCHELLENGER ENCLOSED TERMINAL CONSTRUCTION FOR RESISTANCE ELEMENTS Filed Aug. 27, 1934 fPEEC IYT E0 TH Tia/V" HGT 6.

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NEWTO C JC'HEZLENGFE.

Patented Ma 19, 1936 UNITED STATES ENCLOSED TERMINAL CONSTRUCTION FOR RESISTANCE ELEDIENTS Newton 0. Schellenger,

Elkhart, Ind., assignor to Chicago Telephone Supply 00., Elkhart, Ind., a corporation of Indiana Application August 2'7,

6 Claims.

- This invention relates to a novel terminal device for a variable resistor and particularly to a variable resistor having a circular resistance element of the planar carbonaceous type.

Many of the present day uses to which variable resistors are put impose new requirements, such as, for instance, requiring that the unit be limited to unusually small physical dimensions and at the same time possess a very great range of resistance that must conform closely to a predetermined critical curve. This type of unit is now being employed as a volume control in automobile radio receiving sets and in order for the control to function satisfactorily in such capacity, the range of resistance variation must progress by the smallest possible increments from minimum resistance value of a mere fraction of an ohm, up

to a million ohms or more. In other words, the

continuity of resistance near the terminals must be as near, perfect as possible.

'I am not aware that terminals of the prior art, that make connection with a filmlike carbonaceous resistance element so that some part of the terminal is in the path'of the movable contactor, have fully met all these requirements in a structure that could be manufactured at a relatively low cost. It is difiicult in a unit of limited size to effect a substantial'and economical terminal connection to a delicate, filmlike carbonaceous variable resistance element of approximately .002 of an inch in thickness, without impairing the continuity of the resistance in the vicinity of the terminal. Frequently when such connections are established, tests reveal no material impairment of continuity, but after the movable contactor has traversed the area for some time in service, the continuity suffers permanent damage. In my present invention, I have overcome these difficulties by providing a relatively large contacting area directly between the terminal member and the resistance element, thereby obtaining a very substantial connection under relatively low unit pres-' sure. The rivet which clamps the projecting portion of the resistance element between the terminal and the base member does not of necessity enter into the electrical circuit as its function is primarily mechanical.

While my application S. N. 711,644, filed February 17, 1934, discloses a terminal and element structure of this same general class, this present invention embodies certain structural and functional improvements which are hereinafter fully described.

In devices of the prior art, the current, in passing about the element from one terminal to the 1934, Serial No. 741,551

other, seeks the path of least resistance. This carries it in approximately straight paths for a distance adjacent the terminals, and thence around the innermost portions of the element. The straight paths are substantially chords to the median arc of the path of travel of the contactor, and, hence, are of less linear magnitude than such arc. This means that the contactor intercepts the current through increments of resistance in commensurate with, and somewhat less than, the actual increments of resistancetraversed by the contactor through any given arcuate movement. Thus, a more abrupt gradation of resistance is'realized, than if the terminal connection were positioned outwardly from the median arc of the path of travel of the contactor, whereby a longer path of current would be provided.

As suggested above, for at least half of the arcuate distance of the element, the current follows concentric, arcuate paths adjacent the inner edge, whereby any given movement of the contactor within that zone would intercept current through increments of resistance which increase or decrease (depending on the direction of movement) proportionally as the degree of arc of rotation increases or decreases.

Therefore, in old types of elements, it can be said that their gradients will reveal uniform change intermediate their ends, with abrupt departures at their ends, whereby a true, gradual curve is not obtained.

It is an object of myinvention to provide a. resistance element having its terminal connections entirely without the contact surface.

Itis' another object of my invention to provide a resistance element having asmooth gradation of resistance throughout its length. It is still another object of my invention to provide a resistance element, which may be connected to, terminals without materially affecting the resistance gradient of the element when the contactor passes over the resistance adjacent such connection.

It is a further object of my invention to provide a resistance element which will permit of a greater area of contact between it and its terminals with negligible short-circuiting of efiective resistance.

With the above objects in view and others ancillary thereto, I prefer to accomplish one embodiment of my invention as follows:

I take a suitable piece of heavy paper or parchment to be used as the base of my resistance element. Any flexible insulatingmaterial may be employed for this purpose, but the above named materials are inexpensive and durable, and either serves the purpose very satisfactorily.

Suitable carbonaceous material may be painted, sprayed or otherwise coated upon one side of this base piece to form the resistance or the conducting contact surface.

From this coated base blank is stamped a resistance element generally circular in shape and which has a contact surface about A wide. From the peripheral edge of the element, at spaced points, project extensions covered also by the carbonaceous material. Between the projections the carbonaceous material is excluded to form an arcuate resistance element having the projections adjacent its ends.

These projections are provided with low resistance paint to insure proper contact between terminals, the element and the contactor.

If a tap" is employed, another projection is provided for in the stamping die to form a circular element with three projections. Low resistance paint is applied in a particular way to this projection and to the adjoining contact surface. This feature will be fully explained in the following specification with reference to the accompanying drawings.

The resistance element isglued to a base member of a variable resistance device, and terminals are positioned upon the projections and secured thereto. The terminals are so positioned to barely give clearance to the contactor without touching.

The novel features which I consider characteristic of my invention are recited with particularity in the appended claims. The invention itself, however, both as to organiaztion and method of operation may better be understood with reference to the following specification when taken in conjunction with the accompanying drawing, in which:

Fig. l is a top plan view of a variable resistance device employing a resistance element and terminal structure made in accordance with my invention.

Fig. 2 is a side elevational view of the variable resistance device shown in Fig. 1 when viewed from the right; illustrated with the contactor in a different position.

Fig. 3 is a top plan view of a resistance element made in accordance with my invention.

Fig. 4 is a side elevational view of Fig. 3.

Fig. 5 is a diagrammatic top plan view of my resistance element illustrating the path of current therethrough, and specifically pointing out the advantages of the present invention over the prior art.

Fig. 6 is a graphic chart illustrating the differences between the resistance gradients of the prior art (shown by a dotted line) and the present invention (shown by a solid line).

Referring more specifically to the drawing in which like reference characters refer to like parts:

In Figs. 1 and2, a resistance element l is glued or otherwise fastened to a base 2 of a variable resistance device. Mounted centrally in the 'base 2 is a bearing thimble 3 having a drive shaft 4 journalled therein. The shaft 4 extends sufiiciently far above the base 2 to permit insulating washers 5, contact arm disc 6, arm 6a, and drive extending into an opening in the contactor arm disc for effecting a. driving engagement therewith, whereby the contactor may be moved over the resistance element by the rotation of the shaft. The contactor assembly is provided with a contacting shoe ill, which is engaged by the spring-pressed arm 6a of the contactor arm disc. The spring-pressed arm is disposed about the shaft 4, and has wiping contact about the shaft with a collector ring II. The collector ring is connected to the central terminal II by means of a rivet l2a, and terminal extension l2h.

For a more detailed disclosure of the variable resistance device shown herein, reference is made to my copending application Serial No. 721,728, filed April 21, 1934.

The resistance element 1 is provided with ears or projections l3 which have rivet receiving perforations I4 therein. Overlying these projections are terminals l5, l6 and H, which are fastened to the projections and base 2 by means of rivets l8. The terminals are afforded lateral support by means of claws l9, which are made integrally with the terminals, and which are clinched around the base 2 within notches 20 and open ings 20a.

The edges 2| of the terminals adjacent the contact surface are curved to conform to the curve.- ture of the resistance element and they project inwardly to barely permit the contactor to pass without touching as shown by the dotted line Ct, Fig. 1. Terminal i6 is provided with a projection 22 to maintain the resistance between it and the contactor at a constant value after the contactor has passed beyond the centerline of the terminal (the point of least resistance) in a clockwise direction; otherwise, the resistance would tend to increase after the contactor had so passed.

To insure good contact between the terminals, resistance element and contactor, areas of the element adjacent the terminals, including the projections I3, are covered with a low resistance paint as indicated at 23.

The low resistance paint is applied to the projection of the tap terminal and extends out upon the contact surface in the form of a taper, which has its vertex outside the center of the contact surface. The purpose of this will be seen by reference to Fig. 5. The current does not pass in evenly spaced concentric paths through the entire breadth of the resistance element, but rather, pursues the paths of least resistance. In Fig. 5, the currents path has been diagrammatically shown at P. It will be seen that the current leaves the terminal in a straight path until it reaches a point of tangency at its innermost boundary with the inner edge of the element, from whence it passes around the innermost portions of the elementuntil it can straighten out to follow a direct course to the other terminal. The farther away from the inner edge, the fainter the current paths become.

When the contactor approaches the low resistance paint at the tap, it first engages a small portion thereof, then, due to the taper, more and more. If it is kept moving it will taper-off the same way;

It is necessary that the contactor eifect good electrical contact with the terminal without actually contacting it. In other words, a small amount of resistance becomes highly material, and if present, might cause trouble, whereas direct contact would materially affect the reproducer circuit adversely, due to short-circuiting of effective resistance.

Since the low resistance paint is disposed substantially without the path of the current, and since the contactor gradually (due to the taper) rides on, over, then bit the paint, no material change in the gradient is realized. Nevertheless, a good electrical contact is efiected through the paint with the tap terminal.

In Fig. 6, the broken line indicates the gradient of a device having its tap terminal rivet in the center of the contact surface. Note the flat or shorted portion in the curve designated atA. The solid line gradient B represents the effect of this invention when applied to an equivalent variable resistance device, having an element of the same degree of resistivity throughout as the old device with which it is compared in the graph.

Differences at the ends of the gradients in Fig. 6 will be noticed, and these may be explained with reference to Fig. 5.

When the ,contactor lies in position diagrammatically illustrated by dotted line C, it lies along a radius passing through the new terminal mounting at l3, and through the former location of the old terminal mounting T. The current P passes straight to the terminal at l3 along path illustrated at .X. The current P would pass to the old terminal T along-path Y. The contactor at C, if caused to travel counter-clockwise around the resistance element to the position indicated at K, more smoothlyvaries the effective resistance when the current passes along path X than it could if it passed along path Y, since X is greater than Y. This means that for the same are of rotation, the current is intercepted through a path of effective resistance greater than the median arc of travel of the contactor, which results in a smoother variation of resistance.

For the old terminal mounting at T, resistance paint would have to be provided up to approximately the distance indicated by the broken line R; instead of to line S as in the new type. Thus, a greater amount of resistance is short-circuited at both ends of the old type, which is utilized in the new type. From this description it will be seen that the resistance gradient of an element made in accordance with this invention will be smooth, gradual, and true, with minimum departures from the predetermined gradient curve when the contactor is adjacent the terminals.

I claim: 1. A resistance element of the planar carbonaceous type having a plurality of terminal engaging v projections extending therefrom, one of'said projections adapted for engagement with a tap terminal, alow resistance paint area overlying said projection extending away therefrom onto the contact surface of said element in the form of a taper having its vertex adjacent-the center of the width of said element.

2. In a variable resistance device, a base member of generally circular configuration having a plurality of projections extending therefrom, a

' resistance element mounted upon said base of the shape to conform thereto, a movable contactor carried by said base member to engage said element, a plurality of terminals connected to said base projections to contact portions of the element, said terminals disposed entirely without the path of the contactor, and a quantity of low resistance material adjacent one terminal extending onto said resistance element in the form v of a taper, having its greatest area disposed beneath said terminal and having its vertex substantially within the margin of said .contactors path.

3. In a variable resistance device, a circular base member having integral radially extending projections in the plane thereof, said projections having a plurality of openings therein, a fiat resistance element disposed upon said base member having extensions lying upon said projections, and a plurality of terminal members superimposed upon said extensions on said projections having a plurality of laterally extending fingers registering in said openings to secure said terminals in position.

4. In combination with a flat base having perforations therein, a planar, arcuate resistance element upon said base and having a plurality of extensions of the contact surface projecting therefrom, a terminal adapted to contact one of said extensions comprising a base portion of substantially the same size and shape as said extension, a shank portion integral with said base projecting outwardly therefrom, and laterally extending fingers integral with said base and shank, and substantially normal thereto extending into perforations in the base to position the terminal relative to the resistance.

5. In a variable resistance device, a base member, a flat resistance member on the base member, the base member and resistance member having registering openings therein, a terminal member having a perforatedflat body portion seated on the resistance member with its perforation registering with the openings in the base and resistance members, a. securing member passing through the registering openings andperioration, the base having cut away portions adjacent the body of the terminal and fingers extending from the terminal normal to the body portion enity of terminal members superimposed upon said extensions on said projections and having perforations registering with said' openings, securing means extending through the registering openings and perforations, and laterally. extending fingers on the terminals extending into said notches in the base member to secure said terminals against rotation about the securing means.

NEWTON C. SCHELLENGER.

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