US2597674A

US2597674A - Precision resistance device

Info

Publication number: US2597674A
Application number: US124337A
Authority: US
Inventors: Alexander G Robbins
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1949-10-29
Filing date: 1949-10-29
Publication date: 1952-05-20
Anticipated expiration: 1969-05-20

Description

y 1952 A. G. ROBBINS 2,597,674

PRECISION RESISTANCE DEVICE Filed Oct. 29, 1949 Inventor": I AIexander-GQobbms,

b @4 WOLF.

' His Attorney.

Patented May 20, 1952 PRECISION RESISTANCE DEVICE Alexander G. Robbins, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application October 29, 1949, Serial No. 124,337

4 Claims,

This invention relates to resistance devices, and more particularly to precision resistance devices such as precision potentiometers and precision resistors and methods of manufacturing the same.

One conventional form of precision resistance device comprises a base such as a cylinder of predetermined diameter and length, an electrical conductor of uniform cross-section throughout its entire length spirally wound on the base, contact means such as an electrical terminal connected to one end of the conductor, and a movable contact means arranged to slidably engage successive turns of the conductor as the former is moved from one position to another.

In such a conventional form of precision resistance device the ratio of maximum to minimum resistance gradient, that is, the ratio of maximum change in resistance for a given change in position of the movable contact means to the minimum change in resistance for a comparable change in position of the movable contact means. is attained by spacing successive turns of the wound conductor. Those skilled in the art will understand that a precision resistance device as above described cannot be wound to produce a high ratio of resistance gradient. To increase, by conventional means, the ratio of resistance gradient in a conventional wire-wound resistance device using wire of uniform cross-section, the base may be specially shaped. For example, a frusto-conically shaped base may be used in place of a cylinder. By this means a ratio of maximum to minimum resistance gradient of approximately 10 to 1 is obtainable. Also, further increases in such ratio may be accomplished by employing a wire of tapered cross-section in addition to a specially shaped base. However, although such expedients increase the ratio of resistance gradient to some extent, the resistance 7 gradient is still controlled by the spacing of the successive turns of the winding, and this characteristic limits theextent to which the ratio of resistance gradient may be increased. This characteristic also prevents the attainment of a zero resistance gradient, that is, no increase in resistance for a change in the position of the movable contact means.

Moreover, in such a conventionally wound resistance device resistance changes accompany the post-winding operations thereon. That is, after the wire is wound on the base, it is secured thereto by a bonding cement, and, due tothe pressing of the turns of wire during the curing of the bonding cement, the turns of wire are moved, thereby changing the resistance characteristics of the resistance device. Also, during the bonding operation, heat and pressure are applied to the turns of wire, thereby changing the stress in the wire which in turn changes the resistivity thereof.

It is, therefore, an object of this invention to provide a precision resistance device in which the ratio of maximum to minimum resistance gradient is appreciably increased as compared to conventionally wound resistance devices having wire of uniform cross-section.

It is a further object to provide such a precision resistance device as above described without the employment of tapered wire or a special- In general, my invention comprises in a pre-' cision resistance device a base which may be of any suitable shape, uniform in cross-section or otherwise, an electric conductive coating fixed to a surface of the base, a first and second plu-' rality of slots in the conductive coating, the slots being cooperatively positioned to establish a conductive path of a desired resistance characteristic through the conductive coating, and relatively movable contact means engaging the conductive coating to vary the resistance of the precision resistance device.

For a complete understanding of my invention reference should be had to the following specification and to the accompanying drawing, in

which Fig. l is a diagrammatical view'of a pre-" cision resistance device illustrative of my invention; Fig. 2 is a diagrammatical view of a precision resistance device illustrative'of my'invention and used to describe a method of manufacture thereof; and Fig. 3 is a diagrammatical view of a precision resistance device illustrative of my invention and used to describe a second method of manufacture thereof. It should be noted that each of the figures of the drawing for clarity are considerably enlarged as compared to the actual device and are not drawn to scale.

Referring to the drawing, in Fig.- 1 I have shown for the purpose of illustrating my invention a precision resistance device I comprising a base 2 made of glass or other suitable insulation material and having an upper flat surface 3 to which an electric conductive coating 4 is applied and fixed thereto, and a pair of relatively movable contact means 5 and 6. The conductivecoating 4 may be titanium, tungsten,

zirconium or any other suitable conductive material and may be applied to the surface 3 by any suitable means, as for example, by any ofthe conventional processes of vapor deposition, cathode sputtering, chemical reduction, electroplating or the Schoop process. Contact means 5 is illustrated in the drawing as a fixed electrical terminal connected to the base 2 adjacent an end I of device I and engaging the conductive coating 4 to provide electrical connection between the contact means and conductive coating. Contact means 6 is arranged for slidable engagement with conductive coating 4 adjacent an edge 8 thereof. Device I may be connected to an electrical circuit through a pair of conductor means 9 and ID respectively connected to contact means 5 and 6.

To'increase the resistance of device I and to provide desired finite resistance values of the conductive coating4 between the contact means 5 and 6, a first andsecond plurality of slots num bered II-I5 and IB-I9 respectively are cut or otherwise formed in the conductive coating 4. The slots II'I9 may be of any desired shape and may perforate the conductive'coating '4 or extend partially therethrough, as desired. Slots II-I5 emanate from an edge 20 of the conductive coating 4 and extend in the direction of the opposite edge 8 thereof. Slots I6--I9 are interspaced each between adjacent of theslots I II 5, 3t

as shown in the drawing, and emanate from edge8 of conductive coating 4 and extend in the direction of edge 20 thereof. Thusa winding 'or zigzag path of conductivity as indicated by the arrows 2| isestablished between the relatively movable contact means 5 and 6.

By narrowing the path of conductivity 2I, and by lengthening the same, the resistance between contact means 5 and 6 is increased. Thus byadjusting the length of the slots IB-I9 and the as I8, and a relatively short slot, 'such as IS, the

ratio of resistance gradient of the device I is appreciably increased over the maximum-ratio of resistance gradient attainable in a conventional wire-wound potentiometer having wire of 'uniform cross-section. That is, assuming the distance between

points

22 and 23 and the distance between points Hand 25 to be equal, the ratio of change in the resistance of the device I, resulting from a movement of the contact means 6 from point 22 to 23 to the change in resistance resulting from moving the contact means 6 from point 24 to point 25, is greater than can be provided in a conventional wire-Wound resistance device having wire of uniform cross-section. Therefore, by my invention I provide a simple means for attaining a desired'high ratio of resistance gradient without the necessity of a specially shaped base and tapered wire.

A'further advantage of my resistance device lies in its ability to provide-any resistance gradient down to substantially a zero resistance gradient, that is, no appreciable change in resistance for a given change in the position of the movable contact means. For example, consider the points 25 and 21 and a movement of the contact means 6 from point 26 to point 21. In such a movement of contact means 6 no appreciable resistance change will be introduced because there are no slots in the conductive coating 4 between points 28 and 21 and a broad path of conductivity is presented. Therefore, my resistance device can .bemade to produce more minute resistance changes for a given motion of the movable :contact,means and thereby provide more accurate adjustments than conventional wirewound resistance devices.

Another advantage of my precision resistance device is that having formed the necessary slots in the conductive coating 4 the device requires no further treatment except for a protective coating of synthetic resin or other suitable material, which coating produces .no detrimental change in the resistance of the device. Thus, the undesirable resistance changes which accompany the post-Winding operations on wirewound resistance'devices are eliminated and the accuracy of my device is retained.

Another advantage of my device is that a conductive coating may be applied to any shape of base, as for example,-to cylinders or toroids, and appropriately'slotted to producethe same result.

As a modification of my invention, to increase the number of resistance changes for a given movement of the movable contact mean 6 and thereby provide a still more accuratelyadjustable resistance device, the contact meanst may be arranged slidably to engage the conductive coating '4 along-a path indicatedby-the arrows 28 and the slots III9 may-bearranged each to cross the path 28 as exemplified by slots-.12 and I1. That is, considering the points -29 and 30 the distance between which is equal to the distance between

points

25 and 22, if the contact means 6 is moved along'the path'28 between points 29 and. there isaresistance .changelas the movable contact 6 passes over each of the slots 12 and I1; whereas for an equivalent movement of the contact 6 along the edge 8 of the conductive coating 4 between the

points

25 and 22 only one resistance change takes place becauseonly one slot, namely II,.is crossed. Thus. by arranging the oppositely disposed first and second plurality of slots II-I5 and I6--I9 in overlapping positions, each slot crossing the path 28, as exemplified by slots I2 and I'L'and by arranging for the contact means '6 to follow a path crossing each slot the number of resistance steps in the total resistance of the device 'I is doubled and the accuracy of adjustment is thereby measurably increased.

To illustrate a first method of manufacture and further advantages of my precision resistance device, attention is drawn to Fig. 2 wherein Fig. l is reproduced with appropriate additional references to describe the'above-mentioned first method of manufacture. To manufactures precision resistance device of my invention, as for example, device I, the base '2, which may be of any shape, is coated with a conductivematerial to form an area of conductive coating 4ha-ving

opposite edges

8 and 20. Considering the contact termina15 and the point 25 on the conductive coating 4 and neglecting all of theslots II-I9, if current is passed between terminal 5 and point 25 through the conductive coating 4, it may follow various paths as indicated by the arrows 3| and the resistance between the terminal 5 and point 25 is some finite value depending upon the thickness of the conductive coating 4 and the area of the conductive path indicated by the arrows 3|. Assume next that it is desired to increase the resistance between terminal 5 and point 25 to a predetermined finite value, this may be accomplished by providing the slot [6 of predetermined length emanating from edge 8 of the conductive coating 4 in the direction of the edge 20 thereof and thereby interrupting a portion of the conductive paths 3!. Next assume that it is desired to establish to finite resistance between terminal 5 and point 22. This may be accomplished by providing the slot I! of predetermined length emanating from the edge 8 of conductive coating 4 and extending in the direction of the edge 20 thereof. However, although the slot [1 establishes a desired finite resistance between terminal 5 and point 22, it also destroys the previously established finite resistance between the terminal 5 and point 25 because slot ll interrupts certain of the conductive paths 3| not interrupted by the slot [6. 7

To avoid this undesirable result and thereby provide means for establishing a finite resistance between terminal 5 and point 22 without destroying a previously established finite resistance between terminal 5 and point 25, in accordance with my invention the first plurality of spaced slots lll5 of predetermined length emanating from the edge 25 of the conductive coating 4 and extending in the direction of the edge 8 thereof, are

provided before the slots and I"! are made,

16 and then slot l7. By forming slot 16 a desired finite resistance is established between terminal 5 and point 25. Thereafter, by forming slot I! a finite resistance is established between terminal 5 and point 22. However, the forming of the slot I! does not change the desired finite resistance between terminal 5 and point 25 because slot l1 interrupts no conductive paths between terminal 5 and point 25. The paths 3! which might have been interrupted by forming the slot [1 have previously been interrupted by the slots H and I2 which slots were formed before the finite resistance between terminal 5 and point 25 was established by the slot IS. The same reasoning holds for finite resistances established between terminal 5 and successive points on the conductive coating 4. Therefore, by first forming the plurality of slots Hl5 and thereafter successively forming slots along the edge 8 of conductive coating 4, a plurality of finite resistance values of the device I may be established for each position of the movable contact means 6, and the establishment of each succeeding resistance will have no effect on a previously established finite resistance.

To determine the length of each of the slots l6-|9, the device I is electrically connected in a Wheatstone bridge or other suitable electrical measuring circuit and each slot is out or otherwise formed from the edge 8 of the coating 4 in the direction of the edge 20 thereof until the correct resistance value is attained. For example, to form the slot l5, terminal 5 and point are electrically connected to a suitable electrical resistance measuring circuit (not shown) and a cutting tool (notshown) is applied to the conductive coating 4 at the edge 8 thereof and moved toward the edge 20 thereof until the measuring circuit indicates that the resistance between terminal 5 and point 25 is a desired value. Thereafter, the measuring circuit is electrically connected between terminal5 and point 22 and'the" process repeated to form the slot [1. Additional slots are formed along the edge 8 of conductive steps as previously described the slots emanating from edge 8 of conductive coating 4 and the slots emanating from edge 20 thereof may be arranged to overlap and the movable contact means 6 arranged to follow a path crossing the slots emanating from both

edges

8 and 20 of the conductive coating 4.

However, in the manufacture of a precision resistance device of the conductive coating type, it may be desirable to provide additional thickness to the portion of the conductive coating which is slidably engaged by the movable contact means to allow for attenuation thereof. Such portion shall hereinafter be referred to as the contact path. In following the first method of manufacture above described, such additional thickness of the contact path must be cut by the cutting tool. This is undesirable because a cutting tool comprising a cutting member, such as a diamond, may readily be arranged to cut the thin conductive coating, but when applied to the thicker contact path, may tear the conductive material adjacent to the desired cut.

Therefore, to avoid cutting through the contact path, a second method of manufacture employing photo-engraving may conveniently be applied. For example, referring to Fig. 3 in which I have shown a precision resistance device 32 of the conductive coating type illustrative of my invention, such precision device may be conveniently manufactured in the following manner. First, a special glue 33 such, for example, as topping :enamel, Which i soluble in cold water if unexposed to light and otherwise soluble only in warm water, is coated on a base 34 of insulation material such as glass. Base 34, for simplicity of description is shown rectangular in shape with the understanding that any suitable shape of base may be employed, as for example, a cylindrical or disk-shaped base. Thereafter, the special glue 33 is exposed to a light pattern of desired shape by any suitable conventional means (not shown) such as a light source and screening means to expose to light that portion of the coating of glue 33 defined by the shaded areas 35. The base 34 is then washed with cold water which removes all of the glue 33 from the base with the exception of the portion adhering to the shaded areas 35. The latter portions are not removed by the cold water washing because the exposed glue is not soluble in cold water. A conductive coating 36 is thereafter applied to the base 34 by any suitable process such as cathode sputtering, vapor depositing, electroplating, the Schoop process, or chemical reduction and when applied adheres strongly to the base only where there is no glue. The conductive coating 36 is thereafter masked by any conventional means such as masking paint to leave a contact path indicated by numeral 31 unmasked. An additional thickness of conductive coating is thereafter applied to the contact path, by any of the above-mentioned processes,

the masking paint or other material is removed,

and the base is washed in warm water to remove the exposed glue from the shaded areas 35 thereby also removing the conductive coating from the area 35. The result of the operations above described is the formation of a blank 38 comprising the-base 34, the-conductive coating 36, thecontact' path 31 of additional thickness of conductive coating and a first and second plurality of slots 38-41 and 42--44 emanating respectively from opposite edges 45 and 4B of the conductive coating 35 through the contact-path 31, the slots 39-44 being formed when the exposed glue is washed from the base 34 by warm water.

To form a precision resistance device ofpredetermined resistance characteristics such as device 32 from the blank 38 a stationary contact means 41 is connected to the base 34 adjacent an end 48 thereof to engage the conductive coating of the contact path 3'! and a movable contact means is arranged to slidably engage the contact path 3?. To establish a finite resistance between the contact means 41 and 49 for various positions of the contact means 49 along the contact path 31 it is necessary only to cut or otherwise form an additional length to the slots 39 in successive order from the terminal 41. That is, a finite resistance between contact means 41 and each of a plurality of points 5055 as indicated in Fig. 3 along the contact path 31 is established by cutting or otherwise forming an additional length to each of the

slots

39, 42, 40, 43, 4i and 44 successively in the order named. For example, to establish a desired finite resistance between contact means 4! and point 59, slot 39 is extended asufficient distance as indicated by the dotted portion5$ to provide the desired finite re- Thereafter. a desired finite resistance ween contact means 41 and point 5| is established in the same manner by extending the slot 42 a indicated by the dotted portion 51 a suillcient distance to provide the second mentioned desired finite resistance. The establishment of each successive finite resistance will effect no change in previously established finite resistance value for reasons previously mentioned with respect to Fig. 2. In order to align the cutting tool. or other means of forming the additional length to each slot, the relative position of the cutting tool (not shown) and the blank 33 is controlled by photo-electric or other suitable means (not shown) using the slots 39-44 in the blank 38 as a register.

This method of forming a precision resistance device is particularly advantageous in that the method is readily adaptable to high speed manufacturing technique and is reliable in producing accurate results.

Moreover, by this method the disadvantages previously described with reference to cutting through the heavy thickness of. conductive material of. the contact path 3-! are avoided since no cutting of the material of the contact path is required;

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

An electrical potentiometer comprising a baseof clecti insulationmaterial, a coating of electrical resistance material on said base, a sec- 0nd coating of resistance material extending: from one end of said base tov the otherandcov ering a narrow portion of the. central area of said first coating to define a brush path of double thickness of conductive coating, said coatings including transverse slots alternately extending inwardly from opposite side edges of said device completely across said second coating but not completely across said first coating to provide a zig zag resistance path having a predetermined desired resistance characteristic.

2. A precision potentiometer resistor comprising a, strip of electrical insulation. material. a coating of electrical conductive material extending from one end to the other of said strip in a narrow band along the central section of one face thereof to define a brush path, a second coating of conductive material extending across the entire area of said face, said conductive coatings including slots alternately extending inwardly from opposite edges of said strip, each slot extending entirely across said narrow coating strip and partially through said second coating.

3. A precision potentiometer resistor device comprising an insulating base, a, path for a movable contact extending along the central portion thereof, a coating of electrically conductive ma.- terial on the surface of said base including said contact path, said coating including an extra thickness of conductive material in the area of said contact path, said coating also including transverse slots alternately extending inwardly from the opposite edges of said base, each slot extending at least through the area of said contact path.

4. A method of manufacturing a precision resistor potentiometer of the slotted conductive coating type having a contact path of greater resistance material thickness than the remainder of. the potentiometer comprising the steps of applying a photosensitive adhesive to an insulating base, applying a pattern of light to said photosensitive adhesive corresponding to the pattern of the resistor desired, applying a first uniform coating of conductive resistance material, applying a coating of masking material over the entire surface not included in the contact path, applying a second uniform coating of conductive material on said contact path and then removing. the portions of the photosensitive adhesive as determined by the light pattern together with the portions of the first and second coatings of conductive materials upon the portions of the insulating base covered by the adhesive which is removed.

ALEXANDER G. ROBBINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,061,107 Schellenger Nov. 17, 1936 2,159,106 Richter May 23, 1939 2,273,941 Dorn i- Feb. 24.. 1942 FOREIGN PATENTS Number Country Date 406,634 Great Britain May 23, 1932 461,275 Great Britain Feb. 1'5, 193'?