US 3601744 A
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Description (OCR text may contain errors)
United States Patent 2,597,674 5/1952 Robbins lnventor Felix Zandman Philadelphia, Pa.
Appl. No. 841,322
Filed July 14, 1969 Patented Aug. 24, 1971 Assignee Vishay Intertechnology Inc.
VARIABLE RESISTOR WITH STRAIN-REDUCING ATTACHMENT MEANS FOR THE SUBSTRATE 4 Claims, 6 Drawing Figs.
US. Cl 338/183, 338/292, 338/308 Int. CL 1101c 9/02 Field oiSelrch 338/118,
Reterenees Cited UNITED STATES PATENTS 3,271,721 9/1966 Gordon 338/180 3,405,381 10/ 1968 Zandman 338/308 FOREIGN PATENTS 406,634 5/1932 Great Britain. 333/138 Primary Examiner--Lewis H. Myers Assistant Examiner-Gerald P. Tolin AttorneyThomas M. F erril, Jr.
ABSTRACT: An adjustable resistor wherein a predetermined pattern of thin metallic film is deposited upon a substrate, a thin isolating layer being interposed between the substrate and film, and a movable contact system with suitable linear or rotary mechanical drive means is arranged for movability parallel to the substrate surface on which are fixed the isolating layer and metallic film, the contact system providing connec tion to the metallic film at a distance from one end thereof dependent on the position of the drive means. The pattern in the metallic film compels the electric current to flow through a path of limited width and of efiective length much greater than the straightline length along the midline of the pattern.
I sum 1 ur 3 162 54 r 7 K4 ;////'////y//////// 141/ /1 .f/ I f/ INVENTOR f/a'z Z animal? ATTORNEYS SHEET 2 BF 3 PATENTEB M1824 |97| INV NTQR fe /(x Za men/ 8 hkm z ATTORNEYS BACKGROUND OF THE INVENTION The present invention relates to electrical components and variable resistors. It is particularly concerned with very stable variable resistors for use as rheostats and potentiometers.
Two prevalent types of variable resistors for use as rheostats or potentiometers are wire-wound resistors and composition resistors. In some examples of the former, a narrow, elongated card of fiberboard is wound with wire to the configuration of a flattened helix and is then curved into an arcuate shape and mounted so as to have the resistive wire winding contacted by v a rotatable contact arm. For use of the apparatus as a rheostat,
one connection is made to one end of the resistive wire winding and the other connection is made to the rotatable contact arm. In order for the apparatus to be usable as a potentiometer resistor, an additional connection is provided to the other end of the wire winding. In some examples of composition resistors, a resistive coating on a supporting card is arranged to be contacted by a wiper, and connections are made to the ends of the composition resistive element.
Both of these types of variable resistors have disadvantages. The wire-wound resistors usually lack smoothness, mechanically and electrically. Their resistance changes in a substantially step-by-step fashion. Their wear life, while sometimes better than that of variable resistors with composition elements, is limited. Neither type resistor is readily adjusted to a precise predetermined function of resistance versus control position. Each such unit is subject to appreciable variation during manufacture. Each type provides resistance values greatly dependent on ambient conditions including temperature.
It is an object of the present invention to provide a high precision variable resistor.
It is another object of the present invention to provide a very stable variable resistor.
A further object is to provide a variable resistor wherein a desired characteristic of resistance as a function of displacement of a movable contact element can be achieved with great precision and be stably retained throughout substantial changes of conditions including changes of temperature.
SUMMARY OF THE INVENTION In accordance with the present invention, an improved variable resistor is provided wherein the resistor element consists of a thin film of a resistive bulk metal, deposited upon a thin isolating layer of a dielectric such as a thin epoxy substance on the surface of a substrate. The film, isolating layer and substrate are strongly adherent to each other. A contact unit is arranged for movement along a predetermined path, so that it is caused to establish electrical contact with the thin film at any desired distance from one end thereof and thereby provide a variable resistance value between said end and the contact unit. In order to use the variable resistor as a potentiometer resistor, it may be provided with a terminal for each end of the resistive film in addition to its contact unit terminal. Essentially zero temperature coefficient of resistance is obtained by proper choice of coefficient of thermal expansion and thickness of substrate, isolating layer and bulk metal film, and temperature coefficient of resistivity of the bulk metal film itself.
BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the invention, reference will be made in the following description to the accompanying drawings, wherein:
FIGS. 1 and 2 are vertical and horizontal sectional views, respectively, of an embodiment of the present invention having a linearly movable contactor;
FIG. 3 is a diagram of several alternative patterns for the re sistive metal film;
FIG. 4 is a cross-sectional view of an embodiment of the present invention having a rotary contactor cooperating with resistive film on a flat substrate, the housing and control shaft being shown in section;
FIG. 5 is a sectional view of the embodiment of FIG. 4, taken on line 5-5 thereof, and
FIG. 6 is a cross-sectional view of a further embodiment of the present inventionhaving a rotary contactor cooperating with a resistive film on a substrate in a cylindrical arcuate configuration.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS OF THE INVENTION Referring now to FIGS. 1 and 2, a high precision variable resistor is shown which comprises a housing 11, control knob 13, and a lead screw 15 arranged to be rotated by means of the knob 13. Situated parallel to the lead screw 15 is a substrate 17, supported as by cement bodies 16, 18. A coating 19 which may comprise an epoxy resin or polyimide or equivalent is provided on the substrate 17. A metallic film 21 of resistive bulk metal is securely held in place on'the substrate 17 by the adherent isolating layer 19 and is so arranged as to have its upper surface exposed in order that it can be reliably contacted by a movable contact member.
A lead screw follower 25 is provided on the lead screw 15. Follower 25 has a series of contact elements 27, 29; 31 and 33 carried thereby, preferably arranged on a resilient sheet metal member 35 which may, if desired, have the lower portion thereof slitted for maximum flexibility and for independence of spring action of each of the contact elements.
The substrate 17 may be either a hard insulative substance such as glass, or may be a metal body, if desired. The coating layer 19 thereon holds the bulk'metallic film 21 in place upon the substrate 17, providing the necessary adhesion and electrical isolation to both the metallic film and substrate, whether the substrate is glass or metal or yet other material.
As described in US. Pat. No. 3,405,381, issued Oct. 8, 1968 to Felix Zandman and Branin A. Boyd, the bulk metal film 21 may be made from a resistive alloy such as one of the Nichrome alloys, wherein nickel and chromium are the principal component metals. This film may be thin, for example,'it may be of the order of 0.00002 to 0.0004 inches thick.
In order to provide a substantial value of resistance between the two ends of the metallic film 21, the current conduction path therein is caused to have a length many times longer than the physical length of the substrate 17. One way in which this may be accomplished is by photoetching the film 21 to a pattern such, for example, as the pattern illustrated in FIG. 2. With the pattern as shown here, the current paths from one end terminal junction 37 to the opposite end terminal junction 39 involve traversals between the centerline of the film 21 and the outer edges thereof. The current is forced to flow through such paths by the elongated, narrow gaps etched in the film 21. The first such gaps proceeding from the left end toward the right end of the film 21 are the gaps 41 and 41', between which the current is forced to flow through the narrow middle portion of the film 21. Immediately to the right of these elongated gaps 41 and 41' which extend out to the edge of the film 21 is a further elongated gap 43 which is bounded at both its ends by the film. Next to the right of this elongated gap 43 is a further pair of gaps 45 and 45' extending to the edges of the film 21, at which these gaps are open, and yet further to the right is another narrow elongated gap 47 bounded at its ends by the film. This pattern is repeated throughout the length of the film 21 between the end portions thereof to which are secured the end terminal junctions 37 and 39. The current path from end terminal junction 37 extends approximately to the longitudinal centerline of the metallic film 21 (between the mutually adjacent ends of gaps 41 and 41'), and extends outward toward the opposite edges of the film 21, around the ends of gap 43, and back toward the longitudinal centerline, where the path continues between the mutually adjacent ends of gaps 45 and 45' and immediately extends back toward the opposite edges of the film 21, around the ends of the gap 47.
Using this basic pattern, it is possible to achieve any desired resistance value in a wide range by the selection of the thickness of the bulk metal film and the closeness of spacing (and consequently the number) of the elongated narrow gaps. The thinner the film, and the narrower the film between gaps, in the pattern of FIG. 2, the higher the resistance value.
The contact elements 29, 31 and 33 are arranged to contact the film 21 at any desired distance from the left end thereof,
according to the adjustment of the lead screw 15 and follower 25 by means of knob 13. Contact elements 29, 31 and 33 may be arranged in a straight line pattern as shown, or arranged in a slightly diagonal straight line, if desired. If preferred, the middle element may be positioned slightly the line of the other two. Also, if desired, the number of elements in contact with the film 21 may be greater or less than three.
Although it would be possible to arrange the lead screw and follower 25 to provide the electrical path to the contact elements 29, 31 and 33 through the spring means 35, it is preferable to provide circuit means independent of the frictional contact between the lead screw and follower. For that purpose, a conductive strip 51 preferably is provided and arranged to be contacted by contact element 27 on one end of I the resilient metal member 35. The strip 51 may, if desired, be a narrow strip of a bulk metal film deposited on substrate 17 with an epoxy layer 19 between it and the substrate. Alternatively, the strip 51 may be a metallic strip otherwise secured upon, or merely arranged parallel to, the substrate 17, or even in a plane nonparallel to the substrate.
Terminal leads or lugs 53 and 55 are provided, connected to the junctions 37 and 39, respectively, by internal connecting wires or ribbons or pigtail connectors" 57 and 59. A similar terminal (not visible in FIGS. 1 and 2) may be connected by a pigtail or ribbon 61 to a junction 63 on the conductive strip 51. The latter terminal is provided for a circuit connection to the variable tap of the resistor.
Depending upon how exacting are the requirements for stability of the variable resistor unit, the substrate 17 may be pro vided with a further coating on the bottom thereof substantially symmetrical with the coating 19 which is relied on to isolate and hold firmly the film upon the top thereof. The way in In FIG. 33, none of the narrow elongated slits is shown extending to the edge of the metallic film, but the current paths are compelled to extend through convolutions generally in the same manner as those of FIG. 2. The pattern of FIG. 3B, by N virtue of the several incremental path portions effectively in parallel with each other, provides a high degree of reliability and excellent immunity toserious detrimental effect from any localized defect in or injury to the film. It is of interest to note that with the pattern of FIG. 2 or the pattern of FIG. 3B, just as with the aforedescribed pattern of FIG. 3A, an adjustment maybe made in the graduation of the resistance with the position of the traveller 25 by scribing out one or more gaps in the bulk metallic film, in a similar manner to the scribing illus- 3 trated and explained in the aforementioned U.S. Pat. 'No.
3,405,381 to FelixZandman and Branin A Boyd. Thus, for example, the film in the pattern shown in FIG. 38 may be scribed out to 'the edge along the dotted line 61, or scribed along dotted line 63 in the region of the centerline of the film, or
which the stability of the resistive element is enhanced by the v V useof symmetrical layers of coatings such as epoxy coatings to inhibit bendingtendencies and control the related strains is explained in said US. Pat. No. 3,405,831.
FIG. 3 illustrates some of the alternative pattern configurations which may be used for the resistive film. The version illustrated in FIG. 3A comprises the bulk metallic film having holes etched therein, such as circular holes, for example. For a given thickness and width of the film, and a given number of holes, the diameter of the holes may be decreased to provide a lower resistance value, or increased to provide a greater resistance value.
If desired the pattern of FIG. 3A may include arcuate gaps such as are shown in dotted lines at 58, 58'. Such arcuate gaps may be established in the regular course of making the patterned metal film, as by their inclusion in a photographic master used for photoetching. Alternatively, the pattern may be photoetched without the arcuate gaps at the edges, and such gaps may be cut wherever needed in the course of trimming the pattern to make it coincide precisely with a predetermined resistance distribution. Another type of trimming adjustment which may be relied on if desired is the cutting or scribing, by a stylus, for example, of linear gaps extending outward to the edges from the circles in the pattern. Examples of this are illustrated in dotted lines at 60, 60' in FIG. 3A. To facilitate minute increases of the incremental resistance gradient at any desired point, a few very small openings such as square openings may be provided in the border of the metal film between the end of each linear gap and the edge of the film. This admits of scribing the intervening metal in one, two, or more places in line with any such linear gap according to the amount by which the incremental resistance is to be increased.
scribed at a plurality of places chosen by the operator. With each such scribing, the incremental resistance value per unit length along the substrate in the immediate vicinity of the scribed gap is increased.
FIG. 3C shows a pattern in which a longer path for current conduction may be achieved in a given metallic film for the same number of elongated gaps per unit length thereof. The current path in FIG. 3C, unlike the current paths illustrated in FIGS. 2, 3A and 3B, lacks the feature of multiple mutual parallel incremental paths for redundancy of the current flow,
so that the version in FIG. 3C lacks plural paths substantially symmetrical about the longitudinal centerline of the film. One advantage of the use of the redundant path configurations is the addedprotection against failure of the resistance in the event of a defect such as a rupture or fissure in the metallic film. Another advantage of the illustrated configurations is the I minimization of the inductive component, although all of the illustrated bulk metallic film configurations provide extremely low inductive components, difiicult if not impossible to equal with wire-wound variable resistors.
FIG. 3D illustrates the arrangement of elongated slots or slits in the bulk metallic film in a somewhat slanted or slightly diagonal pattern relative to the centerline of the metallic film, the slanting being exaggerated for clarity. Such a configuration may, if desired,- be used to provide enhanced smoothness of the change of resistance with the movement of the adjustable traveller along the length of the substrate.
FIGS. '4 and 5 illustrate an embodiment of the invention in which the contact member is angularly adjusted rather than longitudinally adjusted relative to a substrate having a bulk metallic film established in a pattern thereon.'The housing 71 is arranged to enclose a substrate 73 upon which is deposited, and held in place by means of an epoxy isolating layer 75 (FIG. 5), a bulk metallic film 77 having a pattern of slits or openings therein for causing current to flow in a tortuous path around said film. A control shaft 79 and a knob 81 thereon are arranged to provide for angular adjustment of a rotor arm 83 from which'depend a series of contact elements 85, 87, 89 and 91. These contact elements preferably are arranged upon a a resilient thin metal member 93 extending downward from the arm 83, and preferably provided with slits extending part way from the lower edge thereof up toward arm 83 in order for each of the contact elements 85, 87, 89 and 91 to be assured of being held in contact with the fixed portion of the variable resistor system. The substrate 73 is supported from the base of the housing 71 by means of spaced supports such as those illustrated at 95 and 97, which may either comprise screwdown holding means or bosses provided with suitable cement.
Terminal junctions 99, 101 and 103 may also be provided (FIG. 4), in the same manner as the terminal junctions 37, 39 and 63 in the embodiment illustrated in FIGS. 1 and 2.
For the purpose of maintaining a stable, trouble-free connection to the movable wiping contact system carried upon rotor arm 83, a contact ring 105 preferably is provided. Ring 105, which is arranged to be contacted by movable contact element 85, may, if desired, be a thin flat ring of bulk metallic film fixed in place upon the epoxy layer 75 on the upper surface of the substrate 73.
The bulk metallic film 77 (shown in FIG. 5) may be etched to flow in elongated paths. One example of such a pattern is illustrated in FIG. 4, this being a pattern having general similarity to the pattern shown in FIG. 2 in the first illustrated version of the present invention. Ifpreferred, the width of the slits may be tapered in order to have the edges of the conductive paths bounded thereby approximately parallel to each other.
With respect to the structure shown in FIGS. 4 and 5, an epoxy layer may be provided upon the opposite surface of the substrate for substantially balancing stresses exerted thereon, and thereby controlling the bending strains produced in the patterned metallic film.
FIG. 6 illustrates schematically in cross section yet a further modification of the present invention. In this embodiment, as in the embodiment of FIGS. 4 and 5, angular adjustment of the contact arm is used. In this version, the substrate 111 is not flat but is in the form of an arcuate segment of a cylinder. Accordingly, the contact arm 113 carried upon shaft 115 may be provided with a series of contact elements such as element 117 spaced apart along a vertical line substantially parallel to the axis of the shaft 115. Such structure may be provided within a housing 119 which, if desired, may be a housing of any desired shape, one example being a cylindrical housing similar to that shown in FIG. 6. I
In the structure illustrated in FIG. 6, the inside cylindrical wall of the substrate provides the support for the patterned bulk metal film. As will be apparent, this structure is adaptable to arrangements with the patterned bulk metal film on the outer (convex) surface of the substrate 111 having the form of an arcuate segment of a cylinder. In the latter case, the angularly adjustable resistance control part is arranged to reach to the outer surface and contact the patterned film.
In any of the illustrated structures, metal film paths for electric current may be provided on both the opposite surfaces of the substrate. As one example, a tortuous high-resistance path may be provided on one surfaceand a lower resistance path for establishing contact for the adjustable means may be provided on the opposite surface.
In making the structure illustrated in FIG. 6, one may prefer to establish the resistance pattern in the bulk metal film while it is flat, as by photoetching it. If so, the substrate 111 may initially be flat and may be retained so until after the pattern has been established in the bulk metal film thereon, and thereafter the substrate 7 may be curved into its cylindrical arcuate geometry. Such a flexible substrate may be made of a thin aluminum sheet or steel sheet and the epoxy layer provides isolation and holds in place the bulk metal film.
The bulk metal film and the substrate conform to I-lookes law, and other classic properties of elastic bodies subjected to stress. Accordingly, in the variable resistors of the present invention, as in fixed resistors having bulk metal film upon a substrate, the response to changes of conditions such as temperature changes can be determined by taking into account the expansion characteristics and modulus of elasticity and thickness of each of the layers of material including not only the substrate and the bulk metal film but also the dielectric adhesive layer. The layer provides for faithful strain transmission from substrate surface to metal film and the layer is substantially free of creep at any normal operating temperature.
In the manufacture of the substrate-supported resistive element included in the present invention, the bulk metal film may be deposited on the substrate and may thereafter be photoetched to the desired pattern to achieve the desired substantial resistance value between the ends thereof. Altema' tively, the bulk metal film may be supported on a carrier such as a flexible or semiflexible sheet of material until after the film has been photoetched to the desired pattern, and it may thereafter be transferred to the substrate to be cement bonded thereon before removal of the carrier sheet. It is also feasible to retain the carrier sheet, if it is made of a dielectric, bonding it to the substrate and thus having it kept in place between the bulk metal film and the substrate.
In each of the embodiments shown, the end portions of the patterned metal film may be rendered more secure by epoxy imbedment. Such reinforcement may also be added along the sides of the pattern if desired, the surface of the patterned metal film remaining exposed in those areas through which the contact elements are movable.
In the foregoing description, it has been explained that finishing adjustments of the resistance gradient with respect to contactor position along the film pattern can be performed by cutting or scribing slits or other gaps in portions of the film pattern. Alternatively, any portions of the pattern where an increased resistance gradient is desired may be rubbed with a fine abrasive such as pumice powder, or may be rubbed with an etchant, or may be subjected to electroetching. By any of these procedures, or combinations of selected ones, the resistance characteristics may be tailored to a desired pattern.
In addition to the end terminals shown, it will be recognized that fixed intermediate resistance taps may be included if desired.
As to the mode of attachment of the substrate in the housing, one or more of the illustrated bodies of cement may be made of material having a low modulus of elasticity or the cementing with high modulus cements is made so as to minimize any strains induced in the substrate by differences of expansion characteristics of the substrate and the housing, in response to temperature changes, for example. Such material as-a polyurethane plastic may be used to provide the degree of flexibility needed, or a low modulus epoxy may be used. High modulus cement can be used if attachment is made on one end only so as to permit relatively free expansion of the substrate.
Features of the adjustable resistor described herein include its superior resolution and great reliability, especially with the patterns characterized by redundant current paths. Further features attainable are the minimization of the dependence on temperature, a substantial temperature coefficient of re sistance being avoided unless other circuit factors make it desirable to do otherwise. Typically, a temperature coefficient of resistance of :3 parts per million per C. over the range from 55 C. to+l25 C. is attainable. One way of attaining a high temperature coefficient of resistance where such is needed for a special circuit is by mismatching the coefficients of thermal expansion of the substrate with the characteristics of the metallic film and the isolation layer. With these features, in addition to its low electrical and mechanical noise characteristics and its suitability for a wide frequency range by virtue of its very low inductance and verylow distributed capacitance, the adjustable resistor of the present invention is particularly suited for a variety of precision potentiometer, rheostat and bridge circuit applications.
As many changes may be made and many widely different embodiments may be constructed without departing from the spirit of the present invention, the accompanying drawings and description are intended not by way of limitation but rather illustration of forms which may be taken by the invention, the scope of the invention being indicated by the claims.
What I claim is:
1. An adjustable electrical resistor device having a temperature coefiicient of resistance within :3 parts per million per C. over the range of -55 C. to +l25 C., which resistor device consists essentially of:
an isolating layer of a dielectric on one face of said substrate, said isolating layer being substantially free of creep under normal operating temperatures;
a metallic film 0.00002 to 0.0004 inches thick, made from resistive bulk metal held in place adjacent to one face of the substrate by the isolating layer which provides faithful strain transmission from the substrate to the metallic film, wherein said metallic film is provided with openings for restricting and rendering tortuous the conductive path 7 8 therein and thereby impeding the flow of electrical curduced in the substrate by differences of expansion rent from one end of the metallic film to the opposite end characteristics of the substrate and housing, said nonconthereof; ducting attachment means contacting only a small area of a conductive strip held in place on one face of the substrate the substrate and said nonconducting attachment means by the isolating layer adjacent to the metallic film without being noncontinuous between the substrate and the houscontacting the metallic film; ing.
a multifingered movable contact means for contacting the The adjustable l l re r device Of claim 1 conductive trip and the metallic and thereby wherein the attachment means interposed between the subestablishing an electrical connection between the con- Straw and the housing has 310W modulus of elastisitl ductive strip and the metallic fil at a variable distance 10 3. The adjustable electrical resistor device of claim 2 f one end f the resistor device; wherein the attachment means are present at opposite ends of a housing for the adjustable electrical resistor device; the substrate-.
a shaft rotatably mounted in said housing, said shaft having T adjustable electrical resistor device of claim 1 a threaded section for engaging and moving the multifin- 'fl the attachment means a h modulus of gered movable Contact means; and elasticity and are attached only to one end of the substrate so nonconducfing attachment means interposed between the as to prevent relatively free expansion of the substrate substrate and the housing which minimizes strains in-