US3530421A - Wire wound potentiometers with printed circuit terminals - Google Patents

Description

Sept. 22, 1970 G. DION 3,530,421

:WIRE WOUND POTENTIOMETERS WITH PRINTED CIRCUIT TERMINALS Filed Oct. 9, 1968 5 Sheets-Sheet 1 INVENTOR. Geo fies Dion 'IATTORNEY G. DION 3,530,421

Sept. 22, 1970 WIRE WOUND POTENTIOHETERS WITH PRINTED CIRCUIT TERMINALS 3 Sheets-Sheet 2 Filed Oct. 9, 1968 m2 32 2o 4O 22 66 Sept. 22,1970 6. DION 3,530,421

WIRE WOUND POTENTIOMETERS WITH PRINTED CIRCUIT TERMINALS Filed Oct. 9, 1968 3 Sheets-Sheet 3 FIG.4

I I I l 160 156 166 174 176 144 United States Patent 3,530,421 WIRE WOUND POTENTIOMETERS WITH PRINTED CIRCUIT TERMINALS Georges Dion, New York, N.Y., assignor to Societe dlnstrumentation Schlumberger, a corporation of US. Cl. 338-174 6 Claims ABSTRACT OF THE DISCLOSURE The ends of a potentiometer resistance element, and especially a resistance coil, are soldered to the potentiometer output terminals without having been previously unwound. The electrical connection is effected by printed circuits formed on an insulating substrate. Each printed circuit includes a contact strip which underlies a portion of the ditferent ends of the resistance element, an adjoining junction strip which terminates at each output terminal and a heating plate of relatively large surface area designed to support at least the tip of a conventional soldering tool. The heating plate is formed contiguous the two strips but is spaced from each output terminal a distance great enough to prevent the disconnection of a soldered connection between a terminal and a junction strip when solder is applied to the contact strip. Preferably, the junction strip is relatively flat and narrow and while providing good electrical conduction between it and the contact strip, provides relatively poor heat conduction from a heating plate to an associated soldered output terminal.

The invention relates to potentiometers and more particularly, to precision potentiometers which utilize a helical resistance element.

Potentiometers of the type presently under consideration typically comprise a housing, a resistance element in said housing and having the ends thereof connected to output terminals and a wiper contact which moves on and along the resistance element. The resistance element is comprised of a resistive wire wound helically on an insulating core of circular cross section. This core may be straight or circular depending upon whether the potentiometer is of a linear or of a circular type.

In manufacturing this type of potentiometer a problem often arises in obtaining a satisfactory attachment of the helical resistive element to the potentiometer output terminals. In order' to obtain an electrical signal proportional, in absolute value, to the distance or angle of move ment of the wiper on the resistance element, the distance, or the angle, between the two end turns of the resistance wire must be very accurately established. In prior art potentiometers the extremities of the resistive wire are unwound until an unwound wire portion is obtained which has a length suflicient to permit the wire extremities to be soldered or otherwise connected to the potentiometer output terminals. This operation, which usually must be performed on resistance wires having very small diameters, is a delicate, tedious and costly proposition. Moreover, since the lengths of unwound wire are not contacted by the wiper and hence do not serve as a useful length of the resistance element, such lengths of unwound wire becomes sources of residual resistances which often introduce not readily determinable but nevertheless significant errors into measurements made with the potentiometer.

It is an object of this invention to provide a potentiometer including a helical resistance element, in which the potentiometer output terminals can be readily and inexpensively connected to the extremities of the resistance element.

Another object of the invention is to provide a potentiometer which includes a helical resistance element wherein the useful length of the resistance element is very accurately established and wherein any residual resistance at the ends of the resistance element is of very low magnitude.

In accordance with one embodiment of this invention, a potentiometer embodies a helical resistance element along which a wiper can be moved and includes an insulating support member having one face thereof mounted adjacent the resistance element. This one face of the support has printed circuits formed thereon and at least two of these circuits are formed into ribbon-like conductive strips hereinafter referred to as contact strips which extend under portions of the resistance element ends for the purpose of making electrical contact therewith. Using conventional printed circuit techniques, the edges defining the ends of the contact strips can be printed onto the insulating support very accurately. Hence, the useful length of the resistance element, considered with reference to these edges of the contact strips may also be determined with a high degree of accuracy. Each of the contact strips may be soldered to a corresponding portion of a resistance element end by applying molten solder to a more remote portion of the contact strips and then relying upon capillary action to etlect solder flowage over the surface of the strip to the coils of the resistance element. Each of the aforementioned two printed circuits further comprises a heating area connected to an output or terminal pin by a junction strip of relatively narrow width. The junction strip introduces thermal losses and delays in heat conduction to the contact strips soldered to the resistance element and therefore an output lead can be soldered to a printed output contact without fear of the resistance element becoming unsoldered from a corresponding contact strip.

The features and advantages of the invention will be best understood by the following detailed description when taken in conjunction with the accompanying drawings on which:

'FIG. 1 is a sectional side view of a circular or rotary potentiometer constructed in acocrdance with the invention,

FIG. 2 is an enlarged section view of a part of the potentiometer of FIG. 1,

FIG. 3 is a plan view of a portion of FIG. 2,

FIG. 4 is a plan view of a particular embodiment of an insulating support on which the resistance element of the potentiometer may be mounted,

FIG. is a plan view of another embodiment of the insulating support-of FIG. 4,

FIG. 6 is a view of a third embodiment of an insulating support for a resistance element which can be used for a linear potentiometer.

Referring now to FIG. 1, a rotary potentiometer constructed in accordance with this invention is designated by the numeral 1, and includes a base 2 of overall cylindrical shape and a circular cover 4 suitably attached to the base 2 by, for example, screws 5. A cylindrical control shaft 6 is mounted for rotation on coaxial ball bearings 8 and 10 having their outer races mounted stationary in circular recesses formed in the base 2 and the cover 4, respectively.

An insulating sleeve 12 is mounted with an interference fit on the shaft 6 and, in turn, has an electrically conductive wiper 14 fixedly secured thereto. The wiper 14 includes two resilient metal contact arms 16 and 18 of different lengths, the longer arm 16 resiliently contacts a circular resistance element 20 and the shorter arm 18 is in wiping electrical contact with a collector ring 22 mounted in concentric alignment with the circular element 20. Extending radially from the base 2 are three insulating sleeves, one such sleeve being indicated at 26. Each sleeve receives an output conductor or lead such as 28 which passes through the wall of base 2. The three output conductors, such as 28, are individually connected to the extremities of the resistance element 20 and to the collector ring 22, respectively, by means of printed circuits which will be described in greater detail hereafter.

The resistance element 20 is mounted on an insulating support plate or disk 30, as seen more clearly in FIGS. 2 and 3, and is comprised of a resistance wire 32 wound helically on an insulating core 34 of circular cross-sectional shape. The wire 32 may be an enamelled copper wire, for example, and the core is typically obtained by helically winding the wire 32 on a straight, cylindrical mandrel which is then bent into a circular shape. During this latter operation, the end turns of the wire 32 are made to adhere to the core by applying an adhesive, for example varnish, between certain of the turns of the wire and the mandrel. The insulating support 30 is of overall circular shape and has a circular aperture 36 formed centrally therein.

Printed circuits 38 are formed on the flat, internal surface of the support 30 by utilizing, for example, conventional photo-engraving techniques. The resistance element 20 is surrounded by an annular ring 40, composed of a suitable insulating material, such as Teflon. The support 30 rests on an internal circular shoulder formed on the base 2. The assembly formed by the support 30, the insulating ring 40 and the wound resistance element 20 is held stationary in the base 2 by a retaining ring 24 having an inwardly curving tip 2411 obtained by initially cutting a substantially V shaped groove in the internal surface 27 of the sidewall of the base 2 and then forcing the tapering end of the thusly formed ring 24 downwardly and inwardly of the base 2. The inwardly bent or wedged tip 24a forces the adjoining portion of the ring 40 radially inward and downward against the component 20 and the component is thus clamped tightly against the printed circuit 38.

The printed circuits engraved on the insulating support are shaped as illustrated by FIG. 3. Two circuits 46 and 48 are located symmetrically relative to a radius 50 taken through the support 30 perpendicular to the axis of rotation of the control shaft 6. Each of these circuits may be described as comprising a narrow, elongated or ribbon-like contact strip 52 extending approximately parallel to the diameter 50, a heating area 54, a narrow, elongated or ribbon-like junction strip 56 and an output contact 58. The outer contact end of the strip 52 extends radially to a postion beneath the resistance element 20 and is tangent to one or several coils of the resistance wire, depending upon the width of the contact strip. The contact strip is then electrically connected to one or more turns of the resistance wire 32 by solder 60. The width and spacing of these contact ends of the strip 52 can be defined with great accuracy utilizing conventional photoengraving techniques. A potentiometer is thus obtained in which the useful electrical travel, limited by the blade width of the wiper, is very accurate. Furthermore, the residual resistance at the two extremities, that is to say the resistance between the contact arm 16 and one of the potentiometer outputs, when the wiper is in its initial position (that is, in contact with the first coil of the resistance wire) is less than the resistance of one half of a turn of the resistance wire. The dimensions of the heating area 54 are large enough so that the area 54 can completely accommodate a conventional soldering iron placed thereon, the rectangular shape of this surface being merely illustrative of a' configuration which conforms to the shape of a conventional rectangular soldering iron tip. The length and width dimensions of the junction strip 56 are selected so as to offer relatively low electrical resistance but relatively high resistance to heat transfer between the output contact 58 and the area 54. This junction strip consists, preferably, of two straight, narrow portions 56a and 56b oriented at right angles with respect to each other so as to occupy as little space as possible.

The output contact 58, which is of generally circular shape, includes a circular aperture '62 at its center which extends perpendicularly through the support 30. The diameter of the aperture is large enough to receive one of the conductors 28 (FIG. 1) which is inserted through the aperture 62 for soldering onto the contact 58.

A third printed circuit 64 of annular shape is also formed on the base 30 so as to form the circular collector ring 22 (FIG. 1). The different printed circuits may be made as a continuous pattern to enable the electrolytic application of a protective layer and then separated from each other by cutting an annular groove 66 through the conductive layer of the printed circuit.

To mount the resistance component 20 into the potentiometer, the insulating support 30, the insulating ring 40 and the resistance component 20 are first positioned in the base 2 with the air gap between the two ends of the component 20 approximately midway between the two spaced apart contact ends 52. The upper extremity of the ring 24 is then wedged inwardly causing the insulating ring 40 to clamp the resistance element 20 against displacement. To solder the spaced apart ends of the resistance component 20 to the ends of the contact strips 52, a soldering iron is placed upon the heating areas 54 and solder is placed on both of those portions of the contact strips 52 which are slightly inwardly of the resistance element. The liquid solder then flows by capillarity only over the surface of the contact and ultimately flows into the area between the extremities of the contact strips and the resistance wire. It is then possible to solder conductors 28 to output contacts 58 without accidentally unsoldering the resistance Wire because the thin, ribbon- like strips 56 and 52 offer a high thermal resistance to the transfer of heat between the contacts 58 and the outer extremities of the strips 52.

FIG. 4 illustrates another embodiment of this invention which enables a fixed potential to be taken ofi at a predetermined point along the length of the helical resistance element. In this embodiment, the insulating support disk is designated by the numeral and has four printed circuits formed thereon. The first three printed circuits designated 102, 104 and 106, respectively, are identical to those illustrated by FIG. 3 and described hereinabove. The fourth circuit 108, like circuits 102 and 104, comprises a contact end 110, a heating area 112, a junction strip 114 and an output contact 116. The contact end is angularly displaced from the contact end of circuit 104- by a predetermined angle, for example, 30 degrees and is also soldered onto the resistive element in the manner described hereinabove. Thus, a fixed intermediate potential take-off is available which can be connected to an additional output terminal on the potentiometer. It is also possible to connect, with a shunting resistance, contact 116 to one or the other of the extremities of the resistance element. The resistance element can, of course, be divided into sections by several other printed circuits thus making it possible to obtain an accurate stepping switch or a plurality of individual voltage sources.

FIG. illustrates another embodiment of this invention. In this embodiment, the insulating disk is referred to by the numeral 118 and has a circuit 120 printed thereon which includes a contact end 122 that covers relatively large, continuous arcuate conductive surface on which the resistive element is soldered. This contact end thus forms a continuous thermal zone with its associated heating plate. This arrangement makes possible the provision of a zone or region of constant potential at the corresponding end of the resistance element.

The instant invention, in addition to having application to circular potentiometers is also applicable to linear potentiometers. FIG. 6 illustrates an application of the instant invention to a linear potentiometer which comprises a rectangular insulating support 130 on which five printed circuits are photoengraved. A first circuit 132 extends along the longitudinal axis of symmetry of the support 130 and is formed by a linear strip conductive 134 and an output contact 136. The strip 134 and the contact 136 serve as a return collector for the potentiometer. Two other printed circuits 138 and 140 are symmetrically located relative to the longitudinal axis of the base, each of these circuits including a transverse end 143', 144, a junction strip 146, and an output contact 140. Finally, two double circuits 154 and 156 are located adjacent opposite respective edges of the support 130*. Each of these double circuits 154 and 156 includes a respective output contact 158 and 160 connected by two junction strips 162, 164 and 166, 168, respectively, to two contact ends 170, 172 and 174, 176, respectively.

This embodiment makes it possible to select different contacts which will provide linear resistance elements of different relative lengths and hence a linear potentiometer of variable length. If a relatively long potentiometer length is required, such as 80 mm., the element may be mounted and connected between the contact ends 170 and 142. For a potentiometer of shorter length, for example 60 mm., the resistance element may be connected between the ends 172 and 142, for a still shorter length of, for example 40 mm., the resistance element may be connected between the ends 174 and 144 and for a relatively short potentiometer length, such as mm., the resistance element may be connected between the ends 176 and 144.

It will be noted that, in this embodiment, each contact end and its heating area form a continuous rectangular plate. In such a potentiometer, the wiper has a linear movement parallel to the longitudinal axis of the base 130, this wiper insuring an electrical connection between the resistance element and the collector strip 134. It will also be appreciated that the same advantages discussed hereinabove in regard to circular potentiometers are also obtained in potentiometers of the linear type.

What is claimed is:

1. A potentiometer comprising, a support having one surface formed of an electrically insulating material, an elongated, helical resistance element mounted adjacent the one support surface, at least one electrically conductive strip mounted on said one surface and having two ends, one end of said strip extending between said one support surface and a portion of said resistance element, a terminal electrically connected by soldering material to the other end of said strip and said one end of said strip being connected by a soldering material to said resistance element and a thermally conductive plate mounted on said support surface adjoining said strip for supporting at least a portion of a heated soldering tool, said plate when heated thereby conducting heat to said strip and being spaced different distances from the two strip ends such that the soldered connection of the more remote strip end remains intact when the temperature of the less remote strip end is increased to that required for soldering.

2. A potentiometer comprising, a housing, a support member mounted on said housing and having at least one face thereof composed of an electrical insulating material, an elongated helical resistance element mounted adjacent the one support face, at least one circuit comprised of a continuous, elongated, electrically conductive strip printed on said one support face and having at least two ends, one end of said circuit extending between a portion of said resistance element and said one support face, whereby said one end of said circuit may be soldered to an overlying portion of said resistance element, an elongated collector circuit printed on said one support face and extending substantially parallel to an adjacent portion of said resistance element, a wiper member engaging said resistance element and said collector circuit and providing an electrical circuit therebetween, means fordisplacing said wiper member relative to said resistance element and said collector circuit, a terminal soldered to the other end of said conductive strip, and a thermally conductive pad on said one support face joined in heat conducting relationship to said strip for receiving a heated soldering tool, said pad being located different predetermined distances from each end of said strip to prevent the unsoldering of said terminal from the other strip end by heat conducted thereto when the one strip end is being soldered to said resistance element.

3. A potentiometer comprising, a housing; an insulating support mounted on said housing; an elongated helical resistance element mounted adjacent one face of said support and having two ends and a longitudinal axis; the one support face having at least two electrical circuits printed thereon; each of said printed circuits including, an elongated contact strip having one end thereof extending between the one support face and a different turn of said resistance element, each of said circuits further comprising, an elongated junction strip for individually electrically connecting a terminal to a different one of the contact strips, each terminal being soldered to one end of a junction strip remote from a corresponding one of the contact strips, a collector mounted on said one support face and having an axis substantially parallel to the longitudinal axis of said resistance element; a wiper arm movable upon said resistance element and said collector in a direction parallel to the axes thereof, for establishing an electrical circuit therebetween, means for displacing said wiper arm relative to said resistance element and said collector and a plurality of thermally-conductive pads having surfaces shaped to conform substantially to at least a portion of a heated soldering tool, each of said pads joining a portion of a contact strip remote from said one end thereof to a portion of a corresponding junction strip and being sufficiently remote on a heat transfer basis from a terminal soldered to a corresponding junction strip to prevent the inadvertent unsoldering of such terminal by heat transmitted thereto from said pad during the application of heated and molten solder to a corresponding contact strip.

4. The potentiometer according to claim 3 wherein the contact strips are of ribbon-like shape and include substantially coextensive, laterally spaced-apart edges for accurately defining the length of the electrical path between different turns of said resistance element to which the contact strip edges are soldered.

5. The potentiometer according to claim 4 wherein the portions of said junction strips joined to said contact strips are of ribbon-like shape and have edges in coalignment with corresponding ones of said contact strip edges.

6. The potentiometer according to claim 5 wherein said v 7 8 portions of said junction strips and the one ends thereof 2,842,647 7/1958 Khouri 338-163 connected to different ones of the terminals are electrical- 3,132,321 5/ 1964 Kuhlman 338-174 1y connected by individual ribbon-like'elernents extend- 3,271,721 9/1966 Gordon 338-180 XR ing laterally from individual ones of the junction strip 3,354,418 11/1967 Casey et al. 338174 portions in a direction away from the spacing defined by a 5 3,358,259 12/ 1967 Kirkendall 338183 corresponding one of said junction strip edges.

THOMAS J. KOZMA, Primary Examiner References Cited UNITED STATES PATENTS 1,877,926 9/ 1932 Mattern 338-174 XR 10 2,266,188 12/1941 Foley et a1 338168 XR

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