US 3652970 A
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United States Patent Hubert [451 Mar. 28, 1972  VARIABLE RESISTOR HAVING AN IMPROVED WIPER MEMBER Primary ExaminerThomas J. Kozma Assistant Examiner-D. A. Tone Attorney-Harry G. Martin, Jr. and J. Raymond Curtin A variable resistor of the rotary type is illustrated having a rotatable wiper member, a substrate member carrying an arcuate resistance element and a collector element, all coaxially secured in a cylindrical housing. The wiper member comprises a relatively rigid carriage element having a screw driver slot therein for rotating the wiper member from the exterior of the housing. A resilient pad element is adhesively secured to the carriage element. A flexible contact element is secured to the face of the pad element facing the resistance and collector elements so that the contact element makes electrical contact between the collector and the desired location on the resistance element. The spacing between the substrate member and the carriage member within the housing is less than the uncompressed thickness of the pad element so that the pad urges the contact element into firm engagement with the resistance and collector elements. The resistor may be designed so that the pad element physically wipes the resistance element to keep it relatively clean and free of foreign material. Preferably, the variable resistor is designed so that a portion of the pad element forms a seal with the substrate member to prevent the passage of contaminates to the resistance element.
ABSTRACT 7 Claims, 4 Drawing Figures PATENTED MR 2 8 I972 FIG.4
INVENTOR. GUSTAV HUBERT M flaw,
ATTORNEY BACKGROUND OF THE INVENTION This invention relates to variable resistors and more particularly to improved wiper elements suitable for potentiometers.
Variable resistors utilizing elongated cermet resistance elements are finding increasingly greater acceptance because of their fine electrical and mechanical characteristics and relatively low cost. In such units, it is usual to provide an internal wiper member which makes electrical contact with the surface of the cermet resistance element at a desired location thereon and which at the same time contacts a collector element to which electrical contact is made from the exterior of the resistor to the desired resistance point. It is desirable that the wiper member be designed to make good electrical connection with the resistance element, but at the same time, to pro vide a low electrical noise" factor which is defined as contact resistance variation. For example, a single point contact between the wiper and the resistance element tends to result in a high noise value because the variation in the potential field on the surface of the resistance element, occasioned. by various factors such as inhomogeneity of the cermet material, surface roughness, moisture absorption, metal particle spacing within the resistance element and surface contamination. These variations in surface characteristics of the resistance elements tend to produce minute perturbations in the resistance value as the wiper traverses the resistance element. It is therefore customary to provide a number of parallel contacts between the wiper member and the resistance element in order to minimize contact resistance variation by averaging out the individual contact resistance variations of the conwas In addition to having a low contact resistance variation, it is desirable that a suitable wiper member exhibit good settability, long life, and only moderate pressure on the resistance element to improve the life and reduce wear of the resistance element. These factors are often incompatible with each other since increasing the contact pressure typically reduces the contact resistance variation at the expense of wiperand resistance element life and may reduce the settability of the wiper.
Accordingly, it is a principal feature of this invention to provide a variable resistor having an improved wiper member which exhibits good life and settability combined with exceptionally low contact resistance variation while at the same time being inexpensive and simple to manufacture and assemble.
SUMMARY OF THE INVENTION An insulating substrate member is provided with an elongated resistance element and collector element thereon. A movable wiper member is provided with means for adjusting its movement. The wiper member comprises a relatively rigid carriage element having a relatively resilient pad element secured thereto. A flexible wiper contact element, which may comprise a conductive epoxy resin bar or a conductive metallic ink, is secured to the face of the pad element which faces the resistance and collector elements. The substrate member and the carriage element are secured so that the distance therebetween is less than the normal uncompressed thickness of the pad element. Consequently, the pad element urges the flexible wiper contact into firm electrical engagement with the resistance element and the collector element. It has been found that this arrangement provides a simple, inexpensive, and reliable wiper member which exhibits exceptionally low contact variation with cermet resistance elements.
Preferably, the pad element overlies and contacts at least a portion of the collector and resistance elements so that movement of the wiper member causes the pad element to clean the collector and resistance elements and to form a seal therewith which resists contamination of the resistor and collector elements with foreign material. A suitable lubricant can be applied between the pad element and the resistance element to further improve the characteristics of the variable resistor.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a central cross-sectional view through a variable resistor in accordance with this invention;
FIG. 2 is an isometric view of improved wiper member in accordance with this invention;
FIG. 3 is an isometric view of the substrate member showing the resistance and collector elements thereon; and
FIG. 4 is a cross-sectional view taken substantially on line IV-IV ofFIG. 1.
DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawings, there is shown a variable resistor 10 which in the preferred embodiment may be a trimmer potentiometer. Variable resistor 10 is provided with a housing member 11 having a hollow, cylindrical interior cavity which is preferably circular in cross-section. A rotatable wiper member 12 and a substrate member 13, which are also preferably circular in cross-section, are coaxially disposed within the circular cavity in the housing 11. Terminal members l4, l5 and 16 are provided for making electrical connections with the resistance element. The variable resistor illustrated is of the preferred type wherein interior cavity 17 is circular in cross-section but the resistor may take other configurations wherein, for example, the cavity is rectangular in cross-section and the wiper member is mounted for linear rather than rotational movement within the cavity.
One face 19 of substrate 13 is provided with an elongated arcuatc resistance element 20 having its ends deposited over and electrically contacting suitable conductive terminations 21, 22. Resistance element 20 is preferably of the cermet type which comprises a plurality of metal or metallic alloy particles dispersed throughout a ceramic glass coating which may be deposited on face 19 of substrate member 13 by screen printing and subsequent firing. Substrate member 13 is preferably composed of a ceramic insulating material such as alumina. Conductive terminations 21, 22 may also be deposited on face 19 of substrate 13 by screen printing and comprise a highly conductive metallic coating. Terminal members 15 and 16 are inserted through appropriate holes in substrate 13 and soldered or brazed to make electrical contact with terminations 21 and 22 which form the electrical ends of resistance element 20.
A suitable collector element 25 is formed in the center of substrate 13. Collector element 25 may comprise a conductive metal coating 26, similar to terminations 21 and 22, formed around a central hole 27 through substrate 13. Terminal member 14 isbent to a desired shape to provide the desired final terminal pin configuration and one end thereof inserted through the hole formed by conductive coating 26. Solder or other conductive brazing material 25 is then applied to electrically and mechanically secure terminal 14 to conductive coating 26 and at the same to form a conductive surface region about the central area of substrate 13 as shown in the drawings. Alternatively, collector element 25 may be formed by depositing or securing a suitable conductive coating or element onto surface 19 of substrate 13 to provide any desired shape for the collector element. Similarly, the resistance element may be of any desired type such as a conductive resin or solid metal resistance material and may be arcuate or linear in shape if desired, and may be secured by any desired means to substrate 13.
Wiper member 12 is cylindrical in the embodiment illustrated, and is mounted coaxially within interior cavity 17 of housing 11. Wiper member 12 comprises a relatively rigid carriage element 30 which is provided with means for adjusting its position from the exterior of housing 11. In the embodiment shown, carriage element 30 has a cylindrical, axial projection 31 which extends through a correspondingly shaped aperture in housing 10 to allow rotational adjustment of the carriage element from the exterior of the housing by insertion of screw driver in screw driver slot 32. Alternatively, the carriage element may be provided with gear teeth so that it can be rotated by an externally projecting worm screw or the carriage element may be disposed within a rectangular housing and in engagement with the threads of a lead screw if the variable resistor is of the linear type.
Carriage element 30 has one face 33 which faces toward substrate member 13 when the members are inserted in the housing. The other face 34 extends radially outwardly from cylindrical projection 31 and abuts against an opposing annular shoulder 35 on housing 11. If desired, an O-ring or other seal means (not shown) may be provided between shoulders 34 and 35 to resist the entrance of dirt, moisture or other contaminates into the interior of housing 1 1.
Wiper member 12 also comprises a pad element 40 which is made of a resilient material such as silicone rubber. Pad element 40 may be made of numerous resilient materials such as felt, poly-urethane foam or neoprene. Preferably, the material selected for pad 40 is relatively impervious to moisture and is capable of withstanding whatever heat the variable resistor may be subjected to without loosing resiliency. Resilient pad 40 is circular in cross-section in the illustrated embodiment and is coaxially mounted within the interior of housing 11 with one surface 41 thereof facing substrate member 13. The other surface 42 of pad 40 is secured to carriage element 30 by adhesive 43, such as an epoxy glue or other suitable means such as molding.
A flexible, conductive wiper contact element 45 is disposed on one surface 41 of pad element 40. Wiper contact element 45 comprises a relatively deformable bar of finely divided conducting metal particles which are flexibly bonded together by a suitable material such as a flexible or deformable resinous plastic binder. Preferably, contact element 45 comprises a flexible epoxy plastic filled with finely divided silver particles. Alternatively, element 45 may comprise what is commonly called a conducting metal ink, consisting of conductive metal particles, deposited on face 41 of pad element 40. Wiper contact element 45 must be sufficiently deformable, flexible, thick and electrically conductive so that the contact element can follow the resilient deformation of pad 40 as it wipes across the surface of substrate member 13 and makes good electrical contact between the resistance and collector elements. It has been found that it is desirable for contact element 45 to present a surface for contact with the resistor and collector elements to produce a low noise factor. A silver filled epoxy plastic bar having a thickness of about 0.04 inches and a width of 0.05 inches has been found to produce very good electrical characteristics.
Substrate member 13 is coaxially secured in housing 11 by any desired means. It is preferred to secure substrate 13 in housing 11 by use of an insulating epoxy potting agent 50. Substrate member 13 is secured within the housing 11 so that the distance between face 33 of carriage element 30 and the upper surfaces of resistance element 20, collector element 25 and face 19 of substrate member 13 is less than the uncompressed axial thickness of resilient pad element 40. Consequently, pad element 40 axially urges wiper contact element 45 into firm electrical and mechanical engagement with resistance element 30 and collector element 25.
It is preferred to assembly the elements and members within housing 11 and to apply an axial force of about one pound on substrate member 13 by a suitable device to initially compress resilient pad element 40. As a small amount of the epoxy potting agent 50 is then poured into cavity 17 and cured sufficiently to secure substrate 13 in the desired position to hold pad 40 compressed. Thereafter the device exerting pressure on substrate 13 is removed and additional epoxy agent is poured into cavity 117 and finally cured to complete the assembly.
In the preferred embodiment shown, pad element 40 extends radially outward from the axis'of wiper member 12, an extent greater than resistance element 20 and wiper element 45 so that it completely overlies the resistance and wiper elements. Consequently, an effective seal is formed adjacent the peripheral edges of pad element 40 and substrate member 113 to effectively resist the entrance of foreign material between them. A suitable lubricant, such as petroleum grease having a lithium anti-friction compound therein may be provided between pad element 40 and substrate member 13. The lubricant not only serves the function of improving the settability of the wiper member by reducing its friction with the substrate, resistance and collector elements, but in addition, it
serves to coat the surface of any foreign particles with a dielectric film to reduce noise. It is preferred that pad element 40 comes into actual contact with resistance element 20 and collector element 25 so that the pad element physically wipes across their surfaces and cleans them as the wiper element position is adjusted.
It will be seen that a construction in accordance with this invention possesses the advantage of being simple and inexpensive to manufacture and easy to assemble in either a manual or automated production process. In addition, it has been found that a contact resistance variation on the order of one quarter of one percent of the total resistor value is easily achievable by the use of a vinyl pad in combination with a'conductive metallic ink wiper element. This range of contact resistance variation represents a significant improvement over prior contact elements utilizing resilient metal spring fingers.
It is preferred to employ a circular variable resistor of the type illustrated in the drawing because of the additional sealing and wiping advantages which accrue from the pad element completely overlying the resistor and collector elements at all times. However, the principles of this invention are easily applicable to a lineally movable, lead screw operated wiper member having a transverse wiper contact element which moves axially along parallel resistor and collector elements in a rectangular housing. In that event, the advantages of very low contact resistance variation are also achieved and by having the pad element overlie at least a portion of the resistor and collector elements, a wiping and cleaning function is automatically achieved each time the wiper member is moved.
While for purposes of illustration, a preferred embodiment of this invention has been described. It will be apparent that the invention may be otherwise embodied within the scope of the following claims.
l. A variable resistor comprising:
A. a substrate member comprising an electrically insulating material, one face of said substrate member being substantially flat, an elongated resistance element disposed on said one face of said substrate member, and a collector element disposed on said one face of said substrate member;
B. a movable wiper member, said wiper member having means associated therewith for adjusting the position of said wiper member, said wiper member comprising:
1. a relatively rigid carriage element;
2. a relatively resilient, pad element secured to said carriage element in overlying relation with at least a portion of said resistance element and said collector element;
. a flexible wiper contact element secured to said pad element, said contact element comprising a relatively deformable bar of flexibly bonded metal particles overlying and contacting portions of both said resistance element and said collector element;
4. said carriage element being spaced from said resistance and collector elements by a fixed distance less than the uncompressed thickness of said resilient pad element so that said resilient pad element is compressed to resiliently urge said contact element into firm electrical and mechanical engagement with said resistor and collector elements; and
C. terminal means for making electrical contact with said collector element and said resistance element.
2. A variable resistor as defined in claim 1, wherein said pad member comprises rubber.
3. A variable resistor as defined in claim 1, wherein said flexible wiper contact element comprises flexibly bonded metal particles dispersed in a deformable resinous material secured to said pad element.
4. A variable resistor as defined in claim 1 including a lubricant disposed between said substrate member and said pad element.
5. A variable resistor as defined in claim 1 wherein said pad element directly contacts at least a portion of said resistance element and said collector element to physically wipe said elements as said wiper member is adjusted.
6. A variable resistor as defined in claim 1 wherein said pad element completely overlies and is compressed against said resistance element and said collector element and is compressed against said one face of said substrate member to form a seal therewith to resist the passage of foreign material to said resistance and collector elements.
7. A variable resistor comprising:
A. a housing member, said housing member having an axially extending cylindrical, interior cavity therein;
B. a substrate member comprising an electrically insulating material, one face of said substrate member being substantially fiat and facing the interior cavity of said housing member, an arcuately extending resistance element disposed on said one face of said substrate member, and a collector element disposed on said one face of said substrate member;
C. a rotatable wiper member, said wiper member being coaxially disposed for rotation within said housing member and having means associated therewith for adjusting the rotational position of said wiper member from the exterior of said housing member, said wiper member comprising:
1. a relatively rigid carriage element coaxially aligned in the cylindrical cavity in said housing member;
2. a relatively resilient, pad element coaxially secured to said carriage element, said pad element having one substantially flat face compressed against said one face of said substrate member, said pad element overlying the entire extent of said resistance element and said collector element to sealingly engage said one face of said substrate member to resist the passage of foreign material to said resistance and collector elements;
3. a flexible wiper contact element secured to said one face of said pad element, said contact element comprising a radially extending relatively deformable contact bar of flexibly bonded metal particles overlying and contacting portions of both said resistance element and said collector element;
4. said carriage element and said substrate member being spaced apart in said housing by a distance less than the uncompressed thickness of said resilient pad element so that said resilient pad element is compressed between said substrate member and said carriage element to urge said one face of said pad element into sealing engagement with said one face of said substrate member and said resistance and collector elements, and to resiliently urge said contact element into firm electrical and mechanical engagement with said resistor and collector elements; and
D. terminal means for making electrical contact with said collector element and said resistance element from the exterior of said housing member.