US 3324439 A
Description (OCR text may contain errors)
'June 6, 1967 A. J. WRIGHT ET AL ELECTRICAL TERMINATIONS FOR CERMET RESISTANCE ELEMENTS 2 Sheets-Sheet 1 Filed Feb. 27, 1964 W 5 WWW mfg Q5 0 W 0 M4 5% aw United States Patent 3,324,439 ELECTRHIAL TERMENATIONS FOR CERMET RESISTANCE ELEMENTS Alan J. Wright, Orange, Calif., and Ronald C. Vickery,
Saxonburg, Pa., assignors to Beckman Instruments, Inc.,
a corporation of California Filed Feb. 27, 1964, Ser. No. 347,855 3 Claims. (Cl. 338-162) The present invention relates to improved electrical terminations for electrical resistance elements and more particularly for resistance elements of the type commonly known as cermets which employ Combinations of metals and glasses as the resistive material, and which are deposited upon a dielectric support base.
Cermet resistance elements presently known in the art are exemplified by US. Patent 2,950,995 of Thomas M. Place, Sr., et al., entitled, Electrical Resistance Element, and US. Patent 2,950,996 of Thomas M. Place, Sr., et al., entitled, Electrical Resistance Material and Method of Making Same, both of which are assigned to Beckman Instruments, Inc., assignee of the present invention. These patents describe resistance elements formed by a layer of resistance material comprising a heterogeneous mixture of non-conductive material and conducting metals fixed to a base, preferentially formed from a refractory material such as steatite. The non-conducting material is a ceramictype material such as glass and the layer is formed by heating the metal-glass mixture at least to the melting point of the ceramic materal but not to the melting point of the metals, so as to create a smooth glassy phase.
Many potentiometers, variable resistors, rheostats and the like are presently constructed by forming the resistance element in a substantially closed path, usually shaped in the form of an annulus. Such components are commonly denoted as single-turn potentiometers, variable resistors, etc. Many of these single-turn components are so utilized that the movable contact or wiper is rotated through 360 or more. In this type of component, it is conventional practice to provide an electrically non-conductive segment in the resistance element of sufficient length, e.g. 10, so that the potentiometer wiper will not cause an electrical short circuit between the conductive ends of the resistive element when traversing that area. The resistance element is commonly terminated on one side of this non-conductive area for rheostats and on both sides in potentiometers.
Heretofore, certain problems have been encountered in constructing resistance elements employing cermet resistance materials. Thus, as the wiper traverses the element, it will, if drawing a significant amount of current, tend to arc as it leaves the electrically active portion of the element and traverses the non-conducting bridge segment or dead space. The arcing will cause erosion of the resistance film with a resulting reduction in the useful life of the element.
Another problem commonly associated with single-turn potentiometers having continuous rotation in either or both directions is that the bridge or dead space between the electrically active ends of the deposited resistance element produces a varying torque when the wiper passes through this dead space. Thus, any difference in height between the deposited element and the base, even as small as .001 inch, may cause an undesirable increase in the torque required to rotate the potentiometer shaft.
A still further problem is that of obtaining a linear resistance element. Thus, the absolute linearity of a variable resistance is determined by several factors, one of which is the physical location of the fixed electrical contact with respect to the resisance element. It, for example, the fixed contact is so positioned that there is an electrical resistance between the movable wiper and the Patented June 6, 1967 fixed electrical contact when the movable wiper is positioned mechanically to obtain the minimum resistance at the contact being measured, this resistance, known in the art as end resistance or residual resistance is a deviation from the desired linear output.
The present invention provides a successful solution for each of the problems enumerated above. In one embodiment described in more detail hereinafter, the base includes a step having the approximate thickness of the cermet resistance element. Conductive elements are located on the oppositely disposed sides of this step by, for example, coating the sides with a conductive material which, when fired, is converted into a continuous conductor film. The cermet resistance material is then deposited upon the base by a silk screen or like method to the height of the step and in intimate contact with the respective conductive portions of the raised step. The cermet then has, when fired, the entire cross-sectional area of each of its end surfaces in contact with a conductive termination. Electrical leads or other conductor members are easily soldered or welded to the exposed conductive portions of the step to complete the electrical termination.
An alternative embodiment of the invention also hereinafter described comprises first forming the resistance element and leaving a void or dead segment which is subsequently filled with a non-conductive ceramic, such as glass. A pair of slots are then formed along the juncture of the glass resistance element members, which slots are filled with a conductive material which intimately contacts the entire cross-sectional end surface of the cermet resistance element.
Accordingly, the present invention provides a non-conductive bridge segment or dead space integral with an electrical termination structure making electrical contact with the entire end cross sectional surface of the cermet resistance element. It has been dscovered that this structure provides a substantial reduction in end resistance, thereby improving the absolute linearity of the variable resistance element.
Another significant advantage derived from these structures is that the wiper contact moves from the resistance element onto the electrically conductive terminating means before a discontinuity between the wiper and the conductive element occurs. Accordingly, any arcing which occurs will be from the wiper to the terminating elements rather than from the wiper to the resistance element, thereby avoiding deleterious erosion of the resistance material.
A further advantage of the terminating structures of the present invention is that the height of the resistance element and that of the bridge or dead space can easily be made equal so that there is no variation in torque required when traversing between the dead space and the resistance element.
A more thorough understanding of the invention may be obtained by a study of the following detailed description taken in connection with the accompanying drawings in which:
FIG. 1 is an isometric view of the base member used to form one embodiment of the present invention;
FIG. 2 is an isometric view of the completed resistance element employing the base element of FIG. 1;
FIG. 3 is a fragmentary sectional view taken along line 3-3 of FIG. 2;
FIG. 4 is an isometric view of the base member of another embodiment of the invention;
FIG. 5 is a fragmentary sectional View taken essentially along line 5-5 of FIG. 4;
FIG. 6 is a cross-sectional view of a representative 7 potentiometer employing the resistance element formed by the present invention;
FIG. 7 is a plan view of the potentiometer of FIG. 6;
FIG. 8 is an enlarged fragmentary view taken essentially along line 8-8 of FIG. 7; and
FIGS. 9, 10, 11 and 12 are fragmentary sectional views illustrating four steps in the formation of another embodiment of the present invention.
In the structure of FIG. 1, a base 10 advantageously "formed of a refractory material, such as steatite, has the circular configuration generally associated with singleturn variable resistance elements. This base comp-rises a substantially flat support surface 11 and a step 12 providing an elevated planar portion 13. The sides of the step 12 are covered by an electrically conductive material to form the respective conductor elements 14, 15. It will be understood that for variable resistance elements such as rheostats, only a single one of these sides would be provided with the conductive material. Advantageously, conductive elements 14, 15 are obtained by coating the sides with a noble metal paste. The base 10 is then fired at a temperature which will convert the metallic paste into a continuous film or the paste may be fired at the same time as the resistance element.
The electrical resistance element is completed by silkscreening or otherwise depositing cermet resistance material onto the flat support surface 11 to approximately the height of the elevated planar surface 13 as shown, with the ends of the resistance material in intimate contact with the conductive elements 14, 15. Base 10 with the applied resistance material is then fired to convert the cermet layer into a resistance element having a hard, smooth, glossy surface. The thickness of the base and element is of the order of to A3 inch and the thickness of the cermet resistance element is of the order of .001 inch.
Leads to external circuitry are made by affixing conductive wires or ribbons 21, 22 to the respective conductive elements 14, 15. Generally, soldering or spot welding is utilized for this operation.
FIGS. 4 and 5 show'an alternative embodiment of the invention. The steatite base is formed initially with a continuous flat surface 31, but while still in the green or unfired state, a steatite slip 32 is deposited on the surface and the entire base fired. A contact material 35 such as a noble metal paste, is then applied to the sides of the slip 32 and the paste is then fired to convert the metal paste into a continuous conductive element. A cermet film 36 is then deposited to the approximate height of the slip 32 and fired to yield a resistance element. As in the previous embodiment, substantially the entire end surface of this resistance element is in electrical contact with the terminating structures provided by the conductive elements 35.
FIGS. 6 and 7 illustrate a potentiometer having a resistance element formed according to the present invention. This potentiometer includes a housing 40 and cover 41 with a shaft 42 journaled in bushings 43, 44 carried in the housing and cover, respectively. The resistance element constructed in the manner of the present invention, comprises a circular support base 51 and annular cermet resistance element 52 and is fitted in the housing 40. A bushing 53 may be fitted through an annular opening 54 of the resistance element base 51 for receiving the shaft 42. The electrical wiper contact is supported on an arm 61 projecting from a collar 62 carried on and affixed to the shaft 42 for rotating the contact element 60 with the shaft to traverse the resistance element.
An enlarged view of the movable wiper 60 and resistance element 50 is shown in FIG. 8 and more clearly shows a significant advantage derived from the present,
invention. Thus, as the contact wiper 61 moves from the resistance element 52, it first engages the conductive element 65. Since there is a very low electrical resistance between the conductive element 65 and the resistance element 52, there is no possibility of arcing until the wiper is moved from the conductive element 65 onto the bridge segment or dead space 66. In this way, any arcing which occurs will be from the wiper to the termination structure rather than from the wiper to the resistance element, thereby avoiding deleterious erosion of the resistance element.
Another advantage illustrated in FIG. 8 is that the elevated planar portion 67 may be located at the same level as the top surface of the resistance element 52 so that there is no increase in torque required to advance the movable wiper 61 across the bridge segment 66. Generally, the resistance material will be deposited to the equivalent height of the step 66, or for very precise matching, the top of the step may be planed down to match the height of the element.
A still further embodiment of the invention and the method of constructing same is illustrated in FIGS. 9-12. Initially, a cermet resistance element 75 is deposited upon a flat planar base 76, with a gap being left between opposite ends of the resistance element 75 as shown. After the cermet resistance material has dried, a non-conductive glass or ceramic slurry is deposited in the gap 77 and dried, after which the base 76 with its supported elements is fired to yield a smooth, hard, glossy surface of both the resistance material and slurry. This yields the configuration shown in FIG. 10.
The junction between the resistance element 75 and the non-conducting element 80 is then removed by means of a diamond wheel or other appropriate cutting means to leave a pair of slots 85, 86 between the resistance element 75 and the non-conducting material 80. Preferably, the upper surface of the non-conducting material 80 is reduced to the level of the resistance element 75 in this step also.
The element is completed by filling the respective gaps 85, 86 with a conductive solution such as a conductive metal slurry which hardens at room temperature to yield the end terminations 90, 91.
Although exemplary embodiments of the inventio have been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.
1. An electrical resistance element for a single-turn variable resistance device comprising a non-conductive base;
a cermet resistance element affixed to said base, said element having a gap between juxtaposed ends thereof interrupting the continuity of said element;
a non-conductive bridge affixed to said substrate between said juxtaposed ends of said cermet resistance element, the sides of said bridge juxtaposed the ends of the cermet resistance element being somewhat wider than the width of the ends of said cermet resistance element; and
an electrical termination for said cermet resistance element comprising a conductive element disposed between an end of said cermet resistance element and an oppositely disposed side of said non-conductive bridge and making electrical contact with substantially the entire end cross-sectional surface of said cermet resistance element, said conductive element being affixed to said non-conductive bridge and extending laterally beyond the outer edge of said resistance element, and
an electrical conductor affixed to said conductive element at a point beyond the end surface of said cermet resistance element.
2. An electrical resistance element for a variable resistance device comprising a non-conductive base having a flat support surface;
a cermet resistance material deposited upon said base to form a resistanceelement having a gap between juxtaposed ends thereof interrupting the continuity of the element;
a non-conductive material deposited in the gap between said ends of said cermet resistance element, said nonconductive material being deposited somewhat wider than the width of said ends of said cermet resistance element;
a pair of slots being formed at the respective junctions between said ends of said cermet resistance material and the non-conducting material, said slots having the same height as the cermet resistance element;
a conductive material filling said slots to the height of said cermet resistance elements so that the ends of said cermet resistance material have substantially their entire cross-sectional surface in contact with said conductive material, said conductive material affixed to the side of said non-conductive material and extending adjacent said non-conductive material laterally beyond the projected extent of said resistance element; and an electrical conductor atfixed to said conductive material at a point beyond the end surface of said cermet resistance element.
3. In a rotary potentiometer employing a cermet resistance element and so constructed to prevent arcing between the movable contact of a potentiometer and the cermet resistance element to thereby avoid deleterious erosion of the cermet resistance material, and further having a very low end resistance and .a substantially uniform torque over a 360 rotation of the potentiometer wiper, said potentiometer comprising,
a non-conductive support base mounted in said housa cermet resistance element aflixed to said support base;
a non-conductive bridge afiixed to said support base between juxtaposed ends of said cermet resistance element, said bridge providing an elevated planar surface substantially coplanar with the surface of said cermet resistance element, said non-conductive bridge being wider than the width of the ends of said cermet resistance element;
an electrical termination for the respective ends of said cermet resistance element comprising conductive elements respectively disposed between the ends of said resistance element and a juxtaposed side of said non-conductive bridge and making electrical contact with substantially the entire end cross-sectional surface of said cermet resistance element, said electrical termination being afiixed to the side of said nonconductive bridge and extending laterally beyond the projected extent of said resistance element;
an electrical conductor aflixed to said electrical termination at a point beyond the end surface of said cermet resistance element; and
a movable wiper contact mounted in said housing for traversing said resistance element, said contact moving from the cermet element to the electrical termination before a discontinuity between the wiper and cermet element occurs thereby avoiding any arcing between the wiper and the cermet resistance element.
References Cited UNITED STATES PATENTS 2,005,922 6/ 1935 Stoekle 338- 3,200,010 8/1965 Place 33830*9 X 3,201,737 8/1965 Mathison 338-184 X 3,206,702 9/1965 Greenwood 338162 3,237,140 2/ 1966 Barden 338184 RICHARD M. WOOD, Primary Examiner. J. G. SMITH, Assistant Examiner.