US2978663A - Improved variable resistor - Google Patents

Description

sta s Patent 2,978,663 IMPROVED VARIABLE RESISTOR No Drawing. Filed Aug. 12, 1959, set. No. 833,129 Claims. or. 338-160) This invention relates to an improvement in electrical contacts. More particularly, it relates to a means for prolonging the satisfactory life of electrical contacts in which one member makes contact with a second member by wiping the surface of the second member, as is common in rheostats and potentiometers.

One object of the invention is to prolong the life of such contacts particularly when exposed to service conditions involving elevated temperatures.

Another object of the invention is to preserve the integrity of the contact materials in service, and to minimize Wear of the contact materials, without impairing the efficiency of the electrical connection between them and without concurrently reducing the pressure of the wiper member on the surface wiped.

Still a further object is to reduce the torque which is required to move the wiper along a conductive track member without altering the design of the parts, and while permitting the use of a simple wiping type of contact.

These and other objects will become apparent from the description of the invention which follows.

In general the devices to which the invention is applicable include an electrically insulating base portion formed preferably of a high melting point glass or ceramic material on which there is supported a thin film of an electrically conductive material having any suitable configuration. The problem of galling is particularly acute with the extremely thin films commonly used in resistors. and like devices, particularly when these devices are operated at relatively high temperatures such as 500 C.

Hence the invention is applicable to devices in which the electrically conductive portion may comprise a thin film, for example one deposited by evaporation of a metal, metal alloy, or other suitable compound or composition; or it may comprise an electrically conductive glaze wherein a continuous metallic network pervades a ceramic matrix as described in an application, Serial Number 833,127 filed of even date herewith; or it may comprise any of the various electrically conductive metal oxide compositions.

For purposes of illustration the invention will he described with reference to thin films consisting of tin oxide, containing antimony oxide in small amounts, but it is to be understood that such films are illustrative of but one preferred embodiment and are not to be construed as limitative.

Thus, in this instance, the electrically conductive portion is formed of a thin film of tin oxide or a thin layer of a composition based on tin oxide, deposited as a film about 0.0001 inch or thinner on the electrically insulating substrate by techniques already known and described for example in United States Patent 2,118,975 and 2,567,331, among others.

Such coatings, and more particularly, tin oxide and tin oxide-antimony oxide coatings as the electrically conductive track are known to be capable of carrying high cur- 2 rents and to possess many advantages indicated in United States Patent 2,564,706 and have been found to possess, in addition, superior stability at temperatures in the vicinity of 500 C.

The second portion of such devices includes a wiper, which may be eithersingle or multiple and mounted by any suitable means so that at least a portion of the wiper is urged firmly into physical contact with the surface of the electrically conductive portion. With the electrically conductive portions formed of thin films as described above, and particularly in service applications at elevated temperatures, the wiper materials usually employed, e.g., platinum alloys or gold alloys or other precious metal alloys, when pressed against and moved along the electrically conductive portion have been found to gall the film material and to seize thereon, and the actual detachment of pieces of the extremely thin film of the electrically conductive material from the base has been experienced. As a result, when using sufiicient pressure to obtain a good electrical contact, the Wiper may often destroy the coating and consequently the resistor unit itself, by such seizing and galling.

One previously devised approach to the solution of the problem has been to diminish the amount of pressure bringing the wiper arm into physical contact with the base. This has not always been satisfactory because it often produces noise as the contact moves past microscopic irregularities and unevennesses in the surface of the material.

Other previously proposed solutions to the problem have involved a redesign of the track or the wiper, but this is objectionable for, one or more of the following reasons: it may add to the cost of manufacture; increase the noise developed by the device in use; reduce the reliability of the contact system; impair resolution; and magnify effects of dirt and wear particles.

By the present invention the problems of sticking, seizing and galling are overcome, either by the addition of suitable materials to the electrically conductive film or by the addition of the same materials to the wiper. The material may be added by rubbing or burnishing it onto the already formed members, or'it may be otherwise incorporated into the members when they are being formed, for example by sintering a shaped mixture comprising particles of a precious metal alloy admixed with lead oxide or other suitable anti-galling material.

Other materials found to possess suitable physical, chemical and electrical properties for the present purposes include the following: graphite; boron nitride; litharge (lead oxide); and molybdenum disulfide. The molybdenum disulfide and graphite are effective for the present invention for somewhat lower temperatures up to about 275 C., but the remaining materials may be used to SOD-600 C.

As indicated above, a preferred method of application of these and other materials possessing similarly suitable properties is by rubbing or burnishing fine particles of these materials onto the electrically conductive film of the otherwise completed device against which a wiper rubs in the normal service of the device. Preferably the fine particles are impalpable powder, the particles of which have no dimension larger than about 5 microns.

No vehicle or dispersing agent is required to facilitate a uniform distribution, but may be used if the method of application chosen includes spraying the particles onto the resistor surface.

The amounts applied are insuflicient to impair the electrical contact between the wiper and the electrically conductive material, but still must be suflicient to diminish any tendency to seize or gall.

Resistors embodying the present invention have been formed as follows: i I

A coating composition has been prepared by mixing the following in a beaker:

25 g. SHCl4'5H2O 0.75 g. SbCl 10 Cc. H20

5 cc. HCl (concentrated) An electrically conductive track has been laid down on an electrically insulating ceramic base material by heating the base to between 600 C. and 1200 C., depending on the specific base material used, and spraying a solution of the above composition onto the heated base.

With a thin ceramic wafer substrate, to obtain an even distribution of the coating, it was found desirable to mount the wafer on a rotatable support in an electrically heated kiln and to spray the coating mist and onto the water while it is in the furnace and after it had been held at temperature for between 5 and minutes.

The wafer and adherent irridescent coating were permitted to cool and thereafter, finely divided (minus 5 microns) litharge (PbO) was rubbed into the tin oxide coating using a piece of felt as the applicator. In alternative embodiments at least about by weight and preferably between 20% and about 50% by weight of lead oxide was admixed with a precious metal alloy, based on either platinum or gold, then shaped to form a contact and sintered to obtain the wiper.

With a resistor consisting of a glass substrate and a tin oxide-antimony oxide coating thereon formed as described above, and a wiper composed of 90% Pt-l0% Rh operated with the wiper spring pressed on the track with a force of -60 grams, and with the wiper oscillated at a uniform speed between spaced terminals at either end of the conductive track, the following comparative results were obtained:

If any tendency to gall or seize is noted, additional powder may be applied to the electrically conductive film track, from time to time by merely repeating the application described above.

Having now described the invention with the particularity required by the patent statutes, we claim:

1. In a variable resistor comprising an electrically insulating substrate, a composition consisting principally of tin oxide supported on said substrate as an electrically conductive coating and a metal wiper adapted to move along and rub against said composition, the improvement which comprises diminishing the seizing and galling between said wiper and said coating by providing a material of the group consisting of lead oxide (PbO), boron nitride, molybdenum disulfide and graphite at the interface between said wiper and said coating.

2. The device of claim 1 wherein the material is provided as a constituent of a sintered metal wiper.

3. The device of claim 1 wherein the material is provided on the surface of the electrically conductive coating.

4. In a variable resistor comprising an electrically insulating substrate, a composition consisting principally of tin oxide supported on said substrate as an electrically conductive coating and a metal wiper adapted to move along and rub against said composition, the improvement which comprises diminishing the seizing and galling between said wiper and said coating by providing lead oxide (PhD) at the interface between said wiper and said coating.

5. In a variable resistor comprising an electrically insulating substrate, supporting and integral with a thin film of electrically conductive coating and a metal wiper adapted to move along and rub against said coating, the improvement which comprises diminishing the seizing and galling between said wiper and said coating by providing a material of the group consisting of lead oxide (PbO), boron nitride, molybdenum disulfide and graphite at the interface between said wiper and said coating.

References Cited in the file of this patent UNITED STATES PATENTS 2,620,555 Lenz Dec. 9, 1952 2,632,831 Pritikin et a1. Mar. 24, 1953 2,699,839 Hamel Ian. 18, 1955 2,702,092 Douglass Feb. 15, 1955 2,750,527 Katz June 12, 1956 2,760,036 Raymer Aug. 21, 1956 UNITED STATES PATENT OFFICE CERTIFIQATE (H @QRRECTWN Patent No, 2 978 663 April 4; 1961 Arnold 55., Lewis et :11,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed speeifieetiom line 4 address of inventors for e/e Market Company read mm c/o Markite Company Signed and sealed this 5th day of September 19610 (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L LADD Commissioner of Patents USCOMM-DC UNITE STATES PATENT FHCE @ETEMQATE @F CQREQHGN Patent No, e em eee April 1%1 Arnold So Louis ei; e10

It is hereby certified that error appears in the above numbered pat ent requiring correction and that "the said Letters Patent should read as corrected below,

In the heading to the printed specificetion line 4 address of inventors for 0/0; Market Company read 0/0 Markite Company e Signed and sealed this 5th day of September 19610 (SEAL) Attest:

ERNEST W. SDER DAVID L. L :l:

Attesting Ufficer Commissioner of Patents USCOMM-DC

Claims (1)

1. IN A VARIABLE RESISTOR COMPRISING AN ELECTRICALLY INSULATING SUBSTRATE, A COMPOSITION CONSISTING PRINCIPALLY OF TIN OXIDE SUPPORTED ON SAID SUBSTRATE AS AN ELECTRICALLY CONDUCTIVE COATING AND A METAL WIPER ADAPTED TO MOVE ALONG AND RUB AGAINST SAID COMPOSITION, THE IMPROVEMENT WHICH COMPRISES DIMINISHING THE SEIZING AND GALLING BETWEEN SAID WIPER AND SAID COATING BY PROVIDING A MATERIAL OF THE GROUP CONSISTING OF LEAD OXIDE (PBO), BORON NITRIDE, MOLYBDENUM DISULFIDE AND GRAPHITE AT THE INTERFACE BETWEEN SAID WIPER AND SAID COATING.