US1992529A - Variable resistor - Google Patents

Description

Feb. 26, 1935. A. c.- HENRY VARIABLE RESISTOR Filed Aug. 17, 1932 A TTORNEY Patented Feb. 26,l 1935 UNITED STATES VARIABLE RE SISTOB Albert C. Henry, Ironton, Ohio, assignor to Henrite Products Corporation, Ironton, Ohio, a corporation of Ohio Application August 17, 1932, Serial No. 629,100

2 Claims. (Cl. 20L-76) This invention relates, generally, to resistors or resistance elements; and the invention' has reference, more particularly, to a novel variable resistor and to the process of producing the same,

the said resistor being suitable for various uses and being especially adapted for use in potentiometers and rheostats employed in radio units and for other purposes.

Heretofore, in making resistors and especially those used in potentiometers and the like, it has been common to coat or paint an absorbent base, such as blotting paper, with graphite or other suitable material. In order to produce variable resistors, the graphite or other material is applied to the base in several coats, the number of coats applied varying over different portions of the base. Such resistors are very friable and cannot stand the rubbing of movable contact elements thereover, nor can they stand any rough handling without irreparable damage. Also, these resistorsas well as those of the articial stone type, such as disclosed in the patent to Bachmann et al, #686,246, are quite hygroscopic with the result that the actual ohmic resistance of such resistors varies with climatic conditions and with humidity changes from day to day, and therefore the action of such resistors is unreliable, rendering the same more or less unsatisfactory in use.

In my copending application, Serial No. 513,093, there is disclosed a novel variable resistor comprising a molded unitary body having a plurality of consecutively arranged sections of varying ohmic resistances, said sections consist- `ing of non-hygroscopic intermixed insulating and conducting materials held together by a suitable non-hygroscopic binder, such as a phenolic condensation product. The present application is a continuation-in-part of my said copending application.

The principal object of the present invention is to provide a novel variable resistor or resistance element and the process of producing the same, which resistor will have relatively low contact resistance with a movable contact sliding thereover, thereby substantially preventing sparking at the point of engagement of the movable contact with the resistor, the said resistor being of sturdy and strong construction and being capable of withstanding rough handling and the rubbing of the movable contact thereover without injury or appreciable wear, the said resistor being impervious to moisture and being unaffected by climatic changes.

Another object of the present invention lies in the provision of a novel variable resistor of solid and rigid construction, the said resistor being made up of a relatively thick foundation or supporting layer or region of electrically insulating or semi-conducting character, and a relatively thin top layer of electrically conducting character, the said thin layer preferably consisting of consecutive sections of varying ohmic resistance, the said layers and sections being A combined in a unitary structure.

Another object of the present invention is to provide a novel variable resistor of the above character and a process for producing the same, whereby the upper and lower layers may be either formed cold as by pressing, and thereafter assembled together and bonded under pressure and heat; or the lower layer may be hot molded and the upper layer formed cold, whereupon the two layers may be combined as by painting the hot molded lower layer with a binder and then bonding the two layers with heat and pressure.

Other objects of this invention, not at this time more particularly enumerated will be clearly understood from the'following detailed description of the same.

The invention is clearly illustrated in the accompanying drawing, in which:

Fig. 1 is a plan view of a typical example of the novel resistor of the present invention, illustrating with legends the extent of the several resistance sections of the top layer thereof.

Fig. 2 is a sectional view taken along line 2--2 of Fig. 1 looking in the direction of the arrows.

Fig. 3 is a diagrammatic sectional view showing a mold developed or extended into lineal form and illustrates a step in the formation of the lower layer of the resistor.

Fig. 4 is a view similar to Fig. 3 but illustrates a step in the formation of the upper layer of the resistor.

Fig. 5 is a plan View of a lower moldv half, and illustrates a loading device in section applied thereto, the said loading device being adapted to load the severalfsections of the top layer into the lower mold half.

Fig. 6 is a diagrammatic sectional view oi a developed or extended mold, and illustrates the iinal step in the formation of the top or upper resistor layer.

Fig. 7 is a developed sectional view of a mold showing the final step in the formation of the resistor; and

Fig. 8 is a diagrammatic sectional view illustrating the process of producing a modified for of resistor.

Similar characters of reference are employed in all of the above described views to indicate corresponding parts.

In carrying out the process of the present invention to produce one form of novel resistor, a plurality of mixes are prepared, one of which mixes contains one or more non-hygroscopic electrically insulating materials, such as silica, felspar or mica, together with a suitable non-hygroscopic binder,v such as a phenolic condensation product, a resin or a pitch. Other mixesconsisting of electrically insulating material, electrically conducting material and a binder are also prepared. Carbon black and/or graphite serve very well as conducting materials. The insulating and conducting materials are ground into powdered form and then thoroughly intermixed y in the desired proportions together with the powdered binder to produce the various mixes used.

The mix consisting oi' insulating material and a binder is then placed in a mold and pressed into the desired shape either with or without the application of heat. Likewise, the mixes coninsulating and electrically conducting materials -together with a hinder, are then placed .in the same mold on top of the lower layer, thereby forming an upper layer, and these two layers are then adhered lay the application ci heat and presn sure. '11. 'the event that either the layers hot molded initially, the same will "ne coated with a hinder solution on its side toward the other laye so that the "here together la ers will etant electric -l5 .7 in making un tne upper layer, the percentage oi" conducting 'terial is preferably higher than that disclosed in my above mentioned copending application in order to reduce the contact resistance oi 'the rheostat. It will be apparent that the relative proportions of conducting and vnon-conducting materials used together with the thickness of the upper layer will determine the ohmic resistmce of its several sections. Thus, by making the upper layer relatively thin, the same may be made considerably more conducting per unit of cross sectional area than the sections of the resistor of my preceding application without the resistor of this invention carrying any more current than that carried by the resistor disclosed in my preceding application, while at the same time the present resistor will have a greatly reduced contact resistance.

In preparing another form of resistor of the present invention, the lower layer may he formed, described in my copending before mentioned application, of a plurality' of sections consisting of differing mixes of conducting and nonmconducting materials held together hy a binder. These sections may be molded either hot or cold and a very thin top layer of uniform mix throughout is then applied over these sections, which top layer is preferably of a relatively higher conducting nature so as to therebyl reduce contact resistance to a minimum. The thickness of the top layer may be varied so as to correspondingly vary the maximum resistance of the resistor to satisfy the various requirements of users, while enabling the use of but one or more standard lower layers.

Lacassey Referring now to Figs. 1 and 2 of the said drawing, there is illustrated one form of the novel resistor of the present invention. This resistor, designated as a whole by the reference numeral 1, is illustrated as of C-shape and consists ci a lower layer 2 which is relatively thick and an upper or top layer 3 which is relatively thin and is bonded integrally to the lower layer 2. The lower layer 2 is entirely non-conducting. A typical mix used in preparing this lower layer 2 consists oi 89% insulating material, such as silica, Ielspar, and mica and 20% binder, such as bakelite or resin, the percentages being by weight. 'Ihis mix is placed in a lower mold half providing a C-shaped recess therein. A developed or ex tended view of such a mold half is illustrated in Fig. 3 and is designated by the reference numeral 4. In this figure the insulating mix for forming the lower layer is shown inserted into the lower mold half 4 and the upper mold half 6 is shown applied thereto. Preferably the mix 2 for forming the lower layer 2 is pressed cold with suiilcient pressure so that the saine will retain its shape.

The relatively thin upper layer 3 is composed of a plurality of sections, the number or' which sections will vary in accordance with the requirements ci the purchaser. 'in Fig. l ci the drawing the top layer is illustrated as having nine sections, the limits of which sections are indicated hy the legends je. to I .disch il of the top layer E will consist depending upon nl...

iicaliy :lnsulat e con material, material percent in making up 3 :lor a typical Section Silica `G raphite Bakelite 2) 50 2. 10 i0 .'20 2i 20 (l0 20 E 84 16 20 G6 lf3 20 60 20 20 Al0 40 20 20 60 20 Each of the sections making up the layer 3 may be preformed as by cold pressing in molds and then these several sections placed in proper order in a mold on top oi the preformed layer 2 as illustrated in Fig. 7 of the drawing. The several sections of the upper layer are then molded together and to the lower layer by the application of heat and pressure. Thus, a molding tem perature of 350 F. and molding pressure of rive thousand pounds per square inch may be applied to the upper and lower layers contained in the mold shown in Fig. "1.

Preferably, the several sections of the upper layer 3 are preformed simultaneously. To accomplish this result, a mold having a lower mold half 9, such as illustrated in Fig. 5 and shown diagrammatically in developed form in Fig. 4, is employed. This mold has the form of the nished resistor layer, and a loading device 10 is used for placing the several mixes into the mold half 9. The loading device 10 has sectional passages 11 to 19 for conveying into mold half 9 the several mixes used in forming sections A to I.

Fig. 6 shows the several mixes for forming the upper layer arranged consecutively in the lower CII f. l'. u U

mold section 9 after the loading device has been removed from the mold. 'Ihe upper mold half 20 is pressed against the several mixes so as to cold form these mixes into the upper layer. This upper layer is then placed upon the preformed lower layer 2 within mold section 4 as shown in Fig. .7. 'I'he upper mold half 6 is now applied together with heat upon the assembled upper and lower layers so as to complete the molding and the curing of the bond ln the same to thereby form the resistor shown in Figs. 1 and 2. The outer contacting surface of the upper thin layer is preferably ground, abraded or machined somewhat so as to remove any excess or lim of binder which tends to collect on such outer surface of the upper layer during the molding process.

It will be apparent thatif desired either the lower layer or upper layer could be hot molded initially and then painted on one of its surfaces with a bakelite or other binder solution and thereafter applied to the other layer and bonded thereto under heat and pressure. Thus, either or both of the two layers may be preformed by cold molding and then the two layers united by hot molding, or either or both of the two layers may be initially hot molded and thereafter secured together into an unitary resistor by use of a liquid binder cured by heating the two layers together under pressure.

In the completed resistor, the two layers thereof, as well as the several sections of the upper' layer, are all held together in an integral manner, forming a one piece structure which is very strong and wear resistant, the same being impervious to moisture and having physical properties which will always enable the same to maintain a constant ohmic resistance or graduated resistances as the case may be. The great mechanical strength of this resistor enables the same to withstand any amount of handling or the abrasive action of movable contacts without injury. It will thus be apparent that the novel resistor of this invention may be relied upon to maintain its ohmic resistance characteristics indefinitely, as is actually found to be the case in practice.

Owing to the relatively high conductivity of the upper layer, the electrical resistance between a movable contact and the surface oi the upper layer is reduced to-a minimum, thereby doing away with arcing and the burning of contacts. It will be apparent that any combination of ohmic resistances may be obtained by merely varying the percentages of the ingredients used in the several mixes of the upper layer. The lower layer in this form of the invention serves as a base or support for the'relatively thin upper layer and prevents any injury thereto.

In the form of the invention shown in Fig. 8 the. lower layer 21 is preformed in the same manner as that disclosed in my copending application above referred to, the said lower layer consisting of a plurality of sections of varying ohmic resistances depending upon the requirements of any particular installation. Thus, each of the sections of the lower layer 2l -of Fig. 8 will comprise a mixture of insulating material, such as silica or felspar, conducting material, such as graphite or carbon black, and binder such as bakeliteor resin. The several sections of this lower layer are illustrated as intermixing slightly at the several planes of contact as indicated at 22. This slight intermixing of the several sections will take place to a limited extent in the formation of the upper layer in the form of the invention shown in Figs. 1 to 'i of the drawing.

A relatively thin layer 23 of a highly conducting mix is applied to the lower layer 21 and these two layers are bonded together under heat and pressure by means of the mold having a lower mold half 24and an upper mold half 25. The lower layer 21 may be either preformed cold or by hot molding andthe same is true of the upper layer 23. Preferably both of these layers are cold formed and then cured together by hot molding. By varying the thickness of the highly conducting upper layer 23, the ohmic resistance of the resistor may be correspondingly varied without the necessity of changing the mixes of the lower layer 2l. This is highly desirable where a plurality of somewhat similar resistors are desired, and also enables the use of one or more standard lower layers with upper layers of varying thickness and mixes. Preferably the entire upper layer 23 is of the same mix. Although the novel resistor of the present invention is illustrated as of C-shape, it willy be apparent that the same may be of any shape desired, such as rectangular, annular or other shape. Preferably. the outer contacting surface of the upper layer 23 is ground or abraded after the two layers have been molded together in order to remove excess binder which tends to collect in the form of a. film on such outer surface during the molding process.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, as defined by the following claims, it is intended that all matter contained inthe above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A process for forming a non-hygroscopic variable resistor having a low contact resistance, said resistor comprising united layers, which process consists in preparing a plurality of dissimilar powdered mixes of electrically insulating and electrically conducting materials together with a binder, arranging said mixes consecutively in a mold and forming the same into a relatively thick resistor layer of variable resistance, preparing a powdered mix of electrically conducting and insulating materials together with a binder, thel percentage of electrically conducting material predominating, placing said mix in a mold and forming the same into a relatively thin resistor layer of relatively high electrically conducting character and of uniform resistance, and thereafter combining said first and second layers to form a unitary resistor.

2. Avariable non-hygroscop'ic resistor comprising, a molded unitary body having a lower relatively thick base layer having a plurality of consecutively arranged sections, each of said sections comprising non-hygrscopic, electrically insulating and electrically conducting materials held together by a binder, the mix of each of said sections differing from those of the remaining sections, whereby the electrical resistance of said layer varies from point to point of its length, and an upper relatively thin layer comprising nonhygroscopic electrically resistant and electrically conducting materials held together by a binder, saidelectriealiy conducting material being in such proportion as to render said upper layer relatively highly conducting, the-electrical resistance of said thin upper layer being substantially constant all

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