US3010092A - Variable resistor - Google Patents

Description

Nov. 21, 1961 R. E. cADDocK VARIABLE REsIsToR Filed Aug. 5, 1958 waw r WEN 1 a w 4 w United States Patent Office Patented Nov. 21, 1961 The present invention relates to variable resistors, and more particularly to a new and improved resistance element for variable resistors and potentiometers and method of producing the element.

The primary object of the invention is to provide a element which excels in the characteristics of W temperature coefficient, high stability of total resistance, low contact resistance, good linearity, stability against chemical change, and resistance to the effects of hurnidity.

Another object of the invention is to provide 'a method of producing resistance elements having the characteristics mentioned above, which method is simple and inexpensive and can easily be Controlled to mass produce elements within close tolerances.

These and other objects and advantagesof the present invention will vbecome apparent to those skilled in the art upon consideration of the following detaled description of the preferred embodiment thereof, reference being had to the accompanying drawings, wherein:

FIGURE 1 is a side elevation of a composition resist ance element em-bodying the principles of the invention;

FIGURE 2 is an enlarged cross sectional view of the element taken as indicated at 2-2 in FIGURE l; and

FIGURE 3 is an enlarged cross sectional view taken as indicated at 3-3 in FIGURE 1, with portions removed in the interest of clarity of illustration. i

In 'the drawings, the element is designated in its entirety by the reference numeral 10 and it comprises an elongate base 11, which is sometimes referred to as a ucard or mandrel. The base ll'l is preferably provided in the form of a fiattened Ceramic tube, which may 'be glazed or unglazed, but I have found that glazed Ceramic is the preferred material. Adherent upon the outer surface o; the base 11 at opposite ends thereof are respective films 12 of metal. Preferably, the films 12 are of platinum, which is almost entirely inert to the corrosive effects of air and moisture, and which also has excellent characteristics for soldering terminal wires directly thereto. Adhering to the top edge of the base 11 and overlying the films 12 at both ends of the element is a thin layer 13 of carbon particles dispersed in a resin binder. The thin layer 13 of carbon and resin lbinder is the high'resistance conductive material on which the movable wiper of the variable resistor or potentiometer Wipes. It will be noted in FIGURES 2 and 3, that the layer or stripe 13 is relatively narrow in width, and extends only a short distance 'below'the crest of the tube. This narrow width of the resistance layer contributes to the good linearity characteristics of the element.

When an element 10 is to be ins'talled in a variable resistor or potentiometer, the terminal wires may be soldered directly` to the films 12, which eliminates many of the troublesome problems of terminating composition elements, as made in the past. That portion of the conductive stripe 13 overlying the metal films 12 is known I as a dead band since movement of the wiper over this portion of .thel conductive stripe produces no change in the voltage between the wiper and the adjacent film 12.

The method of producing an element as described above is substantially as follows: The ends of a Ceramic base ll are painted with an organic compound containihg a metallic salt. Preferably, a metallic salt of platinum is used, although salts of gold or pal'ladiurn may also be employed. By way of example, the following organic platinum compound may be used:

Parts by volume Collodion in methyl alcohol, 6 percent solution 3 'Chlorplatinic acid (HzPtCl) in denatured alcohol,

6 percent solution 3 Denatured alcohol 3 Bismuth Chloride, 1 percent solution in alcohol `1 This mixture is painted on the ends of the base, and the Ceramic base is then fired to approximately 1300 F. The Vorganic material decomposes, leaving the reduced platinumV in an exceedingly fine state of division. With continued heating, the deposit becomes adherent, and eventually becomes integrally bonded with the surface of the Ceramic base, forming at each end a continuous metal film. This process can be repeated, if desired, to build up multiple deposits so as to form a film of any desired thickness.

Continuing with a description of the method, a resincarbon mixture, made of acetylene black dispersed in epoxy resin in any desired proportion, is then painted on the top edge of the base and overlying the metal films at the ends of the element. dentally, that an improved conductive film or stripe is produced which is extremely tough and durable, when benzyl alcohol is used as a diluent. The painted elements are then `baked for approximatcly 15 minutes at a temperature of 3(l0 C., which gives a substantially complete cure of the resin, while allowing a certain' amount of quality control adjustment by means of a 'rebake, if necessary. A cure of less than 15 minutes at 300 C. usually results in an unsta'ble Vtotal resistance; while a cure in excess of 35 minutes causes the film to become quite brittle.

At the end of the cure, the elements are classified as to total resistance, and those elements which fall below a predetermined total resistance range are given a rebake according to a predetermined schedule, which shifts the total resistance up into the desired range.

While I have shown and described in considerable detail what I believe to be the preferred form of my invention, it will be understood that various changes may be made without departing from the scope of the claims.

I claim:

l.. A resistance element for variable resistors comprising; an elongate base of non-conductive material, two separate thin films of corrosion-resistant metal adherent upon said base adjacent respective ends thereof, and a thin narrow stripe of carbon dispersed in a resn binder adherent upon said base and overlying at least portions of said thin films of corrosion-resistant metal.

2. The invention as defined in claim 1, Wherein said base is a fiattene-d tube of glazed ceramic material, and said thin narrow stripe of carbon and resin l'bioder is restricted to one of the narrow edge portions of said flattened tube.

3. The invention as defined in claim 1, wherein said thin films of corrosion-resistant metal are esrsentially Platinum.

4. The invention as defined in claim l, wherein said carbon Vis acetylene black, and said resin binder is .a

-` thermosetting resin.` n

5. The method of making a resistance element for variable resistors comprising the steps of: providing an elongate Ceramic base, painting end portions only of the base with 'a heat-reducible compound comprising a nob-le metal, firing the painted base at a temperature sufiicient to decompose and reduce the lcompound to thin films of metal, painting on said base a narrow longitudinally-extending stripe consisting essentially of a thin layer of carbon particles dispersed in aresin 'binder with the stripe I have discovered, incioverlying the films of metal at the ends of said base, and baking said base for a time and at a temperature sufficient to effect curing of said resin binder.

6. A method including the steps defined in 'claim 5, in which method a plura'lity of base members is provided and each member is su'bjected to the stated treatments and the elements are baked for a period 'of approximately 15 minutes at 300 C., then classifying said elements according to total resistance, removing those elements which fall below a standard resistance value range and rebaking the removed elements for a period of time and at a temperature suflcient to cause shifting of the total resistance for such elements up into the standard resistance -value range.

7. A resistance element for variable resistors, comprising: an elongate flattened tubular base of nonconduetive ceramic material, said base having a pair of end portions and a mid portion; ad'nerent films of noble metal each disposed upon and encircling a respective end portion of References Cited in the file of this patent UNITED STATES PATENTS 2,0S3,88l Treacy Sept. S, 1936 2,330,7S2 Mo-relock Sept. 28, 1943 2,473,183 Watson June 14, 1949 2,481,589- Hansell Sept. 13, 1949 2,614,944 Lytle Oct. 21, 1952 2,692,321 Hicks Oct. 19, 1954 2,781,277 Dwyer Feb. 12, 1957 2,7911680 Peck June 11, 1957 2,803,566 Iohannsen Aug. 20, 1.957 2,838,639 Planer et al Inne 10, 1958

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