US2588994A - Precision variable resistor device - Google Patents

Description

March 11, 1952 B. B. SCOTT PRECISION VARIABLE RESISTOR DEVICE Filed Feb. 25, 1950 Fig.1.

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Patented Mar. 11, 1952 PRECISION VARIABLE RESISTOR DEVICE I Benjamin B. Scott, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application February 25, 1950, Serial No. 146,269

3 Claims. (Cl. 201-48) This invention relates to precision variable resistor devices and has for its object the provision of a simple and reliable resistor having a predetermined resistance characteristic. -By the term variable resistor device, I mean to include precision potentiometers.

A further object is to provide a precision variable resistor device having reduced resolution and loading errors.

A still further object is to provide a precision variable resistor device of increased gradient accuracy, load capacity and heat dissipation.

In carrying out my invention in one form, I providea plurality of selected separate resistors having different resistance characteristics which resistors are electrically interconnected to form parallel circuits together with a plurality of spaced gang-operated movable contact means, each contact means engaging a particular one of the separate resistors, arranged for simultaneous translatory movement along the resistors.

For a complete understanding of my invention, reference should be had to the following specification and the accompanying drawing in which Fig. 1 is a plan view of a precision resistor device illustrative of my invention and Fig. 2 is a chart showing typical resistance characteristics of the separate resistors. I

One ftype of conventional precision'resistor is made by winding a wire of uniform cross section and selected resistance value about a base, as for example, a card of insulation material, spacing the" turns of wire to provide a desired gradient accura'y and providing a brush arranged to engage successive turns of the wound resistor in moved longitudinally along a single wire thereby providing an infinite number of voltage steps and a relatively smooth resistance curve. The resolution error varies inversely, therefore, with the number of turns of wire engaged by the brush for a given movement thereof.

By resistance gradient characteristic I mean the change in resistance between one end of a resistor and a brush contact means moved therealong in relation to the movement of the brush contact means. The gradient resistance characteristic is dependent, among other factors, on the spacing of adjacent turns of wire, the accuracy of uniformity of the wire and the dimensional accuracy of the card on which the wire is wound. By resistance gradient error, I mean the discrepancy in resistance change for a given movement of the brush from a desired ratio of brush movement to resistance change.

Load capacity and heat dissipation are dependent upon the size of wire, the area of wire and card exposed to dissipate heat by radiation and conduction and the temperature differential with respect to ambient temperature.

By loading error, I mean the change in resistance characteristic of the resistor due to load changes thereon. Those skilled in the art will understood that the effect of loading error can be minimized by providing in cascadeoperation a ratio of at least 10-1 between the resistor device winding current and load current. In cascade operation of precision resistors, as for example precision potentiometers, this means that the load resistance must be ten times the resistor resistance. However, as the load resistance is decreased depending upon the application, a point is reached Where the resistance ratio of 10-1 cannot be maintained because to do so results in a diameter of wire on the resistor so large as to introduce undesirably high resolution error. Therefore, application of the conventional wire wound resistor is limited because of loading error and resolution error. 7 Y

Further, in attempting to increase accuracy of a conventional wire wound resistor, it is found that limitations of accuracy are imposed by the accuracy of materials available. That is, commercial wire is uniform in cross section only within certain percentage limits. The card or base on which the wire is wound can be made to dimensions accurate only to certain percentage limits depending upon the material used and upon the skill of the artisan. Also, the spacing of the wire on the card is limited to certain percentage errors depending upon a variety of features. The limitations above described being beyond the control of the manufacturer, in practice a trial and error method of manufacture is required to produce a precision resistor of desired resistance gradient accuracy and considerable cost is thereby entailed.

Therefore, to reduce the cost of manufacturing a precision resistor device and to avoid the inherent limitations described above with respect 3 to conventional wire wound precision devices, I provide a precision resistor device of a multiple unit type of which the precision potentiometer of Fig. l of the drawing is illustrative.

The potentiometer shown in Fig. 1 comprises three conventional Wire wound selected card type variable resistor units numbered 2-4 each comprising a coil of wire 5 of uniform cross section wound on a card or base 6 of insulation material. To obtain the selected units 2 i a plurality of units (not shown) are wound to produce in each unit the maximum gradient accuracy, with respect to a desired. resistance gradient curve, attainable in view of the uncontrollable factors above recited. In the plurality of units thus wound, it is probable that few, if, any, have resistance gradient of satisfactory accuracy with respect to the desired resistance gradient curve. In accordance with my invention the resistor units 24 are selected or matchedwith opposite or compensating resistance radient errors and electrically connected in parallel, as will be hereinafter described to provide a potentiometer, as shown in Fig. 1, having a resistance gradient characteristic approaching .a predetermined characteristic. For example, in Fig. 2 I have shown curves with percentage resistance gradient error plotted as ordinates against brush position with respect. to .one common terminal,,such as the terminal. 22;. In Fig. 2- the zero. line indicates zero gradient error, that is, complete conformity of resistor unit to adesired resistance characteristic. The curve 2a represents the resistance gradient error of the resistor unit 2, While the curves 3a andAa n represent the resistance gradient error of selected units a and 4 respectively. Considering curves'za; 3a and 4a. it is. seen that the, positive and negative errors when combined provide the resistance gradient characteristic curve 8 which shows marked improvementwith respect to each of .the individual curves 2a,, 3a and 4a, i. e. conforms quite closely to the zeroline.

To combine the selected units 2-& to provide a precision resistor device having a. gradient error corresponding to the improved curve 8, the selected units 24 are connected in parallel electric circuit as shown in Fig. 1. That is, associated ends 9-H of respective units 2-4 are respectively provided with terminals 12-44. The opposite ends [5-11 of the respective units 2-4 are provided respectively with terminals Iii-20. Terminals l2l4 are electrically 'connected through a common electrical conductor 2| to a common terminal 22- to provide for electrical connection of the potentiometer l to one side-of a suitable electrical source (not shown). Terminals Iii-20 are electrically connected through a common electric conductor 23 -to a common terminal 24 by whichthe potentiometer I may be'connected to the other side of the electrical source. v

A gang-operated contact means shown as comprising three brush contacts 26--28 mounted on a common electric conducting bracket 29 in electrically conducting relation therewith, is ar ranged for translatory movement on a suitable guide 30 along the units 24 and simultaneous brushing of the wires 5 of the units 24 by the respective brush contacts 28-43. The bracket 29 is electrically connected through a flexible conductor 3| to a common terminal 32. The brush contacts 25'-28 being connected to the common terminal SI and brushing the units2-4 simultaneously, the percentage resistance gradient error for translatory movement for the brush means corresponds to the improved curve 3, the individual resistance errors largely compensating each other by reason of the parallel connection of the resistors. Therefore, by selectively grouping precision units having compensating errors a precision device having an im-' unit resistor devices because of gradient inaccuracy and therefore considered wastage units, are reclaimed. by selective grouping for compensating errors to form the multiple'unittype precision resistor device of my invention.

Other advantages in addition to improved gradient accuracy and lower cost are provided by my invention. That is, the fundamental. limitations of the conventional precision resistor device relating to the obtaining of low end-toend resistance values, reduced resolution error, reduced loading error and increased wattrating are reduced in my multiple unit precision resistor device. For example, in a conventional precision potentiometer to obtain a lower endto-end resistance value it is necessary to increase the wire size and reduce the. number of turns'of wire. This process can be continued only to a point at which the resolution error becomes undesirable. Thereafter, further .reduction in end-to-end resistance is impractical. However, in my precision resistor device the end-to-end resistance value is reduced to any value depending upon the number of units connected in parallel without changing the wire size or the number of turns. .For example, consider unit 2 as a conventional single unit precision. resistor device having an end-to-end re-. sistance of 30,000 ohms. To decrease the. end-. to-end resistance value of unit 2 by conventional means the wire size must beincreased-with a consequent decrease in the number of turns for the same length. However, if the resolution error is on the border line of satisfaction it would not bepractical to attempt to reduce the end-to-end resistance by decreasing the-number of turns of wireand thereby introducing; additional resolution error. Therefore, a limitation is placed-on the end-to-end resistance value of unit 2 which unit has been assumed for themoment to represent a conventional precision .resistor device.

On the other hand, in accordance with my invention, instead. of increasing the wire size and decreasing-the number .of turns of wire to obtain a lower end-to-end resistance value and thereby unavoidably increasing the resolution error, a lower end-to-end resistance value is .01?-

tained by adding additional units 3 and 4 each number of winding turns of each unit and thereby avoiding the introduction of additional resolution error.

The reduction of end-to-end resistance value made possible by my invention also introduces an improvement in reduced loading error. For example, in electrical computing networks, particularly networks of the cascaded type, load errors are produced for which compensation cannot readily be made when conventional prevision potentiometers are employed. That is, as previously explained, a ratio of at least -1 between load resistance and potentiometer resistance must be maintained to reduce the loading error to a satisfactory value. However, where the load resistance is sufliciently small, a point is reached at which the end-to-end resistance value of conventional precision potentiometer cannot be re duced sufiiciently to maintain the 10-1 ratio desired because such reduction would necessitate a reduction in the number of turns and an increase in wire size introducing an undesirable resolution error. Therefore, loading errors become unavoidable. However, since the end-to-end resistance of my precision resistor device, as for example the potentiometer shown in Fig. 1 may be controlled without introducing additional resolution error no substantial difficulty is experienced in maintaining the desired 10-1 ratio of load resistance to potentiometer resistance and out changing the wire size and the number of turns on each of the units.

Heat dissipation has been found to be chiefly a function of card area and temperature differential to ambient. Accordingly, heat dissipation is considerably improved by the increased radiation surface presented by the several cards.

In Fig. 1 of the drawing, linear resistance units are shown to illustrate my invention. However, it should be noted that a plurality of selected non-linear resistance units formed by varying the spacing of the turns of wire 5 or the shape of the base 6, may be connected in parallel circuit to produce a precision variable resistor device of predetermined desired non-linear resistance characteristic.

It should be noted further that in the several resistor units from which selection of units 2-4 is made, the end-to-end resistance values may vary between individual units because of the unavoidable manufacturing limitations previously described. Therefore, in selecting units 2-4 desired end-to-end resistance value is made a part of the predetermined desired resistance characteristic of the precision resistor device.

Further, in Fig. 1 of the drawing, the brush contact means 2628 are shown positioned substantially equidistant from the ends 9-H of respective units 2--4, and arranged for simultaneous movement along the units 2--4 maintaining substantially equal the percent of each resistance unit introduced in the circuit between the terminals 22 and 3|. However, in certain cases it is desirable to shift the resistance error curve of one selected unit to the right or left to reduce the percentage resistance error at a particular portion of the variable resistor device at the expense of increased error at another portion. For example, in precision resistors, the percentage resistance error allowable at the center of the precision resistor device is generally less than the percentage error allowable at the ends of the device.

Therefore, to obtain a minimum percentage resistance error over the selected portion of the precision resistor device, the percentage resistance error curves, as, for example, curves 2a, 3a and 4a of Fig. 2, are shifted one with respect to the other until the desired shape of resulting curve 8 is obtained. To shift the curve 2a, for,

example, to the left, the brush contact 26 is advanced to the right with respect to the brush contacts 2'! and 28 or the resistor unit 2 is moved to the left with respect to units 27 and 28. Thus, in certain cases as described above, the distances between brushes 26-28 and respective resistor unit ends 9-H are not maintained equal in the simultaneous movement of the brush contacts 25-48.

Thus far, for the purpose of illustrating my invention, I have shown and described the combination of three resistor units, namely 2-4, wound each to produce the same resistance characteristics and selected for combination on the basis of compensating resistance errors. However, it should be noted that the same advantages of increased accuracy at lower cost is obtained by combining units wound for diiferent resistance characteristics to give a desired characteristic and selected for compensating errors with respect to each other in combination.

While I have shown only a certain preferred embodiment of my invention by way of illustration, many modifications will occur to those skilled in the art, and I, therefore, wish to have it understood that I intend, in the appended claims, to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A precision variable resistance device com prising a plurality of variable resistor units, electric contact means for each of said resistor units, means mounting said contact means for engagement each with one or more turns of its said resistor unit and for movement together over said resistor units to vary the amount of resistance included between one end of each of said resistor units and its said contact means, electric connection for connecting said ends of said resistor units together, and electric connection for connecting said contact means together, said resistor units having different mutually compensating resistance gradient errors with respect to a predetermined resistance gradient such that when said contact means are moved together over said resistor units the resulting resistance gradient for parallel connection of said resistor units is nearer to said predetermined gradient than the resistance gradient of each of said resistor units.

2. A method of producing a precision variable resistance device having a low resistance gradient error comprising the steps of winding a plurality of similar resistors each having a total resistance equal to the product of the total resistance desired in said resistance device and a predetermined number, matching a number of said resistors equal to said predetermined number for compensation of the resistance gradient errors of each by opposite incremental resistance gradient errors of the others, connecting said matched resistors in parallel and arranging said resistors for electrical contact of each by a single slidable contact device.

3. A method for producing a variable 10W resistance devicahav'ingsmallresistance gradient errors comprising the steps of fabricating a pluraIity" of resistors to ordinary tolerancesffeach having a resistance equal to the product of the total resistance desired in saidresist'ance "device and a predetermined number, selecting a number of saidresistors equal to said predetermined number" for approximate matching and mutual compensation'of resistance gradient errors. af-

ran'gin'g said resistors in parallel alignment, es'-" tablishing parallel electrical connections for'said resistors by means of a common sliding contact device'having' separate Contact elements making of the other" resistors by ax'ial "'adjustiriehtsbe' wi tween each sliding contact elementand itjs'as'sM",

REFERENCES CITED' The following references are of record in the file of this patent: H h V UNITED STATES PATENTS Number 7 Name Date? 1,504,483 Klopsteg'lnn Aug. 12,1924

V FOREIGN PATENTS 15 Number Country Date 495,799 Great Britain No'v.'21," 1938

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