US2347796A - Electrical resistor - Google Patents

Description

y 2, 1944- 1.. PODOLSKY 2,347,796

ELECTRICAL RESISTOR Filed Feb. 19, 1943 BYLeon pod'oh/'y 9 25 AT ORNEYS Patented May 2, 1944 ELECTRICAL RESISTOR Leon Podolsky,

Pittsfield, Mass.,

assignor to Sprague Products Company, North Adams, Mass., a corporation of Massachusetts Application February 19, 1943, Serial No. 476,383

9 Claims.

My present invention relates generally to electrical resistors, and has particular reference to resistors of very high ohmic Values, i. e., in the megohm range.

There is a wide-spread current demand for resistors of this general type, for use in various electrical and radio communication systems, signalling apparatus, and the like. One of the difficulties heretofore encountered in meeting this demand has arisen from the particular requirement that these resistors shall operate successfully for prolonged periods under impressed voltages in the multi-kilovolt range, and at high operating temperatures. By the term operating temperature I mean to refer to the actual temperature to which the resistor is subjected during use, part of which is the ambient or air temperature surrounding the equipment, and part of which is the temperature developed within the resistor itself by virtue of its operation. By high operating temperatures I refer generally to temperatures in excess of approximately 80 C.

The probl m is complicated by space limitations. To comply with desired requirements, a resistor must not only have high ohmic value and be capable of functioning at high voltages and high operating temperatures, but it must be confined to relatively small and practical dimensions,

Generally speaking, attempts to achieve the higher ohmic values have heretofore involved the use of resistance wire of high resistivity, wound in various ways into many thousands of turns.

Quite apart from the fact that the current supply of resistance wire, and of the necessary dies,

drawing equipment, and winding machine capacity are extremely limited, the use of resistance wire for achieving the desired result is relatively expensive, ineillcient, and beset with difficulties. For example, to produce even a megohm resistor, using .0015" diameter resistance wire, approximately 16,000 feet of wire are required, and the winding of this wire into the many thousands of turns which are necessary is a costly time-consuming procedure. Moreover. in order to prevent short-circuiting, it is nccessary to insulate the wire, and thi becomes a difficult problem when the insulation is to withstand operating temperatures exceeding even the relatively low temperature of 70 C. Where sufficiently high ohmic values are not achieved, be" cause of space limitations, the problem is further complicated if the resistor is to be operated on high voltages of the order of 5 to 30 kilovolts, be-

cause under such circumstances very high in ternal temperatures are developed by the relatively great amount of power which is dissipated in a confined space. For example, notwithstanding a recent demand for a resistor of 25 megohms to operate on a 15 kilovolt circuit, the maximum resistance which could be placed in the allowable space, employing the best available wire-wound resistor technique, was only 8 megohms and as a result the wattage dissipation was so great that the generated operating temperature was too high for nearby equipment to withstand, not to mention its destructive effect upon the resistor itself.

In an effort to achieve the desired results by other means, recourse has been had to composition materials consisting generally of an inert filler, a finely divided conductive substance such as carbon, and a thermoor chemo-setting binder. However, though molded resistor bodies of this general type have been widely known and used at relatively low temperatures and voltages, they have never been successfully adapted for operation at high temperatures or high voltages. This inability of composition resistors to operate successfully under high-voltage and high-temperature conditions has not been due merely to the well-known susceptibility of molded bodies of this type to the effects of absorbed moisture, nor to the general failure of protective impregnating or coating media, such as organic waxes and varnishes, to withstand temperatures above approximately 0.

Extensive investigations I have made in connection with carbon-compound resistor bodies leads me to the belief that these bodies have heretofore failed because certain basic necessary relationships between voltage gradient (i. e., potential drop per unit length), current density (1. e., amount of current carried per unit of cross-sectional area), and maximum operating temperature have not been properly observed. For example, because of space limitations, the resistors have not been made long enough; and because of the desire to achieve high resistance values in a small space, the resistors have not been made large enough in cross section.

So far as carbon-compound resistors are con cerned, I have found that satisfactory performance, stability, and durability can be achieved only if the resistor body has a predetermined minimum length and a predetermined minimum cross-sectional area, depending upon a predeterminable relation between the voltage impressed, the current to be carried, and the maximum opcrating temperature to which the resistor is to be subjected.

These findings have forced me to the conclusion that compactness in physical dimensions cannot be successfully achieved by the means heretofore resorted to whereby the length of the resistor and its cross-sectional area have been reduced to the maximum possible degree, without resulting in complete failure of the resistor as an electrical instrument of reliable and durable character. Realizing therefore, that the resistor must have a certain minimum cross-sectional area and a certain minimum length to enable it to perform satisfactorily at all, I have devised an improved structure whereby a moldedcomposition resistor may be produced within the confines of entirely practical dimensions and may nevertheless embody an adequate effective length and an adequate cross-sectional area.

It may therefore be stated to be the primary object of my invention to produce a resistor of the molded-composition type which is compact in size, simple and inexpensive to manufacture, and which is capable not only oi": embodying a resistance value in the multi-megohm range, but also of performing satisfactorily at high voltages and high operating temperatures.

Briefly stated, I achieve this general objective by the employment of a plurality of resistance elements arranged in the form of a stack, each element defining a conductive path transverse with respect to the longitudinal axis of the stack, these elements being electrically connected in series, and the structure being of such a character that the current travel is restricted to the transverse paths. By this arrangement, I am enabled to impart an appreciable cross-sectional area to each resistance element, and to impart to the resistor as a whole an appreciable and adequate effective length far in excess of the longitudinal axis of the stack and of the physical length of the unit itself.

The present improved resistor is therefore essentially a composite device, made up of a series of separate elements. These elements may be flat or substantially so or may assume any other convenient form enabling them to be arranged in closely nested or stacked relationship. Each of the elements has spaced edge portions serving as contact portions and defining between them a conductive path which is substantially transverse with respect to the longitudinal axis of the stack. In each case, the cross-sectional area of this path is uniform throughout its extent. This cross-sectional area is so chosen, with respect to the current to be carried, and the operating temperature to be encountered, that the current density does not exceed the maximum amount allowable under such conditions. Sinailarly, the number of elements employed is so chosen, with respect to the length of the current path through each element, the voltage to be impressed and the operating temperature to be encountered, that the overall effective length is adequate to keep the voltage gradient below the maximum amount allowable under such conditions.

In a preferred embodiment of the invention, the individual resistance elements are substantially u shaped, and the ends of U are employed for interconnecting adjacent elements for the purpose of establishing them in. electrical series.

Regardless of the particular shape which the component resistance elements may assume, the

resultant stack is suitably maintained in an assembled relationship to form a composite resistance unit, and this unit is preferably enclosed within a hermetically sealed container.

I achieve the foregoing objects, and such other objects as may hereinafter appear or be pointed out, in the manner illustratively exemplified in the accompanying drawing in which:

Figure 1 is a longitudinal view of a resistor constructed in accordance with the present invention, the enclosure and the supporting elements being shown in cross section;

Figure 2 is a transverse cross-sectional view taken substantially along the line 2-4 of Figure 1;

Figure 3 is a perspective view of one of the resistance elements shown by itself;

Figure 4 is a perspective view of a number of resistance elements and intermediate insulating discs showing their relationship to one another in the structure of Figure 1;

Figure 5 is a view similar to Figure 4 showing another relationship which may be optionally employed;

Figure 6 is a perspective view of one of the insulating discs by itself;

Figure 7 is a perspective view of a modified type of resistance element;

Figure 8 is a fragmentary view of a composite resistor unit employing the elements of Figure 6; and

Figure 9 is a cross-sectional view taken substantially along the line 9-9 of Figure 7.

I have shown in Figure 3 a preferred type of resistance element or segment. It consists of a ring-shaped body l0 provided with a gap II. This provides spaced edge portions II which define between them (i. e., through the body of the element 10) a conductive path of circuitous character which is transverse to the longitudinal axis of the stack which results when a number of elements are arranged side by side as indicated in Figures 1, 4, and 5.

Along the conductive path, the cross-sectional area of the element II is uniform throughout its extent, and in the preferred construction illustrated, this cross-section is substantially rectangular in character.

This general shape is referred to herein and in the appended claims, for the sake of convenience and simplicity, as a U -shape and it will be understood that this term includes within its scope not only an element (as shown) in which the inner and outer peripheries are truly circular, but also any element in which the body is curved or configured to bring two "U-ends" (such as the ends I!) into relative proximity.

The element fl is preferably of molded character, composed of any suitable composition material having a high resistivity and containing finely divided conductive particles such as carbon. Any of the well-known compositions may be employed for the purpose, consisting, in essence, of some inert filler, the conductive particles themselves, and a suitable binder.

Merely by way of example, and to illustrate the true dimensional nature of the present structure. I may point out that an element constructed as shown in Figure 3 may have an external diameter of approximately of an inch, and a thickness between the parallel side faces of approximately A; inch, the gap II in such a case being approximately inch across.

To enhance the utility of the U-ends I 2 as contact portions, these ends are preferably metalized by subjecting them, in well-known manner, to a spray of finely divided metal particles. This nietalization is intended to be indicated, in the accompanying drawing, by stippled shading,

In constructing a resistor of the present character, employing resistance elements of the type shown in Figure 3', the elements are arranged in the form of a stack, with a relatively thin insulating disc interposed between adjacent elements. One such disc is indicated by the reference numeral l3 in Figure 6.

These discs may be composed of mica or equivalent material, and each one is of such size and shape that it effectively prevents electric current from passing through the body of one resistance element to the body of the next adjacent element in a longitudinal direction with respect to the stack. Preferably, the insulating discs [3 extend laterally beyond the resistance elements III to make sure that the undesired short-circuiting of the resistance elements will not take place.

The spacing of the outer edges of the insulating discs l3 an appreciable distance outward from the peripheries of the adjacent elements 10, not only prevents direct short-circuiting therebetween, especially at the unconnected contact portions l2, but such relative sizes of the insulating discs and the elements III also prevents arcing between the elements. The insulating disc between adjacent U-shaped or arcuate resistance elements preferably extends circumferentially from one side of the direct connection ii to the opposite side thereof, particularly between the contact portions I2 that are not connected directly together.

The elements or segments to are so arranged with respect to one another, that each-contact portion l2 will lie closely adjacent to one of the contact portionsoi' the next resistance element. One such arrangement is shown in Figure 4, in which the gaps II are substantially aligned. To permit the desired series connection to be establlshed, each of the insulating discs [3 is provided with a cut-out portion or gap l4, as shown in Figure 6, and during the process of assembling the stack, the cut-outs I are so arranged that a direct electrical connection may be established, by means of solder or otherwise, in the particular regions where the series interconnection of the resistance elements is to be established. These masses of solder or equivalent electrical bonding means are indicated in Figures 1 and 2 by the reference numerals I5. They are deliberately omitted from Figures 4 and 5, for the sake of clearness of illustration.

In the assembly of pieces as shown in Figure 1,

- the parts are arranged as shown most clearly in is thus constrained to travel through the resistance element We in a clockwise direction (as viewed in Figure 4), then in a counter-clockwise direction through the resistance element Ilib, then in a clockwise direction again through the resistance element 100 and so on. Each element thus defines a conductive path which is transverse with respect to the longitudinal axis of the stack, and the insulating discs restrict the current travel to these transverse paths, the resultant path having an effective length which is considerably greater than the physical length of the resistor unit as a whole.

Another way of arranging the elements is shown in Figure 5. In this case, the gaps H of the resistance elements are arranged along the path of a helix. During the assembly of the stack, the contact portion 12c is soldered or electrically bonded to the contact portion [2 the contact portion l2g is soldered or electrically connected to the contact portion l2h, and so on; so that the current travels in a clockwise direction (as shown in Figure 5) through each successive resistance element III.

A convenient way of assembling the parts and holding them in stacked relationship consists in mounting the elements on a non-conducting core which I have illustratively shown in the form of a ceramic tube I6 extending longitudinally through the aligned center openings of the various elements and discs, the size of this core being so chosen that it projects slightly from the opposite ends of the resultant stack. Suitable clamps I! are mounted on the core l6 to hold the parts together, and electrical connection pieces l8 are mounted in association with the resultant unit in any suitable manner. This assembly is preferably encased within an enclosure l9 whose ends 20 are hermetically sealed around the elements i 8. Insulating spacers 2| may be employed. to advantage at the opposite ends of the enclosed unit to hold it in proper spaced relationship from the walls of the enclosure l9.

While I have illustratively shown the enclosure IS in the form of a single tubular piece of glass or the like, it will be understood that any other structure may be employed for this purpose, either with or without special end caps, and that the hermetical seal may be accomplished in any convenient or desired manner. Similarly, the space within the enclosure and around the resistor unit itself may bepartially evacuated, if desired, or may be partially or completely filled with a suitable inert spacing material. The illustrated mode of enclosure of the resistor unit within a hermetically sealed container is offered merely by way of example.

In Figures 7, 8, and 9, I have shown the possibility of achieving the advantages of the present invention by means of resistance elements which are not U -shaped. In Figure 'l I have illustratively shown a plate-like resistor element 22 of fiat substantially rectangular or square shape, the opposite edges 23 being metalized to serve as contact I portions. The path of current travel between the edge portions 23 is transverse with respect to the stack which results when a number of elements 22 are arranged side by side as shown in Figure 8, and this path of current travel is of uniform cross section throughout its extent.

In constructing a resistor unit of elements such as those shown at 22 in Figure 7, it is preferable that these elements be so arranged that all the metaliaed contact portions face in the same directions, as shown in Figures 8 and 9. Intermediate insulating discs 24 are arranged between the resistance elements, and these discs, in this case, are preferably of rectangular or square shape, extending laterally beyond the elements 22 to prevent short-circuiting, the discs being provided with cut-outs or being so arranged, that the desired electrical bonding may be eifected to establish an electrical series interconnection between the' resistance elements. The reference numerals 25 are intended to indicate masses of solder or equivalent electric bonding material whereby the resistance elements are thus established in an electrical series, The current travels through one element in one transverse direction, then through the adjacent element in the opposite direction, and so on, and the total effective length of the re sistor unit is thus much longer than. the physical length of the resultant assembly.

The stack of pieces assembled as indicated in. Figures 8 and 9 may be maintained together as a unit by any suitable clamping means (not shown) and the composite resistor unit is preferably sup ported and accommodated within a suitable hermetically-sealed enclosure, means being provided for establishing electrical connection, from the exterior, with the opposite ends of the resistor unit.

It will thus be seen that my invention affords a convenient and simple means for creating a resistance path of high ohmic value within a relatively confined space, while the cross-sectional area of the path is nevertheless of appreciable size so that a maximum current density will not be exceeded.

Where the resistance elements are of a carbonaceous composition, the parts are preferably so designed, with respect.to the voltage that is to be impressed, the current that is to be carried, and the operating temperature that is to be endured, that the voltage gradient and current density will not exceed the maximum values given to an ade quately approximate degree by the following formulas:

where V is the voltage gradient in volts per inch, D is the current density in mllli--amperes per square inch, and T is the operating temperature in degrees centigrade. Extensive research has shown that high-resistance carbon-composition resistors in which these maximum values are exceeded will not give satisfactory performance and will fail prematurely when operating at high voltages (i. e., in the multi-kilovolt range).

By way of example, it may be stated that an employment or the present structural arrangement of parts has enabled the successful construction of a resistor of multi-megohm rating which operates satisfactorily and reliably for long periods of time under impressed voltages up to 50 kilovolts and at operating temperatures up to 180" C., the unit as a whole being no more than approximately 10 inches in length and 1 inch in diameter, a size which is well within the confines of normally allowable space requirements.

In general, it will be understood that the details herein described and illustrated may be modified by those skilled in the art without departing from the spirit and scope of the invention as expressed in the appended claims. It is therefore intended that these details be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, and il lustrated its use, what I claim as new and desire to secure by Letters Patent is:

1. In a resistor of the character described, a

edge portions serving as ontact g defining lie-itween them a ccmiurtu verso with respect to the lmr'itm mi ash; or the stack, means intereonnecw tions for electricall calabl hing aid elements in eries, and insulating moans. twiwccn lh claimants to i'cclrici. the current travel to new tramway-so paths, said insulating means w m i ua a diet of insulating material. between mch atliaceni, pan."

lig said contact nor mamas of resistance elements, said disc extending latorally beyond said elements throughout the major portion of the periphery of said elements.

2. In a resistor oi the character described, a plurality of flat resistance elements arranged in the form of a stack, each element having spaced edge portions serving as contact portions and defining between them a conductive path transverse with respect to the longitudinal axis of the stack, means forming a direct connection between said contact portions for electrically establishing said elements directly in series, and insulating means between the elements to restrict the current travel to said transverse paths, said insulating means comprising a disc of insulating material sandwiched between each adjacent pair of resistance elements, said disc extending laterally beyond said elements throughout the major portion of the periphery thereof except in the region where the contact portions are electrically connected together.

3. In a resistor of the character described, a plurality of resistance elements arranged in the form of a stack, each element being substantially U-shaped, the ends of the U serving as contact portions and defining between them a substantially U-shaped conductive path transverse with respect to the longitudinal axis 01 the stack, means interconnecting said contact portions for electrically establishing said elements in series, and insulating means between the elements to restrict the current travel to said U-shaped paths, said insulating means comprising a disc of insulating material sandwiched between each adjacent pair oi resistance elements, said disc being of appreciably greater area than the elements with the periphery thereof spaced laterally outward from the peripheries of the elements throughout the non-connected portions of said elements.

4. A resistor of the character described, comprising a plurality of individual split rings of resistance material arranged in axial alignment, the ends of each ring being spaced apart and serving as contact portions, the contact portion of one ring being arranged adjacent a contact portion of the adjacent ring axially of the resistor, means forming a direct electrical connection between said adjacent contact portions of adjacent rings, an insulating member between each pair of adjacent rings insulating said rings from each other throughout the portions thereof spaced from the connecting means, said insulating member having the peripheral edge thereof spaced appreciably outwardly from the peripheral edges of the adjacent rings throughout the circumference of said rings from one side of said connecting means substantially to the opposite side thereof.

5. In a resistor of the character described, the combination with the elements set forth in claim l, of means for retaining said elements in stacked relationsl'iip andv enclosing the resultant unit within a hcrnictically-sealcd enclosure.

6. in a resistor of the character described, the combination with the elements set forth in claim l, of a b.eat-resistant non-conductive core upon which said elements are mounted, means for rctalning said elemruts in stitched relationship upon said, core, and a hermetically sealed enclosure lorthe resultant unit.

7. in a resistor oi? the characlcr described, the combination of elements set .[orlh in claim i. said elements being ol carbon ronmotiiion, the dimensions of each element being such. that the current density does not exceed the approximate value given by the formula where V is the voltage gradient in volts per inch, and T is the operating temperature'in degrees centigrade.

9. In a resistor 01 the character described, the combination of elements set forth in claim 1, said elements being of carbon composition, the dimensions or each element and the total number of series-connected elements being such that the v current density and the voltage gradient do not exceed the approximate values given by the formulae where D is the current density in milli-amperes per square inch, V is the voltage gradient in volts per inch, and T is the operating temperature in degrees centigrade.

LEON PODOLSKY.

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