US2962685A

US2962685A - High voltage resistor

Info

Publication number: US2962685A
Application number: US787602A
Authority: US
Inventors: Robert C Langford
Original assignee: Daystrom Inc
Current assignee: Daystrom Inc
Priority date: 1959-01-19
Filing date: 1959-01-19
Publication date: 1960-11-29
Anticipated expiration: 1977-11-29

Description

K kovsnLoAa FoLLolr/Na /vo/iwAL LOAD R. c. LANGFORD HIGH VOLTAGE RESISTOR Filed Jan. 19, 1959 YI/I/l/I Il /I/ Nov. 29, 1960 0 Y mm E Fm N E a uw c. u r L R u. n M m w x w 1 n x l l 1 l Tlv l i I l l .i I2 6 r/ r m m m l I P, Nkbkkmnwk u n.

United States Patent 'O mon VOLTAGE Rasisron Robert C. Langford, West Orange, NJ., assignor to Daystrom, Incorporated, Murray Hill, NJ., a corporation of New Jersey Filed Jan. 19, 1959, Ser. No. 787,602

8 Claims. (Cl. SSS-260) This invention relates to a resistor and more particularly to a high voltage resistor which is capable of withstanding severe thermal shock, without subsequent failure, which has a low `operating temperature under over- 'load conditions, and which has a uniform voltage gradient distribution along the resistor.

The high voltage resistor contemplated by my invention includes a plurality of resistor elements connected together in series circuit arrangement and embodied in a unitary structure including, generally, a tubular body member of insulating material within which the resistance elements are suitably mounted, and metallic end caps closing the ends of the tubular body member and providing electrical terminals for the resistor. It has been found, however, that some separation invariably takes place between the insulating tube and the metallic end caps when the resistor is subjected to thermal shock. In prior art high voltage resistor construction, it is then possible for moisture to enter the tube with deleteriousv effects on performance, accuracy and life. In the high voltage resistor construction of my invention, the resistor tube having plastic end caps thereon. The metallic end caps are positioned lover the said plastic end caps. The plastic mounting strip, tube and end caps are all made of the same plastic material h-aving the same temperature coellicient of expansion. Therefore, even if the metallic end caps separate from the plastic tube, the tube remains sealed by the plastic end caps, and moisture cannot enter therein. The mounting of the resistance elements on a plastic mounting strip, and the encapulation thereof in a plastic tube with sealed plastic end caps, in which all of the above components are of the same plastic material, comprises an important aspect of my invention.

During overload current conditions, the excessive temperature rise by heat generated in high voltage resistors of prior art constructions either results incomplete failure of the resistor, 'or results in excessive change in resistance of the resistor due to the temperature coefficient, for elective failure thereof. In the resistor of my invention, the

being asphalt, for example.

solid. As the temperature rises under'overload condi,- tions, the excess heat developed is absorbed in the heat of fusion of the lilling material as the material turns from a solid to a liquid. The temperature inside the tube Will therefore remain at the melting point of the iilling material until the whole of such material is melted. This provides a much lower operating temperature under overload conditions which, in turn, with a usual temperature coefficient of resistance, results in a smaller change in the resistance thereof. The filling of the tubular housing, within which the resistor elements are mounted, with an insulating material which is solid under normalele' vated ambient working conditions, and which melts under overload conditions, comprises an important feature of my invention.

Another important feature of my invention involves the positioning of the resistor elements on the insulated mounting strip. The elongated resistance '-elements are laid out in a parallel extending, uniformally spaced, arrangement whereby the stray capacitance of each resistance element to ground is identical. Further, the metallic end caps closing the ends of the tubular body member are positioned as close together as'possible to make the shunt capacitance therebetween large, and the series capacitance between each resistor element small. Preferably, the shunt capacitance is about ten (l0) times as great as the said series capacitance. This results in a more nearly uniform voltage gradient distribution along the resistor, which is of considerable importance in high voltage resistor construction, used under alternating voltage conditions.

An object of this invention is the provision of a high voltage tubular resistor which is able to withstand severe thermal shock and to withstand overload conditions following a full load run with a minimum of deleterious elects.

An `object of this invention is the provision of a high voltage resistor including a plurality of resistance elements connected in series circuit arrangement and each having substantially identical stray capacitance from the ends thereof to ground.

An-olbject of this invention is the provision of a hig voltage resistor construction which includes a plurality `of resistor elements mounted on a strip of plastic insulatelements are mounted on a plastic strip within a plastic ing material, a tubular member of the same insulating material within which the mounting strip is mounted, end

caps of the same plastic material closing the ends of the tube, and metallic lend caps mounted over the plastic ends and electrically connected to the resistor elements.

An object of this invention is the provision of a high voltage resistor construction comprising a container, a resistance element embedded in an insulating material. which is solid under normal ambient working tempera -ture Within the said container, the said insulating matesistance elements positioned in an evenly spaced, parallel, l, position within a container of insulating material whereby the stray capacitance from such resistance element end to ground is substantially identical.

These and other objects and advantages will become` apparent from the following description when taken with It will be understood that the accompanying drawings. the drawings are for purposes of illustration and are not intended to be construed as defining the scope or limits of the invention, reference being had for the latter purpose to the appended claims.

In the drawings, wherein like reference characters l refer to like parts in the several views:

" for clarity;

Figure 2 is a cross sectional view of the resistor taken on line 2-2 of Figure l;

Figure 3 is a cross sectional view of the along the line 3-3 of Figure 1;

Figure 4 is ga fragmentary view of one end of. the'- resistor showing the plastic which forms'the end cap in' position in the tube prior to turning the metalliczendzl.;

cap onto the tube end; f Figure 5 is a fragmentary view which' is resistor 'taken' similar to 3 Figure 4 only showing the metallic end cap turned onto the tube;

Figure 6 is a chart of temperature versus time for resistor operation under normal and overload conditions; and

Figure 7 is a fragmentary view of the resistor element mounting strip and schematically showingr in.l broken lines stray capacitance existing thereon.

Reference is rst made to Figures l', 2. and 3 of. the drawings whereinA the resistor of my invention is shown comprising a tubular housing 1t) of insulating material, preferably epoxy resin, having internally threaded metallic end caps 11,v i1 tbreadedly attached to the tube ends. External,

annular flanges

12, 12 are formed on the end caps for mounting the high voltage resistor by any suitable means notY shown in the drawings. A plurality of resistor elements: 13 are attached to a mounting strip 14 of insulating material, within the tubular housing 10, by any suitable means not shown.

The resistor elements 13 are not limited to any particular type, and therefore, are not shown in detail in the drawings. Resistor elements of the wire wound or film type, for example, may be employed. For high voltage use, however, film type resistor elements are preferably utilized in which the said elements each comprise a/ cylindrical ceramic tube 16 with a resistive film disposed on the internal surface thereof. Preferably, the said film is an alloy of the nickel-chromium family, which is deposited on a glaze on the internal surface of the said ceramic tube, which is then fired at a high temperatureV thereby dispersing theA resistance film throughout the body of the glaze. Adjustment of the resistance of the dispersedv lrn ismade in a conventional manner by cutting a groove in the form of an internal helix therein until the desired value of resistance is obtained. Metallic resistor.

element end caps

17, 17 and leads are pressed over thev endsy of the cylindrical tube 16 and suitably connected to the resistive lilm within the tube. Although not shown in the drawings, each resistor element may be encased in a suitable plastic shell of epoxy resin, or theV like. Resistor elements of this construction exhibit good stability and good high frequency performance, and are therefore desirable for use in the construction of a precision, high voltage, resistor made in accordance with my invention. The maximum voltage stress in resistor elements of the type wherein a resistance film is deposited on the internal surface of the ceramic tube, obtains in the said ceramic tube, which tube may be made of closely compacted material which is void of air pockets. Such resistance elements are capable of withstanding high voltages without breakdown. Resistance elements, which include a resistive film, or the like, which film is merely coated or covered by an insulating material, often includes air pockets between the coating and film. Resistance elements of this type are not suited for high voltage use since the air in the pockets ionizes under high voltage stress, and nitric acid is thereby formed which destroys the resistive film.

Asbest seen in Figure l, the resistor elements are connected together in a series circuit arrangement. The end resistor elements are connected through lead wires 18 to the metallic end caps 11, the wires 18 extending through a suitable hole in the end caps andsoldered to the ends of the capsv as. at 19. (The connection to only one end cap 11 is shown in the drawing.)

'Plastic end caps, or sealing members, 21, 21 of insulating material close the ends of the tubular housing 10. The plastic end caps areV provided with integrally formed

radial flanges

21a, 21a positioned between the ends of the tubular housing and the metallic end caps 11,.Which flanges are formed during the construction of the resistor in a manner described hereinbelow. The ends of the mounting strip 14 are embedded in the cylindrical. body portion ofthe said plastic end caps, or sealing

members

21, 21, for support Within the tubular housing 10.

In accordance with my invention, the tubular housing 10, mounting strip 14 and plastic end caps, or sealing

members

21, 21 are made of material having the same temperature coefficient of expansion. To this end, the above components are made of the same kind of plastic material and, preferably, the material comprises an epoxy resin. Although other insulating material may be used, epoxy resinhas at least two properties making it particularly well suited for use therein. ln the first place, epoxy resin, when polymerized during the manufacture of the resistor, does not. release water, or water vapor, `which would be highly deleterious in high voltage use. Secondly, the epoxy resin materials, upon polymerizing, form a firm bond with everything with which they are in contact, including other epoxy resin material. Therefore, by making the tubular housing, sealing members and mounting strip of epoxy resin material having the same temperature coefficient of expansion, no separation of these elements results under thermal shock conditions. With my construction, even if the metallic end caps 11, 11 do separate from the tube, moisture, and the like, cannot enter the tubular housing since it remains sealed at the ends.

All resistance elements have a temperature coefcient of resistance; that is, the resistance thereof changes with temperature. Although the temperature coefficient may be either positive or negative, most resistance elements display a positive temperature coefcient. However, regardless of whether the temperature coecient of the resistor elements in the resistor is positive or negative, it will be understood that to function as a precision resistor large resistance changes under various load conditions cannot be tolerated. In the resistor of my invention novel means are provided whereby, during limited periods of overload conditions, the temperature of the resistance elements is prevented from rising above a predetermined value. This, in turn, limits the change in resistance due to the usual temperature coeiiicient. In accordance with my invention, the tubular housing between the insulating end caps 21, 21 is filled with a filler 22 of insulating material which is solid under normal ambient working temperatures, and which melts under overload conditions. Such a filler may comprise, for example, asphalt.

Before describing the operation of the resistor under normaland overload conditions, reference is rst made to Figures 4 and 5 of the drawings wherein successive steps in the assembly ofv end caps to one end of the tubular housing 10 by one. method of assembly are shown. The mounting strip 14, with the resistor elements 13 attached thereto, is positioned in the tubular housing 10 and a mass of epoxy resin material, designated 2lb, is placed in the tubular housing end, with a rounded portion of the material extending beyond the housing end. The epoxy, resin 2lb is preferably in a pliable, semisolid, condition which is easily handled and worked, and comprises the plastic end cap 21 after metallic end cap 11 is turned onto the tubular housing 19, as shown in Figure 5. As the metallic end cap 11 is turned onto the threaded tubular housing end, the epoxy resin mass is shaped to conform to the surrounding structure whereby the flanges 21a are formed between the tubular housing end andthe metallic end cap.

The partially assembled resistor is then placed in a vertical position with the open. end of the tubular housing at the. top, and the liquid asphalt 22 is poured therein to the desired level. Plastic and metallic end caps are then placed over the upper free end of the tubular housing in substantially the same manner as the caps at the other end, as described above. It will be noted that the ends' of the mounting strip 14 are embedded in, and supportedby, the plastic end caps, or sealing members 21,

21. After polymerization, the tube 10, end caps 21 and the ends of mounting strip 14 are bonded together into a substantially unitary structure of like material.

As mentioned above, the asphalt filler 22 is solid under normal ambient working temperature. If, then, for eX- ample, a normal ambient Working temperature of 85 C. obtains when the resistor is dissipating full wattage, the ller remains solid if its melting point thereof is above 85 C. A graph of the plot of temperature versus time for both normal and overload conditions is shown in Figure 6, thc normal load condition being illustrated in solid line and the overload condition in broken line. Under normal full load condition, it will be noted that the resistor reaches an ambient temperature of Temp. 1, which, in the above example, may be 85 C. The melting point of the filler 22 is Temp. 2, which is above the normal operating temperature, and may be, for example, 100 C. Thus, under normal operating conditions, the liller remains solid. If, however, at time T1 during normal load conditions, a substantial overload is placed on the resistor, the temperature will proceed to rise until the melting point of the filler is reached. When the melting temperature of the ller is reached, at time T2 on the graph, energy is dissipated in the filler liquid heat of fusion as the filler turns from a solid to a liquid. Thus, the temperature of the resistor will remain at the melting point of the ller until the whole of the liller is melted. When all of the ller is melted at time T3 on the graph, the temperature will again rise. Between time T2 and T3, under overload conditions the temperature remains constant. Only a relatively small temperature rise is encountered between the normal operating temperature and the melting point of the filler, between times T1 and T2. With a usual temperature coefcient of resistance, the resistance will change only a relatively small amount during brief overload conditions since the temperature rise is small. The use of such filler material is, as mentioned above, of particular value in precision resistors where only small resistance changes may be tolerated.

Film type resistance elements of the type preferably used in the construction of the resistor of my invention are particularly adapted for high frequency use since the capacitance thereof is small as compared with wire wound resistors, for example. With good quality precision resistance elements 13, the only reactive component of any consequence is due to distributed capacitance between the metallic resistance element end caps 17, 17, which is on the order of several micromicrofarads. Where a plurality of resistance elements are utilized in the construction of precision high voltage resistors, not only must the capacitance of the individual elements be small for high frequency use, but substantially identical capacitance between the ends of the resistors in the assembly thereof is necessary so that the inevitable stray capacitance of each resistor end to ground will be the same. This results in a more uniform voltage gradient distribution, necessary for high voltage use.

Reference is made to Figure 7 of the drawings wherein there is shown a fragmentary view of the mounting strip 14 with resistors 13 attached thereto. The resistors are arranged in a uniform manner on the strip with identical stray capacitances 26 (shown in broken lines) between the leads from each resistor element. The capacitance between resistance element end caps 17, 17 is designated 27. By using all of the same type resistor elements whereby the capacitance 27 for each resistor is identical, and by positioning the resistors on the board 14 in a manner wherein the stray capacitance 26 between the ends of each of the individual resistors is identical, the inevitable stray capacitance to ground from each lead of the resistors (designated 28) is also substantially identical. For high voltage, alternating current use, a substantially uniform voltage gradient distribution along the resistor is obtained.

The stray capacitance 26 by the ends of the resistor elements are made identical by positioning the resistor elements in an aligned and uniformly spaced relation, within a single plane. In the illustrated arrangement, this is accomplished by mounting the resistance elements at an acute angle with a straight line, designated 31, through the aligned resistors. Obviously, the resistor elements may be placed coaxially with, perpendicular to, or at any desired angle with the line 31, so long as they are located in a common plane and are uniformly spaced to provide substantially equal stray capacitances 26 between ends.

Having now described my invention in detail, in accordance with the patent statutes, various changes and modications will suggest themselves to those skilled in this art, and it is intended that such changes and modifications shall fall within the spirit and scope ofthe invention as recited in the following claims.

I claim:

1. A resistor `structure comprising `a tubular member, end caps closing the ends of the tubular member, an elongated mounting strip disposed in the tubular member and having ends embedded in the said end caps, and resistor elements mounted on the mounting strip, the tubular member, end caps and mounting strip being made of insulating material having the same coefficient of expansion.

2. The invention as recited in claim 1, wherein the said tubular member, end caps and mounting strip yare made of epoxy resin.

3. The invention as recited in claim l, including genera'lly cup-shaped metallic end caps having internally Ithreaded portions threaded to the ends of the said tubular member and covering thc said end ca-ps of insulating material.

4. A resistor structure comprising a tubular member of insulating material, end caps of insula-ting material olosing the ends of the tubular member, a mounting strip of insulating material, and a plurality of series connected resistors attached to the mounting yst-rip, the said mounting strip being positioned within the tubular member with the ends thereof embedded in the said end caps.

5. The invention as recited in claim 4 wherein the said tubular member, end caps and mounting strip are made of epoxy resin.

6. A resistor comprising a container, lling material within the container, and a resistance element embedded in the filling material, the said lling material being solid under full load operating conditions of the resistor and liquid under predetermined overload operating conditions, the temperature of the resistance element remaining substantially constant under overload operating conditions between the time the filling material begins to liquify and is completely liquited.

7. The invention as recited in claim 6 wherein the said lling material comprises asphalt.

8. The invention as recited in claim 7 wherein the said container comprises a tubular housing of insulating material closed at both ends by end caps of insulating ma.- terial.

References Cited in the le of this patent UNITED STATES PATENTS 2,030,460 Morton Feb. 11, 1936 2,466,211 Crockett -a Apr. 5, 1949 2,862,088 Mairs Nov. 25, 1958