US3757152A - Resistor assembly - Google Patents

Abstract

A resistor assembly capable of withstanding mechanical stresses and high frequency as well as d.c. electrical surges greater than the aggregate voltage rating of its component resistor elements. A plurality of resistor elements are divided into sections, each section bridged by a spark gap. The spark gaps are series connected in a linear sequence between the terminals of the assembly and electrically connected to their respective resistor elements by leads positioned orthogonal to the line of spark gaps. The resulting configuration has a ratio of impedance along the spark gaps to impedance along the resistor elements which is high for steady state low frequency voltage applications and low for large, high-frequency voltage surges. The resistor elements are encapsulated in a thermally conductive, electrically insulating block of epoxy, and advantageously employed as a voltage divider in high voltage charged particle accelerators.

Description

United States Patent [191 Charpentier [451 Sept. 4, 1973 RESISTOR ASSEMBLY Robert R. Charpentier, Chelmsford, Mass.

[73] Assignee: High Voltage Engineering Corporation, Burlington, Mass.

22 Filed: June 2, 1972 21 Appl. No.: 259,234

[75] Inventor:

Primary Examiner-Roy Lake Assistant ExaminerDarwin R. Hostetter Att0rneyRussell & Nields [57] ABSTRACT A resistor assembly capable of withstanding mechanical stresses and high frequency as well as do. electrical surges greater than the aggregate voltage rating of its component resistor elements. A plurality of resistor elements are divided into sections, each section bridged by a spark gap. The spark gaps are series connected in a linear sequence between the terminals of the assembly and electrically connected to their respective resistor elements by leads positioned orthogonal to the line of spark gaps. The resulting configuration has a ratio of impedance along the spark gaps to impedance along the resistor elements which is high for steady state low frequency voltage applications and low for large, highfrequency voltage surges. The resistor elements are encapsulated in a thermally conductive, electrically insulating block of epoxy, and advantageously employed as a voltage divider in high voltage charged particle accelerators.

5 Claims, 7 Drawing Figures RESISTOR ASSEMBLY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to electrical resistors, and more particularly to resistor assemblies having selfprotecting means to prevent damage from voltage surges.

2. Description of the Prior Art Resistors capable of supporting high DC. voltage levels are called for in applications such as voltage dividers and calibration elements associated with the voltage source and acceleration sections of charged particle accelerators. When so used the resistors are subjected to tube sparking and other short duration voltage surges encountered in accelerators. The voltage surges typically last for a period of several microseconds and may exceed the normal DC. voltage across the resistor by a factor of 2 or more, causing considerable problems of resistor breakdown and instability in ohmage value. In addition, resistors used as voltage dividers in acceleration tubes operate in an environment of high pressure insulating gas and must be able to withstand several megarads of ionizing radiation. They are also subject to serious mechanical stresses from handling and heating during operation, and must be kept to a size small enough to fit in with the other accelerator elements.

Several excellent types of resistors are available commercially, but none have been found to perform with a satisfactory degree of reliability under the harsh conditions described. High voltage carbon compositions have been used, but are liable to a considerable reduction in resistance with increased voltage. Wire-wound resistors are bulky and expensive and tend to breakdown under high voltage surge conditions. Metal film resistors have problems similar to wire wound and are even more costly. Some success has been achieved with specially compounded carbon inks applied in a spiral on a steatite substrate, but surging conditions have lim- 40 ited their use. Metal oxide films, deposited on steatite substrates, are characterized by a low voltage coefficient, but once again breakdown, have a sputtering tendency during surges.

Problems other than overvoltage breakdown have also been experienced in the accelerator field. Resort has been made to encapsulating single high voltage resistors in epoxy resins to prevent deterioration of the resistor due to corona sputtering and deleterious effects of the high pressure insulating gas getting inside the resistor. While alleviating these problems, the encapsulation process has itself led to other difficulties, such as the creation of harmful mechanical stresses in the resistor from epoxy shrinkage during preparation of the unit. The epoxy must also have a thermal conductivity sufficiently high to dissipate adequately IR losses from the resistor.

SUMMARY OF THE INVENTION The present invention contemplates a resistor assembly with a self-protecting surge voltage bypass mechanism that effectively solves the above problems encountered in the prior art, and which can be conveniently adapted to the particular voltage requirements of various high voltage applications. It is most applicable under conditions of low frequency or DC. steady state voltages subject to transients and high frequency voltage surges, such as encountered in particle accelerator conditioning periods. The assembly is divided into one or more sections, each section consisting of a resistor element bridged by a spark gap. The various sections are series connected between spark gap poles in a configuration whereby the successive spark gaps present a lower impedance for high frequency voltage surges than for low frequency or DC. steady state conditions, while the path along the resistor elements has a lower impedance for low frequency or DC. steady stae conditions than for high frequency voltage surges.

Such voltage surges tend to propagate in a straight line, resisting changes in the direction of propagation. For example, accelerator tube sparks, having a time constant typically in the order of nanoseconds, have been found to exhibit a straight line tendency. This phenomenon has been turned to advantage in the present invention by the use of spark gaps arranged in a straight line between the terminals of the resistor as sembly, and by connecting the resistor element of each station to each pole of its associated spark gap by connections orthogonal to the line of spark gaps. Damaging voltage surges are thereby constrained to the spark gap path. The operating low frequency, or DC voltage, is maintained across the series connected resistor elements, as well as their protective gaps.

In the preferred embodiment the resistor elements each comprise two separate units, each unit having a ceramic core with a spiral line of resistive metal oxide material deposited thereon. The resistor units depend from opposite poles of the bridging spark gap at right angles to the line of spark gaps and are electrically connected to each other at the ends away from the spark gaps by metallic strips.

The resistor elements are encapsulated in a single specially prepared block of insulating epoxy resin to reduce electric field stresses at the edges of the resistor elements and metallic connectors and to shield the resistor elements from deterioration due to immersion in an ambient high pressure insulating gas medium. Resin shrinkage during casting and consequent mechanical stresses are limited by mixing a high level of filler material with the resin. In the preferred embodiment the total resistance of the assembly can be varied during manufacture while the same overall dimensions are retained by the use of a single casting mold. The addition or elimination of an appropriate number of resistorspark gap sections may be effected for wattage requirements. Voids are filled in by the epoxy mixture, and leads are provided to connect electrically the end sections to the assembly terminals. An intimate contact between the resistor elements and case insulation is achieved that aids in heat dissipation and protects the resistor elements from thermomechanical shock.

The resistor assemblies are highly useful in maintaining a voltage gradient between equipotential planes of a particle accelerator. When so employed they are in close proximity to one another and can be provided with conductive collars between the terminals and outer spark gaps, whereby a small gap is left between the collars of adjacent resistor assemblies. Voltage surges entering an assembly are shunted from collar to collar due to skin effect phenomena and bypass the resistor elements and spark gaps entirely.

The resistor assembly described is mechanically durable and can receive repeated surges of more than 10 times the voltage rating of the individual resistor units without damage. Additional advantages will become apparent from the ensuing detailed description.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in cross-section showing the resistor assembly embodying the invention.

FIG. 2 is a side view in cross-section taken along the line 2-2 of FIG. 11 of one of the resistor units of FIG. 1.

FIGS. 3 and 4 are cross-sectional views showing variations in the configuration of resistor elements.

FIG. 5 is an overhead broken view showing a resistor assembly spanning two equipotential planes in a charged particle accelerator. One-half of the upper equipotential plane is shown in the top portion of the figure, and one-half of the lower equipotential plane in the bottom portion of the figure.

FIG. 6 is an overhead view in partial cross-section showing means for attaching a resistor assembly in a charged particle accelerator.

FIG. 7 is a view in cross-section showing a compact resistor assembly with a single spark-gap and collars near the terminals.

DETAILED DESCRIPTION OF THE INVENTION In a preferred embodiment of the invention, a resistor assembly is divided into a number of similar sections, one of which is shown enclosed in a dotted line in FIG. l and generally indicated by the numeral 2. Except for modification at each end of the assembly, the various sections are identical and a description of the selected section 2 is equally applicable to the others.

The section 2 has as its major components a resistor element and a spark gap element 3. The resistor element consists of two resistor units 4, which may be ccramic core structures with a spiral line of resistive metallic oxide material deposited along its surface, and which are electrically joined at one end by a metallic link 6 screwed onto resistor caps 8. A spark gap 3 is formed from a pair of longitudinally aligned spaced metallic bars 10, the adjacent ends of which serve as spark gap poles 12. Each resistor unit 41 is electrically connected to a spark gap pole 12 by means of a conductive resistor cap M, lead I6 oriented at a right angle to the metallic bars 10, and fastening screw K8 in the metallic bar 10.

Additional sections are provided on either side of section 2, connected between spark gaps to form a continuous line of spark gaps, each having an, associated resistor element. As shown in the drawings this can conveniently be accomplished by utilizing as poles for additional spark gaps 20 the ends of metallic bars 10 not employed as poles 12 for spark gap 3. Complementary poles for spark gaps 20 are provided by end portions of metallic bars 22, the other ends of which in i turn function as poles for further spark gaps. A preferred method of accomplishing such an arrangement employs a plurality of repeated resistive structures, each structure consisting of a metallic bar W and two depending resistor units ll. The structures are constructed separately and then joined by links 6 in forming the assembly.

All the resistor units 4 are encapsulated within a single insulating block 24 of thermosetting epoxy resin having a filler content of at least 80 percent. Although such a large proportion of filler, which is preferably tabular alumina, causes some reduction in the electrical insulating capacity of the resin, it has been found to reduce shrinkage during molding to less than one-half percent and consequently to avoid the imposition of harmful mechanical stresses on individual resistor units 4. In practice the amount by which the resins insulating quantity is decreased is not enough to limit the use of the resistor assembly in any accelerator applications.

The action of a high frequency voltage surge is best described with reference to FIG. ll. During steady state D.C. conditions a voltage is established between metallic terminals 26 and 28. Current flows from one end to the other through the resistor units A, metallic links 6 and metallic bars, setting up across each spark gap a proportional amount of the total voltage difference. When a high frequency voltage surge enters one of the terminals, however, the tendency of the surge to propagate in a straight line causes it to bypass the repeated right angle turns, 30 and 32 preceeding the first resistor unit 34 and to jump across the first spark gap 36. Thereafter the orthogonal relationship between the leads l6 and the metallic bars which form the spark gaps force the surge to travel from spark gap to spark gap by flashover across the gaps, rather than along the lower steady state resistance path offered by the resistor units 4 The surge will continue to break down spark gaps until it either exits at the opposite terminal or is dissipated.

The dimensions and electrical characteristics of the various elements can be selected according to the respective magnitudes of the D.C. operating voltage and expected voltage surges. One assembly developed by applicant, incorporating 40 resistor units and 21 spark gaps, had a separation of 0.015 inch between spark gap poles, the resistor units being about one-fourth inch in diameter and 1% inch long. While operating under a normal voltage requirement of 50 kilovolts, it was subjected to and withstood without damage or change in resistance characteristics thousands of surges of over one megavolt and one milliamps each lasting for several microseconds.

It is an advantage of the invention that the resistance of the assemblies can be conveniently modified without changing the over-all assembly dimensions by varying the number of resistor units in the same mold. An assembly having a reduced number of resistor units and spark gaps is shown in FIG. 3. The space previously occupied by resistor units at each end 38 of the assembly is filled in with epoxy resin. Conducting rods 40 connect the terminals 26 to conducting studs 42, which studs are electrically connected to interior resistor units 4 through two right angle turns. Extended metallic bars 44, providing the outside pole of the first spark gaps, are exterior to the encapsulating resin block and electrically join the terminals 26 to conducting studs 42. The assembly can be further varied by adding or eliminating other resistor sections.

Although the ladder-like resistor arrangement thus far described permits a relatively large number of resistor units to be used, other arrangements employing the principles of the invention might also be desired. In FIG. 4, for instance, resistor units d6 are longitudinally aligned, with each end connected to leads 48 that are orthogonally joined to an exterior metallic spark gap bar 50. Each section of the assembly has only one longitudinal resistor unit 66, permitting a reduction in assembly size.

Referring now to FIGS. 5 and 6, a resistor assembly 52 may be placed across adjacent equipotential planes in a charged particle accelerator by connecting opposite ends of the assembly to adjacent equipotential rings 54 and 56. A preferred mode of placement, in an accelerator having a tube 58 and mounting pads 60 located inside equipotential rings 54 and 56, utilizes metallic supports 62 and 64 clamped to adjacent rings 54 and 56 respectively. A rounded plug 66 on one terminal 68 of the resistor assembly 52 lodges against and partially into a socket 70 in support 62. At the other terminal 72 a plug 74 extending from support 64 fits in an opening 76 in the terminal and compresses a spring 78 provided therein to facilitate attachment and removal of the resistor assembly 52. To prevent rotation of the assembly 52 a peg 79 on support 64 fits into another opening 80 in terminal 72. An adequate electrical connection is assured at terminal 68 by bolting thereon a lead 82 from equipotential ring 54.

For low power operations requiring a resistor assembly of smaller dimensions, an embodiment shown in FIG. 7 having a plurality of series connected resistor units encapsulated in epoxy 84 may be used. Only one spark gap 86 is provided, the poles 88 of which are connected to the end resistor units 90 by metallic clips 92. These clips 92 are shaped to require a surge to turn a right angle before entering the resistor units. Additional protection may be afforded by attaching conducting toroidal collars 94 at the assembly terminals. The collar diameters are such that when a plurality of resistor assemblies are emplaced in a particle accelerator a gap of approximately 0.1 inch is left between the corresponding collars of any two adjacent assemblies. With such a spacing electrical surges tend to flash over between the resistor assemblies along the collars 94 instead of entering the assemblies. The spark gaps 86 provide a second line of protection against any surges that might get in.

Having now described the invention and several embodiments thereof, it may occur to one skilled in the art to adopt particular variations or modifications for certain purposes. It is my intention therefore that the embodiments shown herein are for purposes of illustration, and the invention is to be limited only in terms of the appended claims.

I claim:

1. In a charged particle accelerator having an acceleration tube and a plurality of equipotential rings for grading an applied voltage along the length of the tube, a surge protected resistor assembly for maintaining the voltage between two adjacent rings, said resistor assembly comprising:

a. a pair of terminals adapted to be held in electrical communication between said adjacent equipotential rings,

b. one or more series connected spark gaps, said spark gaps disposed in substantially a straight line between said terminals, the outer pole of one of the outermost spark gaps electrically connected to one of said terminals and the outer pole of the other outermost spark gap electrically connected to the other of said terminals,

. each of said spark gaps having a resistor element associated therewith bridging its poles, each said resistor element electrically connected to the poles of its associated spark gap by conducting means stemming from said spark gap at substantially a right angle to said line of spark gaps.

2. The charged particle accelerator of claim 1 wherein said resistor elements are encapsulated in a block of solid electrically insulative material.

3. The charged particle accelerator of claim 2 wherein said resistor assemblies are immersed in an insulating gas having a dielectric constant less than that of said insulative material.

4. The charged particle accelerator of claim 1 wherein the terminals of said resistor assemblies are provided with conducting toriodal collars of sufficient size to leave a small gap between the corresponding collars of adjacent assemblies.

5. The charged particle accelerator of claim 3 wherein said gap between toroidal collars is approximately 0.10 inch.

Claims (5)

1. In a charged particle accelerator having an acceleration tube and a plurality of equipotential rings for grading an applied voltage along the length of the tube, a surge protected resistor assembly for maintaining the voltage between two adjacent rings, said resistor assembly comprising: a. a pair of terminals adapted to be held in electrical communication between said adjacent equipotential rings, b. one or more series connected spark gaps, said spark gaps disposed in substantially a straight line between said terminals, the outer pole of one of the outermost spark gaps electrically connected to one of said terminals and the outer pole of the other outermost spark gap electrically connected to the other of said terminals, c. each of said spark gaps having a resistor element associated therewith bridging its poles, each said resistor element electrically connected to the poles of its associated spark gap by conducting means stemming from said spark gap at substantially a right angle to said line of spark gaps.

2. The charged particle accelerator of claim 1 wherein said resistor elements are encapsulated in a block of solid electrically insulative material.

3. The charged particle accelerator of claim 2 wherein said resistor assemblies are immersed in an insulating gas having a dielectric constant less than that of said insulative material.

4. The charged particle accelerator of claim 1 wherein the terminals of said resistor assemblies are provided with conducting toriodal collars of sufficient size to leave a small gap between the corresponding collars of adjacent assemblies.

5. The charged particle accelerator of claim 3 wherein said gap between toroidal collars is approximately 0.10 inch.

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