US2518225A

US2518225A - High-frequency resistor

Info

Publication number: US2518225A
Application number: US681169A
Authority: US
Inventors: Stanley O Dorst
Original assignee: Sprague Electric Co
Current assignee: Sprague Electric Co
Priority date: 1946-07-03
Filing date: 1946-07-03
Publication date: 1950-08-08
Anticipated expiration: 1967-08-08

Description

ug. 8, 1950 s, o, DQRs-1- 2,518,225

HIGH FREQUENCY RESISTOR Filed July s, 1946 Ha. Z y

INVENTOR ATTORN EY Patented ug. 8, 1950 UNITED STATES PATENT GFFICE Sprague Electric Company,

North Adams,

Mass., a. corporation of Massachusetts Application July 3, 1946, ySerial No. 681,169

3 Claims. 1

This invention relates to a high frequency resistor and more particularly refers to wire-wound resistors possessing negligible reactance at high frequencies.

AWire-wound resistors are widely used for precision and high power applications where the cost of this type of resistor is justified by the many advantages over the cheaper carbon and composition resistors. In general, the resistor is produced by winding a high resistance wire, such as nichrome, about a. ceramic core. If the wire is not insulated, it is generally advisable to employ space-winding, While if the wire is insulated, e. g. by glass fibers, ceramic material and the like, it need not be spaced during the winding. Terminals are usually affixed at the ends of the core, and, in some cases, taps are made in the middle of the winding, Precise resistance values may be obtained, and, by suitable design, the resistor may dissipate large amounts of power, in the form of heat.

Unfortunately, however, resistors of this type possess serious disadvantages at higher frequencies, e. g. from 100,000 cycles per second to about 50 megacycles per second. Over this range of frequencies, the inductance of the winding becomes a, determining factor in the characteristics of the resistor. If the inductance is at all appreciable, the inductive reactance (XL) will cause a phase shift and the resistor will no longe;` act as a pure, non-reactive resistor. This phase shift is undesirable. Since XL is equal to the product of the inductance times the frequency, the situation will get worse as the frequency increases. To minimize this effect, numerous special windings have been devised, to lower the inductance to as low a value as possible. Unfortunately, however, it has not been possible to sumciently eliminate this inductance. It is theoretically apparent that there must be at least as much inductance as there would be in a straight conductor between the terminals.

It is an object of this invention to overcome the foregoing and related disadvantages. It is a further object to produce an improved high frequency resistance element possessing a minimum phase shift at high frequencies. A still further object is to produce an improved high frequency resistance element in which vthe inherent inductance of the winding does not deleteriously affect the high frequency characteristics of the resistance element. A still further object is to produce high frequency, resistance elements capable of dissipating large amounts of energy. Additional objects will become apparent from the following description and claims,

These objects are attained in accordance with this invention wherein a wound resistance element is shunted by a small distributed capacitance. In a more restricted sense, this invention is concerned with a wire-wound resistance element having in parallel therewith a small capacity, distributed along the winding. In a still more restricted sense, this invention is concerned with a resistance element comprising a ceramic core, resistance wire wound thereabout between the ends thereof and a substantially cylindrical metal electrode disposed along at least part of the winding and connected to one end of the winding. In a still more restricted sense, the invention is concerned with a. resistance element comprising a hollow ceramic core, resistance Wire wound thereabout between terminals at the ends of said core, and a metal electrode disposed circumferentially along the inner surface of said core and connected to-one of said terminals. The invention is also concerned with the use of refractory ceramic insulated resistance wire for the winding, and with the use of noninductive type windings. The invention is further concerned with the method of applying the metal electrode foil.

According to the broad embodiments of this invention, I have found that heretofore unattainable high frequency characteristics may be attained in wire-Wound electrical resistors by employing a distributed capacity in parallel with the windings. This capacity is advisably very small, particularly when employed in conjunction with the better non-inductive windings. Further, it is effective only when it is distributed along at least a portion of the winding. By introduction of this distributed capacity, I am able to offset the undesirable effect of the distributed inductance, and substantially reduce or prevent phase shifting during operation of the resistor. I preferably design my resistance element so that the inductive reactance is approximately equal in value to the especially introduced capacitative reactance.

According to one of the limited embodiments of the invention, the resistor core is a hollow cylindrical ceramic tube, about the inner surface of which I dispose my electrode. This electrode may be fixed, as produced by firing a silver enamel to the surface, or it may be adjustable, as produced by introducing a movable metal sleeve or even a solid rod within the core.

According to a further embodiment of the invention, the electrode may be disposed on the outer surface of the winding. In such cases, I prefer to employ a ceramic insulated resistance wire, such as that disclosed in copending application, S. N. 496,978, now Patent No. 2,421,652, died on August 2, 'i943 by P. Robinson et al. However, glass nbre and other insulating coatings may also be employed. Resinous organic or semi-organic and vitreous ceramic insulation may be applied to the winding before application of the electrode, if so desired. The electrode may be sprayed on by use of a metalspraying gun, may be wrapped or slid on or may be fired on, as desired. Increased heat dissipation may result, if the design is properly made.

The invention willbe further discussed with reference to the appended drawing, in which,

Figure i shows the schematic electrical dlagram of the high frequency resistance elements of the invention;

Figure 2 shows a cross-section of a resistor produced in accordance with the invention;

Figure 3 shows a cross-section through the middle thereof;

Figure 4 shows a partial cross-section of another resistor produced in accordance with the invention, and

Figure 5 shows a cross-section at right angle to that of Figure 4.

Referring more specifically to Figure 1, the resistance element is represented between terminals I0 and as a series of resistance elements |2a, |2b, |20, and |211, and a number of small inductance elements |3a, |31), |3c, and i3d, in distributed series therewith. A number of small capacitance elements Ma, Mb, Mc, and Md, are distributed in parallel therewith between the terminals i0 and Il. The common condenser terminal of these small capacitance elements is connected to terminal lll by means of I5. It may readily be seen that the resistors of the invention are in a way similar to a short-circuited openended transmission line. In accordance with my invention I introduce the capacitance elements Ma, Hb, etc., in a distributed relationship, so that these characteristics of a transmission line may be maintained.

Figure 2 shows a vertical cross-section of a completed resistor, produced in accordance with the invention. The resistor core is represented as having wound on the outer surface thereof a layer of electrical resistance wire 2| Each end of the resistor core 20 is provided with

terminal straps

23 and 24 which in turn are provided with lugs and 26 respectively. The type of winding and method of terminating may be any one of the numerous types and methods employed in the wire-wound resistor field, but preferably the winding itself should possess a minimum amount oi inductance. This may be accomplished, for example, by winding the wire by the well-known Ayrton Perry double layer method. Electrode 22 is circumferentially disposed about the inner surface of the resistor core 2|), and is connected electrically to the terminal strap 24 by means of conductor 28. Insulating material 21 covers and protects the resistor winding 2|.

Figure 3 shows a cross-section through the middle and at right angles to the view shown by Figure 2. On the inner surface of core 20 is a metal electrode 22 while on the outer surface of core 20 is wound resistance wire layer 2|. Insulating coating 21 is disposed thereon.

Referring now to Figure 4 a partial vertical cross-section of another type of resistor is shown.

In this dgure 2t represents the resistor core, which may be either solid or hollow, upon which is provided resistance wire layer 2i as described in the previous paragraphs with e. terminal strap 2Q located at one end of the core and winding. .insulating material 2i is again disposed on top of the winding 2i to insulate and protect the winding in the usual manner. Metal electrode dii is disposed about the surface of the insulating layer 2? and is connected by means of conductor 3l to the terminal strap 2d. Electrode layer 3@ may also contact terminal lug 2t. Obviously the layer @El will not contact the terminal at the other end of the resistor winding.

Figure 5 shows a cross-section through the middle and at right angles to the view of Figure 4. Core tti-is provided with winding 2i upon which are respectively disposed insulating layer 2'?! and metal electrode layer i3d.

The last four gures, illustrating preferred forms of the invention clearly show that there will be distributed capacity between the metal electrode layers 22 or d@ to the resistance winding 2|. It is this distributed capacity which leads to the novel improved resistance elements of the invention. The capacity will depend, of course, upon several variables. First, it will depend upon the surface area of the metal electrode layer with relation to the resistance winding 2i, that is, as the area therebetween is increased the total distributed capacitance is increased; and, second, upon the thickness and dielectric constant of the insulating material between the resistance layer 2| and the metal electrode. The distributed f capacity is increased, of course, with higher dielectric constant insulating materials and with decreasing dielectric thicknesses. Further, the distance along the resistance element of the metal electrode layer may be chosen as desired and as previously mentioned may be variable by appropriate structural design.

The amount of distributed capacitance desired depends particularly upon the amount of inductance present in the resistance winding and the general frequency range over which the resistor must operate. With the better non-inductive windings a small distributed capacitance is suflicient to substantially eliminate phase shifting of the current-voltage passing through the resistor. With single layer solenoid windings a greater distributed capacitance value is required.

The structural features of the resistors of the invention are quite variable. The resistor core may be manufactured of steatite, porcelain, and related materials and the resistance wire may 'ne of nichrome or other similar resistance materials. The resistance wire may be insulated as heretofore mentioned, and if so is preferably insulated with a flexible refractory ceramic material. The terminal straps may be of tinned copper, steel, die casting material, etc., as customarily employed in this field. The insulating material 21, if employed, may be any one of several types. For instance, it may be a resinous type of material such as the polymerization products of the hydrolysis products of the alkyl, aryland aralkylchlor silanes or of the pure organic polymerization resins such as the alkyd resins, polyvinyl glass tube provided with metal end caps, permanently sealing the resistor from corrosive atmospheres.

The metal electrode layer may be provided by plating the core with a thin layer of a high conducting metal such as copper, silver, gold and the like, or it may be applied by a suitable baking enamel such as those employing silver with an organic binder, the latter being volatilized during the firing operation. Alternately the metal electrode layer may be formed by spraying with a. metal spray gun. The layer may also be produced by wrapping the resistor with metal foil or providing a metal sleeve within the core. As a general rule, I prefer to silver the inside of the core by means of a suitable silver baking enamel before winding the resistance layer on the core. The silver layer is preferably continued around one end of the core to the terminal strap portion If a variable distributed capacitance is desired a movable metal sleeve, disposed either within the core or about the resistance winding may be employed, suitable adjusting arrangements being provided therefor.

Resistors constructed in accordance with this invention will possess greatly improved high frequency characteristics. In many cases where the metal electrode is provided on the outside of the resistance winding the heat dissipation characteristics are improved. When employing inorganic insulated resistance wire it is possible to provide the metal electrode layer on the outside of the core, with the winding placed thereabout, or, the metal electrode layer may be provided directly upon the winding without need for an insulating layer 21.

As many apparently widely dierent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments described herein except as dened in the appended claims.

. Number What I claim is:

1. A resistance element comprising an insulating core, resistance wire wound thereabout between the ends thereof, and a tubular metal electrode entirely closed throughout its length uniformly disposed along a substantial portion of the winding, but insulated therefrom, and electrically connected to one end of the winding.

2. A high frequency resistance element comprising a hollow ceramic core, resistance wire wound thereabout between terminals at the ends of said core, and a tubular metal electrode entirely closed throughout its length, uniformly disposed along the major portion of the inner surface of said core and electrically connected to one of said terminals.

3. A high frequency resistance element comprising a ceramic core, insulated resistance wire wound thereabout between terminals at the ends of said core, and a tubular metal electrode entirely closed throughout its length, uniformly disposed about a major portion of the winding and connected to one terminal thereof.

STANLEY O. DORST.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 1,026,377 Barringer May 14, 1912 1,881,444 Flanzer 1.... Oct. 11, 1932 2,111,710 Van Loon Mar. 22, 1938 2,251,631 Mayer Aug. 4, 1941 2,403,349 Dolberg July 2, 1946 2,457,212 Di Toro Dec. 28, 1948 FOREIGN PATENTS Number Country Date 220,785 Great Britain 1923