US2397408A - Wire wound resistor - Google Patents

Description

Patented Mar. 26, 1946 WIRE WOUND RESISTOR David R. Crosby, Haddonield, and Willy Carl Kimmich, Maple Shade, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application March 28, 1944, Serial No. 528,374

(Cl. ZIM-67) 6 Claims.

high inductive reactance. For example, at a frel quency of 10 megacycles per second, the inductive react-.ance of the tubing may be fifty times as much as its resistance, In addition to being cumbersome and unwieldy, the tubing has a further disadvantage in that its reactance varies rapidly with frequency.

The object of the invention is to provide an improved, compact and practicable resistor which has low resistance and negligible inductive reactance independent of the frequency of the current applied to it, and which is capable of dissipating effectively large amounts of power.

This object is achieved according to the invention by providing a single-layer, continuous helical winding of resistive material upon an insulating support; the winding is divided into groups of turns spaced along the support and the turns of each group are so connected to each other that each group provides a plurality of parallel paths for current flowing in the resistor, so that the resultant resistance of each group is small. Current in adjacent groups is caused to iiow in opposite directions so that the resultant inductive rea/:tance is minimal. Means are provided for circulating cooling iiuid about the winding, and the heat engendered by the passage of current through the resistor is thereby eiectively dissipated.

The invention may be better understood from a consideration of the following description of a preferred embodiment thereof, when read in conjunction with the accompanying drawing, in which:

Figure l is a top plan view, partly in section, of a resistance unit constructed in accordance with the invention,

Figure 2 is a Sectional view of the unit of Figure l taken along the line II--II of Figure l,

Figure 3 is a fragmentary view in section of the same unit taken along the line III- III of Figure 2, but on a larger scale than Figure 1 (to make the drawing clearer, the proportions ofthe respective elements have been distorted),

Figure 4'is a diagram illustrating the comparative instantaneous directions in which current flows in the winding,

Figure 5 is an enlarged view in elevation of a metal insert which may be used in carrying the invention into effect, and

Figure 6 is a plan view (on the same scale as Figure 5) of the insert shown in Figure 5.

Referring to the drawing, and particularly to Figures 1 and 2, the resistor is mounted upon a cylindrical support I0 which may be of Bakelite or other suitable insulating material. Formed on its outer surface is a continuous helical groove i2 having, for example, 48 threads or complete turns for each inch of the length of the support; the groove is of sufficient depth to hold the wire of the resistor which, in the preferred embodiment, has a diameter of .0()8 inch. The support also has formed on its outer surface a pair of diametrically opposed longitudinal slots I4 and i6, extending along its entire length and dividing each turn of the helical groove into two approximate semi-circles.

Longitudinaliy spaced within the slots I4, I6 are a plurality of inserts Ia, 18h, etc., of conductive material in the shape of an inverted T, as best seen in Figure 5. The space between each insert may be .equal to the width of two turns of the helical groove. Each insert has a base. v20 and a raised portion 22, in which there is formed a diagonal slot 24 to receive the winding. All the dimensions of each insert, as used in a preferred embodiment, are shown in Figures 5 and 6, the width being that of the longitudinal slots I4, I6 into which the inserts t tightly. The inserts in the two longitudinal slots are staggered, so that the raised portion 22 of each insert in one longitudinal slot is opposed by the space between two adjacent inserts in the other longitudinal slot. (See Figure 3.)

The resistor itself consists of a single-layer, continuous helical winding 26, which may be made of the alloy of nickel and chromium known as Nichroma or other material having high resistivity, a low temperature co-efcient, and which is capable of operating at high working temperatures. The winding is divided into groups 28a, 2Gb, etc., formed in the following manner: The winding commences at a terminal 30 at one end of the support I0, makes a half turn in the helical groove I2, spans the base 20 of the insert Ita, and may then make six complete circles in successive turns of the helical groove, spanning the longitudinal slots I4 and I6 and the base of the inserts accommodated therein, twice for each full turn. This completes group 28a, and the winding now jumps two turns of the helical groove, crossing in the diagonal slot 24 of an insert at the bottom of the support and commences to form group 2812 which, like group 28a and all the other groups, may consist of one half turn and six complete turns. It will be seen that the cross-over from one group to the adjacent group is made alternately at the top and bottom of the device. The winding terminates at a terminal 32 at the other end of the support.

The winding is soldered or otherwise connected to each insert at every point at which it spans or crosses the insert. Current in each group, therefore, may have as many as thirteen parallel paths in which to flow and the resultant resistance of the group is, therefore, low. As may be best seen in Figure 4, current in adjacent groups flows in opposite directions, as indicated by the arrows. The magnetic elds set up by these currents tend to cancel each other. The resultant mutual inductance and, therefore, the inductive reactance of the whole winding is accordingly small.

The resistor mounted on the support is surrounded by an envelope 3'4, preferably of glass or other transparent material, and is spaced from the envelope (for example, by the protuberances 36) thereby defining a radial space 38 between the winding and the envelope. Through this space, water or any other suitable cooling iiuid is circulated at a high velocity in any suitable manner, such for example as that described in Patent 2,254,838 for a Resistor, granted September 2, 1941, to James W. Conklin and Alfred Meneely,

and assigned to the same assignee as the instant I application. The winding presents a considerable surface to the layer of cooling fluid circulating about it at high velocity, and the heat engendered therein by the current applied thereto is thus effectively dissipated.

A resistor constructed in accordance with the invention, having an overall length of 411/2 inches, a resistance to direct current of approximately two ohms and low inductive reactance, was found capable of dissipating 13 kilowatts of power at a frequency of 8 megacycles per second. Readings were taken of the characteristics of another resistor constructed in accordance with the invention which had a diameter of 2 inches, an overall length of 8 inches and a resistance to direct ourrent of 6 ohms, as follows:

Inductive reactance in ohms Direct qurrent resist- Frequency in megacycles persccond ance 1n ohms This resistor is easily able to dissipate 50 kilowatts of power at a frequency of 20 megacycles per second.

The resistor of the invention constitutes a multiple series-parallel arrangement, the half turns in each group being electrically in parallel with each other and the various groups being in series with each other. The metal inserts Ita, I'Sb, etc. have been shown as being in the shape of an inverted T, with a diagonal slot 24 in the upright portion '22 to prevent the winding from slipping as it misses two complete turns of the helical groove in passing from one group to the adjacent group. In practice, however, it has been found that the inserts may consist of a rectangular block, with its top face ush with the top of the slots lll, I5 without the raised portion 22 or the slot 24, and that soldering the winding to the metal inserts keeps it suihciently taut.

The device of the invention used as an artificial load for high frequency dielectric equipment together with an air capacitor (since the load of such equipment is the electrical equivalent of a capacitor in series with a low resistance), is superior to an actual work specimen in that (a) it provides means for measuring power delivered by the equipment, the power being a function of the rate of iiow of the cooling uid and of the increase in its temperature; (b) its electrical characteristics are not affected by repeated use, as is the case with the work specimen; and (c) a load so constituted affords a more accurate control of its electrical characteristics.

There has thus been described an improved and compact single-layer Wire wound resistor having low resistance and low inductive reactance, and the impedance of which is more independent of variations in frequency than resistors of the prior art consisting of steel tubing. With the addition of means for circulating cooling fluid about it, the resistor is capable of dissipating large amounts of power at high frequencies and may, therefore, be employed as a constituent of an artificial load for high frequency dielectric equipment. It has, however, other uses; for example, as a coupling resistor in the video modulating circuit of a television system, or in the absorption circuit of a single side band system, or as a loading resistor in the antenna loading circuit of a radio system. It nds special utility wherever its compactness and stability may be employed. 1t is not intended to limit the scope of the invention, except by the prior art and the terms of the appended claims.

We claim as our invention:

l. A high frequency resistor constituted by a simple continuous single-layer helical winding of resistive material divided into groups of turns longitudinally spaced along a common axis and having a substantially regular surface whereby to permit the rapid passage of coolant therepast with a minimum of obstruction, the turns of each group being electrically connected to each other at a pair of diametrically opposed points on each turn, whereby to constitute in each group a plurality of electrically parallel paths for current flowing in said resistor, and each group being electrically connected to the adjacent group in staggered relation about said axis at said points.

2. A high frequency resistor comprising the combination of a single continuous single-layer helical Winding of resistive material divided into groups of turns longitudinally spaced along a common axis, and conductive members electrically connecting the turns of each group to each other at a pair of diametrically opposed points at each turn and electrically connecting each group to the adjacent group in staggered relation about said axis at said points.

3. A high frequency resistance unit comprising the combination of an insulating support, a single continuous single-layer helical winding of resistive material carried by said support on its outer surface, said winding being constituted by groups of turns, said groups being longitudinally spaced on said support, and conductive members electrically connecting the turns of each group to each other at a pair of diametrically opposed points on each turn and electrically connecting each group to the adjacent group in staggered relation about the longitudinal axis of said support at said points.

4. A resistance unit according to claim 3, characterized in that said support has a pair of diametrically opposed longitudinal slots formed on its outer surface and that said members are accommodated in said slots.

5. A resistance unit comprising in combination an insulating support having helical grooves formed on its outer surface in planes normal to the longitudinal axis of said support, said support further having a pair of diametrically opposed longitudinal slots formed on its outer surface, longitudinally spaced conductive members accommodated in said slots in staggered relation about said axis, and a single-layer, continuous helical Winding of resistive material accommodated in said grooves and spanning said members,

lli

said winding being divided into groups longitudi nally spaced from each other at the spaces between said members, and said winding being electrically and mechanically connected to said members wherever said winding spans said members.

6. The combination of continuous groups of helical windings of resistive material longitudinally spaced along a common axis, conductive members connecting the turns of each group to each other at a pair of diametrically opposed points on each turn and connecting each group to the adjacent group in staggered relation about said axis at said points, means defining a radial space about said windings, and means for circulating cooling fluid through said space.

DAVID R. CROSBY. WILLY CARL KIMMICH.

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