US2087736A - Resistor construction - Google Patents

Description

July 20, 1937. H, JR 2,087,736

. RESISTOR CONSTRUCTION Filed July 2, 1935 &

Patented July 20, 1937 UNITED STATES PATENT OFFICE RESISTOR CONSTRUCTION Application July 2, 1935, Serial No. 29,433

15 Claims.

This invention relates to a resistor construction as well as a method oi. producing such structures.

One of the objects of this invention is to provide a resistor which is practical, simple and durable in construction. Another object is to provide a resistor of the above nature characterized by simplicity of individual parts and ease of manufacture andassembly. Another object is to provide a resistor of the above nature whose manufacture may be economically accomplished without departing from rigid operating requirements.

Another object is to provide a resistor of the above nature in which resistor noises'are eliminated or materially reduced. Another object is to provide a resistor of the above nature comparatively small in size for a given value of resistance desired. Another object is to provide a resistor of the above nature in which the length of the path o-fresistance material through which the current must pass is large when compared with the total length of the resistor unit. Anuther object is to provide a resistor of the above nature wherein the voltage requirements per unit length of resistor path for a given value of resistance are substantially diminished. Another object is to provide a resistor of the above nature having high current capacity for the size of the unit. Another object is to provide a resistor of the above nature in which the heat generated by the operationthereof is continually transferred from the unit. Another object is to provide in a resistor with an insulating covering of material having a low critical temperature effective means for cooling the unit and effecting a substantially even temperature along the length of the unit to avoid hot points while the unit is in operation. Another object is to provide a method of making a resistor which may be practiced with easeand which results in quick and eflicient assembly. Another object is to provide a method of the above character which may be practiced with inexpensive materials and tools and which results in a saving of labor. Other objects will be in part obvious and in part pointed out here inafter.

The invention accordingly consists in the featuresoi construction, combinations of elements, arrangements of parts and in the several steps and 'relation and order of each of the same to one or more of the others, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawing, in which are shown several of the various possible embodiments of my invention,

Figure 1 is a longitudinal elevation of a tubular resistance element which may be used in my unit;

Figure 2 is a longitudinal elevation of a terminal which may be used in my unit;

Figure 3 is an enlarged sectional view of some of the parts of my unit as assembled;

Figure 4 is a ve tical sectional view of my unit as completely assembled;

Figure 5 is a longitudinal elevation of my unit, and

Figure 6 is a vertical sectional view of my unit in modified form.

Similar reference characters refer to similar parts throughout the several views of the drawing.

To provide a clearer understanding of certain features of my invention, it might here be pointed out that the production and use of units of this general character are still fraught with annoying dinlculties. For practical purposes it is often desirable to provide units very small in size so that they may be fitted into small places in radio instruments or the like. However, in present constructions, a reduction in the size of the unit necessitates using a material having a comparatively high value of resistance and the use of such material increases the resultant operating noise level. This increase in the noise level is due to an increase in the voltage requirements per unit length of resistance, thus increasing minute arcing between adjacent particles of re= sistance material in the resistor while in operation. Furthermore the current carrying capacity of such small units is reduced due to the comparatively large amount of heat developed by the current passing through the resistor. For many reasons it is desirable to encase resistors of this general character in a molded mass of phenolic condensation product, this material having particular advantages as a seal against outside weather conditions. However, the critical temperatures of these phenolic condensation products are lower than desired and if these products are raised above such temperatures, disintegration thereof results. these small units are encased in such phenolic condensation products the heat generated thereby cannot exceed the critical temperature of the encasing product and consequently the current carrying capacity of the units is reduced. One of the objects of this invention is to provide a re-- sister in which the several difliculties hereinabove It follows that where referred to are successfully and practically overcome.

Referring now to Figure 1, there is shown a tubular resistance core Ill formed from material having good electrical insulating characteristics, glass being admirably suited for this purpose. A continuous spiral strip coating II of resistance material is applied to the outer surface or tube I0. The resistance valueof strip ll may vary within wide limits depending upon the characteristics desired in the completed unit. However, strip II is desirably graphitic or carbonaceous in nature comprising, as a rule, finely divided particles of carbon or graphite (good electrical conductors) evenly dispersed in a binder such as varnish. Generally speaking, the resistance value of strip II is dependent upon the ratio of carbon or graphite and binder.

Strip 1 I may be applied to tube III in any suitable manner although I prefer to add the conducting material, for example graphite or carbon, to the binder while the binder is in liquid form; after mixing, the conducting particles are suspended in the liquid binderfl The resistance material while in this liquid form may then be applied to tube ID in any suitable manner in the the length of the tube. For example, where the tube is three fiftieths of an inch in diameter and the width of the strip is one thirty second of an I inch, the pitch may be such that twenty turns of strip are applied to every inch of tube, making a. total linear length of approximately four inches of strip to every inch of tube or approximately a four to one ratio.

In Figure 2 there is shown a wire terminal generally indicated at l2 comprising an abutment l3 spaced from end I4 thereof. Accordingly there is formed a projection l5 having an end l4 and terminating in abutment l3. Abutment I3 is preferably frusto-conical in shape thus having a surface [6 substantially at right angles to the axis of projection l5 and a rear sloping portion I'I merging in the usual wire terminal Ill. The diameter of projection I5 is preferably slightly smaller than the interior diameter of tube H] for a purpose to be more fully described hereinafter.

Wire terminals 12 preferably extend from the opposite ends of tube Ill, as more clearly shown in Figure 3, and are electrically connected to the opposite ends of strip II. To provide such electrical connection and also to mechanically attach terminals l2 to the tube In, the projection P5 of terminals I2 is preferably dipped in a conductingmaterial which, when so applied to the terminals, is desirably in a liquid or semi-liquid state. This conducting material is a much better conductor of electricity than strip H and generally comprises a finely divided conductor, such as carbon or the like, suspended in a suitable plastic binder, e. g., a composition of varnish and graphitic or amorphous carbon, the proportion of carbontherein being considerably greater than the proportion of carbon in strip l l. After the terminals [2 have been dipped in the conducting material so that projection l5 and, more important, surface I6 of abutment I3, is thoroughly covered therewith, the terminals are slipped into the opposite ends of tube Ill until surface l6 of abutment l3 rests substantially adjacent the opposite ends of tube 10. As the terminals are being moved into the position shown in Figure 3, some of the conducting material on projection 15 is forced outwardly around the ends of the tube and finally against surface ii of the abutment, whence it is forced inwardly over the outer surface of the tube to flow over the opposite ends of strip II. The parts being in this position are next baked in any suitable manner until the conducting material is hardened to provide binding shells I9 interposed between the ends of tube In and terminals l2.

As best shown in Figure 3, shells [9 are in engagement with surface l6 of terminals l2 and also are desirably sufficiently wide to contact the entire circumference of the end turns of strip H. Accordingly shells l9 engage a substantial surface area of each end strip ll thus providing excellent electrical connections between the terminals and the opposite ends of the resistance strip. Shells l9 not only engage portions of the outer surface of tube 10, as well at strip II, but also extend into the opposite ends of the tube to occupy the space between projection l5 and the interior surface of the tube. Accordingly shells 19 contact a substantial portion of the surface of the tube and thus provide a strong mechanical connection between the terminals l2 and tube l0.

As clearly shown in Figure 3, projections I5 extend along a substantial portion of tube l0, ends l4 thereof being preferably spaced just far enough apart to avoid any passage of current therebetween. Terminals I2 are preferably formed from metal having good heat conducting characteristics, such as copper or Hitenso", an

alloy of copper and cadmium. Projections I5 of U the terminals being good heatconductors continually conduct heat away from the tube in the general direction of the terminals and also serve to provide a substantially uniform temperature along the length of the tube, all as will be more fully described hereinafter.

After the terminals l2 have been connected to the tube in the manner described above, the unit thus formed is next placed in a mold substantially similar to the mold described and shown in Figures 7 and 8 of a pending application, Serial No. 701,471, filed by Harold Pender, which has matured into Patent No. 2,052,533, dated August 25, 1936. Phus tube I0 is supported in the mold by the terminals l2, and portions of abutment l3 are preferably outside of the mold cavity for a purpose to be described hereinafter. A mass of insulating material in a plastic condition, preferably phenolic condensation product, is forced into the mold cavity in an axial direction with respect to tube in to encase tube I 0 and portions of terminals l2 and form a molded covering 20 (Figure 4). Covering 20, which may take any -convenient shape but is preferably circular in cross-section, hermetically seals the connecting portions of terminals l2 and tube in in the position shown in Figure 4, and is also of sufficient thickness to protect these parts against weather conditions. Furthermore, covering 20 protects tube l0 against breakage and helps shells I9 hold terminals I2 and tube In in assembled relationship. Thus end portions 20a and 20b of the covering encircle the beveled surface of abutment Hi to provide an interlock therebetween, and further resist any displacement of the parts, if the terminals happen to be pulled in displacing directions or directions away from the ends of tube III.

The completed resistor unit is best shown in Figure 5 thus comprising cover 2|! with the terminals i2 extending from the opposite ends thereof. Portions i 3a of abutment i3 are immediately adjacent the ends of covering 20, these portions having a greater diameter than the wire sections l8 of the terminals. Accordingly if wires [8 are bent in any direction in use, portions l3a of the abutment being relatively stiff will provide a much greater bending are so that the terminals will not be bent sharply at any point. This increased bending arc prevents breakage of the wire terminalsdue to bending and, even if such breakage does occur, it will occur at some point beyond portions i3a so that a new wire may be soldered or otherwise secured to the terminal.

As mentioned above, covering 20 is preferably formed from a phenolic condensation product or the like for such material has many important advantages, for example, it is waterproof and hence protects the resistor against adverse weather conditions, and it is an excellent electrical insulator thus shielding the resistor from outside electrical disturbances and effective insulating the various parts making up the resistor per se as shown in Figures 3 and 4. However, phenolic condensation products cannot stand high temperatures and accordingly if tube it) becomes too hot, covering 20 is apt to be se riously damaged. As pointed out above, the greater the amount of current passing along strip i l, the greater the heat generated thereby and consequently it is important not only to dissipate this heat to avoid disintegration of covering 20 but also to distribute this heat as uniformly as possible along the tube ID for it follows that the capacity of the resistor is directly limited by the value of the highest temperature point in the unit when the unit is in operation.

However, tube I is continuously cooled for, as heat is generated by the passage of current along strip ii, that heat is transferred to projections i5 and, due to the fact that terminals i2 are formed from a good heat conductor, the heat is rapidly conducted'away from the tube and out along the terminals. 7 Furthermore, the good heat conducting characteristics of the terminals provide an even temperature therealong so that the temperature along the length of the tube is substantially uniform. In practice the hottest portion of the resistance element, which in this case is tube i0 and strip ii, is the central portion Illa thereof and it is to be noted that ends I of projections i5 are substantially adjacent this portion of the tube. Consequently the temperature of the hottest portion of tube III is materially reduced so that more current may pass along strip il without developing an interior temperature in the resistor unit sufllciently high to damage covering 20. In practice it has been found that this cooling function of the terminals l2 and the ability of the terminals to maintain a substantially even temperature along tube lfl materially increase the current carrying capacity. of the unit and thus there has been provided a resistor unit small in dimensions and high in capacity.

As explained above, it is desirable for many reasons to provide a. resistor unit which is as short as possible so that it may fit into small spaces in radio assemblies for which it is de- .sizes and yet the linear length of resistance strip ii is much greater than the length of the tube l0. As the value of a resistance unit is a function of the length of the resistance path, it follows that where the length'of such path is comparatively great, the resistance value of the conducting substance forming the path may be proportionately reduced. For example, where strip ii is four times the length of tube In, as described 'above, the resistance value of the substance forming strip I i may have one-fourth the resistance value of the substance necessary to form a resistor of similar value if the entire outer surface of the tube were coated rather than having a spiral strip ii.

The material reduction of the resistance value of the substance forming strip Ii reduces the noise level of the unit for this reduction in resistance value results in a corresponding reduction in voltage requirements per unit length of resistance path. For example, if the resistance value of the substance forming strip I I is reduced to one-fourth of the former value of resistance required, the voltage requirements per unit length of resistance path are also reduced to onefourth of the former voltage requirements per unit length of resistance path. As mentioned above, the substance forming strip ii comprises a plurality of small carbon particles or the like held in a suitable binder. As current passes through the strip,- minute arcs are created and these arcs create operating noises. The greater the voltage requirements of the unit, the greater the number and intensity of these arcs and hence the greater the noise level of the resistance unit. It will thus become clear that by providing strip 8 I, which materially increases the length of the resistance path in a small unit, the operating noise level thereof is materially reduced.

Under certain circumstances, the outer surface of tube i0 may be completely covered with resistance material and accordingly the showing of the spiral strip ii and the explanation of the resultant advantages achieved thereby should not be interpreted as limiting in any sense. Many of the other features of this invention might be employed in combination with a tube having a substantially straight resistance path from terminal to terminal.

Thus in Figure 8 I have shown another emloodiment of my invention in which a tube 2i, substantially similar to tube It (Figure 1), is completely covered by a coating 22 of resistance material. The composition of coating 22 may be substantially similar to that of strip Ii although the resistance value thereof is generally much reater as described above.

A pairof wire terminals generally indicated at 23 are provided, each terminal having a crimp 21 formed therein and spaced from the ends thereof. Accordingly, projecting portions 26 of the terminals 23 are provided and these portions are preferably dipped in a suitable conducting substance preferably substantially similar to the conducting substance from which shells i9 (Figure 3) are formed.

The terminals are then slipped into the opposite ends of the tube 2i and, after baking, the conducting parts 21' are provided. t ese parts extending over portions of coating 22 on tube 2i and portions of the crimps 24 in terminals 23. Parts 21 provide an excellent electrical connection between the terminals and coating. 22 and; furthermore, portions of the parts 21 are disposed between projecting portions 26 and the interior surface of tube 2i to provide a strong mechanical union between the terminals and the tube.

The unit comprising the tube 2i and terminals 23 may then be placed in a mold in a manner similar to that described above, after which the molded covering 28 is formed thereabout. Covering 28, preferably formed from phenolic condensation product, is substantially similar to covering 20 (Figure 4) and hence hermetically seals the connecting portions of terminals 23, as well as tube 2|, from outside weather conditions.

In operation, the heat generated by the passage of current along coating 22 is distributed evenly along the unit by the projecting portions 26, and heat is also continuously conducted away from the unit along the projecting portions and thence along terminals 23, it being understood that terminals23 are preferably formed from a. metal having high heat conductingcharacteristics as, for example, copper or Hitenso. Thus the current carrying capacity of the resistor unit shown in Figure 6 is substantially increased in the manner more fully explained above.

Accordingly it will be seen that I have provided a thoroughly practical and efficient resistor construction and method of making the same in which the several objects hereinabove referred to as well as many others are successfully accomplished.

As many possible embodiments may be made of the mechanical features of the above invention and as the art herein described might be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. In a resistor unit including throughout its length material whose operating characteristics are impaired by excessive temperatures, in combination, a tubular resistance element, a pair of metal terminals electrically connected to the end portionsof said element and having projections which extend into said element throughout the bination, a tubular resistance element, a pair of metal terminals electrically connected to the end portions of said element and having projections which extend into said element throughout the major portion of the length thereof, said projections being in heat exchange relationship but electrically insulated from said resistance element, and a mass of phenolic condensation product moulded about said element and the connecting portions of said terminals to completely encase and seal said element from outside climatic conditions. I

3. In a resistor, in combination, a tubular member having a resistance coating on the exterior surface thereof, a pair of metal terminals each having projections extending into said tubular member throughout the major portion of the length of v said member thereby to draw heat from the central portion of said member, the adjacent ends of said projections being spaced from each other, means forming an electrical connection between said coating and the portions of said terminals adjacent the end portions of said member, and a solid mass of insulating material completely encasing said member and the connecting portions of said terminals.

4. In a resistor, in combination, a tubular resistance element, and a pair of wire terminals having crimps spaced from the ends thereof, said terminals extending into said element along the major portion of the length thereof so that said crimps are located adjacent the ends of said element, means forming an electrical connection between said crimps and the end portions of said element, the enclosed portions of said terminals being electrically insulated from and in heat exchange relationship with said element.

. 5. In a resistor, in combination, a tubular resistance element, a pair of wire terminals having crimps spaced from the ends thereof, saidterminals extending into said element along the major portion of the length thereof so that said crimps are located'adjacent the ends of 'said element, means forming an electrical connection between said crimps and the end portions of said element, the enclosed portions of said terminals being electrically insulated from and in heat exchange relationship with said element, and a solid mass of insulating material having a low critical temperature completely encasing and sealing said element from outside climatic conditicns.

6. In a resistor, in combination, a dielectric tubular member having a resistance coating applied to the outer surface thereof, a pair of metal terminals each having projections extending into said member throughout the major portion of the length thereof, said projections being insulated from and in heat exchange relationship with said member, the adjacent ends of said terminals being spaced from each other and being substantially adjacent the central portion of said tubular member, and material having greater electrical conductivity than said resistance coating contacting said coating at the end portions of said member and contacting the portions of said terminals adjacent the end portions of said member, and a mass of insulating material completely encasing said member, said material be- "thereof, a pair of metal terminals each having abutments at points intermediate the ends thereof and projections, said projections extending into the opposite ends of said tubular member and along the major portion of the length of said member in heat exchange relationship therewith, the ends of said projections being spaced from each other so that said abutments are substantially adjacent the end portions of said tubular member, conducting material having greater electrical conductivity than said coating contacting said coating at the end portions of said member and contacting said abutments, and an insulating mass encasing said member, said mass being capable of deterioration when subjected to excessive temperatures.

' 8. In a resistor, in combination, a glass tube having a resistance coating applied to the outer surface thereof, a pair of wire terminals having abutments adjacent the end portions of said tube and projections extending into said tube a substantial distance along the length thereof, conducting material contacting said coating at the end portions of said tube and contacting said abutments, said abutments being substantially frusto-conical in shape, and a mass of insulating material encasing said tube and portions of said abutments so that portions of said abutments extend,v from the opposite end portions of the unit.

9. As an article of manufacture, an elongated member formed from insulating material and having resistance material coated on the surface thereof in-the form of a continuous spiral, and terminals mounted in the ends of said elongated member connected to said resistance material and having portions extending into and along the major portion of the length of said elongated member and in heat exchange relationship therewith. j

10. As having resistance material coated on the outer surface thereof, and terminals connected to the ends of said resistance material and having portions extending into the ends of said glass tube along the major portion of the length of said tube, said portions being in heat exchange relationship with said tube. i

11. In a resistor, in combination, a dielectric tube, resistance material coated on said tube in the shape of a continuous spiral strip, a pair of metal terminals extending along the major portion of the interior of said tube and in heat exchange relationship therewith, the adjacent ends of said terminals being spaced from each other and adjacent the central portion of said tube, conducting material contacting the end portions of said strip and contacting said terminals, said conducting material having greater electrical conductivity than said resistance material, and a mass of phenolic condensation product molded about said tube and said abutments to completely encase said tube.

12. In a resistor unit, in combination, a tubular resistance element, a pair of wire terminals having abutments adjacent the end portions of said element and projections extending into said element throughout the major portion of the length an article of manufacture, a glass tube thereof, said projections being insulated from and in heat exchange relationship with the adjacent portions of said element, conducting material contacting the end portions of said element and contacting said abutments, said abutments being substantially frustro-conical in shape, and a mass of insulating material encasing said element.

13. In a resistor, in combination, a tubular resistor element, and a pair of metal wire terminals electrically connected to the opposite ends of said element, each of said terminals having projections extending into and along said resistor element, said projections being adjacent the major portion of the length of said element, electrically insulated from said element, and in heat exchange relationship with said element.

14. In a resistor, in combination, a tubular resistor element, a pair of metal wire terminals electrically connected to the opposite ends of said element, each of said terminals having projections extending along said resistor element, said projections being adjacent the major portion of the length of said element, electrically insulated from said element, and in heat exchange relationship with said element, and a solid mass of insulating material completely encasing said element and the connecting portions of said terminals.

15. In a resistor, in combination, a tubular resistor element, a pair of metal wire terminals having abutments adjacent the ends of said element and projections extending into and throughout a major portion of the length of I said element, means forming a goon electrical connection between the abutments and the ends of said element, said projections being electrically insulated from and in heat exchange relationship with said element, and a solid mass of insulating material having a low critical temperature completely encasing said element and portions of said terminals.

ALEXANDER L. PUGH, JR.

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