US3551873A - Resistor - Google Patents

Description

United States Patent O 3,551,873 RESISTOR Lionel E. Weyenberg, N 35 W 23575 Capitol Drive, Pewaukee, Wis. 53072 Filed Nov. 25, 1968, Ser. No. 778,730 Int. Cl. H01c 1/02 U.S. Ci. 338--253 17 Claims ABSTRACT 0F THE DISCLOSURE A resistor comprising an electrically conductive resistance strip which is press fitted within a slot in a rigid insulating casing. Terminals or leads secured to the strip project outwardly through the slot in the casing and the strip is sealed within the casing by an insulating coating.

The invention relates to a resistor, and more particularly to a low capacity resistor for use with a printed circuit board or the like.

The conventional low capacity resistor is produced by winding up to about two turns of wire on a very small diameter insulating core. After winding, the ends of the wire are welded or brazed to terminals located at the ends of the insulating core. Resistors of this type are generally produced to a 1% tolerance and the resistance of the resistor is determined by the length and diameter of wire wound on the core. Under conventional practices, the wire is taken from a reel and the length of the wire wound on the core can vary from resistor to resistor. In view of this, the practice in the past has been to wind the resistors with a slightly lower resistance than the specified resistance and after completion, the wound wire is then ground or sand blasted to reduce the wire thickness and obtain the desired resistance within the tolerance limits. Not only is this a very costly procedure from a time and labor standpoint, but welding of the thin wire to the terminal strips is a very delicate procedure.

The present invention is directed to an improved low capacity resistor which can be produced to precise tolerances and can be fabricated with substantially less labor cost than conventional resistors of this type. According to the invention, the resistor includes an electrically conductive resistance strip of fixed length which determines the resistance of the resistor. Terminal strips or leads are welded to the strip, and the strip is press fitted Within a slot in a rigid insulating casing formed of a ceramic material. The casing being rigid provides the mechanical strength for the resistor. The open ends of the casing as r well as the slot are sealed by an insulating coating so that the resistance strip is completely sealed within the casing.

The resistor of the invention can be fabricated to precise tolerances without the need for expensive grinding procedures for the resistance is determined by the length of the metal strip. Thus the tolerance can be readily held within the desired limits by utilizing a strip of given length. Moreover, the fabrication of the resistor of the invention eliminates the tedious operation of wrapping one or two turns of thin wire around a very small diameter core, as in the conventional practice, and also eliminates the delicate procedure of welding the ends of the wound wire to the terminal strips.

In a second form of the invention for use as a high resistance type of resistor, a resistance wire is wound on a generally iiat, electrically insulating strip and terminals are connected to the ends of the wire and project outwardly of the strip. In this embodiment, the wound strip is inserted within the slot in the rigid casing and the open ends of the casing as well as the slot are sealed by an insulating coating so that the resistance wire is completely encapsulated.

3,551,873 Patented Dec. 29, 1970 Other objects and advantages will appear in the course of the following description.

The drawings illustrate the best mode presently contemplated of carrying out the invention.

In the drawings:

FIG. 1 is a side elevation of a resistor of the invention as used with a printed circuit board;

FIG. 2 is a perspective view of the resistance strip, having leads welded thereto;

FIG. 3 is a transverse section of the resistor;

FIG. 4 is a perspective view of a modified form of the resistor mounted on a support by a metal clip;

FIG. 5 is a section taken along lines 5-5 of FIG. 4;

FIG. 6 is a perspective View of a modified form of the invention in which the resistance element comprises a wire wound around a at insulating strip;

FIG. 7 is a transverse section of a resistor incorporating the resistance element of FIG. 6;

FIG. 8 is a perspective view of a further modified form of the invention with parts broken away in section and showing the strip within the insulating casing; and

FIG. 9 is a transverse section of the resistor of FIG. 8.

Referring to the drawings, FIGS. l-3 illustrate a resistor 1, which is mounted on a printed circuit board 2. The resistor ll includes an electrically conductive strip 3 formed of metal or the like, and a pair of electrical leads 4 are soldered to the enlarged end portions 3a of the strip 3, as shown in FIG. 2. The enlarged end portions 3a service to reduce the temperature in the end portions where the leads 4 are connected and thereby prevent possible overheating and melting of the soldered joint. The resistance of the strip 3 is determined `by the length of the strip, the cross section of the strip and the type of material used.

The drawings illustrate the strip 3 having a generally rectangular cross section. However, it is contemplated that the strip 3 can have any desired cross section configuration, such as circular, oval or tubular. Moreover, the strip can be straight, as shown in the drawings, or it can be curved, or angular in overall configuration.

Thus, the term strip as used in the specification and claims is intended to mean a member having any desired cross sectional configuration, such as rectangular, square, circular, oval or tubular, and is intended to mean a member which can be straight, angular, bent or curved.

According to the invention, the strip 3 is press fitted Within a slot or recess 5, formed in a generally rigid insulating casing l6i. The casing 6 is formed of a material such as ceramic, ber glass reinforced resin, or the like, which will provide the necessary mechanical strength for the resistor. If formed of a ceramic material, the casing 6 is preferably extruded and cut to the desired length.

A coating 7 of an insulating material covers the casing 6 and seals the open ends of the casing as well as the slot 5 so that the resistance strip 3 is completely embedded within insulating materials. As shown in FIG. 3, the leads 4 project outwardly of the insulating coating 7 and extend through the holes 8 in the printed circuit board 2. The ends of the leads 4 are adapted to be connected by brazing or the like to a printed circuit formed on the under surface of the board 2.

To space the resistor 1 from the board 2, and thereby provide greater air circulation around the resistor to dissipate the heat generated, a pair of feet 9 are formed integrally with the casing 6 and project downwardly into engagement with the board 2. The feet 9 can take any desired shape and function to space the resistor from the surface of the board so that air can freely circulate between the board 2 and the resistor 1.

While FIG. l illustrates a resistor in which the leads 4 project laterally from the resistor, it is contemplated that the leads may extend in any direction from the resistor depending on the particular application in which the resistor is to be used. For example, the leads can project axially from the resistor or laterally, as shown in the drawings, or the leads can project at any desired angle from the strip 3. Moreover, terminal strips can be employed in place of the leads.

To fabricate the resistor of the invention, the strip 3 is initially cut to the desired length to provide the required resistance for the resistor. The leads 4 are then welded to the strip 3 and the strip with the leads 4 attached is then inserted within the slot 5 in the casing 6. After the strip has been inserted within the recess, the insulating coating 7 is applied over the casing to seal the strip therein. The coating 7 can be applied in any convenient manner, such as dipping, brushing, spraying, or the like. After the coating has dried, the projecting ends of the leads 4 are inserted through the openings 8 in the printed circuit board and are brazed to the printed circuit.

FIGS. 4 and 5 illustrate a modified form of the invention, in which the resistor 10 is mounted from a supporting surface 11 by a metal slip 12.

In this form of the invention the resistor 10 includes an electrically conductive strip 13, similar to strip 3 of the first embodiment, and a pair of terminal strips 14 are secured to the strip 13. As shown in FIG. 4 one of the terminal strips projects axially from one end of the resistance strip 13, while the other terminal strip 14 projects laterally.

As in the case of the first embodiment the resistance strip 13 is fitted within a slot 15 in a rigid insulating casing 16, similar to casing 6 of the first embodiment. The slot 15 as well as the open ends of the casing 16 are encased within an insulating coating 17, similar to coating 7 of the first embodiment.

The mounting clip 12, includes a generally curved central section 18, which is bent around the central portion of the resistor 10 and the clip 12 is supported from the surface 11 by a series of screws 19 which extend through the flange of the clip 12.

The clip 12 being a metallic material serves a dual function in that it not only supports the resistor 10 above the supporting surface 11 but also serves as a heat sink to aid in dissipating heat generated by the resistor. The central portion of the resistor is the hottest area, and the clip 12, being attached to the central portion of the resistor acts to transfer heat away from the central portion and thereby reduce the highest temperature developed by the resistor. By reducing the highest temperature, the resistor can carry a greater wattage.

To further aid in distributing heat away from the central, high temperature area of the resistor, a coating of metal, such as aluminum, can be sprayed over the insulating coating and functions as a heat sink. The sprayed metal coating can be employed in conjunction with the metal clip 12, or the sprayed metal coating can be used with any other type of mounting means.

The resistor of the invention has distinct advantages over prior art types in that the resistance can be produced to a given tolerance without the need for auxiliary grinding or blasting operations. The resistance is determined by the length of the metal strip 3, and by using a measured length of the strip, the resistance of the element can be maintained within precise tolerances. This differs from the conventional type of wire wound element in which the element is normally wound with a lower resistance and the wire is then ground to reduce the thickness to obtain the final resistance within the tolerance limits.

As a further advantage, the resistor of the invention can be fabricated without the delicate assembly and welding operations, the overall cost of fabricating the resistor is substantially reduced over conventional types.

FIGS. 6 and 7 illustrate a modified form of the invention in which the resistor is used as a high resistance type. In this embodiment, a metal resistance wire 21 is 4 helically wound on a generally fiat strip 22 formed of electrically insulating material. The ends of the wire 21 are welded to leads 23 or terminal strips which extend outwardly from the strip. As in the case of the previous embodiments the leads 23 can project laterally, longitudinally or at any desired angle from the strip.

As shown in FIG. 7, the wound strip 22 is press fitted within a slot 24 in a generally rigid insulating casing 25, corresponding to casing 6 of the first embodiment, and a coating 26 of electrically insulating material, similar to coating 7, is applied over the casing to seal the open ends of the casing as well as the slot 24 to completely encapsulate the `wire 21.

The casing 25 can be provided with feet similar to feet 9, to space the resistor from a printed circuit board or the resistor can be mounted on a supporting structure by use of a clip, similar to clip 12.

FIGS. 8 and 9 illustrate a further modified form of the invention. In this embodiment, the resistance strip 27 is provided with enlarged end portions 28 and the central portion of the strip 427 is provided with a series of corrugations or reverse bends 29 which serve to increase the resistance of the strip over the resistance of a fiat strip of similar dimensions. In general, the resistance of the strip is dependent on the electrical conductivity of the metal, the cross section area of the central portion of the strip and the number and magnitude of the bends or corrugations 29. By varying one or more of these properties, the resistance of the strip can be varied as desired.

As in the case of the strip 3, the strip 27 is adapted to be inserted within the slot 30 of an insulating casing 31, and the casing 31 extends longitudinally outward beyond the junction of the central corrugated portion and the enlar'ged end portions 28, with the outer extremities of portions 28 projecting beyond the casing and functioning as terminals. As in the case of the previous embodiments, the entire casing 31 is capsulated in an insulating coating 32 which seals the open ends of the casing as well as the slot 30.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. An electrical resistor, comprising a rigid electrically insulating casing having a generally iU-shaped configuration including a pair of spaced side walls and a connecting wall conecting the side walls together, the space between the side walls defining a slot having a greater depth than width, an electrically conductive element having a predetermined length to provide a given resistance for the resistor located within the slot and disposed in engagement with at least one of said side walls, terminal means connected to the element and extending outwardly of the casing through said slot, and electrically insulating sealing means disposed within the slot for sealing the element within the slot.

2. The resistor of claim 1, wherein said element is a metal strip.

3. The resistor of claim 2, wherein the length of the strip is less than the length of the casing.

4. The resistor of claim 2, wherein said strip has a generally rectangular cross section and has a greater width than thickness.

5. The resistor of claim 2, wherein said terminal means comprises a plurality of generally flat terminal members secured to said electrically conductive strip.

6. The resistor of claim 1, wherein said casing is a ceramic material.

7. The resistor of claim 1, wherein casing has open ends and said sealing means is an insulating coating disposed within said slot and said open ends.

8. The resistor of claim 1, and including mounting means for mounting the resistor in spaced relation to a supporting structure.

9. The resistor of claim 8, wherein said mounting means comprises a pair of feet connected in spaced relation to said casing.

10. The resistor of claim 8, Iwherein said mounting means comprises a metallic mounting member which substantially surrounds the central longitudinal portion of the resistor.

11. The resistor of claim 1, wherein said element comprises a metal wire wound on an insulating core.

12. The resistor of claim 11, wherein said core is a generally flat strip and the width of the strip is less than the depth of the slot so that the strip is completely ernbedded within said slot.

13. The resistor of claim 2, wherein said strip has enlarged end portions and said terminal means comprises a pair of terminal members, one of said pair of terminal members is connected to each end portion.

14. The resistor of claim 13, wherein said strip includes a central portion extending between said end portions and said central portion is provided with a series of corrugations.

15. The resistor of claim 13, wherein the terminal members are integral with the resistance strip, and the width of said strip is less than the width of said terminal members.

16. The resistor of clairn 2, wherein said terminal means comprises a pair of terminal members with each terminal member being secured flatwise to the side surface of the strip.

17. An electrical resistor, comprising a rigid electrically insulating casing having a generally U-shaped conguration and including a pair of spaced side Walls and a connecting wall connecting the side walls together, the space between the side Walls dening a slot having a greater depth than width, an electrical resistance assembly including an electrically conductive element and a plurality of terminal members connected to the element, said assembly being located within the slot and disposed in engagement with said side walls and said terminal members extending outwardly of said slot, and electrically insulating sealing means disposed within the slot for sealing the element within the slot.

References Cited UNITED STATES PATENTS 917,241 4/1909 Bolling 338-308X 2,021,486 11/1935 McDonell 338-253 2,114,458 4/1938 Schafer 338--269 2,419,655 4/1947 Reiser 338u258X 2,504,146 4/1950 Mossin 338-254 2,518,941 8/1950 Satchwell 338-254X 2,538,977 1/1951 Mucher 338-256 2,896,004 7/1959 Duffy 338-280 3,286,214 11/1966 Kolb 338--269 FOREIGN PATENTS 797,996 10/1968 Canada 317--242 'ELLIOT A. GOLDBERG, Primary Examiner U.S. Cl. X.R.

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