US 3058081 A
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Description (OCR text may contain errors)
Oct. 9, 1962 M. J. BRAUN RESISTOR TERMINAL Filed April 11, 1960 INVENTOR. MARINUS J. BRAUN ATTORNEYS United States Patent flice This invention relates to resistors of the type which are made by compressing a resistor mixture into a solid body with terminals having their ends embedded in the body. It is an object of the invention to provide an improved molded resistor construction in which each resistor has substantially uniform resistance with respect to similar resistors; and to limit to an inconsequential value the differences in resistance thatare caused by the soldering of the terminals in a circuit.
Resistors are made by molding material of uniform composition and cross section, and with the terminals uniformly embedded, but even under careful manufacturing supervision may differ from one another in resistance values. When the terminals of such resistors are connected into circuits by soldering, changes occur in the resistance values, and these changes often are more than the acceptable limit of variation, which is three percent.
It is an object, as indicated above, to provide a molded resistor with wire terminals that has uniform initial resistance and that can be soldered without producing consequential changes in the initial resistance of the resistor.
Still another object is to provide an improved method of making resistors of the character indicated.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.
In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views:
FIGURE 1 is a diagrammatic, isometric view showing resistors, made in accordance with this invention, con nected together in :a circuit;
a FIGURE 2. is a sectional view taken on the line 2--2 of FIGURE -1; and
FIGURES 3 and 4 are enlarged views showing a method of making the resistors shown in FIGURES 1 and 2.
FIGURE 1 shows two resistors 11 and 12. Each of these resistors consists of a block of compressed resistor mix with a groove 14 in the bottom face of the resistor. The portion of the bottom face, on both Sides of the groove =14, is covered with a layer of metal conductor material, preferably copper. This layer of copper is indicated by the reference character 16. There is no copper within the groove 14.
Terminals 18 extend from the bottom face of each of the resistors 11 and 12 at locations substantially midway between the groove 14 and the opposite ends of the re sistors. Each of these terminals '18 consists of a wire having one end embedded in the mix of which the resistor is made.
FIGURE 1 shows diagrammatically the resistors 11 and 12 connected in a circuit represented by conductors 20 shown in dot-and-dash lines. These conductors are connected to the terminals 18 by solder 21. The resistor 11 may be the only resistor in the circuit when a switch 22 is in the position shown. Changing this switch 22 to its other position adds the resistor 12 to the circuit and the amount of resistance added by the closing of this switch is exactly equal to the resistance provided by the resistor 11 because of the uniformity of the resistance provided by these two resistor units '11 and 12.
FIGURE 2 shows the construction of the resistor 11. It is made of a mixture 24 of material having high specific resistance, such as graphite, a filler, and a resin binder. The terminal 18 is a wire, preferably copper, heavily 3,058,081 Patented Oct. 9, 1962 tinned. This wire may have its upper, or embedded, end bent back on itself to form a fold 26, in which case the end face 28 of the conductor 18 is preferably flush with the outside face of the copper layer 16.
FIGURES 3 and 4 illustrate one method by which the resistors of this invention may be manufactured. A supply of copper powder 16 is placed in a die 30. The die 30 has a bottom formed by punch 32 which has openings 34 into which the terminals 18 have been insterted. The end face 28 of the folded part of each terminal 18 rests on the top surface of the bottom punch 32 so that it will be flush with the outside surface of the copper layer after the powder 16' is compressed.
After adding a measured quantity of the copper powder 16' sufficient to cover the top of the punch 32, and leveling the powder in the die, as by rapping the die, the resistor mix, in uncompressed powder form, is added to the die 30. This uncompressed resistor mix is indicated by the reference character 24', and it is added in sufiicient quantity to fill the die. The resistor mix is then scraped even with the top 36 of the die. A movable punch or die closure element 40 (FIGURE 4) then is brought down into contact with the material in the die and this movable die element 40 is forced downwardly to compress the copper powder 16' and the resistor mix 24 under a high pressure. The copper becomes an integrally united part of the resistor, as is described in the C. H. Krellner Patent No. 2,903,666, September 8, 1959, owned by the assignee of the present application.
In a preferred method the powdered material is compressed by a force of about 12.5 tons per square inch, and the product is cured by being baked at 475 F. for onehalf hour in a continuous type oven. These values are given by way of illustration.
Obviously, the procedure of forming the resistor might be reversed. In this case the bottom punch would not have any openings to receive terminal wires, the resistor mix would be placed in the die first to fill the die opening and leveled off, and then copper powder would be placed over the top of the resistor mix. Terminal wires, whether with folded end, upset head, or spade head, then would be forced into the matrix, and the molding of the resistor accomplished in known manner. Double acting presses and/ or free floating dies might be employed.
The block of material, after removal from the die, has a groove 14 (FIGURE 1) cut in its bottom face by a mechanical cutting operation. The resistance can be controlled accurately by regulating the exact depth of the groove 14, since the groove changes the cross section of the resistor between the terminals 18. The cutting may be performed while continuously measuring the resistance value.
The presence of the copper layer 16 prevents the resistance value from being materially affected by the application of solder to the terminals 18. The principle which produces this result is not understood. No coating of copper is necessary in the groove 14, of course, and is purposely omitted so as to make the resistor mix carry the entire current, which passes through the resistor at its mid region where it has its adjusted minimum cross section.
Although the preferred construction and method of manufacture have been illustrated and described, changes and modifications can be made, and some features can be used in different combinations without departing from the invention as defined in the claims.
What is claimed is:
1. In a resistor comprising a block of compressed resistor mixture with a groove in one face of the block dividing said face into two spaced areas, a conductor embedded in the block at each of said areas and extending from the block through each area, and a metal coating covering each area and bonded to the face of the block and to the side of the conductor where the wire emerges from the block.
2. The resistor described in claim 1, and in which the block is a composition of material having high specific resistance, a filler, and a resin binder; and the metalcoating is made of a powdered metal bonded into an integral unit with the other material of the block.
3. The resistor described in claim 2, and in which the metal powder is essentially a copper powder and it is an outside layer of the block molded in situ with the rest of the material of the block.
4. Theresistor described in claim 1, and in which each conductor is ,a wire having its embedded portion anend of the wire bent back on itself and with the ends of the bent-back portion flush with the outside surface of the metal coating.
5. A resistor comprising a U-shaped body of compressed resistor mixture and with end faces of the U in a common plane, terminal wires extending from said end faces in generally parallel directions, each of the terminal wires having one end thereof embedded in a different one of the end faces, and a metallic coating extending in all directions from each wire terminal over the surrounding surface of the end face from which that wire terminal extends.
6. The resistor described in claim 5 and in which the resistor is formed into a .U-shaped body by a groove in the side of the body on which said end faces are located,
-and'the metallic coating covers all of the grooved side A of the body except the surface that is withinthe groove.
7. The combination comprising a plurality of likeresistors connected in a common circuit and each of which comprises a molded block of compressed resistance mixture having conductor terminals extending therefrom and relationto one another and with? corresponding ends extending into a die cavity from the lower side thereof, placing a layer of metal powder in the bottom of the die cavity, adding a resistor mixture to fthe cavity above the metal powder and for a depth above the conductor terminals, compressing the resistor mixture and metal powder under high pressure in the die to form an integral block with the metal powder as a layer over the entire bottom face of the block andrwiththe ends of. the conductor terminals securely anchored in the block, removing the block from the die, and cutting a groove across the bottom face of the block between the conductor terminals and for a depth substantially greater than the depth of thelayer formed by the metal powders.
9. The method described in claim 8, and in which the conductor terminals are formed of wire, and tthe method includes the steps of-bendingthe end of each wire back on itself, and placing the wiresinthe die cavity with the portions of the wire beyond the bent ends extending through openings in the bottom of the cavity and with the endfaces of the bent back wires in contact with the bottom of thedie cavity.
10. The method of making a resistorocomprising the steps of confining a body of resistor mixture in a die cavity with a layer, of metalpowder coextensive with one surface of the said body, locating two conductor terminals in spaced and generally parallel relation to one another and with corresponding ends extending through the layer of metal powder a uniform distance into the I resistor body, compressing the body of resistor mixture and metal powder to form an integral block with one metal surface through which the spaced conductor terminals extend, heat treating the. blo-ckso formed, and cutting a groove across the metal face of the block between the conductor terminals to a depth below the metal face.
References Cited in thefile of this patent -UNITED STATES PATENTS 2,282,328 Herrick et al. May 12, 1942 2,472,801 Barfield et al. June 14,1949 2,629,166 Marsten et al. Feb. 24, 1953 2,668,932 Kliever Feb. 9,;1954 2,693,023 Kerridge et al. Nov.' 2,, 1954 2,883,502 oRudner -Apr..21,. 1959 2,903,666 Krellner Sept..8, 1959