Electrical resistor and method of making same

March 31, 1959 l. h. berkelhamer 2,880,296

ELECTRICAL RESISTOR AND METHOD OF MAKING SAME

Filed Sept. 23, 1954

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United States Patent Office

2,880,296

Patented Mar. 31, 1959

2,880,296

ELECTRICAL RESISTOR AND METHOD OF MAKING SAME

Louis H. Berkelhamer, Chicago, Hl., assignor to David T. Siegel, Glencoe, Hl.

Application September 23, 1954, Serial No. 457,945

Claims. (Cl. 201—64)

The present invention relates to a novel electrical impedance unit and more particularly to a novel electrical resistor and a method for making the same.

Pile resistors have heretofore been made by winding one or more coils of wire on a ceramic core and then impregnating the coils with a suitable varnish or resin. While such resistors have been satisfactory for many purposes they have been unable to meet high specifications requiring them to withstand the effects of rapid temperature changes between wide limits and high humidity and requiring them to have a relatively long operating life and stability when exposed to severe temperature changes. It was suggested that the above difficulties might be overcome by replacing the ceramic core with a plastic core and enclosing the coils in a relatively thick body of plastic material identical to the plastic material of the core to minimize internal strains caused by temperature changes. However, this suggestion is not entirely satisfactory since such plastic cores or bobbins are relatively difficult and expensive to produce. Plastic cores or bobbins have been made by machining which is relatively difficult as compared to similarly forming a ceramic bobbin and, moreover, such plastic bobbins cannot easily be made uniform in size and shape since even thermo setting plastics are deformed under the pressure of a cutting tool so that acurate machining thereof is very difficult.

It is a primary object of the present invention to provide a novel electrical resistor and a novel method for making an electrical resistor so that the above mentioned difficulties are overcome.

More specifically, it is an object of the present invention to provide a novel electrical resistor having substantially improved operating characteristics and a novel method for making the resistor economically.

Other objects and advantages of the present invention will become apparent from the following description and the accompanying drawings wherein:

Fig. 1 is a perspective view illustrating a resistor embodying the principles of this invention;

Fig. 2 is an enlarged cross sectional view taken along line 2—2 in Fig. I; ' ’ .

Fig. 3 is a cross sectional view taken along line 3—3 in Fig. 2;

Fig. 4 is a fragmentary cross sectional view taken along line4—4 in Fig. 3;

Fig. 5 is a perspective view illustrating a partially completed resistor embodying the principles of this invention;

Fig. 6 is a side elevational view illustrating a partially completed resistor after it has been impregnated with a suitable resin;

Fig. 7 is a somewhat diagrammatic view partially in cross section illustrating one step in the production of resistors embodying the principles of this invention;

Fig. 8 is a cross sectional view taken along line 8—8 in Fig. 7;

Fig. 9 is an elevational view illustrating the last step in the production of resistors embodying the principles of this invention;

Fig. 10 is a perspective view of a modified form of the invention; and

Fig. 11 is a cross sectional view of the resistor shown in Fig. 10.

δ Referring now more specifically to the drawings wherein like parts are designated by the same numerals throughout the various figures, an electrical resistor element 10 embodying the principles of this invention includes a ceramic core or bobbin 12 having a continuous wire wound thereon: 10 to provide one or more coils 14. The ceramic core may be easily and economically formed by machining the ceramic material while it is green to provide flanges 16 for confining the coils after which machining operation the ceramic material is fired. In this manner a ceramic 15 core or bobbin may be economically and accurately formed. Metal terminal members 18 and 20 are mounted on opposite ends of the ceramic core or bobbin and ends 22 and 24 of the wire are secured to these terminal members as by spot welding.

In accordance with an important feature of the present invention the ceramic core and the wire coils are encased in a body 26 of plastic material which is thermally compatible with the ceramic core. It should be noted that the body 26 closes not only the outer periphery and the 25 ends of the core but it also encloses the internal surfaces of the core, or in other words, the plastic body extends within the central aperture of the core. With this structure of the wire of the resistor element is hermetically;, sealed within the body 26 which is impervious to moisture 30 and since the plastic material is thermally compatible with the ceramic material of the core or bobbin, the resistor element can withstand relatively rapid and severe temperature changes. It is believed that this substantial improvement in the operating characteristics of the re35 sistor element results from a reduction in internal strains which might cause the plastic body to check or crack or which might otherwise injure the resistor element. It has been found that the substantially improved results mentioned are obtained when the body 26 is formed with 40 an epoxy resin and a suitable hardening agent mixed with a mineral filler such as talc. In order to assure that the mixture for the body 26 will not be porous and pervious to moisture it is important that the filler particles have a fibrous or needle-like structure rather than a granular 45 structure so that they may pack and interlock tightly together. It is believed that the interlocking of the relatively long needle-like or fibrous filler particles also helps to increase the resistance of the plastic body to cracking or checking.

As set forth in the preceding paragraph the body 26 hermetically seals the wire of the resistor and as will be seen by referring to the drawings the only possible, opening in this seal is where prongs 28 and 30 of the terminal members project outwardly from the body;

The plastic material of the body 26 adheres to the metal . of the terminal members so that the seal is complete.

However, in accordance with the present invention any possibility of moisture entering the body 26 through the openings created by the prongs 28 and 30 is reduced 60 to a minimum by reducing the cross sectional area of the prongs at the points where they pass out of the body 26. Thus, the prong 28 is provided with an opening or struck out section 32 which extends above and below the surface 34 of the body 26 and the prong 30 is provided with a 65 similar opening 36. The plastic material enters into the' openings and between spaced sections 38 and 40 of the. prongs as shown best at 42 in Fig. 4. Thus, the opening through the body 26 for each prong is equal to the combined areas of the prong sections 38 and 40 rather than 70 to the entire cross sectional area of the prong. By providing the apertures in the prongs in the manner illustrated in the openings through the body 26 may be kept %880,W at a minimum sjze while, at the same time the prongs will have sufficient strength and rigidity.

In manufacturing the resistor element 10 the ceramic core is preferablj’ formed by machining; the rod of green ceramic material after which the core is fired. A wire 5 having any suitable insulating cQatjng thereon is then wound on the core or bobbin to provide one or more coils and the opposite ends of the wire are, secured to the terminal members which, of course, are fastened to the opposite ends of the core. At this stage of produc- 10 tion the resistor will appear substantially as illustrated in Fig. 5. Subsequently and as an optional step, the coils may be impregnated with a suitable varnish, or resin such as a silicone resin by utilizing known vacuum impregnation methods. In the event the resistor element 15 is to be impregnated with a silicone resin or other suitable material the prongs 28 and 30 of the terminal members may first be dipped into and coated with solder. Then after the resistor element is, immersed in the impregnating material the impregnating material is 20 cleaned from the prongs and the prongs may again, be dipped into solder. The resulting structure is illustrated best in Fig. 6 which shows solder coatings 44 and 46 on the ends of the prongs while, substantially the remainder of the element is provided with a, coating of the 25 silicone resin or other suitable impregnating material. While other means of applying the solder may be employed, the foregoing procedure insures that the solder coatings to which wires may be easily attached will adhere to the prongs, since the initial application of the solder 30 is made while; the prongs are uncontaininated by the silicone resin or other impregnating material.

In preparing the material for the body 26 a mineral filler such as talc is first mixed with a suitable thermoplastic resin such, as an epoxy resin. If desired a coloring 35 material may first be mixed with the filler. Then a hardener agent is added to the mixture and the mixture is; molded or cast around the core or bobbin and wire coils to provide the body 26. In order to reduce any possibility of air bubbles being contained in the final 40 plastic body 26 the mixture is preferably subjected to vacuum prior to or during the casting operation to eliminate air therefrom.

When casting the body 26, a plurality of the bobbins which have been processed to the point illustrated in ⁴⁵either Figs. 5 or 6 may be arranged in a cavity mold 50 as shown in Figs. 7 and 8. A rod 52 is positioned so that it extends through the central apertures of the cores or bobbins and the diameter of the rod is substantially less than the diameter of the core apertures so that the plastic material may extend through the core apertures in the manner illustrated. After the bobbins have been properly positioned within the mold the mixture of; plastic resin and filler- which has been prepared in the manner set forth above is poured into the mold ⁵⁵and again subjected to a vacuum to further reduce, possibility of any air bubbles appearing in the finished product. The mold is then placed in an oven and the plastic material is cured at an elevated temperature. Preferably the plastic material is cured by first heating it for a short ⁶⁰period of time, then chilling it and then reheating to finish curing. This procedure will impart to the plastic material proper resistance to cracking and checking. As will be understood there may be instances in which it is desirable to eliminate the chilling of the mold and ⁶⁵plastic material after curing or at least when this step is unnecessary and also in many cases the plastic material may be cured at room temperature although this usually takes a considerably longer period of time. The final γθ step in the process is illustrated in Fig. 9 wherein the plurality of bobbins have been removed from the mold and the plastic material has been severed to separate the individual electrical impedance or resistor elements.

In Figs. 10 and 11 there is illustrated a resistor unit 60 which embodies a modified form of this invention. The unit 60 includes a ceramic core 62, wire coils 64 and a plastic body 65. These elements are essentially identical and are formed in substantially the same manner as the corresponding elements of the unit 10 and need not be described in detail. The unit 60 differs from the unit 10 in that terminal members* 66 are provided instead of the above described terminal members 18 and 20. Each terminal member 66 includes a metal cap 68 which is applied to the end of the core or bobbin and an axially extending wire or rod 70 butt welded to the cap. The opposite ends of the wire coils are, of course, connected to the caps. This construction simplifies the application of the, terminal members to the core or bobbin and the wires or rods; 70 require the smallest possible opening through the plastic body so that any danger of moisture entering the plastic body is reduced to a minimum.

From the above description it; is seen that the present invention, provides a. novel electrical impedance or resistor element having a substantially increased working life and resistance to thermal shocks and moisture. In addition, it is seen that by utilizing the novel method of this invention the resistor elements may be rapidly and relatively economically manufactured.

While the preferred embodiments; of the present invention have been illustrated and described herein it is obvious that many details may be changed without departing from the spirit and scope of the appended claims.