US3795046A

US3795046A - Method of making a heat sinked resistor

Info

Publication number: US3795046A
Application number: US00241420A
Authority: US
Inventors: G Shirn
Original assignee: Sprague Electric Co
Current assignee: Sprague Electric Co
Priority date: 1972-04-05
Filing date: 1972-04-05
Publication date: 1974-03-05
Anticipated expiration: 1991-03-05

Abstract

A ceramic core has selected resistance alloys wound thereon, and is covered by a vitreous-enamel insulating layer, and finally by a relatively thick metallic outer covering that is bonded to the vitreous-enamel layer. The metallic outer covering can be applied by pouring molten metal over the vitreous-enamel covered resistor that has been preheated. The resultant electrical resistor is designed for the utmost stability under extreme operating conditions, and has a power rating that is increased over fivefold from the power rating without the metal outer layer.

Description

[ Nlar.5,1974

United States Patent- [191 Shirn METHOD OF MAKING A HEAT SINKED 8/1947- Deyrup 11/1949 RESISTOR I Kohring 12/1950 Scott et al..

[75] Inventor:

George A. Shirn, Williamstown, Mass.

[73] Assignee: Sprague Electric Company, North FOREIGN PATENTS OR APPLICATIONS Adams, Mass.

Apr, 5, 1972 1/1948 Great Britain..l...................,.

[22] Filed:

Primary Examiner-Charles W. Lanham Assistant ExaminerVictor A. DiPalma [21] APPL NOQ 241,420 I Related U.S. Application Data [63] Continuation-impart of Ser. No. 104,621

' [57] ABSTRACT A ceramic core has selected resistance alloys wound thereon, and is covered by a vitreous-enamel insulat- Jan. 7,

1971 abandoned.

ing layer, and finally by a relatively thick metallic outer covering that is bonded to the vitreous-enamel layer. The metallic outer covering can be appliedby pouring molten metal over the vitreous-enamel 'cov- 338/269, 29/613 H01c 3/00, HOlc 5/00, H010 17/00 [51] Int. Cl.

0 5 6 6 d 1 6 E a 63 ,4 m Km 92 2 5 1 .5 "W $3 .m .3 f o d l .m F. .l 8 5 References Cited UNITED STATES PATENTS 5 1912 Barrmger 29/618 x 4 (Ilaims, 2 Drawing Figures 111932 Terwilliger.......................... 338/262 METHOD OF MAKING A HEAT SINKEI) RESISTOR This is a continuation-in-part of application Ser. No.

into the metal 104,621, filed January 7, 1971 and later abandoned. BACKGROUND OF THE INVENTION core upon which the resistance wire is wound, or a ceramic core thatis plated or is impregnated, and a coating over the resistance and the core to aid in electrically insulating and dispersing the heat of the resistor, as well as protecting the resistor. When the resistor develops high temperatures under electrical loads, the heat transfers to the coating and dissipates into the atmosphere and, as is the case with heat sinked resistors, chassis upon which the resistor is mounted. I I

Different materials for surrounding resistors have been tried with varying degrees of success. The coating or housing materials should have the qualities of .electrical insulation, heat dissipation, thermal protection, humidity protection, maintaining solidification and shock resistance, break-proof, andbe economical to use in the fabrication of the resistors. While some coatings or housings solve certain requirements, no treatment her'ebefore used has been completely satisfactory.

Sealing resistors in glass is economical, affords high temperature and moisture protection, but produces fragile units that are not suited for particularly heat sensitive elements. Silicone varnish dip-coatings have good thermal stability, good moisture resistance, fair solvent resistance, but are frequently difficult to apply, subject to pinholes, and thus electrical insulation failures, and are not good thermal conductors. Silicone polymers, highly filled with inorganic materials such as alumina, mica, asbestos, silica or metallic oxides ordinarily have good thermal stabiiity,'fair moisture resistance, but have poor solvent resistance and manufacturing difficulties associated with maintaining the tiller in suspension and in proper ratio.

Sleeves or tapes of various polyesters, fluorocarbons, glass fibers, phenolics, and silicones, alone or in combination are expensive to apply, are good electrical insulators, withstand thermal shock, but are poor thermal conductors. These sleeved or taped resistors can be inserted into metallic housings, or thin sleevings such as aluminum, to increase the heat dissipation, but at a greater manufacturing cost, and with attendant difficulties in cente'ring the resistor in the housing andsubsequently filling the remaining space. v

Ceramic tubes have high temperature humidity, and solvent protection, but are difficult to seal hermetically and are susceptible to cracks or breaks due to the brittleness of the ceramic.

Metal housings for wire-wound resistors have the excellent characteristics necessary to provide reliable and stable resistors. HOwever. these units are produced at rather high manufacturing costs, and have heretofore been difficult to work with because of cumbersome cements and/or cement-fillers. Some have mixed beryllia flour with an exposy filler, but beryllia flour can be quite dangerous to work with. Prior art metal housings that were cast on the resistor cores have suffered from poor insulation characteristics and are subject to cracking upon the application of the cast metal housing.

Accordingly, it is an object of the present invention to provide an electrical resistor that has good electrical insulation properties, as well as the ability to dissipate heat evenly and properly.

A further object of this invention is to provide a resistor that is moisture-proof and solvent resistant.

A still further object of the present invention is to provide a means of economically manufacturing a resistor possessing good electrical insulation characteristics that will not crack upon the application of a cast metal outer coat. I

SUMMARY OF THE INVENTION Leads can be welded to extend out of opposite ends of t .ends of the vitreous-enamel and the leads. The mold can be designed in such a manner that tabs are provided for attaching the housing to a chassis, and optionally the surface area of the metal housingcan be increased for cooling purposes by use offins" or flanges thereon.

A vitreous-enamel covered resistor that had a power rating of 5 watts prior to being cast in a metal housing as described herein is transformed into a heat sinked resistor whose power rating is increased over five-fold by casting same in an aluminum housing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded view of the resistor of the present invention; and FIG. 2 is a perspective view of the assembled resistor of FIG. I.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I illustrates a heat sinked wire-wound resistor within the scope of this invention. The resistor core 10 is generally composed of a substantially smooth ceramic material. for example, percelain, magnesium sili' cate or magnesium aluminum silicate. The resistor core 10 is wound with a bare or uncoated electrical-resistance wire 11, in direct contact therewith by conventional winding means. The wire 11 would normally be composed of an iron-chromium-aluminum alloy. Nickel-chromium alloys or nickel-copper alloys can be utilized as the wire I1. The wire 11 is securely held in place by means of nickeled steel end caps 12 or other type of end terminals which are thermally shrunk or I swaged in place to hold them to the ends of the resistor core 10 and to which the ends of the wire 11 are welded to form an electrically conducting unit from one end cap 12 through the wire 11 to the other end cap. Bare terminal leads 13 are connected to each cap 12 as. by welding, brazing or soldering with high temperature solder, so as to extend outward therefrom.

This unit is then insulated by dipping it in a clay slip and drying it in a normal atmosphere. After drying, the unit is fired in a kiln heated to approximately 700 C in order to fuse the enamel 14. This process can be repeated as many times as necessary to achieve a suitable thickness. it is repeated three times herein to obtain the proper amount of insulation. This vitreous-enamel covering should have an expansion coefficient that is closely matched to that of the ceramic core in order to insure positive protection of the resistance wiring.

Another method of applying the vitreous-enamel coating is to flame-spray the solid coatings by feeding the powders into a powder flame-spray burner. Other suitable methods are available if desired, and may be used without limiting the scope of this invention.

The vitrous-enamel layer serves to insulate the turns of bare wire from one another and secure the turns in position in proper spacing on the core. The vitreous layer also provides a continuous barrier against the penetration of the outer metal covering between the turns of the winding. The fusing .of the enamel layer acts to produce a substantially level coating inwhich the elevations'of the turns of the winding are obscured, so that they are not reflected in the outer metal covering.'

This vitreous-enamel protected resistor is now capable of being covered by a layer of aluminum that can be applied in the molten state in a preheated mold.

The aluminum housing of FIG. 2 is cast on the vitreous-enamel 14 by placing the vitreous-enamel covered resistor into a mold capable of withstanding relatively high temperatures, preheating the unit to a temperature of approximately 350 C, and pouring molten aluminum into the mold that is designed in such a fashion that the aluminum 15 will surround substantially all of the enamel covered resistor 14 leaving the outermost portion 18 of the caps 12 exposed. The resistor 20 is then cooled and removed from the mold. The resistor 20 can be polished if necessary to remove any rough edges on the outer surface thereof.

The temperature of the mold and enamel. covered resister is raised to 350 C before aluminum is poured therein to facilitate and ensure a smooth even flow of the molten metal through the mold, and to prevent the vitreous-enamel coating 14 from cracking from thermal shock upon the addition of the molten metal. Aluminum melts at approximately 660 C and flows freely through a mold preheated to 350 C. A bond is formed between the molten metal and the vitreous-enamel, resulting in a compact, stable, protective coating for the resistor, thereby giving the unit a better heat conducting path than electrical resistors have. been able to achieve in prior art resistors.

The mold and resistor unit should be preheated to a temperature not greater than 500C, but at least to that point where the molten metal can flow smoothly through the mold without solidifying or cracking the vitreous-enamel. if the temperature is allowed to go above 500 C, the vitreous-enamel coating will start to liquify and the metal will come in contact with the woundwire.

allowing for the main heat sinking Optionally, the design of the mold may be such as to provide for cooling flanges 16 on the metal housing. The flanges 16 give more surface area to the housing and allows for a more rapid and complete dissipation of heat into the atmosphere. The design of the mold also should provide for mounting tabs 17 so that the flat-bottomed resistor can be secured to a metal chassis contact to the metallic chassis. I 7

it should be emphasized that the vitreous-enamel insulating layer and the precaution of casting the aluminum in a preheated mold permit the desirable results achieved herein.

While aluminum is mentioned as the preferredv metal herein, other high thermally conductive materials may be used with varying degrees of success. Some of these other metals include lead, tin, zinc,'magnesium, and alloys or mixtures thereof.

Any metal used should melt below approximately 800 C, as the metal must be heated about 50 C higher than its melting point to be able to flow freely. If the temperature of the metal is too high-beyond 800-900 C'-then the vitreous-enamel coating will crack or disintegrateupon exposure to these temperatures, or the molten metal may solidify when it comes in contact with the relatively cooler mold; As was prising;

pointed out earlier, the mold should not be heated beyond 500 C. So any highly thermally conductive metal,

or alloy, can be used if it melts below 800 C.

While heat sinked casting of the metal is preferred herein, it is not the only method that can be used.0ther methods giving similar, although less desirable, results include die casting, spraying, and powder sintering; all are well known methods to those skilled in the art.

A resistor thusly formed is thermally and physically more stable than prior art resistors; it is protected from corrosive fumes mechanical damage, and the effects of humidity. As mentioned earlier herein, the addition of the outer metal layer increases the power rating of the vitreous-enamel covered resistor at least five fold. The heat conducting path formed thereby, gives an even dissipation of heat that allows for excellent quick cooling of the resistor unit. 7 V

What is claimed is:

1. The method of forming an electrical resistor coma. winding a bare resistance wire along a substantially smooth ceramic core, said. wire being in direct contact with said core;

b. placing metal terminals on opposite ends of said core and connecting said wire to said terminals to form an electrically conducting unit;

c. applying a vitreous-enamel insulating layer over said conducting unit so as to insulate. the turns of said bare wire from one another and secure the turns onto said cor'e, said layer having a thickness to provide a continuous barrier against penetration of an outer metal covering between the turns'of said winding, said layer having an expansion coefficient closely matched to said core;

d. firing said insulated conducting unit at approximately 700 C to fuse said vitreous-enamel layer; and I e. applying a thermally conductive metal outer cover ing having a melting point below 800 C over substantially all of said insulated conducting unit in direct contact with said insulating layer, said coverto be through -4. A method of forming an electrical resistor comprising: I

a. winding a bare resistance wire along a substantially smooth ceramic core, said wire being in direct contact with said core;

b. placing metal terminals on opposite ends of said core and connecting said wire to said terminals to form an electrically conducting unit, said terminals having bare leads extending outward therefrom;

c. applying a vitreous-enamel insulating layer over said conducting unit so as to insulate the turns of said bare wire from one another and secure the turns onto said core, said layer having a thickness to provide a continuous barrier against penetration of an outer metal covering between the turns of said winding, said vitreous-enamel layer has an expansiori coefficient that is closely matched to that of said ceramic core;

d. firing said insulated conducting unit at approximately 700 C to fuse said vitreous-enamel layer;

e. preheating saidunit in a mold to a temperature that will allow for the free flow of molten metal therethrough, but not to exceed 500; and

f. casting molten aluminum thereon substantially covering said unit and leaving said bare terminal leads out of contact with said aluminum.

Claims

1. The method of forming an electrical resistor comprising: a. winding a bare resistance wire along a substantially smooth ceramic core, said wire being in direct contact with said core; b. placing metal terminals on opposite ends of said core and connecting said wire to said terminals to form an electrically conducting unit; c. applying a vitreous-enamel insulating layer over said conducting unit so as to insulate the turns of said bare wire from one another and secure the turns onto said core, said layer having a thickness to provide a continuous barrier against penetration of an outer metal covering between the turns of said winding, said layer having an expansion coefficient closely matched to said core; d. firing said insulated conducting unit at approximately 700* C to fuse said vitreous-enamel layer; and e. applying a thermally conductive metal outer covering having a melting point below 800* C over substantially all of said insulated conducting unit in direct contact with said insulating layer, said covering being applied by preheating said unit in a mold to a temperature that allows for the free flow of molten metal therethrough but not exceeding 500* C, and casting said covering thereon in its molten state.

2. The method of claim 1 wherein said mold is designed so as to provide cooling flanges on the outer surface of said metal covering and mounting tabs attached to the base thereof.

3. The method of claim 1 wherein said metal outer covering is of aluminum.

4. A method of forming an electrical resistor comprising: a. winding a bare resistance wire along a substantially smooth ceramic core, said wire being in direct contact with said core; b. placing metal terminals on opposite ends of said core and connecting said wire to said terminals to form an electrically conducting unit, said terminals having bare leads extending outward therefrom; c. applying a vitreous-enamel insulating layer over said conducting unit so as to insulate the turns of said bare wire from one another and secure the turns onto said core, said layer having a thickness to provide a continuous barrier against penetration of an outer metal covering between the turns of said winding, said vitreous-enamel layer has an expansion coefficient that is closely matched to that of said ceramic core; d. firing said insulated conducting unit at approximately 700* C to fuse said vitreous-enamel layer; e. preheating said unit in a mold to a temperature that will allow for the free flow of molten metal therethrough, but not to exceed 500*; and f. casting molten aluminum thereon substantially covering said unit and leaving said bare terminal leads out of contact with said aluminum.