US 3887893 A
A cermet resistive film is deposited on an alumina substrate to form a fixed resistance between a pair of spaced termination points. A portion of the resistive film is removed to form a pair of resistor sections separated by an insulating gap. A fusible link which includes a layer of cadmium material is deposited in the gap to provide electrical continuity between resistor sections and the resistor body is then covered with a protective coating. When a maximum temperature is reached due to an overload current, the fusible material melts and opens circuit.
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Description (OCR text may contain errors)
United States Patent Brandt et al.
[ FUSIBLE RESISTOR  Inventors: Ivan L. Brandt, Fox Point; Theodor von Alten, Grafton, both of Wis.
 Assignee: Allen-Bradley Company,
 Filed: Sept. 24, 1973  Appl No.: 400,236
 US. Cl. 338/262; 337/296; 337/297; 338/215; 338/273; 338/295; 338/334  1nt. Cl H011: 1/00  Field of Search 338/334, 308, 295, 273, 338/215, 262; 317/256, 242, 260; 337/405, 413, 296, 297, 159, 163, 190; 219/517  References Cited UNITED STATES PATENTS 1,208,448 12/1916 Arthur 337/290 1,255,597 2/1918 0116s 317/242 2,263,752 11/1941 Babler 337/297 2,537,959 l/1951 Beverly 317/260 1 June 3, 1975 2,672,542 3/1954 Fisher 338/215 2,973,418 2/1961 Whitman 337/163 3,386,063 5/1968 1146656616 1111 337/405 3,423,574 1/1969 5116111 5116... 337/405 3,441,804 4/1969 K161111661 338/308 3,766,508 7/1972 Wada 337/296 Primary Examiner-E. A. Goldberg Auorney, Agent, or Firm-Ouarles & Brady [5 7] ABSTRACT A cermet resistive film is deposited on an alumina substrate to form a fixed resistance between a pair of spaced termination points. A portion of the resistive film is removed to form a pair of resistor sections separated by an insulating gap. A fusible link which includes a layer of cadmium material is deposited in the gap to provide electrical continuity between resistor sections and the resistor body is then covered with a protective coating. When a maximum temperature is reached due to an overload current, the fusible material melts and opens circuit.
16 Claims, 5 Drawing Figures 3 I ,1, jtx A 6 Ill/II] I I FLU/A16 LA YER FUSIBLE RESISTOR BACKGROUND OF THE INVENTION The field of the invention is resistors used in electrical circuits, and more specifically, resistors having a fusible link which opens the circuit when an electrical overload occurs.
Fusible resistors are used in electrical circuits to protect components in the circuit from overload currents. They are typically much larger than conventional fixed resistors, substantially more expensive, and usually available in only a few resistance values. Except for power supplies and power output circuits of electrical systems, the use of fusible resistors has hitherto been minimal.
Prior fusible resistors have either been constructed of resistance wire which burns out at a predetermined temperature or of a resistance material which is located in close proximity to a fusible link that melts at a predetermined temperature. For example, in US. Pat. No. 2,966,649 issued to A. Haynman on Dec. 27, 1960, a fixed resistor is disclosed and a metallic band is disposed around the resistor body with one of the resistor lead wires connected to it through a fusible link. The lead wire springs free of the band when the fusible link melts to open the electrical circuit at a preselected temperature.
SUMMARY OF THE INVENTION The present invention relates to a fusible resistor and method of manufacture in which a fusible link is integrally formed with a film of resistive material. More specifically, the fusible resistor is formed by depositing a resistive film on a substrate between a pair of spaced termination points, providing a gap which interrupts electrical continuity between the termination points, and depositing a fusible link on the substrate within the gap to provide electrical continuity across the gap at normal operating temperatures. The fusible link includes a fusing layer which melts at a preselected temperature, and which separates by surface preferred wetting to open-circuit the resistor.
A general object of the invention is to provide a fusible resistor which is competitive in size, operation and cost with a fixed resistor of comparable wattage rating. The shape and size of the substrate may be identical to that used to make a corresponding fixed resistor, and the carbon, metal or cermet resistive film which is deposited thereon may be identical to that used to make a corresponding fixed resistor. Additionally, the method and means used to attach the lead wires to the substrate and to insure electrical contact with the termination points is identical to that used in manufacturing a corresponding fixed resistor. In other words, the manufacturing process is substantially the same as that used to make a corresponding fixed resistor and only a few additional steps are needed to form a gap and deposit the fusible link.
Another object of the invention is to provide a fusible resistor in which it is economically feasible to manufacture a variety of standard resistance values. After the resistive film is deposited on the substrate, it is adjusted in value by cutting a helical groove in the resistive film. Because the shape and size of the substrate may be the same as that of a corresponding fixed resistor, the same manufacturing process and equipment can be used to form and adjust the resistance element. The minimal additional cost necessary to add the fusible link to a wide selection of resistance values, not only makes it technically feasible, but also, economically feasible to substitute the fusible resistor for fixed resistors in many electrical circuits where an additional measure of protection is desired.
Another object of the invention is to insure that the fusible link opens the circuit at the selected temperature. The fusing layer is deposited on the substrate and spans the gap to provide electrical continuity. To insure electrical connection of this metallic layer, the gap is bounded by ohmic contacts formed of a conductive material such as silver. An important aspect of the invention is that the substrate, fusing layer and ohmic contact materials are selected such that when the fusing layer melts, it is drawn from the gap to the ohmic contacts by surface preferred wetting and, therefore, separates to open circuit the resistor.
Another object of the invention is to provide a selection of fusing temperatures. The fusing layer includes a low melting point material such as cadmium, tin or lead which may be alloyed in selected amounts with other materials to obtain the desired melting point. To prevent oxidation, and thus to prevent the melting point from rising during use, a layer of non-acidic flux is deposited over the fusing layer.
A further object of the invention is to provide a structure which may be formed in a number of convenient configurations. The substrate may be flat or cylindrical, the resistive film may be deposited on a portion of the substrate and the fusible link located in close proximity and in circuit with the resistive film.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, and reference is made to the claims herein for interpreting the breadth of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view with parts cut away of a fusible resistor embodying the present invention,
FIG. 2 is a perspective view of the fusible resistor during an early stage of its manufacture,
FIG. 3 is a partial view in cross section showing the gap in which the fusible link is formed,
FIG. 4 is a partial view in cross section of the completed fusible resistor, and
FIG. 5 is a partial view in cross section of the fusible resistor when open circuited.
In the drawings the various fusible resistor elements are not necessarily drawn to scale in order that all the elements described hereinafter can be shown. Reference is therefore made to the following description for specific dimensions and thicknesses.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of the drawings, the fusible resistor is embodied in a conventional one-quarter watt fixed resistor package which includes a circular, cylindrical body 1 having a pair of lead wires 2 extending from each of its ends. The resistor body 1 includes an insulating substrate which is shaped in the form of a circular, cylindrical core 3 having a central opening 4 that extends through its entire length. The substrate material is comprised of approximately 96 percent aluminum oxide which is extruded or pressed, and then sintered to form an alumina-ceramic material which forms a hard base to which the lead wires 2 are attached and upon which the resistance material is deposited as described hereinafter. For a more detailed description of the method of manufacturing the core 3, reference is made to U.S. Pat. No. 3,329,922.
A resistance film 5 is deposited along the entire length of the outer cylindrical surface of the core 3. Al though metal or carbon can be used, a cermet resistance material is used, and it is deposited on the surface of the core 3 by rolling the core across an applicator which contains the cermet material in a paste, or ink, form. The organic carrier which forms a part of the paste is first dried and then decomposed with heat and the cermet coated core 3 is then fired at approximately 900C. The glass constituent of the cermet is melted at this temperature and the metal materials which are dispersed throughout the glass form the metal and metal oxide system which is characteristic of cermet resistive materials.
To obtain a broad range of resistance values, a number of cermet inks are used. Each ink is formulated in a manner well known to the art and each exhibits a different ohms per square. For example, to obtain a full range of standard resistance values up to K ohms on a core 3 having a length of 0.270 inches and a diameter of 0.093 inches, two cermet inks having 3 ohms per square, and 100 ohms per square are used. As will be explained hereinafter, final calibration of the resistance value is made by cutting a helical groove in the cermet resistance layer.
Referring to FIG. 2, the resistive film 5 is terminated at each end of the body 1 by ohmic contacts 6 and 7. The ohmic contacts 6 and 7 provide a conductive path between the resistive film 5 and the ends of the core 3, and are formed by dipping the ends of the core 3 into a silver-glass mixture. A number of silver-glass mixtures are commercially available such as DuPont Silver Paste 8706 in which the silver comprises approximately 66-69 percent of the mixture, the glass approximately 3.7 to 5.9 percent, and the remainder is an organic carrier which forms an ink. In addition to the terminating ohmic contacts 6 and 7, a layer of the same silver-glass mixture is applied around the core 3 to form a center band 10 which is from 80 to 100 mils in width. The center band 10 is processed currently with the ohmic contacts 6 and 7 by first drying and then decomposing the organic carrier with heat.
As shown best in FIG. 1, a coating 13 of an organic, acid resisting material such as phenolic epoxy, is next applied to the resistor body 1 over all but the center band 10. Referring particularly to FIG. 3, the center band 10 and resistance film 5 are then divided into two sections by removing a 30 to 50 mil band of material from the center band 10 down to the alumina substrate 3. An electrically insulating gap is thus formed and is bound on each side by ohmic contacts 8 and 9 which are formed by the remaining portions of the center band 10. The cut is made sufficiently deep to clean the surface of the alumina substrate 3.
Referring particularly to FIG. 4, the exposed alumina substrate in the gap and the surfaces of the adjacent ohmic contacts 8 and 9 are now prepared to receive the fusible link. This is accomplished by applying a sensitizing material to the gap consisting of Ag-l-neodecanioc acid being dispersed in a vehicle of ethyl cellulose as a binder, di-n-butyl phthalate as plasticizer, and toluene and pine oil as solvents. The applied sensitizing material is dried and fired in air at about 600C. leaving the silver on the exposed substrate in the gap and on the ohmic contacts 8 and 9. The fusible link is then formed by immersing the resistor body 1 in a cyanide electrolyte containing cadmium ions. The resulting cadmium fusing layer 12 is about 0.00025 inch to 0.00050 inch thick and spans the gap between the resistor sections and makes electrical contact therewith through the ohmic contacts 8 and 9.
The next step is to adjust the resistor to its final value by making a helical groove 14 in the resistance film 5. This method and the apparatus for performing it are well known in the art as exemplified by U.S. Pat. No. 3,329,922, which discloses a method of manufacturing metal film resistors, and by U.S. Pat. No. 2,597,338, which discloses a method of manufacturing carbon film resistors. The groove 14 is made in both sections of the resistor,and it cuts through the coating 13, the resistance film 5, and into the alumina substrate 3 approximately 5 mils.
The fusible link is formulated to melt and open circuit at a predetermined temperature. To stabilize the melting point, however, it has been found that a flux layer 17 is necessary to prevent the cadmium fusing layer 12 from oxidizing at high temperatures. The flux layer 17 inhibits oxidation of the cadmium and prevents the melting point of the fusible link from rising. A l to 2 mil layer of a non-acidic, rosin type flux is used, and is painted over the entire surface of the fusing layer 12. More specifically, the flux is comprised of 42.5 percent water white rosin flux, 6.3 percent heat stabilizer such as that commercially available as Dupont Elwax No. 250, and 51 percent organic solvents such as butanol and methylene chloride.
The lead wires 2 are now attached to the resistor body 1. Each lead wire 2 includes a knurled head 15 on one of its ends, and an associated collar 16 which extends radially outward therefrom. As described in more detail in our U.S. Pat. No. 3,808,575, the knurled heads 15 of the two lead wires 2 are driven into the openings 4 at the ends of the body 1, and the lead wires 2 are soldered in place with -10 solder.
The remaining step is to apply a protective, conformal coating 18 to the entire outer surface of the resistor body 1. A coating material which is particularly suited for this purpose is disclosed in detail in the above-cited copending patent application. It includes as major constituents an epoxy resin and a phenolic resin along with a silica filter. The conformal coating 18 may be applied in several layers. A coloring pigment may be added and after adding an appropriate number of coats color coding is applied. Surface heat is applied after every layer is applied to initiate polymerization and to cure the resin.
Referring particularly to FIG. 4, the completed fusible resistor includes a pair of spaced termination points 19 and 20 which are located at each end of the resistor and which are joined by a conductive path that includes the two resistor sections joined by the fusible link indicated generally as 21. The power dissipated by the resistor sections when current flows through this path generates heat which causes the temperature of the substrate 3 and attached fusible link 21 to rise. When the melting point of the fusing layer 12 is reached, it liquifies. The liquified cadmium has a relatively high surface tension which causes it to bead. Additionally, the cadmium does not wet" the exposed surface of the substrate 3 as well as the silver-glass ohmic contacts 8 and 9, and as a result, it divides, or separates, and flows toward the respective ohmic contacts 8 and 9 where it beads and solidifies as shown in FIG. 5. Electrical continuity between the termination points 19 and 20 is thus abruptly interrupted when the melting point of the fusing layer 12 is reached.
It should be apparent to those skilled in the art that the fusible link 21 can be formed of other electrically conductive materials to obtain different melting temperatures. For example, tin, lead, zinc, indium, silver, or combinations thereof may be substituted for the cadmium fusing layer of the preferred embodiment. Cadmium and cadmium-silver alloys are preferred, however, because they resist oxidation at elevated temperatures. Also, cadmium and cadmium-silver alloys are drawn rapidly by surface preferred wetting toward the ohmic contacts located on each side of the gap when their melting temperature is reached, with the result that the fusible link opens circuit quickly.
It should also be apparent to those skilled in the art that the fusible resistor may be manufactured in a number of ways. For example, the resistive film 5 may be applied as two separate sections which are spaced apart to form a gap therebetween and in which the fusible link is deposited. The order in which the manufacturing steps are performed may also be varied. For example, the point at which the lead wires 2 are attached may be changed to suit the available assembly equipment. And finally, although the invention offers many advantages when embodied in the cylindrical configuration described herein, the shape of the substrate and position of the resistive element and fusible link may take a number of forms. For example, in an integrated circuit formed on a chip" of substrate material, the resistive film is deposited thereon and the fusible link is deposited in close proximity to the resistive film and in electrical circuit therewith. In fact, the fusible link may be positioned on the side opposite the substrate surface upon which the resistive film is deposited. Regardless of the configuration, however, the fusible link must be connected in circuit with the resistive element and must be positioned sufficiently close to receive an accurate indication of temperature. Other variations in the disclosed fusible resistor and method of manufacture can be made without departing from the spirit of the invention, and reference is made to the following claims for a definition of the invention.
1. A fusible resistor, the combination comprising:
a cylindrical substrate made of an electrically insulating material and having ohmic contacts disposed at its ends;
a pair of lead wires fastened to the ends of said cylindrical substrate and in electrical contact with said ohmic contacts,
a resistive film deposited on the surface of said cylindrical substrate between said ohmic contacts, said resistive film being divided into two resistor sec tions by a gap which interrupts electrical continuity between said lead wires; and
a fusible link deposited on said substrate and within said gap to provide electrical continuity between said resistor sections at normal operating temperatures, said fusible link including a fusing layer which melts at a preselected temperature to open circuit.
2. The fusible resistor as recited in claim 1 in which said fusing layer is coated with a layer of flux which inhibits oxidation of the fusing material at elevated temperatures.
3. The fusible resistor as recited in claim 2 in which a protective coating covers said resistive film and said fusible link.
4. The fusible resistor as recited in claim 1 in which said fusing layer is cadmium.
5. The fusible resistor as recited in claim 2 in which said flux includes water white rosin flux and a heat stabilizer.
6. A fusible resistor the combination comprising:
a substrate made of an electrically insulating material and having a pair of spaced termination points;
a resistive film deposited on the surface of said substrate between said termination points, said resistive film being divided into two resistor sections by a gap which interrupts electrical continuity between said termination points; and
a fusible link deposited on said substrate and within said gap to provide electrical continuity between said resistor sections at normal operating temperatures, said fusible link including a fusing layer made from a material having substantially less resistance per square than said resistive film and which is responsive to the heat generated in said resistive film to melt at a preselected temperature and said fusing layer material has a relatively high surface tension when in the liquid state and is drawn out of said gap by surface preferred wetting when said preselected temperature is exceeded to open circuit.
7. The fusible resistor as recited in claim 6 in which said fusing layer material is selected from the group consisting of cadmium, silver, indium, tin, lead, and zinc.
8. The fusible resistor as recited in claim 6 in which a layer of flux is disposed on said fusing layer to inhibit the oxidation of said fusing layer material at elevated temperatures.
9. The fusible resistor as recited in claim 8 in which a protective coating is placed over said resistor film and said fusible link.
10. The fusible resistor as recited in claim 8 in which said fusing layer is formed by depositing metal ions on said substrate.
11. The fusible resistor as recited in claim 10 in which said fusing layer includes cadmium ions which are deposited by an electrolysis process.
12. A fusible resistor the combination comprising:
a substrate made of an electrically insulating material and having a pair of spaced termination points;
a resistive film selected from the group of carbon composition and cermet resistive materials and deposited on the surface of said substrate between said termination points; and
a fusible link deposited on said substrate in series connection with said resistive film to provide electrical continuity between said termination points, said fusible link including a metallic fusing layer 7 8 which has substantially less resistance per square a resistive film deposited on the surface of said subthan said resistive film material, which is supported strate between said termination points; by the substrate and which melts at a preselected a fusible link deposited on said substrate in series temperature to open circuit by surface preferred connection with said resistive film to provide elecwetting. trical continuity between said termination points, 13. The fusible resistor as recited in claim 12 in said fusible link including a fusing layer which which said fusing layer is coated with a layer of nonmelts at a preselected temperature to open circuit acidic flux. by surface preferred wetting; and
14. The fusible resistor as recited in claim 13 in ohmic contacts disposed to each side of said fusing which said fusing layer material is selected from the to layer and when said fusing layer material melts, it group consisting of cadmium, silver, indium, tin, lead, flows to said ohmic contacts. and zinc. 16. The fusible resistor as recited in claim 15 in 15. A fusible resistor. the combination comprising: which said substrate is an alumina-ceramic material a substrate made of an electrically insulating material and said ohmic contacts include silver.
and having a pair of spaced termination points; 15