US3136972A - Encapsulated resistor - Google Patents

Description

June 9, 1954y w. J. RANDOLPH ENCAPSULATED -REsIsToR Filed Aug. 16, 1961 INVENTR. Mu rf? J. ,Qq/vamp# BY 4m/yens United States Patent() t 3,136,972 v ENCAPSULATED RESISTOR Walter J.'Randolph, Bedminster, NJ., assignor to Consolidated Electronics Industries Corp., New York, N.Y., a corporation of Delaware Filed Aug. 16, 1961, Ser. No. 131,898

3 Claims. (Cl. SSS-273) This invention relates to encapsulated resistors in which the resistive element' is a thin film on the outer surface of an insulating body. The purpose of encapsulated resistors is to prevent adverse environmental conditions, such as water vapor or other matter in the air, from attacking the resistor and changing its resistance either permanently or temporarily. Encapsulation of electrical components is usually accomplished by inserting the component into a suitable container, frequently of the same material as the encapsulating material, and then filling the space around the component with encapsulating material in a liquid form. After the encapsulating material hardens, the component is effectively sealed in, and if this is properly done, subsequent changes in the environment should have little or no effect on the electrical value of the component.

The main problem in encapsulating resistors of the type in which the resistance element is a thin film on the surface of a ceramic body is that, if the encapsulated resistor is subsequently heated and cooled, the difference between the coefficients of thermal expansion of the ceramic and of the encapsulating material may squeeze or abrade the thin film and cause it to change resistance by a considerable amount. However, simply enclosing the resistor within a sleeve does not completely prevent undesired changes, and in accordance with the present invention it has been found necessary to coat the encapsulated elements with a mold release which has the desired effect of rendering the encapsulated resistor almost entirely immune to external environment, including changes in the temperature of the component.

The'invention will be described in greater detail in connection with the drawing in which the only figure is a crosssectional View of an encapsulated resistor constructed according to the invention.

In the drawing a cylinder 11 of some suitable insulating material such as porcelain or another known form of insulating material is coated with a thin, resistive film 12. The resistive film 12 is. usually of carbonaceous material and may be helically grooved in accordance with known techniques to increase the accuracy of pre-setting of the resistance and for other purposes whichy are well known. A pair of metal end caps 13 and 14 attached to opposite ends of the insulating body 11 covers the ends of the resistive film 12 to make connection therewith, and a pair of wire terminals 16 and 17 is attached to the end caps 13 and 14, respectively, to extend therefrom. A coatingrofvarnish 18 is applied to the resistor to cover the resistive film itself and to prevent any changes therein and to cover the end caps 13 and 14 and a small portion of the adjacent wire terminals 16 and 17. y Prior to encapsulating the resistor just described, a sleeve 19 of encapsulaing material is slipped over the end caps 13 and 14. This sleeve is preferably made of an irradiated polyolefin which has the characteristic of shrinking to a predetermined diameter when heated to a predetermined temperature. It also has exceptionally good resistance to tearing.

After the sleeve 19 is slipped over the end caps, the whole thing is heated to a temperature which will cause the sleeve to shrink down and` grasp the end caps 13 and 14 tightly and thereby seal off a space 20 immediately surrounding the critical area between the end caps. Be-

3,136,972 Patented June 9, 1964 cause the amount of shrinkage can be predetermined, it

`caps 13 and 14, or more properly, the outer surface of the thin layer of varnish thereover.

Following the shaping of the sleeve 19, both the sleeve and a short section of the terminals 16 and 17 are coated with a layer 21 of mold release'. For this purpose, I have found it advantageous to dip the sleeved unit into a suitable silicone mold release. It is convenient at this time to bend the terminal 16 back more or less parallel to the terminal 17 so that these two terminals mayL then be inserted through apertures 22 and 23, respectively, in the bottom of an open-topped receptacle 24, This receptacle may be made of any suitable insulating material, such as one of the epoxy resins used widely for encapsulation purposes. Thereafter, the remaining space in the receptacle 24 is filled with an encapsulating material, and, particularly if the receptacle is made of an epoxy resin, it is desirable to fill the receptacle with more epoxy resin material in a liquid form. This completes the encapsulation process and forms a monolithic mass with only the two terminals 16 and 17 extending from it. By properly spacing the apertures 22 and 23, the distance between the terminals 16 and 17 may be set to a figure commensurate with established spacings for printed circuits.

While it has been attempted heretofore to encapsulate film resistors without the layer 21 of mold release, it has been found that the encapsulating material clings to the film 12 or to the varnish 18. The addition of the mold release overcomes this problem and permits satisfactory encapsulation of such resistors.

While this invention has been described in limited terms as it relates to a specific embodiment, it will be understood by thse skilled in the art that the true scope is determined by the following claims and that modifications may therefore be made from the specific embodiment described without departing from the scope of the invention.

What is claimed is: f y

1. An encapsulated resistor comprising: an insulating support cylinder; a resistive film coated on the cylindrical surface thereof; a pair of end caps attached to the ends of said cylinder and making electrical connection with the ends of said resistive film; a pair of wire terminals extending from said caps; a thermally shrunk insulating sleeve surrounding the cylindrical surfaces of said support cylinder and said end caps, the inner surface of said sleeve being shrung to a diameter slightly greater than the diameter of said resistive film so as not to make mechanical contact with said film; a mold release coating covering the outersurface of saidsleeve and the exposed ends of said caps; a hollow container for said resistor, said container comprising a hollow cylindrical body and an integral end having two holes therein through which said wire terminals extend; and epoxy filler substantially filling the remaining space within said container and encasing said resistor.

2. An encapsulated resistor comprising: an insulating support cylinder; a resistive film coated on the outer cylin drical surface thereof; a pair of end caps attached to the ends of said cylinder and making electrical connection with the ends of said resistive film; a pair of wire terminals extending from said caps; a thermally shrunk sleeve of irradiated polyolefin material surrounding the cylindrical surfaces of said support cylinder and said end caps and extending therebeyond, the inner surface of said sleeve being shrunk to a diameter less than the diametetr of said caps and slightly greater than the diameter of said resistive film so as not to make mechanical contact with said film; a mold release coating covering the outer surface of said sleeve and the exposed ends of said caps; a hollow epoxy container for said resistor, said container support cylinder; a resistive lilm coated on the cylindrical surface thereof; a pair of endcaps attached to the ends of said cylinder and making electrical connection with the ends of said resistive film; a pair of wire terminals extending from said caps in opposite directions and substantially parallel to the axis of said support cylinder, one ofsaid wire terminals being bent back on itself and extending approximately parallel to the other wire terminal and in substantially the same direction; a thin varnish coating over said caps and the portion of said lrn therebetween; a thermally shrunk insulating sleeve surrounding the cylindrical surfaces of said support cylinder and `said end caps and extending therebeyond, the inner suri face of said sleeve being shrunk to a diameter less than the diameter of said caps and slightly greater than the diameter of said resistive iilm so as not to make mechanical contact with said film; a mold release coating covering the outer surface of said sleeve and said varnish coating on the exposedends of said caps; a hollow epoxy container for said resistor, said container comprising a hollow cylindrical body and an integral end plate having two holes therein through which said Wire terminals extend; and epoxy filler substantially filling the remaining space within said container and encasing said resistor.

References Cited in the tile of this patent UNITED STATES PATENTS 2,130,156 Rollefson Sept. 13, 1938 2,664,487 Gorden Dec. 29, 1953 2,685,016 Blackburn July 27, 1954 2,742,551 Kohring Apr. 17, 1956 2,803,729 Kohring Aug. 20, 1957

Claims (1)

1. AN ENCAPSULATED RESISTOR COMPRISING: AN INSULATING SUPPORT CYLINDER; A RESISTIVE FILM COATED ON THE CYLINDRICAL SURFACE THEREOF; A PAIR OF END CAPS ATTACHED TO THE ENDS OF SAID CYLINDER AND MAKING ELECTRICAL CONNECTION WITH THE ENDS OF SAID RESISTIVE FILM; A PAIR OF WIRE TERMINALS EXTENDING FROM SAID CAPS; A THERMALLY SHRUNK INSULATING SLEEVE SURROUNDING THE CYLINDRICAL SURFACES OF SAID SUPPORT CYLINDER AND SAID END CAPS, THE INNER SURFACE OF SAID SLEEVE BEING SHRUNG TO A DIAMETER SLIGHTLY GREATER THAN THE DIAMETER OF SAID RESISTIVE FILM SO AS NOT TO MAKE MECHANICAL CONTACT WITH SAID FILM; A MOLD RELEASE COATING COVERING THE OUTER SURFACE OF SAID SLEEVE AND THE EXPOSED ENDS OF SAID CAPS; A HOLLOW CONTAINER FOR SAID RESISTOR, SAID CONTAINER COMPRISING A HOLLOW CYLINDRICAL BODY AND AN INTEGRAL END HAVING TWO HOLES THEREIN THROUGH WHICH SAID WIRE TERMINALS EXTEND; AND EPOXY FILLER SUBSTANTIALLY FILLING THE REMAINING SPACE WITHIN SAID CONTAINER AND ENCASING SAID RESISTOR.

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