US2685016A - Hermetically sealed resistor - Google Patents

Description

July 2 7, 1954 Filed Sept.- 16, 1952 K. BLACKBURN 2,685,016

HERMETICALLY SEALED RESISTOR 2 Sheets-Sheet 1 $5.514- 32E. E/B

oQi-AuR M 27, 1954 H. BLACKBURN 2,685,016

HERMETICALLY SEALED RESISTOR Filed Sept. 16, 1952 2 SheetsSheet 2 OOOOOOO OOOOOOO OOOOOOO OOOOOOO OOOOOOO OOOOOOO OOOOOOO L1 E; I 2/ INVENTOR.

l/sesaerz zncksueu otmm r am Patented July 27, 1954 HERMETICALLY SEALED RESISTOR Herbert K. Blackburn, 1

The I-T-E Circuit Brea phia, Pa.

eadon, Pa., assignor to ker Company, Philadel- Application September 16, 1952, Serial No. 309,860

7 Claims. 1

My invention relates to hermetically sealed resistors of the type wherein it is desirable to retain the inherent characteristics in its original construction.

In many electrical applications, it is essential that the resistive component of the circuit maintain a constant resistive value, continuity and dielectric strength irrespective of temperature or atmospheric variations.

Thus, it becomes necessary to provide a sealed coil to insure the exclusion of vapor, Water and other foreign matter to prevent the loss of these desirable characteristics and to prevent the absorption of water in the insulation and windings which would cause a reduction of the resistance of the coil.

It is well known in the art that failure of resistive components to retain proper continuity may be caused by either chemical action when vapors and liquids are not excluded from the coil or by electrical action when water containing ionized materials are absorbed.

Heretofore, the hermetically sealed resistors, designed to retain the above mentioned characteristics, had several shortcomings and disadvantages.

The prior art hermetically sealed resistors were constructed by winding the resistive coil on a ceramic bobbin or coil form with the ends of the wire extending from respective ends of the coil form.

A circumferential edge at the outer ends of the coil form was then cut at a bias or chamfered to increase the adherence surface for metalization, as hereinafter noted.

After the bobbin was wound, a hollow cylindrical ceramic tube having a diameter slightly greater than the outside diameter of the barriers on the coil form was positioned around the bobbin or the coil form.

The ends of the hollow cylindrical housing are metalized by electro-chemical process to enable the sealing agent, such as lead solder, to more readily adhere thereto.

After the hollow cylindrical housing is sealed over the coil form, terminal connectors are at-- tached to the respective ends of the coil form and the ends of the coil are attached thereto. Lead solder is then poured into the resulting cavity at each end to seal the hollow cylindrical housing, the terminal connectors at each end and the coil form. This construction thus results in a resistive component which is sealed against adverse atmosphere conditions.

Many disadvantages were enconntered in this prior art hermetically sealed resistor component. For example,

(1) The use of two diiferent materials, such as a ceramic and metallic lead solder which have different coefficiencies of thermal expansion, resulted in dangerous stres and strains of the unit when exposed to temperature extremes and thereby increased the possibility of fracturing the seal;

(2) The ceramic coil form and housing have an inherent large thermo coefficiency of expansion which are undesirable characteristics for a resistive component of an electric circuit which is subjected to large temperature changes;

(3) The use of an electrical conductive material such as lead solder as the sealing agent could result in shorting in the windings of the coil in the event that molten solder was crossover flux and seep through;

(4) Since the ends of these prior art hermetically sealed resistors consisted primarily of solidified solder, it constitutes an electrical conducting surface. Hence, if the resistive component is mounted on edge on a metallic base, the possibility of a short circuit is greatly enhanced, Accordingly, it becomes necessary when mounting the resistor on end to provide additional insulation means;

(5) Due to the necessity of requiring metallic solder as a seal, the resulting resistive component is usually unduly heavy and bulky for use in electrical circuits having minimum space and weight requirements;

(6) The large number of separate and individual operations such as chamfering, grooving, metalizing, sealing, etc., are undesirable since they increase the cost of the component and result in excess handling thereof;

(7) The use of a ceramic material, such as steatite which is frequently used, as a shell or housing for the sealed unit i highly undesirable since ceramics of low porosity are undesirable from ceramics of high porosity except by means of complex and highly technical tests.

The hermetically sealed resistors of my invention overcomes all the aforementioned disadvantages and provides many additional advantages hereinafter noted.

My novel resistor is manufactured by placing the windings of the resistive coil on a resin coil form which is a nonhygroscopic compound.

The coil form is provided With a plurality of disc barriers and alternate sections of the coil are wound in opposite directions to reduce the in ductive eifect thereof.

The windings of the coil are positioned on a coil form in spaced relationship to obtain a high ratio of air to wire to thereby obtain a cushion effect to compensate for differences in coefficiency of expansion of the windings and the resin material of the coil form and seal.

After the windings of the coil are properly positioned on a coil winding, the terminal connectors are attached to the respective end thereof. This subasseinbly, consisting of the winding form and its associated coil and terminal connectors, is then positioned in a mold and a liquid resin or sealing compound is molded there around. Either hot or cold pour or hot or cold cure liquid resin may be used.

The liquid material has the properties of adherence to the solid material of the coil form, the ability to be converted to a solid at a safe temperature for the material of the coil form, high adherence to the terminal connectors and a similar coefilciency of thermal expansion to the material of the coil form. Thus, the molding liquid may be the same or different material to that used for the coil form.

By this method, I am able to obtain a hermetically sealed resistor which retains the inherent characteristics of its original construction irrespective of severe temperature and atmosphere changes and conditions.

Accordingly, the primary object of my invention is to provide a hermetically sealed resistor having a resinous seal which has the same thermal coefiicient of expansion as the winding form.

Another object of my invention is to provide a precision resistor requiring no metaliz-ing such as silver, copper or tin for the sealing.

Still another object of my invention is to provide a hermetically sealed resistor in which the terminal connectors may be connected to the coil from before the coil is wound thereon to thereby greatly reduce unnecessary handling of the wound coil form.

A still further object of my invention is to provide a precision resistive component in which the coil windings are positioned in unique spaced relationship, resulting in a high ratio of air to wire to thereby provide a cushioning effect to compensate for itself in the coe ciency of thermal expansion of the coil and the resin seal.

Another object of my invention is to provide a hermetically sealed resistor which is light in weight and compact.

A still further object of my invention is to provide a resistor utilizing a minimum number of materials thereby susbtantially reducing undue stress and strains to eliminate the possibility of fracturing the seal.

Still another object of my invention is to provide a non-conducting hermetic seal for a resistive component.

Still another object of my invention is to pro vide a novel precision resistor which is manufactured without a conducting seal so that it may be mounted on its side or ends without the necessity of additional insulating material.

A still further object of my invention is to provide a low tolerance resistance in which the sealing material is a non-conductive material to thereby eliminate the possibility of shorting the windings of the coil as occurred in the prior art resistors which were sealed with a conductive material.

A still further object of my invention is to provide a hermetically sealed resistor which is economical due to its simplicity of manufacture and the availability of the materials used.

A still further object of my invention is to provide a resistive component completely surrounded by a non-porous material to insure the exclusion of vapor, water and other foreign materials.

Another object of my invention is to provide a hermetically sealed resistor which will retain the inherent characteristics of its initial construction.

Still another object of my invention is to provide a resistive component which has constant resistance value, continuity and dielectric strength irrespective of temperature and atmospheric changes.

Another object of my invention is to provide a non-hygroscopic coil form which prevents distorted windings, breaking of the hermetic seal and breakdown of the dielectric.

Another object of my invention is to provide a precision resistor in which the coil form is made of the same non-hygroscopic compound as the outer seal.

A still further object of my invention is to provide a novel hermetic seal for a resistive component in which the compound is impervious to atmospheric conditions and fungus.

Still another object of my invention is to provide a resistor with relatively low tolerance when subjected to the temperature cycling test (-65" C. to 95 C.) salt water immersion tests, humid cycle test and overload test.

These and other objects of my invention will be apparent from the following description when taken in connection with the drawings in which:

Figure 1a is a side view of the coil form used with my invention.

Figure 1b is an end view of Figure 1a taken along the line lblb.

Figure 2 is a side view of the terminal connector used with my novel resistor.

Figure 3a is a side view of the coil form shown in Figures 1a and lb with the terminal connectors of Figure 2 attached to the respective ends thereof.

Figure 3b is an end view of Figure 3a taken along the line 3b-3b.

Figure 4 is a side view similar to Figure 3c showing the coil form with the coil wound there- Figure 5 is a side view similar to Figure 4 showing the wound coil form with an insulating tape wound on the outer circumference of the coil thereof, which may be used with my invention.

Figure 6a is a side view showing the assembly of a plurality of wound coil units, as seen in Figures 4 or 5, assembled with stoppers and spacers in preparation for the molding operation.

Figure 6b is a side view of a spacer shown in Figure 6a.

Figure 6c is a side view of a spacer shown in Figure 6a.

Figure '7 is a perspective view showing the assembly of Figure 6a mounted in a molding form.

Figure 8 is a cross-sectional view of the molding form shown in Figure '7.

Figure 9a is a cross-sectional view of the staggered spaced relationship of the windings of my novel resistor.

Figure 9b is a cross-sectional view of the prior art method of parallel spaced relationship of the windings of a resistive coil.

Figure is a perspective view of my novel precision resistor.

Figure 11 is a cross-sectional view of the completed resistor of Figure 10.

Figure 12 shows one method of mounting the completed resistor of Figure 10 to a base.

Referring now to Figures 1a and 1b, the winding form H is made of a solid non-hygroscopic compound which may or may not be similar sealing compound.

A winding form H is preferably made of a solid epoxy-type of heat-curing resin which may be sold commercially as a hysol. This type of compound has a high volume resistivity, high surfaced resistivity, low water absorption, high heat distortion and chemical inertness.

The winding form may be machined from a rod of solid resin material or may be molded in the form shown in Figure 1.

The winding form i has a main shaft 20, with a hollow cylindrical portion 4!, and is provided with a plurality of discs or barriers l5, l6 and ii which may or may not be integral therewith.

The barriers 5, l5 and H are placed on the winding form I! so that portions of the electromagnetic field can be set up in opposition to reduce induction as will hereinafter be apparent.

The electrical barriers I5, 55 and H are provided with a radial notch It to provide a path through which the wire of the coil may pass for the purpose hereinafter described.

The shaft 29 of the winding form H is provided with a keyed flat section 2| to receive a corresponding section 23 of the terminal connector 2| shown in Figure 2. The terminal connectors 2| are made of a non-corrosive tin-dipped material to be attached securely and permanently to the winding form The keyed flat section 23 of the terminal connector 2! engages the keyed surfaces 4| of the winding form H, as best seen in Figure 3b.

The terminal connector 2| is also provided with a notch section 28 to receive the ends of the coil winding. In a preferred method of manufacture and construction, the terminal connectors 2| are attached to the winding form 5 prior to the time that the coil is wound thereon, as seen in Figures 3a and 3b.

The terminal connector 2| may be temporarily secured to the winding form II by an adhesive compound which may be the same material as used for the winding form El. Wire i then wound on the shaft 26 between the barriers i5 and It in a clockwise or right hand direction. The left end of the wire is passed through the notch is of the barrier I5 and secured to the terminal connector 23 at the notch 23.

The right hand end of the coil wire is then passed through the notch I8 of the barrier it and wound on the shaft 29 between the barriers I6 and I! in an opposite direction such as counterclockwise or a left hand winding. The right end of the wire of the coil 3| is then passed through the notch 8 of the barrier I! and secured to the ri ht hand terminal connector 2| at the notch 28.

It will be noted that a small quantity of solder may be used to secure the respective ends of the coil 3li-3-| to the right and left hand terminal connectors 2|.

It will be noted that Figure 3a shows a preferred embodiment of my invention. In the example shown, the terminals 2| are connected to the ends of the winding form H and extend radially therefrom. However, it will be apparent to those skilled in the art that the terminal con-- nector may be connected to extend longitudinally or parallel to the axis of a winding form I I.

It will also be apparent to those skilled in the art that the terminal connectors 2| may be secured to the winding form after the wire has been wound into the coils 3|l3| on the winding form As best seen in the cross-sectional view of Figure 9a, the wire of the coils 303| are wound in staggered spaced relationship to achieve a high ratio of air to wire.

Figure 91) illustrates the conventional spaced parallel winding of resistive coils to illustrate a low ratio of air to wire.

With the staggered spaced relationship of the windings, as illustrated in Figure 9a, a cushioning effect can be obtained to compensate for the difference in coefliciency of thermal expansions of the coil 30-3l and the resin of the winding form H and the seal hereinafter described.

It will be noted that after the wire has been wound in the winding form I I, illustrated in Figure 4, insulating tape 60 may be wound concentrically on the outside circumferences of the coil 3D3|, as seen in Figure 5. The tape at is more flexible longitudinally than laterally and i noncorrosive. Hence, the insulating tape 56, which may be an acetate film cloth, serves as additiona cushioning means to compensate for the difference in coefiiciency of expansion of the two materials of the coil and seal.

The semi-assembled unit of Figure 4 may be molded with a sealing component either individ ually or in groups with other similar semi-assembled units. One preferred method of applying the hermetic seal to a plurality of semi-assembled units of Figure 4 is shown in Figure 6. Insulating conducting wires are inserted in the hollow portion 4| of the winding form H. Disc shaped stoppers 42 are provided on each side of the semi-assembly of Figure 4 with spacers 43 positioned adjacent thereto.

The stoppers 42 and spacers 43, as seen in Figures 60 and 6b, respectively, are circular in form and have a diameter equal to the inside diameter of the mold 5|. The stopper 42 is provided with a radial protrusion 6! which extends through the opening of the mold 5i to prevent overflow of the sealing compound from one resistor to another.

The completed unit, as seen in Figure 6a, has a pancaked assembly in which two stoppers 42 and spacers 43 are sandwiched between adjacent winding forms, as shown in Figure 4. The disc shaped stoppers 42 serve to prevent the flow of resin to adjoining winding forms, serve as a side support for the mold to insure a flat, smooth surface for the resistor and also serve as a mold release.

The spacers 43 serve to provide a means of insuring that the stoppers 42 are parted to the axis 4| of the resistor to be seal d, also reduce the possibility of blow holes by providing a large volume and surface to dissipate the extreme heats generated by the hermetic reels, serve positioners for the winding bobbin H to insure that they are spaced concentric with the inside circumference of the mold form 5|.

The support wire 46 serves as a common mounting for the sandwiched assembly of the winding forms H and their associated stoppers 4i. spacers 43. The wire 40 may be a rigid member having a mold release substance applied to the external surface thereof. However, it has been far more desirable to make the unit All of flexible wire with an insulating material on the outside thereof so that it will not adhere to the liquid resin.

It has been found that the insulated wire 50 provides the further advantage of providing a cushioning means to prevent creepage of liquid resin into the center hole ii of the Winding form H.

.As best seen in Figure 7, the pancaked assembly of Figure 6a is inserted in the cylindrical mold form iii of Figure 8. In a preferred embodiment, the mold form I, having a diameter corresponding to the diameter of the spacers 33 and stoppers a2, is cylindrical in form and has a longitudinal slot 55. However, it is apparent that either the interior or exterior of the mold form 5% may assume any desirable configuration.

The mold form 5! is preferably mad of a material which will not adhere to the liquid resin when it hardens. However, an a ternate method of construction may be used whereby the mold form 5! may be made of any mold material wherein the interior of same is coated with a mold release substance.

After th pancake assembly of Figure 6a is inserted in the mold 51, as seen in Figure '7, the liquid resin or sealing material is pumped into the mold through the opening 55 to fill the circumferential area surrounding the winding form l i.

It will be noted that the sealing mate ial may be either a hot or cold pour or hot or cold cure resin. The resin which is used as a sealing material for my novel resistor may be the same material such as hysol or a different material from that used for the winding form l but hav ing the same coefiicient or" thermal expansion. However, it must have the properties of (1) high adhesion to the solid material of winding form i i, (2) ability to be cured at a safe temperature for the other material used in the resistor such as the insulation of wire 40, the mold of the winding form H, the mold of the terminal connector 2i and the small amount of solder used to attach the end of the coil 3il-3l to the ter inal connectors 2 i, (3) the same thermal coefficient of expansion as the material of the winding form.

After the sealing material has been allowed to cure or set, the pancak assembly may be removed from the mold 5| and in turn, the resistors it and their associated spacers and stoppers s3 s2 removed from the wire form 40.

Figure is a perspective View illustrating the novel resistor of my invention. The cross-sectional View of Figure 10, as seen in Figure 11, illustrates the sealing compound surrounding. the coils 36-3! and its adhesion the winding form H.

Thus, it may be seen that the end results in a precision resistor in which a winding ii and the hermetic seal form an integral unit in which the coils 8I are completely embedded therein.

As heretofore noted, winding form I i and seal are preferably made of the same material in order to obtain identical or substantially similar coefficiency of thermal expansion of the two A solid epoxy-type of heat curing resin having superior heat stability chemical resist ance and electrical properties and sold as l-l'ysol 5500, which is hot or acid set may be used for the solid material H. The epoxy-type resins to product which I am referring are essentially reaction products of epichlorohydrin and Bis-phenol A,

but any polyphenol or polyhalohydrin may be used. It has been found that this type of epoxy resin with the addition of 25% slate makes an ideal winding form. This material combination of seven parts resin and three parts hardener is cured by heating at 100 C. for two hours, 200 C. for four hours, with corresponding temperatures for intermediate time intervals.

An addition of 25% slate increases the heat distortion by approximately 20 and the above described material results in a winding form which has the properties of high volume resistivity, high su face resistivity, low water absorption, high heat distortion and chemical inertness.

The resin material used for the seal is a cold or amine set type of epoxy known as Hysol 6020 with the addition of five percent slate. This material, a combination of four parts resin to one part hardener is cured for twenty four hours at 20 (3., two hours at 60 C., with corresponding temperatures for intermediate time intervals.

This sealing material has high adherence to the winding form and to the metal terminals, ability to be converted into a solid at a safe temperature for the remaining parts of the resistor and has substantially the same coeflicient 1 thermal expansion as the winding form ll.

Since the sealing material readily adheres to the solid winding form H, after it is cured, it is difilcult or impossible to determine the line of joining between the seal and the winding form.

Figure 12- illustrates one preferred method for mounting my novel hermetically sealed resistor. For example, the resistor F0 may be mounted on its end and attached to a base plate l2 by means of screw H which is screwed into the hollow opening 5% of the winding bobbin ii.

Accordingly, I have provided a novel precision resistor in which the sealing mat ial readily adheres to the winding form on WhlCl'l a coil has been wound in a staggered spaced relation.

Thus, my novel resistor is composed of a minimum number of materials and is provided with adequate compensation to insure against damage of the seal due to unequal coefliciency of thermal expansion.

1' have further provided a precision resistor which is relatively simple in design, economical to manufacture and maintains its accuracy in temperature cycling tests (65 C. to 0.), salt water test, humidity cycle test and overload test.

In the foregoing, I have described my invention only in connection with preferred embodirents thereof. Many variations and modifications of the principles of my invention within the scope of the description herein are obvious. Accordingly, I prefer to be bound not by the specific disclosure herein but only by the appending claims.

I claim:

1. A hermetically precision sealed wire wound resistor comprising a winding form consisting of a solid epoxy-type heat-curing resin and twentyfive per cent slate, having resistor wire wound thereon and an enclosure therefor comprising a cold-set type of epoxy resin having five per cent slate, said enclosure being integral with said winding form and completely sealing said winding form.

2. A precision wire-wound resistor comprising a winding form consisting of a solid epoxy-type heat-curing resin, said winding form having a resistor wire wound thereon and a resin of substantially the same composition integral with and completely enclosing and sealing said winding form.

3. A precision resistor comprising a winding form consisting of a solid epoxy-type heat-curing resin and having added thereto twenty-five per cent slate and having a resistor winding thereon, and a thermoplastic resin having substantially identical thermal coefficient of expansion as that of the winding form integral with and completely enclosing and sealing said winding form.

4. A precision resistor comprising a winding form composed of a solid epoxy-type heatcuring resin, a resistor winding thereon and an enclosure comprising a non-hygroscopic resin integral with said winding form and completely enclosing and sealing said winding form, said housing having substantially the same thermal coeflicient of expansion as said winding form.

5. A precision resistor comprising a winding form composed of a solid epoxy-type heat-curing resin, a resistor winding thereon, and an enclosure comprising a non-hygroscopic resin integral with said winding form and completely enclosing and sealing said winding form, said enclosure having substantially the same thermal coefiicient of expansion as said winding form, the enclosure being integral with the winding form throughout the winding forms area and spaced therefrom only by the interposed resistor winding on said form.

6. The method of constructing a precision resistor which comprises winding a resistor on a winding form, comprising a solid epoxy-type heat-curing resin, inserting said winding form in a mold and admitting a thermal setting resin having the same co-efficient of thermal expansion as said winding form into said mold and permitting the thermal setting resin to cure for a period of twenty-four hours to two hours at twenty degrees centigrade to sixty degrees centigrade With corresponding temperatures at intermediate time intervals.

7. The method of making a precision resistor comprising winding a resistor wire on a form consisting of a solid epoxy-type heat-curing resin containing about twenty-five per cent slate, inserting said winding form in a mold and admitting thereto epon resin containing about five per cent slate, and curing said resinous mixture between 20 and centigrade for a period ranging between two and twenty-four hours.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,265,821 Siegel Dec. 9, 1941 2,286,161 Rights et al June 9, 1942 2,460,795 Warrick Feb. 1, 1949 2,500,449 Bradley Mar. 14,1950 2,547,405 Mitchell et a1 Apr 3, 1951 2,558,798 Thom July 3, 1951 2,586,609 Burle Feb. 19, 1952 2,668,867 Exstein Feb. 9, 1954 FOREIGN PATENTS Number Country Date 796,138 France Mar. 30, 1936

Claims (1)

1. A HERMETICALLY PRECISION SEALED WIRE WOUND RESISTOR COMPRISING A WINDING FORM CONSISTING OF A SOLID EPOXY-TYPE HEAT-CURING RESIN AND TWENTY-FIVE PER CENT SLATE, HAVING RESISTOR WIRE WOUND THEREON AND AN ENCLOSURE THEREFOR COMPRISING A COLD-SET TYPE OF EPOXY RESIN HAVING FIVE PER CENT SLATE, SAID ENCLOSURE BEING INTEGRAL WITH SAID WINDING FORM AND COMPLETELY SEALING SAID WINDING FORM.

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