US3199058A

US3199058A - Precision resistor

Info

Publication number: US3199058A
Application number: US178631A
Authority: US
Inventors: Howard A Cramer; William E Mclean
Original assignee: Electra Manufacturing Co
Current assignee: Electra Manufacturing Co
Priority date: 1962-03-09
Filing date: 1962-03-09
Publication date: 1965-08-03
Anticipated expiration: 1982-08-03
Also published as: GB1023064A

Description

Aug 3 1965 H. A. CRAMER ETAL 3,199,058

PRECISION RESISTOR Filed March 9. 1962 RN w.. l SE n w Qm` .GWJ WML A l "1 \\\\C\Q Mmmm V .T MAE D M V M A W U 0 L .www .d N u W N NN m7 X s 2, .m vm x hm @n m j I m Nm, mm., ov CQ \\\\\N\\\ CX Ov y s mi N 3V www m United States Patent C) M 3,199,053 PRECISIN RESHSTR Howard A. Cramer and William E. McLean, Independence, Kans., assignors to Electra Manufactdring Company, Kansas City, Mo., a corporation of Missouri lFiled Mar. 9, 1962, Ser. No. 178,631 S Claims. (Cl. 33m- 237) The present invention relates to t-he encapsulation of electrical components and more particularly to precision resistors of the hermetically-sealed type.

It is .an object of the present invention to provide an encapsulated resistor in which the resistance element is hermetically sealed in a rigid nonconducting cylinder but which is capable of withstanding wide and abrupt changes in temperature without failure or without the setting up of linternal strains producing a condition of incipient failure. It is a general object of the .invention to provide an encapsulated precision resistor having reliability of the lhighest order, approaching absolute, and which is suitable for use wherever continued operation in the face of sudden environmental changes is essential fas, for example, in guided missiles, satellites, or the like. It is an object, therefore, to produce an encapsulated resistor which is capable of enduring extreme physical or thermal shock and which is capable of withstanding electrical overload.

It is one of the specific objects of the invention to produce an encapsulated resistor which is immune to relative dimensional changes occurring in the resistance element and in the cylinder which encloses it.

Other features and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

FIGURE 1 shows a resistor of the wire pigt-ail type to which the present invention has `been applied.

FIG. 2 is an enlarged axial section of the resistor shown in FIG. 1.

FIG. 3 shows the setup employed in making the terminal assembly employed in the device of FIG. 2.

FIG. 4 is an enlarged axial section of a resistor embodying a modied construction.

While the invention has been described in connection with certain preferred embodiments, it will be understood that we do not intend to limit the invention to the embodiments disclosed but on the contrary we intend to cover the various alternative and equivalent constructions included within the spirit and scope of the appended claims.

Turning now to the drawings a resistor constructed in accordance with the present invention has a body 1t) with axially extending lead wires 11, 12, the form factor being similar to resistors of conventional lilm or composition type. The details of construction are revealed in the enlarged axial section, FIG. 2. Here it will be noted that the resistor includes a circuit element, here shown as a resistor element 15, which is cylindrical in shape having a resistive film 16 which may be severed by a shallow helical groove 17. For the purpose of making contact with the film 16, resistor terminals in the form of

metal end caps

21, 22 are provided which are snugly fitted to the ends of the resistor elements.

For the purpose of encapsulating and sealing the resistor element 15, an impervious cylinder 25 is used formed of rigid non-conducting material, the cylinder having a diameter which is sufficiently larger than the resistor element to dene an annular inner space 26.

In accordance with the present invention thin-walled flexible diaphragm members are provided for sealing the ends of the cylinder 25 and which are secured to the resistor terminals at a localized area along the axis so that such members are free to yield, with diaphragm action, in response to relative expansion between the resistor ele- Patented Aug. 3, 1965 ment and the cylinder which surrounds it. In the present embodiment the diaphragm elements are indicated at 3l, 32 respectively, with the elements being welded or otherwise secured to the

resistor terminals

21, 22 at the center as indicated at 33, 34 respectively. To insure freedom of movement of the diaphragm elements in response to endwise forces in both directions, the diaphragm elements are spaced from the terminals which they engage. In the present instance such spacing is accomplished by inwardly dimpling or embossing the diaphragm elements -as at .35, 36. Thus it is not possible for the resistor terminals to bottom, face to face, against the diaphragm elements. If desired, and without departing from the invent-ion, the embossment could be provided on the

resistor terminals

21, 22. :In either event, the embossment not only insures freedom of movement but also serves to localize the weld or connection between the adjacent members so that it occupies a limited area centered on t-he resistor axis. The lead wires 11, 1-2 are preferably buttwelded -in axial position against the outside surface of the diaphragm members 31, 32 as indicated at v37, 38 respectively, -but they may be attached by other suitable means.

For the puprose of providing a good seal and strong mechanical connection between the diaphragm members 31, 32 and the ends of the cylinder 25, the diaphragm -members are preferably provided wit-h walls or

flanges

31a, 32a lrespectively having a 4diameter which is just slightly smaller than the inner diameter of the cylinder. The walls are sealed to lthe cylinder by a suitable sealant 40. The seal between the diaphragm members and the tubing may be brought about using any one of a number of well known glass or ceram-ic to metal sealing techniques. For example, the inner end surfaces of the cylinder may be `metalized and the sealant 40 may consist of a suitable solder. Or, if desired, the sealant may be in the form of frit lcapable of .wetting both the cylinder and the surface of the metal. Finally, when the cylinder is of fusible material sealing may be accomplished by direct fusion; in this case it is desirable to employ a metal for the diaphragm elements 31, 32, which has substantially the same coeiiicient of expansion as the material of which the cylinder 25 is made.

In the preferred embodiment of the invention illustrated in FIG. 2, the resistor terminal element 21 and the associated diaphragm element 31 are preferably in the form of nested cups or wafers having parallel walls and with a small .cle-arance space being maintained between them so that there is minimum interference with relative axial movement.

The terminal assembly at each end of the resistor, for example, the terminal assembly which consists of the terminal 21, diaphragm 31, and lead wire 1.1, may be welded together 'using a simple and effective welding setup as shown in FIG. 3. This ligure shows the parts in exploded relation prior to making the weld. Arranged within the terminal element 21 is a centering jig 41a and a coaxial first electrode 41. A second electrode 42, which is shown diagrammatically and which may be of any desired coniiguration, clamps the lead wire 11. Guide means 43 may be used for centering the diaphragm element 31 to insure that concentricity is maintained. In operation the electrode 42 and the lead wire 1v1 which it engages are brought down into bottomed contact against the diaphragm member 31, thus conditioning the electrical circuit and permitting formation of the welds previously indicated at 33-37.

After the individual electrode assemblies are welded together, they are secured to the opposite ends of a resistor blank or substrate having a conductive iilm deposited `on the surface. The resistance of the blank may be brought to a precise value by grooving in a so-called spiraling lathe or by other suitable means. After the resistance of the blank has been adjusted and the electrode assemblies secured thereto, the protective cylinder is slipped over the assembly and sealed, as heretofore stated, to the diaphragm elements 31, 32.

' While 4the invention has been described in connection with cup-shaped

inner terminals

21, 22 at the ends of the resistor element, it will be apparent to one skilled in the art that other terminal congurations may be ernployed without departing from the invention. For example, in FIG. 4 there is shown an alternate construction in which the resistor electrodes indicated at 2lb, 22h are disc shaped having a

central projecting portion

21a, 22a projecting into an opening 15e which extends longitudinally through the resistor element 15a. The elements may be joined together using techniques similar to those disclosed in FIG. 3. Both structures posess substantially the same features and advantages.

It is found that resistors constructed in accordance with the present teachings are capable of resisting thermal shocks which would fracture or destroy encapsulated resistors of more conventional design. For example, the units disclosed herein have been subjected to sudden changes in temperature between 65 and +150 C., a difference of 215 C., without damage. Conventional encapsulated resistors were found to suffer a 70% failure rate when subjected to this treatment. The high degree of reliability of the present design is considered to result from the fact that any applied temperature change tends to act first upon the cylinder 25 effectively changing its length, with the differential expansion between the cylinder and the resistor element being accommodated by the movement which takes place in the end walls of the diaphragm elements. lf desired, the end walls of the diaphragm elements may be concentrically waved or convoluted to provide additional flexibility. However, this has not been found to be necessary in practice.

lt has been found that resistors of the present design are also capable of resisting severe shock and vibration without failure. Some of the severity of the shock is, of course, taken up by the lead wires 11, 12. but it is believed that substantial added resistance to shock is provided by oating mount provided by the diaphragms.

From an electrical standpoint it is found that the present resistors are capable of handling over-current conditions in excess of those which can be withstood by ordinary precision resistors. The good thermal coupling between the resistor terminals and diaphragm elements assists in conducting the heat outwardly where it can be effectively radiated and conducted into the environment. Moreover, any differential expansion between the resistor element and the cylinder which encloses it brought about by the conducted current is immediately taken up by diaphragm action.

Since the reliability of the present units approaches absolute, such units are ideally suited for use in computers, guided missiles or satellites, wherever the highest order of reliability is essential.

Notwithstanding the above features and advantages it is noteworthy that resistors of the present design may be manufactured at low cost. lt will be appreciated by one skilled in the art, and having in mind the discussion of FIG. 3 above, that welding, assembly, and sealing may all be easily automated enabling the present resistors to sell for little more than resistors of conventional type.

While the enclosing cylinder 25 may be made of glass, it will be apparent that the invention is not limited thereto and that the member may be made of ceramic material or any other rigid non-porous material which permits an effective seal with respect to the diaphragm members.

We claim as our invention:

1. An encapsulated resistor or the like compr-ising, in combination, a circuit element, cooperating inner and outer terminal caps at each end `of the element, a rigid cylinder surrounding said element, said inner terminal caps being connected to the element and providing electrical connection therewith and said outer terminal caps being dimensioned to engage the ends of the cylinder for sealing thereto, the outer terminal caps being in the form of thin exible metal diaphragms having an inwardly projecting central protuberance secured to the cooperating inner caps over a limited area at the axis to Itake up differential expansion of the circuit element and the cylinder which surrounds it incident to abrupt temerature changes, and axial lead wires secured to the outer caps.

2. An encapsulated resistor comprising, in combination, a cylindrical resistance element, inner .and outer nested terminal caps at each end of the element, a rigid cylinder surrounding said element, said inner terminal caps being dimensioned to engage the element and to provide electrical connection therewith and said outer terminal caps being dimensioned to t snugly with respect to the ends of the cylinder for sealing thereto, the outer terminal caps being made of thin flex-ible metal and having an embossed central portion secured to the inner terminal caps over a limited area at the axis so that relative movement between the adjacent caps may occur incident to differential expansion between the element and the cylinder incident to abrupt temperature changes, lead wires engaging the centers of the outer caps, the lead wire, inner cap and outer cap at each end being coaxially fused together.

3. An encapsulated resistor or the like comprising, in combination, a cylindrical circuit element, inner and outer nested terminal caps at each end of the element, a rigid cylinder surrounding said element, sa-id inner terminal caps being dimensioned to engage the element and to provide electrical connection therewith and said outer terminal caps being dimensioned to lit snugly w-ithin the ends of the cylinder for sealing thereto, the outer terminal caps being made of thin flexible metal and having a concave central portion secured to the inner terminal caps over a limited area at the axis so that relative movement between the adjacent caps may occur incident to dilerential expansion between the element and the cylinder incident to abrupt temperature changes, and means for making external electrical connections to the center of the outer terminal caps.

4. An encapsulated resistor or the like comprising, in combination, an elongated circuit element having conductive end terminals axially positioned thereon, a rigid protective cylinder surrounding said element, a cup-shaped diaphragm member at each end of the circuit element, each of said diaphragm members having a generally radially disposed imperforate metallic central portion and a peripherally disposed axially directed portion, said axially directed portion of each diaphragm member being secured to respective opposite ends of said cylinder and sealingly engaged therewith so as to define a totally enclosed chamber surrounding said element, said imperforate central portion of each of said diaphragm members being in the form of a diaphragm sufficiently thin as to deform axially in both directions when force is applied to the center thereof, each diaphragm member being secured to the adjacent terminal over a limited area at the center and allowing for differential expansion between the circuit element and the cylinder, and external electrical Contact means secured to said diaphragm members with freedom for movement therewith as said diaphragm members ex due to differential expansion between said circuit element and said cylinder.

5. An encapsulated resistor or the like comprising, in combination, a cylindrical resistor element having conductive end terminals axially positioned thereon, a hollow cylinder telescoped thereover, a diaphragm member at each end of the resistor, each of said diaphragm members having a generally radially disposed imperforate metallic central portion and a peripherally disposed axially directed portion, said axially directed portion of each diaphragm member being secured to respective opposite ends of said cylinder and sealingly engaged therewith so as to define a totally enclosed chamber surrounding said element, said imperforate central portion of each of said diaphragm members being in the form of a diaphragm suiciently thin as to deform axially in both directions when force is applied to the center thereof, each diaphragm member being secured to the adjacent resistor terminal over a limited area at the center and allowing for differential expansion between the resistor element and the cylinder, and external electrical contact means secured to said diaphragm members with freedom for movement therewith as said diaphragm members flex due to dicrential expansion between said circuit element and said cylinder.

6. An encapsulated resistor or the like comprising, in combination, a cylindrical circuit element, cooperating inner and outer terminal caps at each end of the element, a rigid cylinder surrounding said element, said inner terminal caps being dimensioned to engage the element and to provide electrical connection therewith and said outer terminal caps being dimensioned to engage the ends of the cylinder for sealing thereto, at least one of the caps at each end having an axial projection providing a limited welding area for the cooperating cap and the outer terminal caps being in the form of diaphragms of thin lexible metal so that relative movement may occur between the cooperating caps incident to differential expansion between the circuit element and the cylinder upon abrupt change in temperature, and axial lead wires welded to the caps.

7. An encapsulated resistor or the like comprising, in combination, a cylindrical circuit element, cooperating inner and outer terminal caps at each end of the element, a rigid cylinder surrounding said element, said inner terminal caps engaging the element to provide electrical connection therewith and said outer terminal caps being dimensioned to telescope within the cylinder for sealing to the ends thereof, the outer terminal caps being made of thin flexible metal and secured to the inner terminal caps at the axis and at least one of the cooperating caps at each end being dimpled to provide a limited welding area with the caps spaced apart a small distance axially so that relative movement may occur between them incident to differential expansion between the circuit element and the cylinder upon abrupt changes in temperature, and axial lead wires welded to the caps.

S. An encapsulated resistor or the like comprising, in combination, a cylindrical circuit element, inner and outer nested terminal caps mounted on one end of said element, a rigid protective cylinder of impervious material surrounding said element and spaced therefrom, said inner terminal cap being dimensioned to engage said element so as to provide electrical connection therewith, said outer terminal cap being dimensioned to t snugly within one end of the cylinder for sealing engagement therewith, means for sealing the other end of said cylinder and for providing electrical engagement with the other end of said circuit element, said outer cap being made of thin flexible metal and having an embossed central portion secured to said inner cap over a limited area in the region of the axis thereof so that relative movement between the adjacent caps may occur incident to differential expansion between said element and said cylinder resulting from abrupt temperature changes.

References Cited by the Examiner UNITED STATES PATENTS 2,046,922 7/ 36 Minnium 338-273 2,215,587 9/40 Kerschbaum 338-274 X 2,402,773 6/ 46 Ragatz 338-274 X 2,407,171 9/ 46 McFarren 338-274 X 2,416,599 2/47 Victoreen 338-273 X 2,553,875 5/51 Shaw 338-274 X 3,064,070 11/62 Douglass et al 338-274 X FOREIGN PATENTS 207,597 2/ 40 Switzerland. 580,626 10/52 Great Britain.

RCHARD M. WOOD, Primary Examiner.

Claims

1. AN ENCAPSULATED RESISTOR OR THE LIKE COMPRISING, IN COMBINATION, A CIRCUIT ELEMENT, COOPERATING INNER AND OUTER TERMINAL CAPS AT EACH END OF THE ELEMENT, A RIGID CYLINDER SURROUNDING SAID ELEMENT, SAID INNER TERMINAL CAPS BEING CONNECTED TO THE ELEMENT AND PROVIDING ELECTRICAL CONNECTION THEREWITH AND SAID OUTER TERMINAL CAPS BEING DIMENSIONED TO ENGAGE THE ENDS OF THE CYLINDER FOR SEALING THERETO, THE OUTER TERMINAL CAPS BEING IN THE FORM OF THIN FLEXIBLE METAL DIAPHRAGMS HAVING AN INWARDLY PROJECTING CENTRAL PROTUBERANCE SECURED TO THE COOPERATING INNER CAPS OVER A LIMITED AREA AT THE AXIS TO TAKE UP DIFFERENTIAL EXPANSION OF THE CIRCUIT ELEMENT AND THE CYLINDER WHICH SURROUNDS IT INCIDENT TO ABRUPT TEMPERATURE CHANGES, AND AXIAL LEAD WIRES SECURED TO THE OUTER CAPS.