Abstract:
Sealed relays intended for high-voltage or high-power switching are enclosed in a herrnetically sealed plastic housing or jacket ( 11 ) capable of long-term maintenance of either a high vacuum or a pressurized insulating gas within the relay to suppress contact arcing during switching. The impermeable plastic housing eliminates need for conventional glass or ceramic contact enclosures, and enables use of inexpensive relays ( 71 ) in demanding applications.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation in part of U.S. Provisional Patent Application No. 60/012,337 filed Feb. 27, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     Hermetically sealed electromagnetic relays are used for switching of high electrical currents and/or high voltages, and typically have fixed and movable contacts, and an actuating mechanism supported within a hermetically sealed chamber. To suppress arc formation, and to provide long operating life, air is removed from the sealed chamber by conventional high-vacuum equipment and techniques. In one style of relay, the chamber is then sealed so the fixed and movable contacts coact in a high-vacuum environment. In another common style, the evacuated chamber is backfilled (and sometimes pressurized) with an insulating gas (e.g., sulphur hexafluoride) with good arc-suppressing properties. 
     The sealed chamber is conventionally formed by a glass or ceramic envelope which is fused (glass-to-metal seal) or brazed (ceramic-to-metal seal) to metal components of the relay such as terminal pins and a typically cylindrical or tubular metal base. These fused or brazed junctions are specified by Military Specification MIL-R-83725 with respect to high-voltage sealed relays. 
     Properly selected grades of glass or ceramic provide the essential characteristics of low gas permeability, excellent insulating or dielectric qualities, low outgassing, and mechanical strength. Glass envelopes, however, are handmade by skilled artisans, and are expensive and subject to breakage, and ceramic envelopes are both expensive to press and metalize, and difficult to procure. It is to the solution of these problems that our invention is directed. 
     Our improvement is directed to the replacement of these glass or ceramic chamber-enclosing envelopes with an inexpensive and easily formed vacuum-tight assembly of plastic and epoxy, or in an alternative form, an envelope made entirely of epoxy. We have established that this type of plastic/epoxy or epoxy envelope provides an excellent hermetic seal, good dielectric and outgassing characteristics, and a strong, inexpensive sealed relay for switching high currents and/or high voltage. 
     Attempts have been made in known designs to use plastic materials in relays, and U.S. Pat. Nos. 4,039,984, 4,168,480 and 4,880,947 are examples of the use of epoxy resins as adhesives to secure together relay housing components. Curing of the epoxy to a cross-linked thermoset state shrinks the joint bond and weakens the seal. Certain other designs (e.g., U.S. Pat. 5,554,963) have used thermoplastic (as opposed to cross-linked thermosetting) polymers, but the resulting relay envelope is not a true hermetic seal which can maintain either a high-vacuum or high-pressure environment. 
     For purposes of this invention disclosure, a hermetic seal means a seal which is sufficiently strong and impermeable to maintain for a long term a high vacuum of 10− 5  Torr (760 Torr=one atmosphere) or less, and a pressure of at least 1.5 atmospheres. In contrast to the prior-art designs, the present invention achieves hermetic sealing by encapsulating the relay chamber in a jacket of impermeable epoxy or a comparable thermosetting polymer, the jacket having single-junction epoxy-to-metal bonds. Shrinkage of the epoxy during polymerization is a significant advantage in the invention as it provides a strong and reliable single-junction seal. 
     In one embodiment described below, an unsealed relay is encapsulated in a vacuum chamber, thus eliminating the need for an evacuation tube which characterizes prior relay designs. This same new method can be used to make pressurized relays which are evacuated, backfilled and encapsulated within a properly equipped chamber. 
     SUMMARY OF THE INVENTION 
     This invention is directed to the replacement of glass or ceramic contact-enclosing housings in sealed relays with an economical thermosetting-plastic jacket which is impermeable to inflow of air in a high-vacuum relay, and to outflow of insulating gas in a backfilled and pressurized relay. Epoxy is a presently preferred material because it forms hermetic seals with impermeable metal components (such as terminals) which must extend through the jacket, and is substantially impermeable to gasses of small molecular size such as hydrogen. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a sealed relay according to the invention; 
     FIG. 2 is an enlarged sectional elevation of the relay before encapsulation, on line  2 — 2  of FIG. 5; 
     FIG. 3 is a reduced sectional elevation of the relay on line  3 — 3  of FIG. 5; 
     FIG. 4 is a top sectional view of the relay on line  4 — 4  of FIG. 2; 
     FIG. 5 is a top view of the assembly shown in FIG. 2; 
     FIG. 6 is an elevation of a cylindrical assembly which supports terminal pins and fixed/movable contacts of the relay; 
     FIG. 7 is a sectional elevation on line  7 — 7  of FIG. 6; 
     FIG. 8 is a bottom plan view on line  8 — 8  of FIG. 7; 
     FIG. 9 is an enlarged elevation of detail shown in the lower-right corners of FIGS. 2 and 3; 
     FIGS. 10A and 10B are respectively a sectional side elevation and a top view of second embodiment of the invention using an open-frame relay in a plastic cup supported in an outer metal cup, the assembly being shown before encapsulation; 
     FIG. 11 shows the assembly of FIGS. 10A and B in a closed chamber having evacuation, pressurization and encapsulation-material valves; and 
     FIG. 12 is a view similar to FIG. 11, and showing the relay assembly filled with cured encapsulation material. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a sealed relay  10  using a plastic and epoxy-sealed envelope to enclose the fixed and moving contacts of the relay. A primary external sidewall of the relay is formed by a plastic potting cup  11  which serves as a mold to hold epoxy material  12  poured into the cup and cured to provide a hermetic seal. Insulated electrical leads  13  extend through the epoxy material for connection of fixed and movable contacts to external circuitry. A threaded metal mounting base  14  extends through the underside of cup  11 , and has a lower end closed by a metal cover plate  15  secured by a nut  16 , and through which a pair of actuating-coil leads  17  extend for connection to external circuitry. 
     The concepts of the invention are useful in many different styles of hermetically sealed relays (whether of a high-vacuum type, or a back-filled or pressurized type), and will be described in the context of a double-pole double-throw relay using a conventional and typical electromagnetic actuator and fixed and movable contact assemblies. The invention is not limited to this specific configuration which is illustrated only by way of example, and is equally applicable to other types of sealed relays. 
     Referring to the sectional elevations of FIGS. 2 and 3, base  14  (made of a high-permeability magnetic-metal alloy such as C1018 iron) has a cylindrical sidewall  18 , a central cylindrical pole piece  19 , and an annular space  20  between the sidewall and pole piece into which is fitted a conventional actuating coil (not shown). The upper end of space  20  is closed by a washer-like disk  22  made of a non-magnetic material such as monel metal, and which is brazed to the sidewall and pole piece to provide a hermetic seal. 
     A movable armature  23  is pivotally mounted to the top of the base by a hinge (not shown). A coil spring  25  is seated in an annular space  26  between the upper ends of the sidewall and pole piece above disk  22 , and urges the armature away from the pole piece when the relay is in a nonenergized condition. The armature has an upwardly extending actuating leg  27  with a slot  28  (FIG. 3) at its upper end. The pole piece has a central bore  29  extending to an evacuation tube  30  brazed and hermetically sealed to the pole piece, and through which a sealed chamber  31  of the relay can be pumped down to a high vacuum (and, if desired, backfilled to a pressure of say three atmospheres with an insulating gas such as sulphur hexafluoride). Tube  30  is thereafter pinched off and sealed where it extends through an externally threaded boss  32  which receives nut  16 . 
     Sealed chamber  31  is enclosed by base  14  and a hollow assembly  35  as best seen in FIGS. 6-8. Assembly  35  includes a generally cylindrical plastic sidewall  36 , an upper closure cap  37  press fitted into the upper end of the sidewall, and a metal ring  38  press fitted into the lower end of the sidewall and having at its lower end an outwardly extending flange  39  which is brazed to a metal disk  40  which is in turn brazed to a disk  41  brazed to an inwardly extending annular shoulder  41  in the outer surface of base  14  (FIGS.  2  and  9 ). These brazed junctions hermetically seal the joined components. 
     Six metal terminal pins  44   a-f  are radially spaced apart, and extend through sidewall  36  to form the six terminals of a DPDT switch. Pins  44  are fixtured in an injection mold in which plastic sidewall  36  is formed, and are thereby rigidly supported by the sidewall. Pins  44   a-b  and  d-e  form fixed contacts of the switch, and pins  44   c  and  f  are conductive posts on which a pair of movable contacts  45  (FIG. 4) are mounted. External leads  13  are secured to the pins by connectors  46  secured to the pins. 
     Each movable contact is Y-shaped in plan view (FIGS. 4 and 8) to define a pair of contact surfaces  48  which are urged against or away from one of the associated pair of fixed contacts in seesaw fashion when the relay is energized or deenergized. Each movable contact has a pair of downwardly extending inner and outer tabs  49  and  50  each having a hole at its upper end so the contact can be fitted over associated pin  44 . 
     A lower hole  51  extends through each inner tab  49  to receive an insulated rod  52  which couples the movable contacts together. Rod  52  is fitted into slot  28  of armature leg  27  (FIG.  3 ), and is held captive between the movable contacts by a lower end  53  of each outer tab  50 . This general style of fixed and movable contact assembly is conventional, and is described in greater detail in, for example, U.S. Pat. No. 3,604,870, the disclosure of which is incorporated herein by reference. 
     The relay is assembled by placing assembly  35  against base  14  with ring flange  39  against disk  40 , and insulated rod  52  engaged in slot  28  of the armature leg. With cap  37  removed, proper alignment of the parts can now be checked by actuating the relay coil, and any necessary adjustments are made before welding ring flange  39  to disk  40 . Cap  37  is then press fitted into sidewall  36 , and an O-ring  55  is fitted into an annular groove  56  in the outer surface of base sidewall  18  beneath disk  40  (FIG.  9 ). 
     Open-top plastic (Nylon 6/6 is a presently preferred material) potting cup  11  has a hexagonal sidewall  61  and a bottom wall  62  having a central circular opening  63  which receives the threaded lower end of base  14  as shown in FIGS. 2 and 8. Optional mounting tabs  64  (shown in FIGS. 3-5) may be integrally molded with the potting cup if desired. The potting cup is tightened on base  14  to compress O-ring  55  by temporarily tightening a nut (not shown) on the externally threaded part of the base against the cup. 
     With the assembly fixtured in an upright position, external leads  13  are supported to extend vertically from pins  44 , and uncured epoxy  12  is then poured into a space  66  between the exposed outer surfaces of assembly  35  and base  14 , and the inner surface of the potting cup. The epoxy also covers the top of assembly  35 , and fills the potting cup as shown in FIG.  1 . After conventional curing of the epoxy, the relay is evacuated (and, if desired, backfilled) through tube  30  which is then sealed by cold-weld pinch off, and the relay coil and associated cover plate  15  are secured in place by nut  16 . 
     The body of encapsulating epoxy  12  forms a hermetic seal around all of the components which define sealed chamber  31 . More specifically, hermetic seals are formed at the epoxy-to-metal junctions of the epoxy with pins  44  where they emerge from sidewall  36 , with connectors  46 , with the exposed portions of ring  38 , disk  40  and sidewall  18  of the base. O-ring  55  is not relied on for a hermetic seal, and is instead used only to prevent leakage of uncured epoxy during the pouring and curing cycles. 
     A second embodiment of a sealed relay according to the invention is shown in FIGS. 10-12, and this embodiment uses a simple and inexpensive open-frame relay in an open-top housing assembly which is evacuated, encapsulated and backfilled while positioned within a sealed chamber. This manufacturing method eliminates need for an evacuating and backfilling tubulation, and enables use of an inexpensive relay for high-voltage and high-power applications heretofore handled only by more expensive high-vacuum or pressurized units of known types as described in the introductory part of this specification. 
     Referring to FIGS. 10A and B, a relay assembly  70  is shown prior to encapsulation, and the assembly includes a conventional open-frame relay  71  (illustrated as a single-pole single-throw or SPST type, but other conventional contact configurations are equally useful) secured to and suspended from a generally rectangular header  72 . Elongated metal terminal pins  73   a-d  extend through the header, and pins  73   a  and  b  are connected to a coil  74  of the relay electromagnetic actuator. Pin  73   c  supports a fixed contact  75 , and pin  73   d  is connected to a movable contact  76  which is pulled against the fixed contact when the relay is energized. A coil spring  77  urges the movable contact into an open position in conventional fashion. 
     Relay  71  is positioned within an open-top plastic cup  79 , with the underside of header  72  supported on short spaced-apart lugs  80  which extend inwardly from the inner perimeter of a sidewall  81  of cup  79  slightly below the top of the cup. The header does not make a snug press fit within the upper end of the cup, and there is instead an intentional narrow gap  82  of say 0.002-0.003 inch between the side edges of the header and the inner surface of sidewall  81 . 
     Plastic cup  79  is in turn centrally fitted within an open-top metal cup  84  having a base  85  against which the plastic cup rests, and an upwardly extending sidewall  86 . The plastic cup is smaller in external dimension than the interior of sidewall  86 , creating a space or gap  87  between the plastic and metal cups. Sidewall  86  extends higher than the top of the plastic cup, and pins  73   a-d  in turn extend higher than the top of the metal cup. An acceptable alternative to metal cup  84  is a similarly shaped plastic cup having a separate metal plate resting on the cup bottom for bonding with encapsulation material. 
     The thus-assembled components are next placed in a sealed chamber  89  as shown in FIG.  11 . The chamber has an evacuation valve  90  connected to a high-vacuum pumping system (not shown) of a conventional type using mechanical and diffusion pumps. The chamber also has a pressurization valve  91  connected to a pressurized source (not shown) of an insulating gas such as SF 6 . The chamber further has a third valve  92  positioned above cup  84 , and connected to a piston-cylinder assembly  93  for holding and delivering a metered amount of uncured viscous, but fluid encapsulating material  94 . 
     Evacuation valve  90  is then opened, and the high-vacuum pumping system actuated to withdraw air from the chamber interior to a vacuum which is preferably at least 10− 2  to 10− 3  Torr if the relay is to be backfilled. Ambient air is simultaneously withdrawn from relay assembly  70  through gap  82  between header  72  and sidewall  81 . Valve  90  is closed when a desired vacuum is achieved. 
     Open-frame relays are unsuited for long-term vacuum operation due to outgassing of components such as the relay coil which will eventually contaminate and adversely affect a high-vacuum environment. This problem is eliminated by backfilling and pressurizing the chamber and as-yet-unsealed relay assembly with an insulating gas which is admitted by opening pressurization valve  91 . The gas flows freely through gap  82  to fill and pressurize the interior of the relay assembly. 
     With the chamber interior stabilized in a high-pressure condition, valve  90  is closed, valve  92  is opened, and piston-cylinder assembly  93  actuated to deliver at a pressure exceeding that of the pressurized chamber a metered amount of fluid encapsulating material into metal cup  84  to completely fill gap  87  and cup  84  to a level just beneath the top of sidewall  86  as shown in FIG.  12 . The encapsulating material is too viscous to pass through small gap  82 , and the backfilled environment within the relay assembly remains undisturbed. 
     Preferably, chamber  89  is of a conventional type which includes a heater such as an induction heater, and heat is applied to the now-encapsulated relay assembly to cross link and cure the encapsulating material. With the chamber vented to atmosphere, the completed relay assembly is removed for testing and packaging. In production, many relay assemblies would be processed in a single loading of the chamber, and the methods of the invention can also be adapted for use in a continuous production line. 
     The optimum environment in which the relay contacts make and break is dependent upon the required performance of the relay. Vacuum (less than 10− 5  Torr) is generally a good environment for high-voltage applications, but would not be chosen for applications where relay components in the vacuum environment might outgas. There are many gases that can be used to improve electrical performance of a relay. Sulfur hexafluoride (SF 6 ) is a good dielectric gas which at higher pressure will standoff significantly higher voltages than open air. A relay that will standoff 5 kilovolts in open air will standoff 40 kilovolts if it is pressurized with 10 atmospheres of SF 6 . Another characteristic of SF 6  is that once ionized it becomes an excellent conductor. This makes it a good choice for relays that need to make into a load and keep consistent conduction of current while the load is being discharged. It is not a good gas, however, if that load needs to be interrupted, because the SF 6  will tend to continue conduction, and prevent the load from being interrupted. 
     Hydrogen (and hydrogen-nitrogen blends) has been shown to effectively cool the electrical arc that is created when the electrical contacts move away from each other while breaking a load. The difficulty with hydrogen is that not only is it the smallest molecule so that it will propagate through the smallest cracks, but it can also chemically propagate through many materials. The design of the present invention using cross-linked polymers, unlike other designs, will hold pressurized hydrogen gas for many years. 
     There are several kinds of epoxy materials which bond satisfactorily with metal and, which are impermeable to prevent leakage of air into a vacuum relay, or loss of insulating gas in a pressurized relay. A presently preferred material is commercially available under the trademark Resinform RF-5407(75% alumina filled) mixed 100:12 by weight with Resinform RF-24 hardener. Alternative epoxy materials should provide these characteristics: 
     a. Low gas permeability (less than 10− 10  standard cubic centimeters of air per second). 
     b. High dielectric strength (greater than 100 volts per mil). 
     c. Low outgassing (to maintain a vacuum of 10− 5  Torr or better). 
     d. Good mechanical strength. 
     e. Thermal expansion characteristics reasonably matched to those of the metal with which the epoxy forms a hermetic seal. 
     There have been described several embodiments of epoxy envelopes for hermetically sealing standard relay designs in a special atmosphere for improved performance. These envelopes provide significant cost savings in the manufacture of vacuum or pressurized sealed relays, and have performance characteristics at least equivalent to relays of this type using glass or ceramic envelopes. The invention is not limited to the specific relay types described above, and is equally useful with other switching devices such as reed-style relays and the like.