Abstract:
A surge protector assembly may be provided with three terminals, a gas discharge tube having a first conductive end and a second conductive end opposite the first conductive end, a pair of metal oxide varistors conductively connected to the ends of the gas discharge tube, and a pair of bracket arms associated with the ends of the gas discharge tube. The bracket arms are movable between a steady-state position in which they cause the first and second terminals to be conductively separated from the third terminal and a shorting position in which they cause the first and second terminals to be conductively connected to the third terminal. The bracket arms are spaced from the ends of the gas discharge tube by a first distance when they are in their steady-state positions and by a second distance when they are in their shorting positions, with the first distance being greater than the second distance, and the bracket arms are spring-biased towards their shorting positions.

Description:
BACKGROUND OF THE INVENTION 
     The invention is directed to an electrical surge protection device of the type used to protect equipment and electrical circuits from being damaged by overvoltage surges and/or excessive surge currents. 
     Surge protection devices may be used to protect power lines, communication lines and electrical equipment connected to the power lines and communication lines from overvoltage surges, such as those caused by lightning or power cross conditions, for example. A power cross condition, which may occur as a result of a downed power line, consists of a high voltage and a high current at an alternating frequency, such as 60 Hz. 
     During an overvoltage surge, a surge protection device may provide temporary surge protection by shunting the overvoltage surge to ground. In the presence of a prolonged overvoltage condition, a surge protection device may also be designed to fail in a short-circuit state. Various embodiments of surge protectors are disclosed in U.S. Pat. No. 5,388,023 to Boy, et al., U.S. Pat. No. 5,500,782 to Oertel, et al., and U.S. Pat. No. 5,880,919 to Napiorkowski, et al. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a surge protector assembly having three terminals, a gas discharge tube having a first conductive end and a second conductive end opposite the first conductive end, a first metal oxide varistor conductively connected to the first end of the gas discharge tube, and a first bracket arm associated with the first end of the gas discharge tube. The first bracket arm is movable between a steady-state position in which the first bracket arm causes the first terminal to be conductively separated from the third terminal and a shorting position in which the first bracket arm causes the first terminal to be conductively connected to the third terminal. The first bracket arm is spaced from the first end of the gas discharge tube by a first distance when the first bracket arm is in its steady-state position and by a second distance when the first bracket arm is in its shorting position, with the first distance being greater than the second distance, and the first bracket arm is spring-biased towards its shorting position. 
     The surge protector assembly includes a first holding member associated with the first bracket arm and a first portion of eutectic material, such as solder, that bonds the first bracket arm to the first holding member when the first bracket arm is in its steady-state position so that the first holding member holds the first bracket arm in its steady state position. The first bracket arm is released by the first holding member upon a change of state, such as melting, of the first portion of eutectic material so that the first bracket arm moves to its shorting position. 
     The surge protector assembly also includes a second metal oxide varistor conductively connected to the second end of the gas discharge tube and a second bracket arm associated with the second end of the gas discharge tube. The second bracket arm is movable between a steady-state position in which the second bracket arm causes the second terminal to be conductively separated from the third terminal and a shorting position in which the second bracket arm causes the second terminal to be conductively connected to the third terminal. The second bracket arm is spaced from the second end of the gas discharge tube by a third distance when the second bracket arm is in its steady-state position and by a fourth distance when the second bracket arm is in its shorting position, with the third distance being greater than the fourth distance, and the second bracket arm is spring-biased towards its shorting position. 
     The surge protector assembly further includes a second holding member associated with the second bracket arm and a second portion of eutectic material that bonds the second bracket arm to the second holding member when the second bracket arm is in its steady-state position so that the second holding member holds the second bracket arm in its steady state position, the second bracket arm being released by the second holding member upon a change of state of the second portion of eutectic material so that the second bracket arm moves to its shorting position. 
     Each of the bracket arms may have a hole formed therein, and a portion of each of the holding members may pass through one of the holes formed in the bracket arms. Each of the holding members may be provided with a relatively large-diameter portion and a relatively small-diameter portion, with the relatively small-diameter portion of each holding member passing through one of the holes in the bracket arms. The relatively large-diameter portion of each holding member may be bonded to one of the bracket arms by the eutectic material. 
     Each end of the gas discharge tube may have a hole formed therein, and each metal oxide varistors may be disposed in one of the holes formed in the ends of the gas discharge tube. The surge protector assembly may also include a pair of conductive mounting members that support the ends of the gas discharge tube, with each conductive mounting member having a first end connected to the first end of the gas discharge tube and a second end conductively connected to the first terminal, and the bracket arms may make physical contact with the conductive mounting members when the bracket arms are in their shorting positions. The surge protector assembly may also include a printed circuit board and a support structure that supports the gas discharge tube on the printed circuit board, and the surge protector assembly may be disposed within a housing and provided as a complete surge protector. 
     The features and advantages of the present invention will be apparent to those of ordinary skill in the art in view of the detailed description of the preferred embodiment, which is made with reference to the drawings, a brief description of which is provided below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment of an electrical surge protector in accordance with the invention; 
     FIG. 2 is an exploded perspective view of the surge protector of FIG. 1; 
     FIG. 3 is a perspective view of the surge protector of FIG. 1 with portions of the housing cut away; 
     FIG. 4 is a cross-sectional view of a surge protector assembly shown in FIG. 1; 
     FIG. 5 is an exploded perspective view of a first portion of the surge protector assembly of FIG. 1; 
     FIG. 6 is an exploded perspective view of a second portion of the surge protector assembly of FIG. 1; 
     FIG. 7 is an exploded perspective view of the surge protector assembly of FIG. 1; 
     FIG. 8 is a cross-sectional view of the surge protector assembly shown in a shorted condition on one side; 
     FIG. 9 is a representation of an equivalent circuit of the surge protector assembly of FIG. 1; 
     FIG. 10 is a graph of a waveform illustrating an example of the operation of the surge protector assembly; 
     FIG. 11 is an alternative embodiment of a surge protector assembly in accordance with the invention; 
     FIG. 12 is an enlarged cross-sectional side view of a portion of the surge protector assembly shown in FIG. 11; and 
     FIG. 13 is a bottom view of the surge protector assembly of FIG. 11 with the printed circuit board of FIG. 11 not shown. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates one embodiment of a surge protector  10  in accordance with the invention. Referring to FIG. 1, the surge protector  10  has an insulating housing  12 , which may be composed of plastic, a cover  14  which may be disposed on the housing  12 , a pair of outer protector terminals  16 ,  18  which extend from the housing  12 , and a center protector terminal  20  which extends from the housing  12 . The outer protector terminals  16 ,  18  may be provided in the form of metal pins or screws, and each may be provided with one or more respective nuts  22  and washers  24  to facilitate fastening the terminals  16 ,  18  to conductors (not shown) for which surge protection is to be provided. The center terminal  20  may be provided in the form of a flat metal plate having an aperture  26  adapted to receive a bolt (not shown). 
     A surge protector module  30  is provided within the housing  12 , as shown in FIGS. 2 and 3. Referring to FIG. 7, which is an exploded perspective view of the surge protector module  30 , the module  30  incorporates a gas discharge tube  32  having a pair of end caps  34 , each of which has a cylindrical hole  36  formed therein. Each of the end caps  34 , which are composed of a conductive material such as metal, act as a gas discharge tube terminal. Referring also to FIG. 4, each end cap  34  of the gas discharge tube  32  is generally cup-shaped, having a relatively large-diameter portion  38  and a relatively small-diameter portion  40  in which the cylindrical hole  36  is disposed. 
     The gas discharge tube  32  has a pair of annular housing members  42 , each of which is composed of a dielectric material. The annular housing members  42  are separated by a conductive ring member  44 , which may be integrally formed with or otherwise connected to a conductive post, ring or pin  46  that may act as another terminal of the gas discharge tube  32 . As shown in FIG. 4, a cavity  48  is disposed within the annular housing members  42 , with a gas, such as Argon, disposed in the cavity  48  to provide the electrical characteristics of the gas discharge tube  32 . 
     The gas discharge tube  32  as described above may be a conventional component that has been sold for a number of years prior to the filing date of this patent, such as a gas discharge tube as disclosed in U.S. Pat. No. 5,500,782 to Oertel, et al., which is incorporated by reference herein. 
     Referring to FIGS. 6 and 7, a pair of conductive mounting members or brackets  50  may be connected to the gas discharge tube  32 . Each of the mounting brackets  50 , which may be L-shaped members, may be permanently connected to a respective one of the conductive end caps  34  of the gas discharge tube  32 , via welding for example, at a number of welding points  52 . 
     Referring to FIGS. 4 and 7, the surge protector assembly  30  includes a pair of surge protection elements in the form of metal oxide varistors  60 . The metal oxide varistors  60  are sized to fit within the cylindrical holes  36  formed in the end caps  34  of the gas discharge tube  32  and are disposed within the cylindrical holes  36  after assembly of the surge protector assembly  30 , as shown in FIG.  4 . 
     Referring to the upper portion of FIG. 7, the surge protector assembly  30  includes a metal bracket  70  having a pair of bracket arms  72 , each of which is vertically disposed and generally parallel to the vertical surfaces of the end caps  34  of the gas discharge tube  32 . The bracket  70  has a pair of pinch members  74  integrally formed therewith, with the pinch members  74  being adapted to grip and make conductive connection with the conductive ring or pin  46  that acts as a center electrode of the gas discharge tube  32 . 
     The bracket  70  has a pair of mounting arms  76  that extend downwardly from the upper portion of the bracket  70 . A mounting clip  78  is formed on the lower portion of each of the mounting arms  76 . Referring to FIG. 1, each of the mounting clips  78  may be sized or adapted to clip onto a conductive wall member  80  that extends upwardly from a bottom portion of the interior of the housing  12 . The wall member  80  is conductively coupled to the center protector terminal  20  so that all portions of the conductive bracket  70  are conductively connected to the center protector terminal  20 . 
     Referring to FIGS. 4 and 5, each of the bracket arms  72  has a circular, inwardly recessed portion  80 , with a circular hole  82  formed in its center. A respective holding member  84  is disposed through each of the holes  82 . Each holding member  84  has a relatively large-diameter outer portion  86  and a relatively small-diameter inner portion  88 . Each of the large-diameter outer portions  86  is sized so that it fits within one of the inwardly recessed portions  80  in the bracket arm  72 . 
     Each of the holding members  84  is bonded to one of the bracket arms  72  with a eutectic material, such as solder. The eutectic material may be provided in the form of a solder disk, solder paste, or coil preform  90  having a diameter generally corresponding to the annular surface of the each of the inwardly recessed portions  80  of the bracket arms  72  and generally corresponding to the inner annular surface of each of the relatively large-diameter portion  86  of the holding members  84 . 
     Referring to FIG. 4, the bracket  70  may act as a spring so that each of the bracket arms  72  is spring-biased inwardly. When the bracket arms  72  are bonded to the relatively large-diameter portions  86  of the holding members  84 , the holding members  84  exert an outward force on the bracket arms  72  which prevents them from bending or moving inwardly. 
     The bonding between the holding members  84  and the bracket arms  72  provided by the eutectic material  90  is designed to fail in response to a change of state, such as melting, of the eutectic material  90 . In that case, the inwardly biased bracket arms  72  move inwardly, under the influence of their spring bias, until a bottom edge  92  of each of the bracket arms  72  makes physical and conductive contact with the conductive brackets  50 . As shown in FIG. 1, a portion of each of the outer protector terminals  16 ,  18  extends into the housing  12 , and each of the conductive support brackets  50  may be connected to one of the outer protector terminals  16 ,  18  at a plurality of weld points  94 . 
     Each of the metal oxide varistors  60  may be selected to provide a static or DC clamping voltage that is greater than the static or DC breakdown voltage of the gas discharge tube  32 . For example, the gas discharge tube  32  may be selected to provide a DC breakdown voltage of 330 or 350 volts, and each of the metal oxide varistors  60  may be selected to provide a DC clamping voltage of 430 volts, for example. 
     Each of the metal oxide varistors  60  may have a size that is relatively small, such as about three millimeters in diameter, in order to provide the surge protector assembly  30  with a relatively low capacitance, such as a capacitance lower than about 30 picofarads or lower than about 20 picofarads, which would make the surge protector assembly  30  suitable for high-speed networks, such as DSL networks and existing telephone (POTS) networks, and which would provide the surge protector  10  with a very low insertion loss, such as 0.01 dB, depending on the frequency of the network. 
     Assembly 
     In order to assemble the surge protector assembly  30 , the bracket assembly  100  shown in FIG. 5 may be completed by soldering the two holding members  84  to the bracket arms  72  with the eutectic material preforms  90 . After soldering, the relatively large-diameter portion  86  of each holding member  84  will be bonded to one of the inwardly recessed portions  80  of each of the bracket arms  72  at an annular bonding area. The force required to physically break that bond may be selected to be a minimum force, or breakaway force, such as 60 pounds, so that jarring or other shock to the surge protector  10  will not unintentionally break the bond. 
     After forming the gas discharge tube assembly  102  shown in FIG. 6 by connecting the support brackets  50  to the end caps  34  of the gas discharge tube  32 , the metal oxide varistors  60  may be placed within the holes  36  in the gas discharge tube  32 , and then the completed bracket assembly  100  (after soldering as described above) may be placed over the gas discharge tube assembly  102  so that the holding members  84  press inwardly against the metal oxide varistors  60  to hold them in place, as shown in FIG.  4 . 
     Referring to FIG. 2, after construction of the surge protector assembly  30 , that assembly  30  may be installed within the housing  12  by placing the assembly  30  so that the clips  78  grip the conductive wall  80 , forcing the assembly  30  downwards until the horizontal portions of the mounting brackets  50  make contact with the interior surfaces of the outer protector terminals  16 ,  18 , and then welding the mounting brackets  50  to the terminals  16 ,  18  at the weld points  94 . The cover  14  may then be connected to the housing  12 , such as by ultrasonic welding. A moisture barrier (e.g. a non-conductive gel) may be included in the protector housing  12 . 
     Operation 
     During operation, a voltage transient or a power surge may be induced in one or both of the electrical conductors (not shown) which are attached to the two protector terminals  16 ,  18 . Some of the following operational examples refer to FIG. 9, which is a representation of an equivalent circuit of the surge protector assembly  30  of FIG.  1 . 
     In the example described immediately below, it is assumed that: 1) a fast-rising voltage surge is induced so that a high voltage is present on the protector terminal  16  relative to the center protector terminal  20 , which may be connected to a grounded conductor (not shown), 2) the surge has a rise time that is fast enough to increase the response time of the gas discharge tube  32 , and 3) the gas discharge tube  32  is operable and not vented. 
     In such case, there will be a fast-rising voltage transient indicated by the segment  110  shown in FIG.  10 . The surge voltage will increase (relative to the voltage on the center terminal  20 ) until its magnitude reaches the clamping voltage of the metal oxide varistor  60  associated with the terminal  16  (the metal oxide varistor  60  shown in the left-hand portion of FIG.  9 ), as indicated at a point  112  shown in FIG.  10 . At that point, surge current begins to flow through the metal oxide varistor  60 , and the surge voltage stops increasing as fast, or alternatively may not increase at all, as shown in FIG. 10 by a segment  114 . 
     When the voltage reaches the breakdown voltage of the gas discharge tube  32 , as indicated in FIG. 10 by point  116 , surge current begins to pass through the gas discharge tube  32  and the surge voltage decreases by the breakdown of the gas discharge tube  32 , as indicated by a segment  118 , to a relatively low magnitude, such as 15 volts, as indicated by a segment  120 . 
     Referring to FIGS. 9 and 10, between points  112  and  116  shown in FIG. 10, surge current will pass from the terminal  16 , through the left-hand metal oxide varistor  60 , and to the center terminal  20  of the surge protector  10 . 
     As shown in FIG. 9, the gas discharge tube  32  is in parallel with each of the metal oxide varistors  60 . When the surge voltage reaches the breakdown voltage of the gas discharge tube  32  (i.e. at point  116 ), the impedance of the gas discharge tube  32  becomes significantly lower than that of left-hand metal oxide varistor  60 , and consequently, the surge current flows through the gas discharge tube  32  beginning at point  116  in FIG.  10 . 
     The time required for the gas discharge tube  32  to turn on (the length of time from the start of the surge voltage to the time when the gas discharge tube  32  turns on as indicated at point  116 ) is relatively short, such as several nanoseconds. Since that time is very short, the surge current that passes through the metal oxide varistor  60  will cause no damage to the varistor  60 . 
     The operation described in the above example, and in particular the behavior of the surge protector  10  represented by the graph of FIG. 10, is the same as those of commercial embodiments of the surge protector disclosed in U.S. Pat. No. 5,500,782 to Oertel, et al., which commercial embodiments were sold in the United States prior to 1996. 
     If the rise time of the surge (i.e. represented by the slope of the segment  110  in FIG. 10) is slow enough to not affect the response time of the gas discharge tube  32 , the gas discharge tube  32  will turn on and conduct surge current before the metal oxide varistor  60  turns on and conducts surge current because the DC breakdown voltage of the gas discharge tube  32  is lower than the DC clamping voltage of the metal oxide varistor  60 , as noted above. In that case, no surge current will pass through the metal oxide varistor  60 . 
     In the presence of a prolonged power cross condition, where current passes through the surge protector  10  for a relatively long time, such as several seconds, the heat generated by the current will cause the eutectic material that bonds the bracket arm  72  to the holding member  84  to melt. Consequently, the bracket arm  72  will be released from the holding member  84  and will travel inwardly, due to its spring bias, until the bottom edge  92  of the bracket arm  72  makes contact with the conductive support member  50 , as shown in the left-hand side of FIG.  8 . The selective connection of the bottom edge  92  of the bracket arm  72  to the conductive member  50  is represented in FIG. 9 by the switch  92 . When the bottom edge  92  of the bracket arm  72  makes contact with the conductive member  50 , the protector terminal  16  is shorted to the grounded center terminal  20 , which would be indicated in FIG. 9 with the closing of the switch  92 , which is considered to be a switch-grade fail short. 
     The above examples assume that the gas discharge tube  32  is operable and is not vented. However, in some cases, the gas discharge tube  32  may become vented or inoperable, in which case a relatively large surge current would pass through the metal oxide varistor  60  for a relatively long period of time, such as several seconds for example (if the fail-short release mechanism described above were not provided in the surge protector assembly  30 ). 
     If the gas discharge tube  32  is inoperable, prior to the release of the bracket arm  72  by the holding member  84 , which is caused by melting of the eutectic material, all surge current will pass through the metal oxide varistor  60 . The inoperability or venting of the gas discharge tube  32  of the surge protector  10  does not cause a problem because of the very short time required for the bracket arm  72  to be released by the holding member  84 , which may be less than one second. The release time may depend on the spring force generated by the bracket arms  72 , the type of eutectic material used and its melting point, and the amount of eutectic material used to bond the bracket arms  72  to the holding members  84 . 
     In some prior art surge protectors that incorporate fail-short mechanisms, a pellet or disk of fusible material having a relatively large volume of a conductive or insulating material is disposed between a spring-biased bracket arm to hold a conductive shorting member away from an intended shorting point. The inventors have realized that, in order for such a surge protector to short, the pellet or disk must completely melt before shorting can occur. If the pellet or disk melts only partially, leaving a portion of the disk or pellet intact, shorting may be delayed for a relatively long period of time or prevented altogether. A relatively long delay in shorting may allow undesirable external arcing of surge current to occur within the protector assembly for a relatively long period of time, damaging one or both of the spring-biased bracket arms and preventing the surge protector from shorting at any time, or creating a high resistive path which can cause a thermal “runaway” condition. 
     The above problem is not present with the embodiments disclosed herein due to the design of the fail-short release mechanism, which results in a relatively fast release time and acts as a thermal switch having a switch-grade fail short characteristic. 
     Alternative Embodiment 
     An alternative embodiment of a surge protector assembly  200  is shown in FIGS. 11-13. The surge protector assembly  200  is generally similar to the surge protector assembly  30  described above, except that a bracket  70  of a different construction is used and the surge protector assembly  200  is connected to a printed circuit board  202 . Also, instead of using the conductive support brackets  50  connected to the end caps  34  of the gas discharge tube  32 , the surge protector assembly  200  utilizes rod-like conductors or wires  204 , that act as electrical terminals, that are welded or otherwise conductively connected to the end caps  34 . 
     In FIGS. 11-13, components that are similar, but not identical, in purpose and construction as corresponding components described in connection with FIGS. 2-8 are designated with the same reference numerals. The operation of the surge protector assembly  200  is substantially the same as that of the surge protector assembly  30  described above. 
     Modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. This description is to be construed as illustrative only, and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and method may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.