Abstract:
A surge protection device that includes varistors to absorb surge currents and protect a load from an overvoltage condition. The surge protection device includes a combination of switching elements and diodes. The surge protection device includes a basic cell design that can be easily adapted for higher voltage applications and/or higher discharge current applications and/or higher energy applications by parallel and/or series combinations of the basic cell design. Operation of the device minimizes fatigue to the varistors, thereby extending their useful life. The surge protection device achieves a better ratio of MOV clamping voltage rating versus MOV maximum continuous AC voltage rating, as compared with a conventional MOV installed alone.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to the field of surge protection, and more particularly to a surge protection device that includes varistors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many surge protection devices utilize varistors (also known as voltage dependent resistors or VDRs) as the main component for diverting electrical disturbances associated with industrial distribution equipment. A varistor is an electronic component with a “diode-like” nonlinear current-voltage characteristic. The basic function of a varistor is to conduct significantly increased current when voltage is excessive, while limiting residual voltage to protect circuits. Varistors are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by a high voltage away from sensitive components. 
         [0003]    The most common type of varistor is a metal oxide varistor (MOV). An MOV contains a ceramic mass of zinc oxide grains in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When low voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. Accordingly, the MOV will look like an open circuit. When the voltage applied across the electrodes exceeds the MOVs “maximum continuous voltage” (MCOV) rating, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behavior is a highly-nonlinear current-voltage characteristic in which the MOV acts as an insulator (high resistance) at low voltages and acts as a conductor (low resistance) for transients that exceed the MOV&#39;s maximum continuous voltage. 
         [0004]    The maximum continuous voltage rating defines maximum AC or DC voltage that can be applied before the MOV begins to conduct. Once the maximum continuous voltage is exceeded the MOV begins to heat as conduction current flows. In this state, the MOV will eventually have a catastrophic failure due to thermal runaway. To address this problem, surge protection device (SPD) manufacturers have incorporated thermo-mechanical disconnection systems to disconnect the MOV from the supply voltage before it fails. However, there is a limitation to the use of thermo-mechanical disconnection systems that relates to the magnitude of excess voltage that can be applied. If the voltage is sufficiently high enough, the heating of the MOV occurs too quickly to allow the thermo-mechanical disconnection system to react. This is due to the rate of conduction current rise (di/dt). In this regard, the current cannot be dissipated through the MOV and “hot spots” or areas of concentrated current density rapidly form in the MOV, thereby causing a puncturing of the MOV. 
         [0005]    Moreover, while MOVs offer low costs and relatively high transient energy absorption capability, MOVs progressively degrade with repetitive stress. Accordingly, an MOV will age quicker with more frequent activation. In a typical surge protection device, at least one MOV is employed per mode of protection and sometimes two or more MOVs are used in parallel to achieve higher surge current ratings. The MOV maximum continuous voltage rating is selected to provide maximum protection. However, the MOV maximum continuous voltage rating must be sufficiently high enough relative to the system voltage so that the MOV is only active during a surge event. If the MOV is activated when there is no surge event, then the MOV will age more quickly due to fatigue. One common design convention is to select an MOV having a maximum continuous voltage rating of 115% of nominal system voltage to avoid unwarranted activation. 
         [0006]    The present invention provides a new and improved surge protection device that solves the problems described above and overcomes drawbacks of prior art solutions, such as use of gas discharge tubes (GDTs) or crowbar circuits comprising zener diodes and silicon-controlled rectifiers (SCRs) (also known as a thyristor). 
         [0007]    The present invention provides a new solution to the problem described above, and provides advantages that are not found in existing prior art solutions. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the present invention, there is provided a surge protection device comprising: a surge protection circuit including: a first pair of series-connected varistors; a first pair of series-connected diodes, wherein said pair of diodes are connected with opposite polarity; and a first switching element connected at a first end between the first pair of series-connected varistors and at a second end between the first pair of series-connected diodes. 
         [0009]    In accordance with another aspect of the present invention, there is provided a surge protection device comprising: a first surge protection circuit including: three series-connected varistors; first, second and third series-connected diodes, wherein second series connected diode is connected at a first end to the first series-connected diode and connected at a second end to the third series connected diode, said second series-connected diode having a polarity opposite the polarity of said first and third series-connected diodes; a first switching element connected at a first end between the first and second series-connected diodes and connected at a second end between the first and second series-connected varistors; and a second switching element connected at a first end between the second and third series-connected varistors and connected at a second end between the second and third series-connected diodes. 
         [0010]    In accordance with still another aspect of the present invention, there is provided a surge protection device comprising: (1) a first surge protection circuit including: a first pair of series-connected varistors; a second pair of series-connected varistors; a first pair of series-connected diodes, wherein said pair of diodes are connected with opposite polarity; a first switching element connected at a first end between the first pair of series-connected varistors and at a second end between the first pair of series-connected diodes; and a second switching element connected at a first end between the first pair of series-connected diodes and at a second end between the second pair of series-connected varistors, wherein the first and second switching elements have opposite polarity; and (2) a second surge protection circuit including: a first pair of series-connected varistors; a second pair of series-connected varistors; a first pair of series-connected diodes, wherein said first pair of diodes are connected with opposite polarity and have a polarity that is reverse of said first pair of diodes of said first surge protection circuit; a first switching element connected at a first end between the first pair of series-connected varistors and at a second end between the first pair of series-connected diodes; and a second switching element connected at a first end between the first pair of series-connected diodes and at a second end between the second pair of series-connected varistors, wherein the first and second switching elements have opposite polarity, and a polarity reverse of said first and second switching elements of said first surge protection circuit, wherein a plurality of said first protection circuits connected in an alternating fashion with a plurality of second surge protection circuits, wherein adjacent first and second surge protection circuits share one of said varistors of said first pair of series-connected varistors, one of said varistors of said second pair of series-connected varistors, and one of said diodes of said first pair of series-connected diodes. 
         [0011]    An advantage of the present invention is the provision of a surge protection device that protects a load from transient overvoltages. 
         [0012]    Another advantage of the present invention is the provision of a surge protection device that does not have follow-on current. 
         [0013]    Still another advantage of the present invention is the provision of a surge protection device that is adaptable to address preprogrammed trip levels, required voltage or maximum current capacity, and variable surge current levels. 
         [0014]    Still another advantage of the present invention is the provision of surge protection device that minimizes fatigue to varistors. 
         [0015]    Yet another advantage of the present invention is the provision of a surge protection device that achieves a better ratio of MOV clamping voltage rating versus MOV maximum continuous AC voltage rating, as compared with a conventional MOV installed alone. 
         [0016]    Yet another advantage of the present invention is the provision of a surge protection device that has a basic cell design that can be easily adapted for higher voltage applications and/or higher discharge current applications and/or higher energy applications by parallel and/or series combinations of the basic cell design. 
         [0017]    A still further advantage of the present invention is the provision of a surge protection device that includes modular circuits that can be arranged in series or parallel configurations to achieve needed voltage or maximum current capacity, thereby providing greater flexibility to the system designer seeking to reduce manufacturing costs. 
         [0018]    These and other advantages will become apparent from the following description taken together with the accompanying drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention may take physical form in certain parts and arrangement of parts, an embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof, and wherein: 
           [0020]      FIG. 1  is a schematic illustrating a static surge protection device having a surge protection circuit according to a first embodiment (“basic cell” design); 
           [0021]      FIG. 2A  shows an AC voltage waveform with a positive voltage surge at 90 degrees; 
           [0022]      FIG. 2B  shows an AC voltage waveform with a negative voltage surge at 90 degrees; 
           [0023]      FIG. 3  is a schematic illustrating a static surge protection device having a surge protection circuit according to a second embodiment, wherein the second embodiment is an expanded version of the first embodiment to provide increased energy dissipation capacity; 
           [0024]      FIG. 4  is schematic illustrating a static surge protection device having an alternative circuit arrangement for the static surge protection circuit shown in  FIG. 3 ; 
           [0025]      FIG. 5  is a schematic illustrating a static surge protection device having a surge protection circuit according to a third embodiment; 
           [0026]      FIG. 6  is a schematic illustrating a static surge protection device having a surge protection circuit according to a fourth embodiment, wherein the fourth embodiment is an expanded version of the third embodiment to provide increased energy dissipation capacity; and 
           [0027]      FIG. 7  is a schematic illustrating a static surge protection device having a surge protection circuit according to a fifth embodiment, wherein the fifth embodiment is an expanded version of the alternative arrangement of the second embodiment shown in  FIG. 4 , the fifth embodiment providing increased energy dissipation capacity. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Referring now to the drawings wherein the showings are for the purposes of illustrating embodiments of the invention only and not for the purposes of limiting same,  FIG. 1  shows a static surge protection device  2  comprised of a driver circuit  10  and a surge protection circuit  21  according to a first embodiment (basic cell design). Device  2  is shown connected between an AC circuit (power line) and a load that requires surge protection. 
         [0029]    In one embodiment of the present invention, driver circuit  10  may take the form of an overvoltage crowbar sensing circuit. 
         [0030]    Circuit  21  is generally comprised of a pair of series-connected varistors  40   a  and  40   b , a pair of series-connected diodes  50   a  and  50   b  (connected with opposite polarity), and a switching element (in the form of a thyristor  31 ) connected between varistors  40   a ,  40   b  and diodes  50   a ,  50   b . More specifically, thyristor  31  is connected at a first end between the pair of series-connected varistors  40   a ,  40   b  and connected at a second end between the pair of series-connected diodes  50   a ,  50   b  Driver circuit  10  is used to activate (i.e., turn “on”) thyristor  31 , as will be described below. 
         [0031]    As indicated above, the switching element takes the form of a thyristor-type semiconductor (also referred to as a silicon controlled rectifier or SCR) in the illustrated embodiment. However, it is also contemplated that switching element may take other suitable forms, including but not limited to, a gas discharge tube (GDT), or crowbar circuits having zener diodes and SCR. It should be further appreciated that the thyristor-type semiconductor may be selected from a thyristor family, including but not limited to, gate turn-off thyristor (GTO), reverse conducting thyristor (RCT), MOS-controlled thyristor, etc. 
         [0032]    A thyristor has a gate, an anode and a cathode. When a small amount of current flows into the gate, this allows a larger current to flow from the anode to the cathode. Even when the current into the gate stops, the thyristor continues to allow current to flow from the anode to cathode. More specifically, in the normal “off” state of the thyristor, the current flow is restricted to the leakage current (e.g., &lt;0.1 mA). When the gate-to-cathode voltage exceeds a certain threshold, the thyristor turns “on” (i.e., activated state) and conducts current from the anode to the cathode. The thyristor remains in the “on” state even after gate current is removed, so long as current through the thyristor remains above a “holding current.” The holding current is the minimum amount of load current that is required to hold the thyristor latched in the “on” state. Once current falls below the holding current for an appropriate period of time, the thyristor switches “off.” Typical holding current values range from 1 milliamp to 50 milliamps or more. 
         [0033]    In the illustrated embodiment, varistors  40   a  and  40   b  take the form of metal oxide varistors (MOVs). When a small or moderate voltage is applied across the electrodes of a varistor, only a small current flows through the varistor. When a large voltage is applied across the electrodes of a varistor, a large current flows through the varistor. Accordingly, a varistor has a highly-nonlinear current-voltage characteristic, in which the varistor has a high resistance at low voltages and a low resistance at high voltages. When the varistor is in a conducting state, surge energy is absorbed and the voltage across the varistor is limited or “clamped” to a given voltage level that is relative to the magnitude of surge current being observed. 
         [0034]    Operation of device  2  will now be described in detail. During normal operating conditions, the AC circuit “sees” varistors  40   a ,  40   b  in series. Varistors  40   a ,  40   b  are in a high resistance mode, and thus appear as an open circuit. When driver circuit  10  detects a positive voltage surge (S 1 ), e.g., as illustrated in  FIG. 2A  (depicted at 90 degrees, but the surge may occur at any point on the waveform), driver circuit  10  turns on thyristor  31 . Consequently, current conducts through varistor  40   a , thyristor  31  and diode  50   a , as shown by current flow path A. In this regard, varistor  40   a  conducts current because the AC circuit voltage is equal to or exceeds the varistor maximum continuous voltage (MCOV). Diode  50   a  also conducts current since it is forward biased. Since the current follows the path of least resistance, current does not flow through varistor  40   b . When the current flowing through thyristor  31  is at or near zero, thyristor  31  automatically turns off. Consequently, the AC circuit will again “see” varistors  40   a  and  40   b  in series. 
         [0035]    Circuit  21  responds in a similar fashion when driver circuit  10  detects a negative voltage surge (S 2 ), e.g., as illustrated in  FIG. 2B  (depicted at 90 degrees, but the surge may occur at any point on the waveform). In this regard, driver circuit  10  turns on thyristor  31  causing current to conduct through varistor  40   b , thyristor  31  and diode  50   b , as shown by current flow path B. Varistor  40   b  conducts current because the AC circuit voltage is equal to or exceeds the varistor maximum continuous voltage (MCOV) and diode  50   b  also conducts current since it is forward biased. Since the current follows the path of least resistance, current does not flow through varistor  40   a . When the current flowing through thyristor  31  is at or near zero, thyristor  31  automatically turns off, and the AC circuit again “sees” varistors  40   a  and  40   b  in series. 
         [0036]    Circuit  21  diverts transient current surges through current flow path A or current flow path B, thereby protecting the load. Only one of the two varistors  40   a ,  40   b  is activated during a surge event, thereby resulting in less fatigue to each of the varistors  40   a ,  40   b . In other words, the fatigue is shared by each of the varistors  40   a ,  40   b , since only varistor  40   a  is activated in the event of a positive voltage surge and varistor  40   b  is activated only in the event of a negative voltage surge. Accordingly, only one of the two varistors  40   a ,  40   b  is activated for every positive and negative voltage surge. Thus, during normal operation there are two (2) varistors (i.e., varistors  40   a  and  40   b ) in series so there is no fatigue, but when an event (such as a voltage surge) is detected, the switching element (e.g., thyristor  31 ) is triggered and only one (1) varistor (i.e., varistor  40   a  or varistor  40   b ) is turned on to provide “clamping.” 
         [0037]    Referring now to  FIG. 3 , there is shown a static surge protection device  2 A comprised of a driver circuit  10  and a surge protection circuit  22 A according to a second embodiment. Circuit  22 A is an expanded version of circuit  21  that increases (i.e., doubles) the amount of energy that can be dissipated. Circuit  22 A is generally comprised of first and second pairs of series-connected varistors  40   a ,  40   b  and  42   a ,  42   b , a pair of series-connected diodes  50   a  and  50   b  (connected with opposite polarity), and a pair of switching elements in the form of thyristors  31  and  32 . Thyristor  31  is connected between varistors  40   a ,  40   b  and diodes  50   a ,  50   b , while thyristor  32  is connected between diodes  50   a ,  50   b  and varistors  42   a ,  42   b . More specifically, thyristor  31  is connected at a first end between the first pair of series-connected varistors  40   a ,  40   b  and at a second end between the pair of series-connected diodes  50   a ,  50   b . Thyristor  32  is connected at a first end between the pair of series-connected diodes  50   a ,  50   b  and at a second end between the second pair of series-connected varistors  42   a ,  42   b.    
         [0038]    Operation of circuit  22 A will now be described in detail. During normal operating conditions, the AC circuit “sees” series-connected varistors  40   a ,  40   b  and series-connected varistors  42   a ,  42   b  in parallel. When driver circuit  10  detects a positive voltage surge (S 1 ), e.g., as illustrated in  FIG. 2A , driver circuit  10  turns on thyristors  31  and  32 . Consequently, current conducts through (1) varistor  40   a , thyristor  31  and diode  50   a , and (2) varistor  42   a , thyristor  32  and diode  50   a , as shown by current flow paths A. Accordingly, the surge current is divided between varistors  40   a  and  42   a . When the current flowing through thyristors  31  and  32  is at or near zero, thyristors  31  and  32  automatically turn off. Consequently, the AC circuit will again “see” series-connected varistors  40   a ,  40   b  and series-connected varistors  42   a ,  42   b  in parallel. 
         [0039]    Circuit  22 A responds in a similar fashion when driver circuit  10  detects a negative voltage surge (S 2 ), e.g., as illustrated in  FIG. 2B . In this regard, driver circuit  10  turns on thyristors  31  and  32  causing current to conduct through (1) varistor  40   b , thyristor  31  and diode  50   b , and (2) varistor  42   b , thyristor  32  and diode  50   b , as shown by current flow paths B. Accordingly, the surge current is divided between varistors  40   b  and  42   b . When the current flowing through thyristors  31  and  32  is at or near zero, thyristors  31  and  32  automatically turns off, and the AC circuit again “sees” series-connected varistors  40   a ,  40   b  and series-connected varistors  42   a ,  42   b  in parallel. 
         [0040]    By dividing the current between a pair of varistors during a voltage surge, the amount of energy that can be dissipated by circuit  22 A ( FIG. 3 ) is double the amount of energy that can be dissipated by circuit  21  ( FIG. 1 ), using all varistors of the same maximum continuous voltage rating. 
         [0041]      FIG. 4  shows a static surge protection device  2 B having a surge protection circuit  22 B that is an alternative to the circuit arrangement for surge protection circuit  22 A shown in  FIG. 3 . In this regard, circuit  22 B has the same components as circuit  22 A, except that the polarities of thyristors  31  and  32  are reversed and the locations of diodes  50   a  and  50   b  are exchanged (thereby reversing polarities). As a consequence, current flow path A flows through diode  50   a  and varistors  40   b  and  42   b , and flow path B flows through diode  50   b  and varistors  40   a  and  42   a , as illustrated in  FIG. 4 . 
         [0042]    Referring now to  FIG. 5 , there is shown a static surge protection device  3  comprising a driver circuit  10  and a surge protection circuit  23  according to a third embodiment. Circuit  23  is generally comprised of series-connected varistors  80   a ,  80   b  and  80   c , series-connected diodes  70   a ,  70   b  and  70   c , and switching elements in the form of thyristors  61  and  62 . Diode  70   a  is connected with a polarity opposite to the polarity of diodes  70   b  and  70   c . Thyristors  61  and  62  are connected in parallel between varistors  80   a - 80   c  and diodes  70   a - 70   c . More specifically, thyristor  61  is connected at a first end between diodes  70   a  and  70   b , and connected at a second end between varistors  80   a  and  80   b . In the illustrated embodiment, varistors  80   b  and  80   c  each have a maximum continuous voltage rating generally equal to one-half the maximum continuous voltage rating of varistor  80   a . All of the varistors  80   a ,  80   b  and  80   c  have the same peak current rating. Thus, varistor  80   b , taken together with varistor  80   c , provide a maximum continuous voltage rating approximately equal to the maximum continuous voltage rating of varistor  80   a  taken individually. 
         [0043]    In the event of a positive voltage surge, driver circuit  10  turns on thyristors  61  and  62  causing current to conduct through diode  70   b , thyristor  61 , varistor  80   a  and diode  70   c , as shown by current flow path A. Likewise, in the event of a negative voltage surge, driver circuit  10  turns on thyristors  61  and  62  causing current to conduct through diode  70   a  and series-connected varistors  80   c  and  80   b , as shown by current flow path B. Since varistors  80   b  and  80   c  each have a maximum continuous voltage rating generally equal to one-half the maximum continuous voltage rating of varistor  80   a , the total maximum continuous voltage rating of varistors  80   b  and  80   c , taken together, is approximately equal to the maximum continuous voltage rating of varistor  80   a , taken individually. When the current flowing through thyristors  31  and  32  is at or near zero, thyristors  31  and  32  automatically turn off, and the AC circuit again “sees” varistors  80   b ,  80   a  and  80   c  connected in series. The foregoing configuration results in less stress on the varistors, since varistor  80   a  is only activated during a positive voltage surge, and varistors  80   b  and  80   c  are only activated during a negative voltage surge. Accordingly, not all of the varistors  80   a ,  80   b  and  80   c  are activated each time there is a voltage surge, thus minimizing the number of stress-inducing activation cycles for individual varistors. 
         [0044]      FIG. 6  shows a schematic illustrating a surge protection circuit  123  according to a fourth embodiment. Circuit  123  is an expanded version of circuit  23  (see  FIG. 5 ), and thus provides increased energy dissipation capacity. A static surge protection device  4  is comprised of circuit  123  and an associated switch driver circuit  10 . 
         [0045]    Circuit  123  is an association of circuits  23  described above. In particular, circuit  123  is comprised of a plurality of circuits  23  “overlapping” with a plurality of adjacent mirrored circuits  23  (labeled as circuits  23 A in  FIG. 6 ). In other words, circuit  123  is comprised of a series of alternating circuits  23  and circuits  23 A, wherein adjacent circuits  23  and circuits  23 A share series-connected diodes  70   a ,  70   b ,  70   c  or varistors  80   a ,  80   b ,  80   c.    
         [0046]    During normal operating conditions, the AC circuit “sees” the series-connected varistors  80   b ,  80   a  and  80   c  of each circuit  23  and  23 A in parallel. In the event of a positive voltage surge, driver circuit  10  turns on all thyristors  61  and  62  of circuit  123 , thereby causing current to conduct through diodes  70   b , thyristors  61  and  62 , varistors  80   a  and diodes  70   c , as shown by current flow paths A. Likewise, in the event of a negative voltage surge, driver circuit  10  turns on all thyristors  61  and  62  of circuit  123 , thereby causing current to conduct through varistors  80   c ,  80   b , thyristors  61 ,  62 , and diodes  70   a , as shown by current flow paths B. When the current flowing through thyristors  61  and  62  is at or near zero, thyristors  61  and  62  automatically turn off, and the AC circuit again “sees” the plurality of series-connected varistors  80   b ,  80   a  and  80   c  of circuit  123 . 
         [0047]    Circuit  123  increases the number of varistors as compared to circuit  23  taken alone ( FIG. 5 ). Accordingly, circuit  123  dissipates more energy (i.e., higher voltage) during a voltage surge. Moreover, circuit  123  puts less stress on the varistors, since current flow paths A and B respectively provide parallel current flow paths in response to positive and negative voltage surges, thereby dividing the surge current among multiple varistors during an overvoltage event. 
         [0048]      FIG. 7  shows a schematic illustrating a static surge protection circuit  122  according to a fifth embodiment. Circuit  122  is basically an expanded version of circuits  22 A and  22 B ( FIGS. 3 and 4 ), and thus provides increased energy dissipation capacity. A static surge protection device  5  is comprised of circuit  122  and an associated switch driver circuit  10 . 
         [0049]    Circuit  122  is basically an association of circuits  22 A and  22 B described above. In particular, circuit  122  is comprised of a plurality of circuits  22 B overlapping with a plurality of adjacent circuits  22 A, as shown in  FIG. 7 . In other words, circuit  122  is comprised of a series of alternating circuits  22 B and  22 A, wherein adjacent circuits  22 B and  22 A share one varistor  40 , one diode  50   a  or  50   b  and one varistor  42 . During normal operating conditions, the AC circuit “sees” the series-connected varistors  40 , and series-connected varistors  42  of circuit  22 A and  22 B. 
         [0050]    In the event of a positive voltage surge, driver circuit  10  turns on all thyristors  31  and  32  of circuit  122 , thereby causing current to conduct as shown by current flow paths A. Likewise, in the event of a negative voltage surge, driver circuit  10  turns on all thyristors  31  and  32  of circuit  122 , thereby causing current to conduct as shown by current flow paths B. When the current flowing through thyristors  31  and  32  is at or near zero, thyristors  31  and  32  automatically turn off, and the AC circuit again “sees” the plurality of series-connected varistors  40  and series-connected varistors  42 , in parallel. 
         [0051]    In the embodiment illustrated in  FIG. 7  there are six (6) series-connected varistors  40 , and six (6) series-connected varistors  42 . Accordingly, when all thyristors  31  and  32  are turned ON, current conducts through three (3) series connected varistors  40  and three (3) series-connected varistors  42 . When all thyristors  31  and  32  are turned OFF, the AC circuit “sees” six (6) series-connected varistors  40  in parallel with six (6) series-connected varistors  42 . Therefore, expanding circuit  122  to N series-connected varistors  40  and N series-connected varistors  42 , with all thyristors  31  and  32  are turned ON, current conducts through N/2 series-connected varistors  40  and N/2 series-connected varistors  42 . When all thyristors  31  and  32  are turned OFF, the AC circuit “sees” N series-connected varistors  40  in parallel with N series-connected varistors  42 . 
         [0052]    Circuit  122  increases the number of varistors as compared to circuits  22 A and  22 B, shown respectively in  FIGS. 3 and 4 . Accordingly, circuit  122  dissipates more energy (i.e., higher voltage) during a voltage surge. Moreover, circuit  122  puts less stress on the varistors, since current flow paths A and B respectively provide parallel current flow paths in response to positive and negative voltage surges, thereby dividing the surge current among multiple varistors during an overvoltage event. 
         [0053]    It should be understood that for each embodiment described above it is contemplated that the surge protection circuits may be expanded to accommodate additional varistors. Moreover, the maximum continuous voltage rating of each varistor may be selected to achieve a desired level of surge protection. 
         [0054]    The foregoing description discloses specific embodiments of the present invention. It should be appreciated that these embodiments are described for purposes of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.