Abstract:
A circuit and device protects connected equipment from a broad range of unsafe conditions of over-voltage and under-voltage by disconnecting the line voltage before surge protection components, generally MOVs, are damaged from sustained surges. However, nuisance tripping of the protective circuit is avoiding by discriminating surges that are properly handled by the MOVs and components in the load or protective device. Further, the devise are protected from sustained high voltage line conditions, which would ordinarily result in repeated cycling between the on and off states as the line voltages fluctuates slightly at or about the trip threshold, as the circuit has a deliberate hysteresis such that the turn on voltage is about 10 to 20 V lower than the shut off threshold.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     None 
     BACKGROUND OF INVENTION 
     The present invention relates to transient voltage surge suppressors (TVSS) for protecting electrical equipment connected to alternating current (AC) electrical power lines, and in particular, to TVSS circuits for protecting electrical loads from sustained excessive AC voltages. 
     Electrical power lines are often subject to surges and other transients of high current or voltage caused by various events, such as lightning, switching on or off significant electrical loads, or even occasional short circuits. Such surges or transients can cause permanent damage to electrical equipment connected to the power line, particularly equipment in which electronic devices are responsible for the consumption and use of the power. Transient voltage surge suppressors have long been used to detect and attempt to divert such surges and transients before they reach the devices connected to the power line. The TVSS industry and marketplace generally focus on the limiting of transients of short duration, such as those resulting from lightning strikes. However, additional relatively common conditions exist which can produce sustained over-voltage conditions whereby the AC voltage presented via the power lines is significantly higher than that for which the connected electrical equipment is designed and capable of operating without damage. For example, poor voltage regulation by the electrical utility provider, improper wiring of the facility, use of standby generators, or incorrect or defective bonding of neutral and ground line connections can all lead to sustained AC over-voltage. 
     Under such sustained over-voltage conditions, conventional TVSS devices, which use voltage-limiting components (e.g., metal oxide varistors or “MOV”s) to limit transients of short duration, will either be ineffective (if their limiting, or “let-through,” voltage is more than the peak value of the power line voltage) or simply burn out, since MOV voltage limiters overheat in a very short time when absorbing the excessive power associated with the over-voltage. 
     It is therefore a first object of the present invention to provide an over-voltage protection circuit that disconnects a surge suppressor circuit and the load from the power source before the surge suppressor components are damaged, and then restores the load when the over-voltage conditions is abated. 
     Another object is to provide an under voltage protection circuit that disconnects a surge suppressor circuit and the load from the power source before the critical components in the device that constitutes the load are damaged, and then restores power to the load when the under-voltage conditions is abated. 
     Another object is to provide a variable voltage/time response, which varies in proportional to propensity for the condition to damage a MOV or other surge suppressor components, wherein the threshold for disconnecting the power from the surge suppressor circuit and load varies with the total energy in the voltage transient. 
     It is another objective to also disconnect the load and surge protector components under more sustained over voltage conditions, which would result in damage to the MOV, and/or require a thermal fuse coupled to the MOV. 
     Yet a further object is to provide a combined over and under voltage protection circuit, with the over voltage protection circuit having the aforementioned variable voltage/time response. 
     An additional object is to provide an over voltage protection circuit of the aforementioned character that precludes cycling of the disconnection circuit between the open and closed state at or near the trip threshold. 
     SUMMARY OF INVENTION 
     The invention is for an improved TVSS device which includes circuitry to detect such over-voltage conditions and cause a switch, such as a relay, to disconnect the load (as well as the MOV circuitry) from the power line when there is a sustained over-voltage condition between the power and neutral voltage lines. 
     An unsafe voltage protection circuit of the TVSS, in accordance with the present invention, monitors at least the line-neutral inter-terminal voltage for an over-voltage condition. If the inter-terminal voltage exceeds a predetermined maximum voltage, even for a short time interval, the incoming power line connection is interrupted to protect the load circuitry (as well as any other additional circuitry, such as transient suppression circuitry using MOV devices) from exposure to such excessive voltage. This power interruption is maintained for so long as such over-voltage condition exists. 
     Additionally, in a preferred embodiment the line to neutral inter-terminal voltages is further monitored by the unsafe voltage protection circuit for under-voltage conditions, whereby the load can be protected from exposure to low voltage (e.g., “brownout”) conditions. 
     In the case of an over-voltage condition, the response time varies to avoid nuisance trips caused by lower voltage surges of very short duration that would not damage the other protection circuitry component or the protected equipment. The trip time takes into account the conductive threshold and damage characteristics of MOV or comparable components. 
     Further, in or to avoid nuisance cycling of the protection circuit when the over-voltage condition hovers at about the trip threshold, the device provides for a lower turn on threshold after trip. 
     The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a functional block diagram of an over-voltage protection circuit for protecting an electrical load in accordance with one embodiment of the present invention. 
         FIG. 2  is an electrical schematic diagram of a first embodiment of the voltage protection circuit of  FIG. 1 . 
         FIG. 3  is a schematic diagram illustrating the time response characteristics of the detection circuit of  FIG. 2 . 
         FIGS. 4A ,  4 B,  4 C illustrate power semiconductor devices that may be used as substitutes in place of the electromechanical relay in the circuit of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an unsafe voltage protection circuit  10  for protecting a load  18  against sustained under-voltage or over-voltage conditions includes a voltage detection circuit  12 , a switch  14  and a transient limiter circuit  16 , all interconnected substantially as shown (and discussed in more detail below). The ground  11 , neutral  13  and power  15  lines connect to the voltage detection circuit  12 , switch  14  and transient limiter circuit  16 . The voltage detection circuit  12  is, in the broadest embodiments, capable of monitoring the inter-terminal voltages between any two of the combination of the ground  11 , neutral  13  and power  15  lines, but will be described with respect to the protection for unsafe voltage conditions detected by the inter-terminal voltage between ground and line. 
     In one embodiment, whenever the voltage between the line terminal and the neutral terminal exceeds a predetermined maximum voltage, the voltage detection circuit  12 , via control  17 , instructs the switch (e.g. relay) to interrupt the current path of the incoming power line  15  to the transient limiter circuit  16  and load  18 , thereby preventing the excessive voltage appearing on the power line  15  from reaching the transient limiter circuit  16  and load  18 . Additionally, the voltage detection circuit  12  may detect when the voltage between the neutral  13  and power  15  lines falls below a predetermined minimum voltage whereupon the switch  14  is instructed to disconnect the incoming power line  15  from the transient limiter circuit  16  and load  18 , thereby preventing damage that can be caused by an under-voltage condition. It should be appreciated that other variants of the circuit permit the monitoring of anyone or more of such inter-terminal voltages between ground, line and neutral terminal, and the interruption of the current in response to an unsafe over or under-voltage condition there between. 
     Referring to  FIG. 2 , a specific implementation of the unsafe voltage protection circuit  12  of  FIG. 1  will be described first in the normal state of operation, when the inter-terminal voltage is within the range of the under-voltage and over-voltage trip or protection thresholds. Voltage detection circuit  12  provides a peak detector function operative with respect to the relay driver/controller function, by measuring the peak AC line voltage as a DC value, and compares it against a reference voltage using a voltage divider/zener-diode-comparator. The transient limiter circuit  16  (also labeled as TVSS/Filter Circuit) only receives power when a relay or other switch connected to the line input power is closed in response to the outcome of the voltage divider/zener-diode-comparator result. The relay driver/controller function is fault-tolerant with respect to the shutoff mechanism, as should either of two relay controller switches (Q 2 , Q 3 ) fail, the other one will handle the shutoff function. It also uses semi-isolated DC supplies via rectifier diodes D 4  and D 9  for the peak detector and relay driver (filtered to DC by C 2  and C 4 , respectively). As described in further detail below, the relay driver supply is voltage-regulated by the series zener diodes ZD 3  and ZD 4  to the nominal operating voltage of relay K 1 A  14 . Thus, the circuit can be modified to adjust the different shutoff thresholds and relay configurations as suits the application. 
     The relay driver portion of circuit  12 , is formed by transistors Q 2 , Q 3  and Q 4 , and is powered by half-wave rectified AC voltage to charge up capacitor C 4  to a DC voltage regulated by ZD 3  and ZD 4 . Two series zener diodes are shown here, but a single part of sufficient value (up to the maximum safe operating voltage of the diode) can be used as well. R 10  is used as a current-limiting device to protect Q 3  from transient overloads. R 7 , which supplies base current to Q 3 , is connected to the detector portion of the circuit to supply DC power via rectifying diode D 4 . If detector components C 1  or D 4  fail, the relay(s)  14   a  will shut off. 
     Thus, relay K 1 A  14  constitutes the controlled portion of switch  14  in  FIG. 1 . Diode D 9  half-wave rectifies AC power from the line connection  15  to charge capacitor C 4 . The DC voltage of C 4  is regulated by zener diodes ZD 3  and ZD 4 . Two series zener diodes are deployed in this embodiment, but a single part of sufficient value (up to the maximum safe operating voltage of relay coil in K 1 A  14 ) can be used as well. The supply of rectified power to C 4  and relay coil  14   a  closes switch  14  such that current is supplied to the transient limiter circuit  16  and the load  18 . It should be appreciated that two switching poles, possibly actuated by two separate relay coils or other driving mechanism, can be used in parallel, depending on the needs of the application. In operation, diode D 4  half-wave rectifies AC power from the line connection  15  to charge capacitor C 2 . To the extent the AC power exceed the threshold clamping value of MOV 7 , the excess power in the voltage surge is shunted to neutral, protecting the components in detection circuit  12 , as well as transient limiter circuit  16  and load  18 . It should be appreciated that MOV 7  has a higher switching threshold than the MOVs in the transient voltage surge suppression (TVSS)/filter circuit  16 , being rated at about 275 VAC. C 2  powers the combination of transistors Q 2 , Q 4  and Q 1  that constitute the relay driver/controller function of detection circuit  12 . 
     However, the preferred switching thresholds and the voltage dependent switch time can be expected to vary depending on the nature of the load, or protected circuit, and the surge suppressor components, as the principles of operation are applicable to protecting components that are energized by applications inclusive of three-phase  208 , single-phase  240  and  120 , and the like. 
     The half-wave-rectified AC line voltage, provided by the connection of the line terminal to D 9 , is filtered to DC by C 2 . After filtering, it is compared to a reference voltage set by ZD 2  using a voltage divider to correlate the shutoff threshold. When the peak voltage, exceeds the shutoff threshold, determined by voltage across R 4 , as divided with respect to the value of R 5  plus R 6 , ZD 2  will begin to conduct, forward-biasing Q 2  and turning it “on”. Q 2  creates a “crowbar” to the power supply ground (neutral) and quickly discharges the relay coil(s) and the relay driver storage capacitor C 4  through D 5 , causing the relay(s) to shut off. Simultaneously, Q 3  becomes reverse-biased and cuts off the relay driver supply source. Depending on the conditions necessary to turn on Q 1 , as further discussed below, the relay  14   a  will turn on again once the peak AC voltage drops by predetermined level below the shutoff threshold. 
     However, in the normal state of operation, transistor Q 2  is in an off state, the incoming power line voltage, rectified by diode D 9  and filtered by capacitor C 2 , causes zener diodes ZD 3  and ZD 4  to become conductive (since their thresholds, or breakdown, voltage is exceeded by the rectified incoming power line voltage) and transistor Q 4  to turn on. This causes a base current for transistor Q 3  to be produced (limited by resistor R 7 ), thereby causing transistor Q 3  to turn on and provide a drive current  17   a  for the input coil  14   a  of the relay K 1   a    14 . (No current diversion takes place through diode D 5  since transistor Q 2  is turned off.) As a result, in accordance with well known relay operation, the magnetic energy  17   b  produced by the relay coil  14   a  causes the relay pole  14   c  of the relay output  14   b  to be connected to relay throw  14   d , thereby providing a current path for electrical current from the power line connection  15  to the output switch  26 . 
     With output power thus available, current also flows to light emitting diode LED  1  and diode D 1 , being limited by resistor R 1 . This lighting of diode LED  1  indicates a proper power connection to the output switch, and the provision of surge protected power at line, neutral and ground output terminals  1  and  2  of TVSS/Filter Circuit  16 , where one or more loads  18  of  FIG. 1  is connected 
     In the TVSS/Filter Circuit  16  over-voltage protection for the inter-terminal voltages between the ground  11 , neutral  13  and power line  15  connections is provided by way of multiple varistors MOV 1 , MOV 2 , MOV 3 , MOV 4 , MOV 5 , MOV 6 . Additionally, fuses TC 1  and TC 2  provide backup protection against burning up varistors MOV 2 , MOV 3 , MOV 5  and MOV 6 . It should be noted that in this embodiment TC 1  thermally couples with MOV 1 – 4 , and TC 2  with MOV 5 – 6 , such that overheating of the MOV trips the coupled fuse. 
     Further, it should be appreciated that on supplying power to device  10  at the line, ground and neutral terminal of the TVSS/Filter Circuit  16 , the voltage detection circuit  12  is energized and functioning before power is supplied to the line, neutral and output terminals, as capacitor C 4  must be fully charged before the switch  14   b  of relay  14  is able to close. 
     When the rectified voltage between the cathode of D 4  and the neutral terminal exceeds a threshold value that exceed the breakdown threshold of ZD 2  current is potentially available for supply to the base of Q 2 . However, the speed at which Q 2  opens is moderated by capacitor C 3 . This causes base current to become available for transistor Q 2 , thereby turning transistor Q 2  on in a saturated state. The emitter terminal of transistor Q 4  and cathode of diode D 9  thus become effectively shorted to the neutral line  13 . As a result, transistor Q 4  and zener diode ZD 4  are turned off, thereby preventing the flow of base current to transistor Q 3 . Transistor Q 3  then turns off, thereby eliminating the drive current for the relay coil  14   a . With transistor Q 2  and diode D 9  both turned on, the charge stored in capacitor C 4  is quickly depleted, thereby causing the relay coil  14   a  to become quickly deactivated. 
     With the elimination of the drive current for the relay coil  14   a  and depletion of charge across capacitor C 4  (and, therefore, the collapse of the magnetic field  17   b ), the relay pole  14   c  becomes connected to relay throw  14   e , thereby interrupting the current path between the power line connection  15  and the line output terminals  26 . Instead, current now flows to the flashing unsafe voltage LED circuit  40  that is operative to intermittently turn LED 3  on and off. This lighting of diode LED 3  indicates the interruption of output power due to the unsafe voltage condition. 
     Another condition, which this circuit  10  protects against, is an under-voltage condition between the neutral  13  and power  15  lines. During such a condition, the inter-terminal voltage between the neutral  13  and power  15  lines is insufficient to cause zener diodes ZD 3  and ZD 4  to go into zener breakdown, thereby preventing diode ZD 4  and transistor Q 4  from turning on. In turn, this prevents transistor Q 3  from receiving a base current. As a result, transistor Q 3  is turned off and no current is available to drive the relay coil  14   a  and generate a magnetic field  17   b  to cause the relay output  14   b  to connect pole  14   c  to throw  14   d . Instead, the relay pole  14   c  remains connected to relay throw  14   e , thereby interrupting the current path between the power line  15  and line output terminal  26 , and powering the flashing of unsafe voltage LED 3  circuit  40  which is operative to intermittently turn LED 3  on and off, This lighting of diode LED 3  indicates the interruption of output power due to the unsafe voltage condition. 
     It should be understood that providing both an over and under-voltage protection function to detection circuit  12 , while being a preferred embodiment, is not intended to be limiting as the over and under protection sub-circuit components need not be deployed together 
     Further, an “instantaneous” shutoff is not always needed to protect components or equipment from mild to moderate over-voltages, and can result in an excessive number of “nuisance trips”, which can be quite disruptive to users of information technology (IT) and home theater systems. Generally, potential damage to voltage-limited components can be correlated with the following power event parameters: voltage, duration, and available current. A response delay can be utilized that is voltage dependent: longer (up to 3 seconds) for mild over voltages, decreasing at around 150% of nominal input voltage to an asymptote (controlled solely by the sum of the detector time response and relay contact “crowbar” release time) which should be less than 25 milliseconds. The placement of capacitors C 3  and C 5  across the base-emitted junctions of Q 2  and Q 4  achieves this objective as the switching of these transistors will have an added time response directly proportional to the dv/dt across. It should be further appreciated that this circuit is designed to respond to an over-voltage condition more rapidly than to an under-voltage condition. In the event of an over-voltage condition, as discussed above, not only is the drive current to the relay coil  14   a  from transistor Q 3  terminated, but the existing charge across capacitor C 4  is also quickly depleted via transistor Q 2  and diode D 5 . This causes the relay coil  14   a  to deactivate quickly. In the event of an under-voltage condition, however, the drive current to the relay coil  14   a  from transistor Q 3  is interrupted, but the existing charge across capacitor C 4  is allowed to be depleted more slowly through the windings of the relay coil  14   a . This causes the relay coil  14   a  to deactivate more slowly. 
       FIG. 3  illustrates the result of the optimal selection of C 3  in the detection circuit to control the rate of shut off in over voltage conditions for the circuit  12  of  FIG. 2 . The applied voltage is plotted on the ordinate axis whereas the desired shot-off time, as a function of applied voltage is plotted as curve A. thus, at the lowest shut off voltage threshold of the circuit, 142 VAC the circuit responds to a sustained pulse of 1,000 msec. (1 sec.), however as the peak voltage is broader, that is up to about 180 VAC, it is desirable that the tripping pulse duration decrease proportionally in length, that is to about a single AC half cycle at 60 Hz., or about 8 msec., with the trip voltage duration being the same or lower for surges having a peak voltage over about 180 to about 240 VAC. Such a voltage dependent trip time response avoids, nuisance trips, which would not damage equipment, yet protects MOV in the primary surge protection circuit from being damaged or heated by constant voltage below their nominal breakdown threshold. This further illustrated by a conception damage rating curve for an MOV in the curve labeled “B”, which is offset above the circuit characteristic response curve, “A”, such that the circuit always disconnects the MOV faster than the minimum time or pulse duration that causes damage. 
     Thus, when the detection circuit  12  in  FIG. 2  disconnects the line voltage from a high voltage condition, it is desirable that power is not restored until the line voltage drops to a significant value below the trip voltage, termed the “recovery” voltage. If the recovery voltage is only within 1 to 2 volts of the trip voltage the disconnect circuit  18  and devices or powered equipment that constitutes the load  18 , can cycle between the on and off states when the over-voltage condition hovers about the trip voltage. Accordingly, another aspect of the invention is an increased level of hysterisis wherein the line voltage decreases by a predetermined amount, about 5 volts below the trip voltage in this embodiment, before the power is restored. 
     In detection circuit  12 , absent components Q 1 , ZD 6  and R 11  detection circuit  12  has a “recovery” voltage threshold that is only 1–2 volts lower than the shutoff voltage threshold. This means that for power events where line voltage may be fluctuating more than two volts in the vicinity of the shutoff threshold, the shutoff circuit may be actuated multiple times in a short period of time. As this is not desirable, R 5  is intended to be selectively bypassed or shunted in the circuit such that “recovery” voltage is decreased to a more comfortable level. As will be further explained, R 5  can be inserted into the voltage divider and controlled using a feedback loop provided by Q 1 , R 11  and ZD 7 . When the voltage detector is below the shutoff threshold (and the relays are on), Q 1  is in cutoff and R 5  is part of the voltage divider. When the shutoff threshold is reached, Q 2  begins to go into saturation and turns Q 1  on, bypassing R 5 . Decreasing the effective shutoff threshold, via the selection of the ratio of R 6  and R 5 , provides a level of hysteresis that is needed to achieve the desired “noise immunity” 
     Under normal, that is safe operation, when relay K 1 A  14  is powered, Q 1  is normally in the off, or open circuit state open, Q 4  is open such that current flows through both R 6  and R 5 . Thus, the turn-off or trip threshold voltage of the circuit is determined by R 5  and R 6  in comparison to R 4 , which divide the voltage differential necessary to exceed the breakdown threshold of ZD 2 , and thereby turn on Q 2 . However once Q 1  turns on, via the when Q 4  turns off such that the zener breakdown threshold for ZD 6  is now exceed, the voltage shifts as the voltage divider is now determined by R 4  and R 6 , as R 5  is effectively shunted as current flows to R 4  via Q 1 . Hence to re-energize the detection circuit, the voltage at ZD 2  must drop to a lower value before Q 2  will turn off. Thus, the voltage divider portion of circuit  12  is controlled by operation of the feedback loop provided by Q 1 , R  11  and ZD 6 . Accordingly, zener diode ZD 6  is selected for a breakdown threshold value sufficient to energize the base of Q 1  when the trip condition occurs. Thus, in the circuit of  FIG. 2 , the illustrated values for R 6 , R 5  and R 4  result in a trip voltage of about 198 VDC with a turn on voltage of about 176 VDC, or a difference of about 22 VDC. 
     Referring to  FIGS. 4A and 4B  in regard to the switch  14  used, as will be readily understood and appreciated by one of ordinary skill in the art, depending upon load current requirements, it is possible to substitute the use of a power semiconductor device in place of an electromechanical relay. Suitable examples would include a thyristor device, such as a silicon controlled rectifier (SCR)  114   a  ( FIG. 4A ) or triac  114   b  ( FIG. 4B ), or a power metal oxide semiconductor field effect transistor (MOSFET)  114   c  ( FIG. 4C ). If so, the drive current  17   a  provided by transistor Q 3  (converted to a voltage as necessary) or control signal  17   c , operative on the detection or production of an unsafe voltage conditions, would control the gate terminal of the SCR  114   a , triac  114   b  or MOSFET  114   c  in accordance with well-known conventional techniques. 
     While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.