US3660719A

US3660719A - Transient suppression system

Info

Publication number: US3660719A
Application number: US66934A
Authority: US
Inventors: Aime J Grenier
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1970-08-26
Filing date: 1970-08-26
Publication date: 1972-05-02
Anticipated expiration: 1989-05-02

Abstract

An electrical protective system is disclosed for selectively shunting potentially harmful, temporary, high voltage electrical signals, such as transient signals, from a circuit being protected. The system includes a normally non-conductive, voltage breakdown device, such as a spark gap device, connected in shunt with the input of the circuit to be protected. In response to the occurrence of a transient signal of a preselected voltage level, the spark gap device is rendered conductive and an electrical arc is established thereacross in order to provide a shunt path for the transient signal. In addition, an energy dissipating means is serially connected to the spark gap device for initially dissipating the energy of the transient signal and for then limiting the current passing through the spark gap device subsequent to the establishment of the electrical discharge so as to permit rapid extinguishment thereof upon the expiration of the transient signal.

Description

United States Patent Grenier 1 May 2, 1972 [54] TRANSIENT SUPPRESSION SYSTEM Primary E.raminer-James D. Trammell [72] Inventor: Aime J. Grenler, North Attleboro, Mass. g gfig kmfg im gl g gf fi i"' [73] Assignee: Texas Instruments Incorporated, Dallas,

[57] ABSTRACT [22] Filed: Aug. 26,1970 An electrical protective system is disclosed for selectively shunting potentially harmful, temporary, high voltage electri- [211 P 66934 cal signals, such as transient signals, from a circuit being protected. The system includes a normally non-conductive, volt- 52 us. (:1 ..3l7/l6, 317 31, 317/41, age breakdOw" Such as a Spark gap device connecled 317/50, 3l7/63 in shunt with the input of the circuit to be protected. In [51] Int. Cl. ..H02h 3/22 response to the occurrence of a transient Signal of a [58] Field ofSearch ..3l7/l6, 68, 31,41, 50 Preselected J1me level- Spark gaP device is rendered conductive and an electrical arc is established thereacross in [56] References Cited order to provide a shunt path for the transient signal. In addition, an energy dissipating means is serially connected to the UNITED STATES EN spark gap device for initially dissipating the energy of the transient signal and for then limiting the current passing 2,456,986 12/1948 Paluev ...3l7/4l X through the Spark gap device Subsequent to the establishment 2,703,852 3/1955 Meadorw' X of the electrical discharge so as to permit rapid extinguish- 3,339YI 16 8/1967 Harmon X ment thereof upon the expiration of the transient signal. 3,454,832 7/1969 Hurtle ..3l7/16 X 6 Claims, 4 Drawing Figures 0 g 22 l l 1/0 TRANSIENT SUPPRESSION SYSTEM The present invention relates generally to electrical protective systems and more particularly is directed to a transient suppression system adapted to protect an electrical network.

It is well known that industrial automation has increased to a remarkable extent in recent years. Correspondingly, the necessity for various types of industrial control devices has similarly increased. Particularly with the proliferation of various types of semiconductor devices, which are small and compact in size and consume relatively low amounts of power, industrial control circuits have increased in complexity and sophistication in order to achieve increased industrial automation and efficiency. However, although semiconductor devices are relatively immune to mechanical shock and thus generally considered to be extremely rugged, one problem associated with the installation of many industrial control systems utilizing semiconductor devices has resulted from the high degree of sensitivity of such devices to high voltage electrical transient signals. Such signals, in certain instances may be of a sufi'icient magnitude to seriously damage or destroy critical circuit elements, which may be quite sensitive to the application of excessive voltages beyond rated capacity, and in other instances, may adversely affect operation of the circuit by causing inadvertent turn-on or turn-off of certain circuit elements thereby disrupting proper system operation. Such effects become particularly noticeable in a relatively complex system or when an industrial control system is exposed to an environment wherein other electrical systems are simultaneously in operation so that undesired transient signals may be generated.

Although various means have been proposed for limiting the affect of such transient signals, such as shielding systems, filter circuits of various types, etc., the problem nevertheless remains extremely significant in view of the difficulty of designing proper systems to cover all anticipated situations as well as in the expense and inconvenience of providing such circuit protection. Additional solutions have been proposed involving the connection of various types of shunting devices across the input of a circuit which is to be protected so as to shunt undesired transient signals away from the circuit. However, certain difficulties have been encountered in the provision of such protective devices in view of the severe operational requirements which are imposed to obtain the requisite degree of protection. For example, it may be necessary to protect a circuit against transient signals having a voltage of several thousand volts, which may be sufficient to destroy the protective device itself in certain instances, thereby requiring the provision of further additional protective devices for dissipating the energy of the transient signal or for protecting the protective device.

Accordingly, it is an object of the present invention to provide an improved transient suppression system.

It is another object of the present invention to provide an improved transient suppression system adapted to be connected across the input of a circuit to be protected for selectively shunting transient signals from the circuit being protected.

It is a further object of the present invention to provide an improved transient suppression system adapted to be connected in shunt across the input of a circuit to be protected for selectively shunting transient signals from the circuit being protected.

It is a further object of the present invention to provide an improved transient suppression system adapted to be connected in shunt across the input of a circuit to be protected for selectively shunting and dissipating transient signals.

It is still a further object of the present invention to provide an improved transient suppression system adapted to be connected in shunt across the input of a circuit to be protected for selectively shunting and harmlessly dissipating unwanted transient signals, which transient suppression system is compact and extremely durable in use.

Additional objects and advantages of the present invention will be readily apparent from the following detailed description and accompanying drawings wherein:

FIG. 1 is a schematic circuit diagram partially in block form illustrating the provision of a protective system in accordance with the present invention; and

FIGS. 2-4 are graphical illustrations of electrical signals developed across various portions of a system illustrated such as that in FIG. 1.

Referring generally to the drawings and particularly to F IG. 1, a transient suppression system 10 is illustrated connected across a pair of line terminals 12 and 14 which are arranged to supply power from an a.c. power supply 16. The transient suppression system 10 is connected in parallel across the input of a network 18, which is to be protected, and as shown, is arranged intermediate the power supply 16 and the network [8 so as to selectively shunt undesired transient signals from the network 18, thereby aflording transient suppression protection for the circuit 18. More particularly, the transient suppression network 10 preferably includes a voltage breakdown device 20 connected in shunt with the circuit to be protected 18, as shown, and serially connected with an energy dissipating means 22. In this connection, the voltage breakdown device 20 preferably comprises a spark gap device having a pair of spaced apart

electrodes

24, 26, extending into a suitable enclosure or envelope (not shown). The envelope sealingly encloses a suitable gaseous medium which thus occupies the spacing or gap between the

electrodes

24, 26 and is adapted to support an electrical discharge or arcing therebetween in response to the establishment of a preselected breakover or striking voltage between the electrodes, determined by the design of the electrodes, the gap length, and the particular gaseous atmosphere selected. The energy dissipating means 22 serially connected to the spark gap 20 preferably comprises a ballast resistor which has preselected properties such that it initially presents a relatively low impedance when the spark gap is rendered conductive and caused to arc over so as to rapidly absorb and dissipate the energy of the transient signal, while subsequently presenting a relatively high impedance upon expiration of the transient so as to limit any follow-on current through the spark gap and permit extinguishment of the are at the end of the applied half cycle of a.c. power as the applied voltage signal approaches the zero cross-over point, thereby obviating the need for any further energy dissipation. In this connection, the ballast resistor 22 preferably has a positive temperature coefiicient of resistance, i.e., its resistance increases with increasing temperature, and accordingly, it is formed of a preselected material having the desired characteristics for initially permitting a relatively large current flow therethrough so as to transfer and dissipate the energy initially developed across the spark gap to the ballast resistor in order to avoid damage to the spark gap, as well as to then limit the current flow therethrough in order to permit extinguishment of the are established across the spark gap upon the expiration of the transient signal as the applied a.c. voltage signal approaches through a zero cross-over point at the end of the applied half cycle of a.c. power, thereby permitting power to be supplied in the usual fashion to the circuit 18 being protected.

Although certain types of spark gap devices have been utilized in transient suppression systems, significant problems have arisen in attempting to dissipate the transient energy and effect extinguishment of the arc prior to the occurrence of damage to the device. For example, if a high voltage transient were to occur at a point close to the start of a particular a.c. half cycle, the spark gap would not be extinguished until the next zero crossing point of the applied a.c. power signal. Accordingly, a relatively large amount of energy would appear across the spark gap for this period of time and in many instances could cause irreversible damage to the spark gap. Similarly, attempts to provide additional components to aid in dissipating the transient have also suffered from various deficiencies so that such systems have generally been severely lacking in durability.

However, in accordance with the principles of the present invention, the ballast resistor 22 is provided and is arranged so as to initially remain relatively highly conductive upon the establishment of an arc across the spark gap device 20 so as to cause substantially the entire energy of the transient signal to be transferred to the ballast resistor, which dissipates such energy, while at the same time the ballast resistor is arranged such that it rapidly increases in resistance in response to the passage of increased current flow and consequent heating so as to reduce the current flow through the spark gap device to a safe level, while permitting extinguishment of the arc across the spark gap upon expiration of the transient signal as the applied a.c. voltage signal approaches through a zero cross-over point at the end of the applied half-cycle of ac. power thereby rendering the spark gap non-conductive and precluding the further flow of current therethrough. Thus, the high energy transient may be dissipated in a relatively brief time interval of the order of several microseconds, preventing the occurrence of any damage to either the spark gap or to the ballast resistor in view of the short time interval required for dissipation of the transient signal. Such a mode of operation is particularly advantageous in view of the fact that it has been empirically found that most transient signals which are generated have a time duration of the order of perhaps to 20 microseconds. Consequently, a transient suppression system, which efiiciently dissipates the energy of transient signals during such a time interval, provides the requisite degree of transient suppression protection without any adverse affects on the operation or durability of the circuit components utilized, since occasional transients of somewhat longer duration would be sufficiently infrequent to cause damage to the system.

In a typical example of a system such as that illustrated in FIG. 1, the power supply 16 may comprise a conventional 480 volt, 60 hz power supply such that a peak voltage of approximately 680 volts may appear across the line. Accordingly, the load circuit 18 would ordinarily be designed to be capable of withstanding voltage levels in excess of that magnitude, and assuming the ordinary safeguards in terms of excess capability of electronic components, it has been found advantageous to provide protection for the circuit 18 against high voltage signals, which are in excess of approximately 1,000 volts. Thus, in this example, the spark gap device 20 may be selected to have'a striking voltage of approximately 1,000 volts. Consequently, upon the generation across the line of a transient signal having a voltage amplitude of 1,000 volts or greater, the spark gap is rendered conductive or caused to arc over and a relatively high current flow passes between its electrodes to the ballast resistor 22. The ballast resistor is selected such that is has a relatively low resistance initially of the order of perhaps ohms, and hence, permits this high current to pass therethrough. Accordingly, substantially the entire energy of the transient signal is developed across the ballast resistor 22 which is caused to self-heat as a result of the current flow and the power dissipation thereacross. In response to the heating efiect, the ballast resistor 22 increases rapidly in resistance and thus begins to severely limit any follow-on current flow, which would ordinarily continue to flow through the spark gap while it is in a conductive condition. As the applied a.c. voltage signal approaches its next zero cross-over point at the end of the applied half cycle of ac. power, the spark gap is extinguished and rendered non-conductive. Thus, the high voltage transient signal is efiectively dissipated across the ballast resistor 22 in a relatively short time interval, while prolonged conduction of high level current flow through the spark gap device which could be harmful is prevented. In addition, the generation of additional heating and power dissipation due to follow-on current once the spark gap is rendered conductive is prevented. Similarly, the ballast resistor is essentially self-protecting as a result of its positive temperature coefficient of resistance.

In this connection in accordance with an important feature of the present invention, the ballast resistor 22 is preferably fabricated of a preselected material which has a relatively low resistance in its initial unheated state of the order of between 5 and 40 ohms and preferably approximately 15 ohms, but which increases substantially in resistance in response to the operation of heat due to the passage of current therethrough. Furthermore, this material preferably is capable of being heated to a relatively high temperature of the order of several thousand degrees centigrade in order to effectively dissipate energy without suffering any harm, while its resistance increases substantially when heated to temperature approaching these levels, such that its ratio of hot to cold resistance is of the order of approximately 1521. In this regard, it has been found that a particularly advantageous material for use in fabricating the ballast resistor comprises a filament of essentially pure tungsten such as that utilized in a conventional incandescent light bulb. In this regard, the tungsten filament re sistor may be selected having a desired length and thickness in order to achieve the desired resistance value. In addition, it has been found advantageous in certain instances, particularly when a semiconductor device, which is susceptible to damage by excessive di/dt, is utilized as the protective switch device, to add a small amount of inductance to the transient suppression system to limit the initial surge of current to a level which cannot harm the circuit elements by fabricating the ballast resistor in the form of a helical coil having a plurality of windings. By utilizing such a shape, it is possible to achieve an increased resistance level without the requirements for a large amount of space otherwise needed to accommodate a resistor having the requisite properties as previously described. If desired, the tungsten ballast resistor may be disposed within an evacuated envelope formed of glass or the like or may be disposed in a suitable enclosure having a gaseous atmosphere comprising an inert gas such as argon in order to minimize oxidation of the tungsten filament. It may be noted that, since it is anticipated the transient suppression system illustrated would operate on an intermittent, rather than a steady state basis, such as in response to switching transients generated by associated circuitry or upon initial turn-on or tum-off of the power being supplied to the circuit 18 and in view of the inherent durability of the system, it is contemplated that the transient suppression system will have a relatively long electrical lifetime.

To briefly graphically illustrate the advantageous mode of operation of a transient suppression system in accordance with the present invention, reference is made to the graphical illustrations in FIGS. 2-4, which depict the voltage signals appearing across various portions of the system plotted against time. More particularly, FIG. 2 illustrates an ac. line voltage signal 30 which is developed across the power terminals 12, 14 and applied across the transient suppression network 10 and across the circuit 18. In addition, FIG. 2 illustrates the appearance of a high amplitude transient voltage signal 32 which is generated at the onset of a positive a.c. half-cycle.

FIG. 3 illustrates a voltage signal 34 developed across the spark gap device, as well as a transient voltage signal 36 which initially appears across the spark gap device and effects triggering or arching over of the spark gap, rendering the spark gap conductive. The voltage transient spike 36 has an amplitude approximately equal to the striking voltage of the spark gap, which in the illustrated embodiment is somewhat less than the amplitude of the transient voltage signal 32. Since the spark gap device is rapidly triggered by the transient signal, the voltage thereacross immediately drops to a level of several volts as indicated by the portion 34 of the voltage curve. However, it may be noted that a relatively large potentially harmful current surge would pass through the spark gap device without the presence of the ballast resistor which prevents such current surges, as previously explained. Accordingly, as illustrated in FIG. 4, which shows the voltage developed across the ballast resistor, a spike of voltage 38, which has a slightly lower amplitude than the voltage spike 36, is developed across the ballast resistor 22. Consequently, substantially the entire energy of the transient signal is dissipated by the ballast resistor 22, as indicated by the voltage spike 38, and the voltage across the spark gap device which has dropped to the level indicated by the curve 34, remains substantially constant, while substantially all of the energy of the voltage signal is dissipated by the ballast resistor. in addition, the initial current surge which passes through the ballast resistor, when spark gap is rendered conductive by the transient signal, causes the ballast resistor to self-heat and rapidly increase in resistance. Thus, the voltage across the ballast resistor begins to drop relatively rapidly as the energy of the transient signal is dissipated as heat. As the transient energy is dissipated, a voltage signal 40 which generally corresponds to but is slightly lower in amplitude than the line voltage due to the small voltage drop across the conductive spark gap in series with the ballast resistor is developed across the ballast resistor.

Thus, it may be seen that the high voltage transient signal appears across the spark gap for an extremely brief time interval before it is transferred across the ballast resistor, which in turn, rapidly dissipates the energy of the transient as heat. The self-heating and rapid increase in resistance of the ballast resistor precludes the possibility of damage or stress on any of the circuit components during the remainder of the a.c. halfcycle.

As the applied line voltage approaches the next zero crossover point, the spark gap is fully extinguished and essentially no further voltage signal appears across the ballast resistor as indicated by the portion 42 of the curve of FIG. 4, while full line voltage merely continues to be developed across the spark gap device which is connected across the line terminals, as indicated by the portion 44 of the curve of FIG. 3. Accordingly, it may be seen that the unique arrangement in accordance with the present invention initially presents a low impedance to the transient signal, serves to dissipate the energy of the transient signal relatively rapidly, in order to prevent the occurrence of harm to any circuit components and then functions as a relatively high impedance to the follow-on current which would ordinarily flow through the spark gap device so as to limit currentfiow through the spark gap device, while preventing the occurrence of damage thereto and ensuring extinguishment of the spark gap device at the end of the ac. half-cyle.

Thus, a unique transient suppression system has been provided which serves to selectively shunt transient signals from a circuit being protected and to harmlessly dissipate such signals.

Various additional features of the present invention will be readily apparent to those skilled in the art and are deemed to be within the spirit and scope of the appended claims.

I claim:

1. An electrical protective system for protecting an electrical circuit against undesired, temporary high voltage electrical signals, said system being adapted to be connected in shunt across the input of the circuit to be protected, comprising a spark gap device including a pair of spaced electrodes adapted to support an electrical discharge therebetween in response to a preselected voltage level established thereacross as a result of the occurrence of an undesired, temporary high voltage, transient signal in excess of said preselected voltage level so as to define a relatively high current capacity path between its electrodes for shunting transient signals in excess of said preselected voltage level from the circuit being protected, and

a ballast resistor serially connected to said spark gap device for harmlessly and rapidly dissipating the energy of said transient signal thereacross to protect said spark gap device against excessive heating and for limiting the current passing between the electrodes of said voltage breakdown device subsequent to the establishment of the electrical discharge so as to effect extinguishment thereof subsequent to the expiration of the transient signal, said ballast resistor having a resistance which increases responsive to current flow therethrough and associated energy dissipation thereacross so that a relatively low resistance value is presented upon the initial establishment of the electrical discharge to facilitate dissipation of sub stantrally all of the energy of the transient signal thereacross resulting in a substantially increased resistance level sufficient to limit subsequent current flow therethrough to a level below that required for maintaining the electrical discharge across the spark gap electrodes.

2. An electrical protective system in accordance with claim 1 wherein the resistance level of said ballast resistor when heated in response to the current flow therethrough and dissipation of energy thereacross is approximately 15 times greater than its initial resistance level prior to the establishment of the electrical arc.

3. An electrical protective system in accordance with claim 2 wherein said ballast resistor has a resistance level in the range of between approximately 5 ohms and 40 ohms prior to the passage of heating current therethrough.

4. An electrical protective system in accordance with claim 3 wherein said ballast resistor has a resistance level of approximately 15 ohms prior to the passage of heating current therethrough.

5. A system for selectively shunting and dissipating high voltage electrical transient signals to protect an electrical network, said system comprising a spark gap device connected in shunt with the circuit to be protected, said spark gap device including a pair of spaced electrodes adapted to support an electrical arc therebetween in response to the establishment of a preselected voltage level between said electrodes due to the occurrence of a transient signal in excess of said preselected voltage level, and a ballast impedance serially connected to said spark gap device for substantially completely dissipating the energy of the transient signal to protect said spark gap device against excessive heating, said ballast impedance having a resistance which increases responsive to current flow therethrough and associated energy dissipation thereacross so that a relatively low resistance level current path initially is provided for the transient signal upon the establishment of the electrical arc to facilitate dissipation of substantially all of the energy of the transient signal across said ballast impedance resulting in a substantially increased resistance level, said ballast impedance including a filament of essentially pure tungsten having a preselected length and a preselected thickness sufficient to dissipate the energy of the transient signal and to limit subsequent follow-on current as a result of the increased resistance thereof due to heat generation so as to effect extinguishment of the electrical arc rendering the spark gap device non-conductive subsequent to the expiration of the transient signal.

6. A system in accordance with claim 5 wherein said tungsten filament is arranged to define a helical coil in order to minimize the space occupied thereby and to provide a nominal value of inductance in series with said spark gap device, thereby limiting the initial surge of current flow through said system in response to the establishment of the electrical are between said electrodes.

Claims

1. An electrical protective system for protecting an electrical circuit against undesired, temporary high voltage electrical signals, said system being adapted to be connected in shunt across the input of the circuit to be protected, comprising a spark gap device including a pair of spaced electrodes adapted to support an electrical discharge therebetween in response to a preselected voltage level established thereacross as a result of the occurrence of an undesired, temporary high voltage, transient signal in excess of said preselected voltage level so as to define a relatively high current capacity path between its electrodes for shunting transient signals in excess of said preselected voltage level from the circuit being protected, and a ballast resistor serially connected to said spark gap device for harmlessly and rapidly dissipating the energy of said transient signal thereacross to protect said spark gap device against excessive heating and for limiting the current passing between the electrodes of said voltage breakdown device subsequent to the establishment of the electrical discharge so as to effect extinguishment thereof subsequent to the expiration of the transient signal, said ballast resistor having a resistance which increases responsive to current flow therethrough and associated energy dissipation thereacross so that a relatively low resistance value is presented upon the initial establishment of the electrical discharge to facilitate dissipation of substantially all of the energy of the transient signal thereacross resulting in a substantially increased resistance level sufficient to limit subsequent current flow therethrough to a level below that required for maintaining the electrical discharge across the spark gap electrodes.

6. A system in accordance with claim 5 wherein said tungsten filament is arranged to define a helical coil in order to minimize the space occupied thereby and to provide a nominal value of inductance in series with said spark gap device, thereby limiting the initial surge of current flow through said system in response to the establishment of the electrical arc between said electrodes.