United States Patent fi9J
`Standler
`
`[II] Patent Number:
`[45] Date of Patent:
`
`4,586,104
`Apr. 29, 1986
`
`[54]
`
`[75]
`
`[73]
`[21)
`[22)
`[51)
`[52)
`
`[58]
`
`[56]
`
`PASSIVE OVERVOLTAGE PROTECI'ION
`DEVICES, ESPECIALLY FOR PROTECI'ION
`OF COMPUTER EQUIPMENT CONNECTED
`TO DATA LINES
`Ronald B. Standler, Albuquerque, N.
`Inventor:
`Mex.
`RIT Research Corp., Rochester, N.Y.
`Assignee:
`Appl. No.:
`560,710
`Dec. 12, 1983
`Filed:
`Int. Cl.4 ......................... H02H 9/04; H02H 9/06
`u.s. Cl . ........................................ 361/91; 361/56;
`361/119
`Field of Search ....................... 361/54, 55, 56, 91,
`361/110, Ill, 119, 402, 406; 336/232
`References Cited
`U.S. PATENT DOCUMENTS
`2,789,254 4/1957 Bolle et at. .
`3,372,285 3/1968 Blazek et at. ................... 361/lll X
`3,793,535 2/1974 Chowdhuri ....................... 361/56 X
`3,890,543 6/1975 Jonassen ................................ 361/56
`3,934,175 1/1976 Clark ............................... 361/118 X
`4,068,277 1/1978 Simokat ................................ 361/55
`4,068,281 1/1978 Harnden, Jr . ....................... 361/106
`4,075,591 2/1978 Haas ................................ 336/232 X
`4,325,097 4/1982 Clark ..................................... 361/56
`4,389,695 6/1983 Carpenter, Jr . .................. 361/56 X
`
`FOREIGN PATENT DOCUMENTS
`3027469 2/1982 Fed. Rep. of Germany ........ 361/56
`2382787 11/1978 France .................................. 361/56
`982130 2/1965 United Kingdom .................. 361/54
`725143 3/1980 U.S.S.R. ................................ 361/91
`
`OTHER PUBLICATIONS
`"Grounding, Bonding, and Shielding for Electronic
`Equipment and Facilities", DOD Handbook, vol. 11, of
`2 volumes-Applications, 1/21/82.
`"Transient Voltage Suppression", 3rd Edition-Gen(cid:173)
`eral Electric.
`"Suppression of Fast Rise-Time Transients-Clark,
`General Semiconductor Industries, Inc.
`"Semiconductor Devices in Hostile Electrical Environ(cid:173)
`ments"-Knox.
`"Lighting Protection of Line Repeaters"-Popp.
`Polyswitch Devices-A New Low Resistance Conduc-
`
`tive Polymer-Based PTC Device for Overcurrent Pro(cid:173)
`tection"- Doljack.
`"Electrical Transients in Power Systems"-Green(cid:173)
`wood.
`"PTC-Positive Temperature Coefficient Thermistors",
`Keystone Catalog.
`"Effect of Lead Wire Lengths on Protector Clamping
`Voltages"-Clark et al- 1979, Federal Aviation Ad(cid:173)
`ministration Workshop on Grounding and Lightning
`Technology-report FAA-RD-76-6, Mar. 6-8, 1979,
`pp. 69- 73.
`"Current Protectors Take on Surges Without Resetting
`or Replacement"-Ballog, Electronics, 1/13/81, pp.
`159-162.
`Elektrie 32 (1978), Jrg. 32, No. 7, pp. 377-379.
`IBM Technical Disclosure Bulletin, vol. 21, No. 9,
`2/79.
`Primary Examiner-Harry E. Moose, Jr.
`Attorney, Agent, or Firm-M. Lukacher
`[57]
`ABSTRACI'
`Overvoltage protection device's operative to prevent
`damage to electrical equipment in response to overvolt(cid:173)
`ages in the form of fast transients and also to continuous
`overvoltages utilizes clamping or discharge elements
`which conduct in response to overvoltages or current
`surges. A high voltage responsive element, suitably a
`spark gap, is connected to the input of the device which
`goes to the line, while a lower voltage responsive ele(cid:173)
`ment, suitably an avalanche diode or diodes or a zener
`diode or diodes, is connected to the output of the device
`which goes to the equipment to be protected. A resistor
`is connected between the spark gap and the avalanche
`device in series with one side of the line on the circuit to
`be protected. This resistQr is preferably a positive tern·
`perature coefficient resistor which changes resistance
`rapidly to a high resistance state when the avalanche
`device conducts so as to protect the avalanche device
`while allowing the development of overvoltages suffi(cid:173)
`cient to cause breakdown and conduction in the spark
`gap. Fast overvoltage transients are protected against
`by inductance between the avalanche device and the
`output that is greater than the inductance in the shunt
`path provided by the avalanche device.
`
`25 Claims, 8 Drawing Figures
`
`LINE
`INPUT
`
`PROTECTED
`DEVICE
`OUTPUT
`
`Page 1 Dell Inc.
` Exhibit 1024
`
`

`
`U.S. Patent Apr. 29, 1986
`
`Sheet 1 of3
`
`4,586,104
`
`PROTECTED
`DEVICE
`OUTPUT
`
`PROTECTED
`DEVICE
`OUTPUT
`
`PROTECTED .
`DEVICE
`OUTPUT
`
`26
`
`FIG. I
`
`FIG. 5
`
`FIG. 6
`
`LINE
`INPUT
`
`LINE
`INPUT
`
`LINE
`.INPUT
`
`Page 2
`
`

`
`U.S. Patent Apr. 29, 1986
`
`· Sheet 2 of3
`
`4,586,104
`
`I
`
`·. ,/\
`
`I 0
`
`~ ·
`~~~ ~ 1 v~
`
`30
`
`, . 0 029
`. · ..
`FIG. 2
`
`0 0
`I 0 0 I
`I
`0
`I
`
`I 0 0
`'0
`
`I~·~
`' ~
`o ...
`
`16a
`
`33
`.-FIG. 4a
`
`30
`
`Page 3
`
`

`
`U.S. Patent Apr. 29, 1986
`
`Sheet3 of3
`
`4,586,104
`
`33
`
`FIG. 4b
`
`LINE
`INPUT
`
`PROTECTED
`DEVICE
`OUTPUT
`
`FIG. 7
`
`Page 4
`
`

`
`PASSIVE OVERVOLTAGE PROTECfiON
`DEVICES, ESPECIALLY FOR PROTECfiON OF
`COMPUTER EQUIPMENT CONNECTED TO
`DATA LINES
`
`DESCRIPTION
`The present invention relates to overvoltage protec(cid:173)
`tion devices, and particularly to an overvoltage protec(cid:173)
`tion device which is purely passive and which protects 10
`against both fast transients, as well as continuous over(cid:173)
`voltages.
`The invention is especially suitable for protecting
`computer equipment., particularly computer interfaces
`or terminals which are interconnected by long connect- 15
`ing cables or transmission lines and which operate in
`accordance with standard protocols such as the RS-232
`for serial unbalanced lines and interfaces and the RS-
`422 for serial balanced data lines and interfaces.
`When computer terminals are located a long distance 20
`from the computer, transient voltages can enter the
`connecting cables and damage the interface hardware.
`Such transient overvoltages are commonly caused by
`lightning but they may, however, be due to electrostatic
`discharge or electromagnetic pulses and be very fast, 25
`having rise times of nanoseconds to microseconds. In
`addition, overvoltages may be sustained over long peri(cid:173)
`ods of time. For example, cloud to ground lightning can
`have long continuous currents of the order of 100 am(cid:173)
`peres for the duration between 0.04 and 0.5 seconds. 30
`Overhead power lines may sag or fall and touch the
`lines on which the data is transmitted. This will inject
`sustained overvoltages which may, unless protected
`against, enter the computer directly or through a
`modem. Overvoltages may also result from accidental 35
`connections, for example, of telephone lines to com(cid:173)
`puter data lines. There is also the possibility of malicious
`damage or sabotage by the connection of a high voltage
`or current source (e.g. the 120 V power lines) to a com-
`puter data line.
`The problem therefore presents itself to protect
`equipment, and particularly sensitive computer hard(cid:173)
`ware, from overvoltages across the gamut from ex(cid:173)
`tremely fast transients to sustained overvoltages.
`An approach which has been taken for overvoltage 45
`protection is to use different elements which conduct at
`different voltages. Typically, an avalanche or a zener
`diode is selected to conduct before the voltage across
`the protected equipment exceeds the rating of that
`equipment. An element which conducts at a higher 50
`voltage, typically a spark gap, protects the diode from
`high currents which could destroy that diode. Such
`devices are described in U.S. Pat. No. 2,789,254 issued
`Apr. 16, 1957 and U.S. Pat. No. 3,934,175 issued Jan. 20,
`1976. The device of the latter patent utilizes a delay 55
`circuit including an inductor and a resistor in series
`between the spark gap and the zener diode to allow
`them to respond independently and conduct at their
`respective higher and lower voltages. It has also been
`suggested to use positive temperature coefficient resis- 60
`tors for overcurrent protection in a protected circuit
`(see an article by Frank A. Doljack, IEEE Transactions
`on Components, Hybrids and Manufacturing Technol(cid:173)
`ogy, vol. CHMT-4, No.4, December, 1981, p. 372, and
`particularly p. 377). For fast transients, however, over- 65
`voltages may appear at the output of the circuit to be
`protected before either the avalanche device or the
`spark gap conducts. The failure of the elements to con-
`
`40
`
`1
`
`4,586,104
`
`2
`duct promptly has been attributed to their inherent
`inductance which blocks the flow of transient currents.
`To that end, special avalanche devices in a special pack(cid:173)
`age having low shunt inductance have been suggested
`5 (see U.S. Pat. No. 4,325,097, issued Apr. 13, 1982).
`It is desirable to use conventional elements such as
`spark gaps and avalanche diodes as protection elements,
`while, at the same time, accommodating overvoltages
`which run the gamut from continuous to very fast tran(cid:173)
`sients. The protection device must be kept small in size
`to be compatible with the computer equipment with
`which it is used. Desirably the protection device is used
`directly ahead of the interface. With many standard
`computer interfaces very little space is available, for
`example, the available space may be only a few inches
`long, a few inches wide, and less than an inch in height.
`The space limitations exacerbate the problem owing to
`the possibly large amount of energy to be dissipated.
`It is an object of the present invention to provide a
`protective device which may be connected to the ends
`of lines and cables which interface with the electronic
`equipment for protecting the equipment from overvolt(cid:173)
`ages appearing on the lines, whether fast transients or
`sustained overvoltages, whether with little energy or
`extremely large energy and high current, which is small
`in size so as to be compatible with the space available
`and the connectors used in standard interfaces, such as
`RS-232, and which utilizes standard circuit compo-
`nents.
`A protection device provided in accordance with a
`feature of the invention utilizes inductance, in the path
`between an element which conducts in response to an
`overvoltage and the protected circuit, which presents
`higher inductance in series with the circuit than is pres(cid:173)
`ented in shunt by the protecting elements, such that fast
`transient overvoltages cause the protecting element to
`become conductive before an overvoltage reaches the
`protected circuit.
`In accordance with another feature of the invention,
`the protection device utilizes a positive temperature
`coefficient resistor to protect a low power element,
`such as an avalanche device in shunt with the output of
`the device, which goes to the protected circuit to pro-
`tect the avalanche device from sustained large currents
`while providing a low resistance during normal opera-
`tion (no overvoltage condition).
`A protection device provided in accordance with
`another feature of the invention utilizes a printed circuit
`card on one side of which the components are mounted
`and on the other side of which there are traces or
`printed conductors, which provide the protective in(cid:173)
`ductance between the output and the element (e.g. the
`avalanche device) which conducts at lower voltage, by
`having a number of bends in and a length of the trace
`greater than the bends and the length of the bends in the
`shunt path provided by the conductive element.
`A protection device provided in accordance with still
`further features of the invention utilizes the traces on
`the printed circuit card as well as bus bars to minimize
`inductance which might prevent operation of the ele(cid:173)
`ments of the circuit in their protective modes on fast
`transients.
`Briefly described, an overvoltage protection device
`embodying the invention, for transient overvoltages
`which appear on a line connected to the input of the
`protection device, the line transmitting signals with
`respect to an electrical circuit connected to an output of
`
`Page 5
`
`

`
`4,586,104
`
`4
`3
`the protection device, utilizes an element having a shunt When it fires, the spark gap 20 protects the avalanche
`device 14.
`resistance which decreases when an overvoltage is pres-
`ent on the line to a value much less than the resistance
`In order that the spark gap 20 fires before the ava-
`presented by the circuit connected to the output of the
`lanche device destructs, a resistor 22 is connected in
`device. The protection device is enabled to handle very 5 series between the avalanche device 14 and the spark
`fast transients by means of having an inductance greater
`gap 20. The resistor is desirably connected in the side of
`than the inductance presented to the line by the shunt
`the circuit which goes to the signal side of the line and
`element, which inductance is connected between the
`the protected circuit, not the ground side. The resis-
`element and the output of the device.
`tance value of the resistor 22 is selected such that the
`The foregoing and other features, objects and ad van- 10 firing voltage of the spark gap 20 is reached before the
`current through the avalanche device rises above its
`tages of the invention, as well as the presently preferred
`embodiments thereof, will become more apparent from
`maximum rated current. Preferably the resistor 22 has a
`a reading of the following description in connection
`positive temperature coefficient. The resistor 22 is
`therefore labeled RPTc in the drawing. Suitable PTC
`with the accompanying drawings in which:
`FIG. 1 is a schematic diagram illustrating a protec- 15 resistors are commercially available from Murata Erie
`North America of Marietta, Ga. 30067 (e.g. their part
`tion device in accordance with the invention; ·
`FIG. 2 is a _top vi~w sho_wing a. prot:<:tion dev~ce i~
`numbers PTH60HOIAR330M140) or from Raychem
`accord~nce wtth the t?vent~~n whtch_utihzes the ~trcutt
`Corp. of Menlo Park, CA 94025. (e.g., their part num-
`ber c24Too2H.).
`show~ m FIG. 1 and m addttton prov1d_es protectiOn for
`20 The PTC resistor 22 has the further advantage in that
`four ClrCui!S connected b_y an RS-232 mterf~ce;
`.
`when the protection device is used on the transmitting
`FIG .. 3 IS a bottom v1ew of the protectiOn device
`22 fi
`shown 10 FIG. 2;
`d
`f th 1·
`th
`·
`I
`fil
`·d
`·
`d FIG 4b .
`orms a ow pass 1 ter
`d
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`f
`en o
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`e reststor
`FIG A_ •
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`IS an en VIeW 0
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`th
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`FIGS 5 6
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`h
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`are sc ema c 1 gr
`s us ra g
`(
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`·
`f
`·
`tJ
`600
`
`
`e.g., 2400 to 9
`baud • t _e reSIStance 0 the ~eststor 2~
`protection devices in accordance with other embodi-
`should be as small as ~oss1ble. The PTC res1stor typi-
`ments of the invention.
`cally has values of resistance at ro?m temp~rature be-
`Referring first to FIG. 1, there is shown a protection
`tween 20 and 50 ohms. Self heatmg prov.tded w~en
`device for protecting one electrical circuit that is con-
`nected to one line. In the embodiment shown in FIGS. 30 large currents flow through the . protectJo~ devt~e
`~u~es the tempera~re of the reststor to ns~ to tts
`2, 3 and 4, four such protection devices are incorpo-
`swt~ch t~mperature_ , th~ temperature at whtch the
`rated in a single unit. The unit is capable of protecting
`four lines and four circuits. One side of each circuit is
`PTC s res1stance begms to mcrease by about a factor of
`10 for ea~h 10 ~egrees (celsiu~) incr~e in_temperature.
`common and is connected to local ground. With the
`four sets of protection device circuits, the unit is suitable 35 After bemg swttcb~d the res1~tance 1s typtcally. at least
`3000 o~ms. ~cc~rdmgly, du~mg normal. ope~at10n, the
`for protecting a standard asynchronous RS-232 inter-
`protectt~n c_trCUit does not mte:fere_ wt~h h1gh sp~ed
`face for a computer or a computer terminal device (a
`commumcatlons. Whe~ operatmg _m tts p_rotectlve
`modem, display, printer or keyboard, for example).
`The protection device circuits shown in FIG. 1 is
`mode to suppress sustamed destructtve transients and
`typical of each of the four in a RS-232 protection unit. 40 ov:rvoltages, ~he resistor the_n switches to the high
`The first of the basic components or elements used in
`resistance requtred for protecuve purposes.
`the protection device is an element that decreases in
`The shunt path to the spark gap 20 and avalanche
`diode 14 are shown defined by "V" like indentations in
`resistance to a value much lower than the resistance
`presented by the protected circuit which is connected
`the diagram in order to denote that the shunt path
`to the output terminals 10 and 12 of the device. This 45 through these elements 14 and 20 is made as short as
`element is an avalanche device, illustrated as a bipolar
`possible to minimize the inherent inductance in the
`avalanche diode 14. Two unipolar avalanche diodes
`shunt path. The avalanche device 14 must operate in
`connected in series; for example with their cathodes
`response to transient overvoltages with nanosecond
`directly connected to each other, may be used. The
`risetimes. With such fast transients even the most care-
`avalanche device is connected in shunt with the pro- so ful circuit layouts, with short lead lengths to minimize
`tected circuit and also in shunt with the line which is
`inductance, are insufficient. Accordingly, the transient
`connected to the input terminals 16 and 18 of the pro-
`can bypass the avalanche device 14, because of the
`tection device. The avalanche device becomes conduc-
`relatively high inductance presented to the fast transient
`tive when an overvoltage is present that exceeds the
`by the shunt path including the avalanche device 14.
`avalanche device's breakdown voltage. The conduction 55 The overvoltage can then reach the protected circuit
`and do permanent damage. To prevent this occurrence,
`process is very rapid when the breakdown voltage oc-
`curs. The avalanche device has a limited current carry-
`an inductor element 24 is connected in the signal side of
`ing capacity and typically cannot dissipate more than 5
`the circuit between the output terminal 10 and the ava-
`watts (steady-state) without destructing.
`lanche device 14. The inductance presented by the ele-
`In order to carry higher currents another element, 60 ment 24 is greater than the inductance in the shunt path
`including the avalanche device 14. Fast transients are
`which presents a resistance much less than the resis-
`tance of the protected circuit connected to the output
`then forced through avalanche device 14 and cause
`terminals 10 and 12, is connected in shunt with these
`conduction under overvoltage conditions, thereby pro·
`terminals and with the line. This element has a break-
`tecting the circuit that is connecte9 to the output termi-
`down or firing voltage much higher than the break- 65 nals 10 and 12. The inductor is preferably provided by
`printed conductors or traces having a length and anum-
`down voltage of the avalanche device 14. While the
`metal oxide varistors, neon lamps, or other gas dis-
`ber of bends greater than, and preferably at least twice,
`charge tubes may be used, a spark gap 20 is preferred.
`the length and number of bends in the shunt path includ-
`
`Page 6
`
`

`
`4,586,104
`
`5
`6
`propagating directly from the inputs 16a to 16d to the
`ing the avalanche device 14. Suitable traces are iJlus-
`outputs lOa to lOd.
`trated in FIG. 3.
`To intercept flash over, should it occur, the wide
`Inductance in the shunt path including the spark gap
`ground band 28 is provided, underneath the resistors
`20 may also divert or block voltages which should
`cause the firing thereof during fast transient overvolt- 5 22a to 22d. The band 28 is connected to the common or
`age conditions. To this end the leads from the spark gap
`ground side of the device. It is integral with the com-
`are made as short as possible. The spark gap 20 is
`mon traces 31. A length of solid bus bar wire 33 (22 to
`mounted with its axis vertical to avoid right angle bends
`16 A WG) on the top of the board under the resistors
`in its leads, as described more fully hereinafter. The lead
`22a-22d also guards against flashover. This wire 33 is
`of spark gap 20 that is connected to a printed circuit 10 also connected to the common traces 31 on the printed
`trace is folded over and connected along its length 36 to
`circuit board.
`the trace (see, FIGS. 3 and 4a) so as to distribute the
`A bus bar 29 is also soldered to the common terminals
`heat and increase the current carrying capacity of the
`on the upper side of all of the spark gaps. The braid 26
`trace. FIG. 3 shows bow leads from each of the spark
`is soldered to the bus bar 29. Two parallel pieces of
`gaps (20a through 20d) are coincident with and con- 15 conductor may be used for the bus bar 29. This con-
`nected to the traces. Also terminals at the upper ends of
`struction reduces the inductance in the shunt paths to
`the spark gaps are connected to a bus bar 29 which in
`ground provided by the spark gaps 20a to 20d and their
`turn is connected to the common traces 31 on the
`leads. For convenience of :assembly, bus bars 29, 33 and
`35 are made of the same conductor. Part 35 is between
`printed circuit board thereby providing heavy short
`conductors to minimize inductance and provide high 20 Parts 29 and 33.
`current carrying capacity; the current carrying capacity
`Intense peak currents create intense magnetic fields
`of the bus bar being much greater than that of the traces
`perpendicular to the signal conductors on the printed
`on the printed circuit board. Sti11 further, a braided,
`circuit board and concentrate current in a thin layer.
`flexible conductor 26 is provided which is connected to
`This effect might cause the signal conductors to evapo-
`the bus bar and thence goes to an external ground, 25 rate if the current density was sufficiently great, and is
`thereby assuring that heavy currents are shunted di-
`avoided by extending the spark gap leads 36a to 36d
`rectly to ground before entering the protected area of which go through the board and routing·them for about
`6 millimeters along the printed circuit board trace to the
`the protection circuit, and of course not entering the
`area of the protected circuit.
`input terminals 16a to 16d.
`With high voltage transients at the input terminals, 30 The spark gaps 20 are suitably type 2027-15-B manu-
`factored by the Joslyn Company of Goleta, CA. 93116,
`the possibility exists of a discharge or flashover across
`the series elements. These elements are the resistors 22.
`where these spark gaps have a DC frring potential be-
`To foreclose this possibility, a bus bar 33 that is con-
`tween 120 and 180 volts. Spark gaps are prefered over
`nected to common conductor 31 is contiguous to the
`gas tubes, such as neon lamps, since such lamps of rea-
`cases of resistors (22a through d in FIGS. 2, 3 and 4). A 35 sonable size do not have the requisite current carrying
`capacity and can be shattered by a high current tran·
`trace in a form of a band 28 is located on the side of the
`board opposite from the resistors 22. Trace 28 is integral
`sient such as lightning. Other spark gaps with lower
`firing voltage may be used, however. Spark gaps with a
`with the common traces 31.
`Referring more particularly to FIGS. 2, 3 and 4a and
`150 volt DC firing potential can typically remain non~
`4b, there is shown a printed circuit board unit contain- 40 conducting for about 0.5 to 1.0 microsecond when 300
`ing the board 30 and four sets of components, which
`volts is suddenly impressed across the gap, then operate
`define protection circuits for four signal Jines and a
`in the glow region for about 0.5 to 1.5 microseconds,
`common line. 25 pin connectors 37 and 39 for RS-232
`and then become fully conducting in the arc region.
`interfaces are illustrated in phantom in FIGS. 2, 3 and
`The brief non-conducting period of spark gaps with
`4a. The pins from the connectors 37 and 39 extend 45 potentials across them that are several times their DC
`through the board and are soldered to conductor traces
`frring potential is accommodated by the avalanche de-
`on the bottom side of the board. All of the traces are
`vices 14. The spark gaps provide effective transient
`located on the bottom side of the board. The spark gaps
`protection in shunting large currents, even of the order
`20a through 20d are cylindrical and mounted with their
`of 5 to 20 kiloamperes, away from the protected circuit.
`axes in a vertical plane to minimize the length of the 50
`The avalanche devices suitable for the RS232 appli-
`spark gap leads and remove the right angle bend in each
`cation may be two avalanche diodes connected back to
`lead that would otherwise be present. This minimizes
`back, each of which has a breakdown voltage of ap·
`the inductance in the shunt path (in series with) the
`proximately 18 volts. It has been found preferable to use
`spark gaps 20a to 20d.
`18 volt avalanche devices rather than 30 volt avalanche
`The inductors 24a to 24d are provided by the long 55 devices since they can safely shunt a larger transient
`paths with several right angle bends in the printed cir-
`current. The smaller avalanche device voltage also
`cuit traces between the avalanche diodes 14a to 14d,
`places less stress on the protected circuit.
`where each inductor 24 is on the signal path between
`Referring to FIG. 5, there is shown a protection
`the diode and the output terminals lOa to 10d which go
`circuit similar to the circuit shown in FIG. 1, and like
`to the connector pins. This long trace has a length at 60 parts are identified by like reference numerals. The
`least twice that of the avalanche diode's case connected
`resistor 23, not a like part to FIG. 1 where 22 was a
`thereto plus its leads. There are at least twice as many
`PTC, may be an ordinary 1 or 2 watt carbon composi-
`right angle bends in the traces which provide the con-
`lion or wirewound resistor. A PTC resistor 50 is used in
`ductors 24a to 24d than in the avalanche device shunt
`the signal side of the circuit between the spark gap 20
`14a to 14d between the signal side and common. Ac- 65 and the input signal terminal line 16. When sustained
`cordingly, the inductance in series with the output ex-
`overvoltages, as may be caused by contact with power
`ceeds the inductance of the avalanche device shunt.
`lines, occurs, the PTC resistor 50 switches to its high
`resistance state and protects the spark gap 20. As still
`Therefore, fast rise time pulses are prevented from
`
`Page 7
`
`

`
`7
`further protection against such sustained overvoltages,
`and so called follow current, may be provided by plac(cid:173)
`ing a fuse or circuit breaker in series with the input
`terminal 16.
`For balanced transmission lines, such as RS422 com- 5
`puter data Jines, which are connected to the input termi(cid:173)
`nals 16 and 18 the protection circuit shown in FIG. 6
`may be used. The spark gap 60 preferably has three
`terminals with a common gas chamber. Alternatively, it
`may be two spark gaps connected in series. The center 10
`or common terminal of the spark gaps is connected to
`local ground about which the input lines and the pro(cid:173)
`tected circuit input are balanced. Each signal line has a
`positive temperature coefficient resistor 62 and 64 in
`series therewith between the spark gap 60 and a pair of 15
`bipolar avalanche devices 66 and 68 which are con(cid:173)
`nected between local ground and each of the signal
`carrying paths in the device. These bipolar avalanche
`devices limit the maximum common-mode voltage at
`output terminals 10 and 12. An additional bipolar ava- 20
`lanche device 14 is connected in shunt with the output
`terminals 10 and 12 and between the two signal lines to
`minimize the maximum differential-mode output volt(cid:173)
`age between terminals 10 and 12.
`Inductors 70 and 72 are also provided between the 25
`output terminals and the avalanche device 14 to guard
`against the propagation of fast transients before conduc(cid:173)
`tion in the avalanche devices.
`Referring to FIG. 7 there is shown a protection cir(cid:173)
`cuit which can operate with signals having frequencies 30
`from DC through the radio frequency range. This cir(cid:173)
`cuit includes a spark gap 20 connected in shunt with the
`input terminals 16 and 18 which go to the signal lines,
`and a semiconductor clamp 82 which is an extremely
`fast-acting clamp circuit with very small shunt capaci- 35
`tance. While any fast-acting low-capacitance clamp
`circuit may be used, the illustrated circuit having paral-
`lel connected oppositely polarized fast-recovery rectifi(cid:173)
`ers and avalanche devices is suitable.
`A capacitor 74, having a capacitance value which 40
`may be of the order of 1000 pF, isolates the clamp 82
`from the spark gap 20. Many transients, including light(cid:173)
`ning, have little energy above a few megahertz, but
`other transients such as electrostatic discharges and
`nuclear electromagnetic pulse have a rise time of a few 45
`nanoseconds but may have little total energy. In the
`latter case, the semiconductor clamp 82 can absorb all
`of the transient energy without damage. Connected
`across the capacitor 74 is a PTC resistor 76 in series
`with an inductor 78. The inductor may have a few 50
`microhenries inductance. The PTC resistor may have
`20 to 50 ohms resistance at ambient temperature and
`switch to greater than 3000 ohms as heretofore de(cid:173)
`scribed. The fast transients and radio frequency signals
`pass through the capacitor and the clamp circuit 82 55
`responds within a few nanoseconds to any overvoltage
`condition. The inductor 78 blocks the fast transients and
`radio frequency signals such that the PTC resistor 76
`operates with lower frequency signals and DC. If lower
`frequency or DC response is not desirable, inductor 78 60
`and PTC resistor 76 may be omitted from the circuit.
`The protection device shown in FIG. 7 then operates in
`the same manner as described in connection with FIG.
`1.
`
`From the foregoing description it will be apparent 65
`that it has been provided improved passive overvoltage
`protection devices capable of operating with overvolt(cid:173)
`ages which can range from fast transients down to DC
`
`4,586,104
`
`8
`and with currents which range between tens of milliam(cid:173)
`peres up to tens of kiloamperes. Variations and modifi(cid:173)
`cations in the herein described devices, within the scope
`of the invention, will undoubtedly suggest themselves
`to those skilled in the art. Accordingly, the foregoing
`description should be taken as illustrative and not in a
`limiting sense.
`I claim:
`1. An overvoltage protection device for overvoltages
`which appear on a line connected to the input of the
`device, which line transmits signals with respect to an
`electrical circuit connected to the output of said device,
`said protection device comprising an element having a
`resistance which decreases when an overvoltage is pres(cid:173)
`ent on said line to a value much less than the resistance
`presented by said circuit to said output, said element
`being connected in shunt with said line across said out(cid:173)
`put, and means presenting an inductive reactance
`greater than the inductive reactance presented to said
`line by said element and of sufficient magnitude to en(cid:173)
`able the resistance of the element to decrease to said
`much less value before said overvoltage reaches the
`output of said device and said electrical circuit con(cid:173)
`nected thereto, said inductive reactance presenting
`means being connected between said element and said
`output.
`2. The overvoltage protection device according to
`claim 1 wherein said device comprises a printed circuit
`board on which said element is disposed, a trace on said
`board connecting said element in shunt with said line,
`and a trace on said board connecting said element to
`said output, said trace connecting to said output having
`a plurality of bends greater in number. than the trace
`connecting said element to said line and a length greater
`than the length of said element and its connecting trace.
`3. The overvoltage protection device according to
`claim 1 wherein said device comprises a zener diode, a
`pair of conductors connected at the input of said device
`to said line, said zener diode being connected between
`said conductors, and said inductive reactance present(cid: