United States Patent [191
`Chaudhry
`
`[54] ETHERNET OVERVOLTAGIJ
`OVERCURRENT PROTECTION SYSTEM
`
`[75] Inventor: Nisar A. Chaudhry. West Babylon.
`N.Y.
`
`[73] Assignee: T11 Industries, Inc. Copiague. N .Y.
`
`_
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`[21] APPL No_; 984,593
`_
`_
`Du’ 3’ 1997
`[22] F?cd'
`[51] Int. Cl.‘5 ..................................................... .. H01C 7/12
`[52] us. (:1. . . . . .
`. . . .. 361/119; 361/120; 361/104;
`
`0
`[58] Field 316'
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`361/106
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`‘
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`“
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`'
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`‘
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`’ 364/483’
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`[56]
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`References Cited
`
`U.S. PATENT DOCUMENTS
`
`03 3A
`11500579 6
`[11] Patent Number:
`[45] Date of Patent:
`
`‘
`
`5,790,363
`Aug. 4, 1998
`
`7/1989 Hung et a1, ............................. .. 361/56
`4,849,846
`2/1990 Shannon et a1.
`379/437
`4.903.295
`4,964,160 10/1990 Traube et a1.
`379/412
`5,416,663
`5/1995 Atkins ................................... .. 361/119
`5,539,820
`7/1996 Pistilli ................................... .. 379/412
`5,721,773
`2/1998 Debalko ................................ .. 379/412
`
`Primary Examiner-Jeffrey A. Ga?in
`Assistant Examiner—Michael J. Sherry
`Attorney, Agent, or Firm-Morgan & Finnegan
`
`[57]
`
`ABSTRACT
`
`An overvoltage/overcurrent protection system for Ethernet
`networks using unshielded twisted pair cabling. the system
`Comprising two overvoltage/overcurrent protection circuits,
`one circuit for the twisted pair carrying incoming digital
`signals and one circuit for the twisted pair carrying outgoing
`digital signals.
`
`4,586,104
`
`4/1986 Standler .................................. .. 361/91
`
`7 Claims, 2 Drawing Sheets
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`U.S. Patent
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`Aug. 4, 1998
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`US. Patent
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`Aug. 4, 1998
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`5 ,790.363
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`1
`ETHERNET OVERVOLTAGE/
`OVERCURRENT PROTECTION SYSTEM
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`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to an overvoltage/
`overcurrent protection system for Ethernet networks which
`carry high speed digital signals.
`2. Discussion of the Related Art
`It is known in the prior art to protect telephone lines from
`overvoltage conditions. Telephone lines traditionally carry
`analog voice signals but can also carry relatively low speed
`digital signals. The same two telephone wires are used to
`carry both incoming and outgoing signals. Overvoltage
`protection for telephone lines has been provided by gas
`discharge tubes. Suitable gas discharge tubes are shown. for
`example. in US. Pat. No. 4.212.047 issued to John Napi
`orkowski on Jul. 8. 1980 and U.S. Pat. No. 4.319.300 issued
`to John Napiorkowski et al. on Mar. 9. 1982.
`It is also known in the prior art to protect telephone lines
`from overvoltage and overcurrent conditions with solid state
`devices. with or without gas discharge tubes. For example.
`US. Pat. No. 5.359.657 issued to Dimitris J. Pelegris on Oct.
`25. 1994 discloses protecting telephone lines with a solid
`state overvoltage/overcurrent protection circuit. while US.
`Pat. No. 4.586.104 issued to Ronald B. Standler on Apr. 29,
`1996 discloses using both a gas discharge tube and a solid
`state device for protecting a communications line against
`overvoltage conditions.
`Coaxial transmission lines have been used to carry signals
`requiring large bandwidth. such as analog video signals and
`very high speed digital signals. It is known in the prior art
`to protect coaxial transmission lines from overvoltage con
`ditions using coaxial surge arrestors. One such coaxial surge
`arrestor is shown in US. Pat. No. 5.566.056 issued to Nisar
`A. Chaudhry on Oct. 15. 1996.
`The June 1997 issue of Cabling Business contains an
`article entitled “Cabling For Ethernet (10 Mb). Fast Ethernet
`(100 Mb) And Gigabit Ethernet” (pages 74-75). It identi?es
`IEEE speci?cation 8023i (also known as IOBASE-T) and
`IEEE speci?cation 802.3u (also known as lOOBASE-TX).
`"lhe article states that 1OBASE-T Ethernet networks handle
`10 megabits per second (Mbps) while 100BASE-TX Fast
`Ethernet networks handle 100 Mbps.
`Both IOBASE-T Ethernet and lOOBASE-TX Fast Ether
`net can be transmitted over unshielded twisted pair (“UTP”)
`wires. One twisted pair is used for incoming digital signals
`and one twisted pair is used for outgoing digital signals. In
`50
`addition. there are categories for classifying horizontal UTP
`cables and connecting hardware by performance capabili
`ties. Thus. Category 3 encompasses UTP cables and con
`necting hardware with transmission characteristics up to 16
`MHz. Category 4 encompasses UTP cables and connecting
`55
`hardware with transmission characteristics up to 20 MHZ
`and Category 5 encompasses cables and connecting hard
`ware with transmission characteristics up to 100 MHz.
`IOBASE-T Ethernet (l0 Mbs) can be implemented with
`Category 3. 4 or 5 cables and connecting hardware. whereas
`lOOBASE-TX Fast Ethernet (100 Mbs) can be implemented
`with Category 5 cables and connecting hardware.
`overvoltage/overcurrent protection for high speed digital
`data lines is dilferent from surge protection for standard
`voice telephone lines. Thus. surge protection for high speed
`digital data lines must have low capacitance and low inser
`tion loss to avoid attenuating the high speed data signals.
`
`2
`Moreover. the surge protection for the high speed data lines
`must protect against both high voltage surges and low
`voltage surges. Therefore. a need exists for an overvoltage
`protection system for IOBASE-T Ethernet and IOOBASE
`TX Fast Ethernet networks which does not signi?cantly
`decrease the bandwidth of the networks.
`The present invention ?lls that need by providing an
`overvoltage/overcurrent protection system for both
`IOBASET Ethernet and IOOBASE-TX Fast Ethernet net
`works employing UTP cables and connecting hardware. The
`system of the present invention comprises two overvoltage!
`overcurrent protection circuits. one circuit being connected
`in series with the twisted pair carrying incoming digital
`signals and the other circuit being connected in series with
`the twisted pair carrying outgoing digital signals. Each
`circuit provides overcurrent protection as well as primary
`and secondary overvoltage protection. The present invention
`provides overvoltage/overcurrent protection for IOBASE-T
`Ethernet and IOOBASE-TX Fast Ethernet networks without
`signi?cantly decreasing the bandwidth of the networks.
`SUMMARY OF THE INVENTION
`An overvoltage/overcurrent protection system for
`lOBASE-T Ethernet and IOOBASE-TX Fast Ethernet net
`works according to the principles of the present invention
`comprises two overvoltage/overcurrent protection circuits.
`one circuit connected in series with the unshielded twisted
`pair carrying the incoming digital signals and the other
`circuit connected in series with the unshielded twisted pair
`carrying the outgoing digital signals. Each circuit contains
`overcurrent protection as well as primary and secondary
`overvoltage protection. The primary overvoltage protection
`is provided by a gas discharge tube while the secondary
`overvoltage protection is provided by a diode bridge with an
`avalanche diode connected across the diode bridge. One
`embodiment of the invention employs fuses for overcurrent
`protection. Another embodiment employs positive tempera
`ture coe?icient resistors (P'I‘CRs) for overcurrent protection.
`The subject matter which I regard as my invention is
`particularly pointed out in the claims at the end of the
`speci?cation. My invention. including its method of opera
`tion and its numerous advantages. may best be understood
`by reference to the following description taken in connection
`with the accompanying drawings wherein like reference
`characters refer to like components.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1A is a schematic diagram of a ?rst overvoltage!
`overcurrent protection circuit in accordance with a ?rst
`embodiment of the invention. the circuit being adapted to be
`connected in series with a twisted pair of wires carrying
`incoming digital signals in a lOBASE-T Ethernet or
`IOOBASE-TX Fast Ethernet network.
`FIG. 1B is a schematic diagram of a second overvoltage!
`overcurrent protection circuit in accordance with the ?rst
`embodiment of the invention. the circuit being adapted to be
`connected in series with a twisted pair of wires carrying
`outgoing digital signals in a 10BASE-T Ethernet or
`IOOBASE-TX Fast Ethernet network.
`FIG. 2A is a schematic diagram of a ?rst overvoltage!
`overcurrent protection circuit in accordance with a second
`embodiment of the invention. the circuit being adapted to be
`connected in series with a twisted pair of wires carrying
`incoming digital signals in a IOBASE-T Ethernet or
`IOOBASE-TX Fast Ethernet network
`FIG. 2B is a schematic diagram of a second overvoltage!
`overcurrent protection circuit in accordance with the second
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`Using two avalanche diodes connected in series provides
`two bene?ts: (l) the surge energy handling capability of the
`protector doubles and (2) the capacitance o?ered to the
`diode bridge is halved Two avalanche diodes connected in
`parallel could also double the surge handling capability. but
`diodes do not have the same breakdown voltage. and there
`fore do not share energy equally. Moreover. using two
`avalanche diodes connected in parallel causes the capaci
`tance to the diode bridge to double. which could signi?
`cantly attenuate the digital signal.
`Excessive positive voltages appearing at terminal 40 are
`clamped by diodes 26 and 36 and avalanche diode 38.
`Excessive negative voltages appearing-at terminal 40 are
`clamped by diodes 28 and 34 and avalanche diode 38.
`Excessive positive voltages appearing on terminal 42 are
`clamped by diodes 32 and 36 and avalanche diode 38.
`Excessive negative voltages appearing at terminal 42 are
`clamped by diodes 30 and 34 and avalanche diode 38. If
`diode 38 is a 12 volt avalanche diode. then voltages exceed
`ing about plus or minus 15 volts would be clamped by the
`secondary protection circuit. If diode 38 is two series
`connected 12 volt avalanche diodes. then voltages exceeding
`about 35 volts would be clamped by the secondary protec
`tion circuit. Avalanche diodes are available with many
`different breakdown voltages and the clamping voltage may
`be chosen by selecting suitable avalanche diodes.
`Resistors 22 and 24 isolate the primary and secondary
`overvoltage protection sections and also limit the current
`which can ?ow through overvoltage protection circuit 10.
`Resistors 22 and 24 may be between about 5 and about 6
`ohms and are preferably 5.6‘ ohms. l/z watt.
`FIG. 1B shows a second overvoltage/overcurrent protec
`tion circuit 50 in accordance with the ?rst embodiment of the
`invention. Circuit 50 is adapted to be connected in series
`with a twisted pair of wires (a) in Category 3. 4 or 5 cabling
`carrying outgoing digital signals in a lOBASE-T Ethernet
`network or (b) in Category 5 cabling carrying outgoing
`digital signals in IOOBASE-TX Fast Ethernet network.
`Circuit 50 has an input (transmit) side and an output
`(transmit) side. Terminals 80 and 82 may be connected. for
`example. to a LAN. while terminals 52 and 54 may be
`connected. for example. to an ONU. a computer. a WAN or
`another LAN.
`Circuit 50 provides both primary overvoltage protection
`and secondary overvoltage protection as well as overcurrent
`protection. Overcurrent protection is provided by fuses 56
`and S8. The primary overvoltage protection section com
`prises three-electrode gas discharge tube 60. while the
`secondary overvoltage protection section comprises diodes
`66 through 76 and avalanche diode 78. Circuit 50 also
`comprises resistors 62 and 64 which isolate the primary and
`secondary overvoltage protection sections and also limit the
`current which can ?ow through circuit 50.
`The description of the components in circuit 10 and their
`operation applies to like components in circuit 50. The
`combination of circuit 10 (for incoming digital signals) and
`circuit 50 (for outgoing digital signals) comprise one
`embodiment of the overvoltage protection system of the
`present invention for protecting 10BASE-T Ethernet and
`lOOBASE-TX Fast Ethernet networks.
`FIG. 2A shows a schematic diagram of a ?rst overvoltage/
`overcurrent protection circuit 11 in accordance with a sec
`ond embodiment of the invention. Circuit 11 is adapted to be
`connected in-series with a twisted pair of wires (:1) in
`Category 3. 4 or 5 cabling carrying incoming digital signals
`in a lOBASE-T network or (b) in Category 5 cabling
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`3
`embodiment of the invention. the circuit being adapted to be
`connected in series with a twisted pair of wires carrying
`outgoing digital signals in a lOBASE-T Ethernet or
`lOOBASE-TX Fast Ethernet network.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`FIG. 1A is a schematic diagram of a ?rst overvoltage]
`overcurrent protection circuit 10 in accordance with a ?rst
`embodiment of the invention. Circuit 10 is adapted to be
`connected in series with a twisted pair of wires (a) in
`Category 3. 4 or 5 cabling carrying incoming digital signals
`in a lOBASE-T Ethernet network or (b) in Category 5
`cabling carrying incoming digital signals in a lOOBASE-TX
`Fast Ethernet network. Circuit 10 has an input (receive) side
`and an output (receive) side. The input side is adapted to be
`connected to a source of digital signals at terminals 12 and
`14. The source may. for example. be the output from an
`optical network unit (ONU). a computer. a local area net
`work (LAN) or a wide area network (WAN). The output side
`of circuit 10 may. for example. be connected to a computer
`or a LAN at terminals 40 and 42. One or more servers and
`one or more personal computers (PCs) may be connected to
`the LAN.
`Circuit 10 provides both primary and secondary overvolt
`age protection as well as overcurrent protection. The over
`current protection is provided by fuses 16 and 18 which are
`connected in series with the twisted pair of wires which
`carry the incoming digital signals. Fuses 16 and 18 are
`preferably less than one amp and may be 3/4 amp Littlefuse
`type fuses. The primary overvoltage protection section com
`prises three-electrode gas discharge tube 20 which is con
`nected across the twisted pair of wires which carry the
`incoming digital signals. Gas discharge tube 20 conducts
`when the voltage on either of the twisted wires exceeds a
`threshold value. The breakdown voltage may be between
`about 150 and about 300 volts. with a breakdown voltage of
`on the order of 250 volts being preferred. A suitable three
`electrode gas discharge tube is shown in Napiorkowski. U.S.
`Pat. No. 4.212.047. Suitable three-electrode gas discharge
`tubes are also available from TlI Industries. Inc.. Copiague.
`N.Y. as T11 71 type or 73f75 type with a voltage breakdown
`range of 150-300 VDC.
`As will be understood by persons skilled in the art. two
`two-electrode gas discharge tubes may be used in lieu of a
`single three electrode gas discharge tube and using two
`two-electrode gas discharge tubes is the full equivalent of
`using one three-electrode gas discharge tube. In the present
`invention three-electrode gas discharge tubes are preferred
`to two-electrode gas discharge tubes and. therefore. three
`electrode gas discharge tubes are shown in the drawings.
`The secondary overvoltage protection section of circuit 10
`comprises diodes 26 through 36 and avalanche diode 38.
`Diodes 26 through 32 form a diode bridge and avalanche
`diode 38 is connected across the diode bridge. The diode
`bridge is connected across the twisted pair of wires which
`carry the incoming digital signals and limits the voltage on
`those wires in the event that the voltage substantially
`exceeds. for example. the normal digital signal levels. Typi
`cal digital signal levels are plus or minus ?ve volts. Diodes
`26 through 30 may be type IN4007. 1 amp. 1000 volt PIV
`diodes. Diodes 34 and 36 may be type RL204G. 2 amp. 1000
`volt PIV diodes. Diode 38 may be a type 1.5KE12. 12 volt
`avalanche diode. Alternatively. diode 38 may be two series
`connected low capacitance. 6 volt 1500 watt diodes made by
`Samtech of 650 Mitchell Road. Newbury Park. Calif. 91320
`and sold under part number LC01-6.
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`carrying incoming digital signals in a IOOBASE-TX Fast
`Ethernet network. Circuit 11 has an input (receive) side and
`an output (receive) side. The input side is adapted to be
`connected to a source of digital data signals at terminals 12
`and 14. As with circuit 10 in FIG. 1A. the source may. for
`example. be an ONU. a computer. a LAN or a WAN. The
`output side of circuit 11. may. for example. be connected to
`a computer or a LAN and the LAN may have connected to
`it one or more servers and one or more PCs.
`Circuit 11 provides both primary and secondary overvolt
`age protection as well as overcurrent protection. The over
`current protection is provided by positive temperature coef
`?cient resistors (PTCRs) l7 and 19 which limit the current
`?owing through circuit 11. As current flows through the
`PTCRs. the temperature increases and. as the temperature
`increases. the resistance increases. thereby reducing the
`current ?ow. P'TCRs l7 and 19 may be type TR600-150
`which are available from Raychern Corp. Menlo Park.
`Calif. The primary overvoltage protection section comprises
`gas discharge tube 20 which is connected across the wires
`carrying the incoming digital signals. The operation of gas
`discharge tube 20 in circuit 11 is the same as in circuit 10.
`The secondary overvoltage protection section of circuit 11 is
`the same as the secondary overvoltage protection section of
`circuit 10 and operates the same way.
`FIG. 2B shows a second overvoltage/overcurrent protec
`tion circuit 51 in accordance with the second embodiment of
`invention. Circuit 51 is adapted to be connected in series
`with the twisted pair of wires (a) in Category 3. 4 or 5
`cabling carrying outgoing digital signals in a IOBASE-T
`Ethernet network or (b) in Category 5 cabling carrying
`outgoing digital signals in a IOOBASE-TX Fast Ethernet
`network. Circuit 51 has an input (transmit) side and an
`output (transmit) side. Terminals 80 and 82 may be
`connected. for example. to a LAN . while terminals 52 and
`54 may be connected. for example. to an ONU. a computer.
`a WAN or another LAN.
`Circuit 51 provides both primary and secondary overvolt
`age protection as well as overcurrent protection. Overcurrent
`protection is provided by PTCRs 57 and S9. The primary
`overvoltage protection section comprises three-electrode gas
`discharge urbe 60. while the secondary overvoltage protec
`tion section comprises diodes 66 through 76 and avalanche
`diode 78. PTCRs 57 and 59 function like Pl‘CRs 17 and 19
`and limit the current which can ?ow through circuit 51. The
`combination of circuit 11 (for incoming digital signals) and
`circuit 51 (for outgoing digital signals) comprise a second
`embodiment of the overvoltage/overcurrent protection sys
`tem of the present invention for protecting IOBASE-T
`Ethernet and IOOBASE-TX Fast Ethernet networks.
`It will be understood that various changes in the details.
`materials, arrangement of parts and operating conditions
`which have been herein described and illustrated in order to
`explain the nature of the invention may be made by those
`skilled the art without departing from the principles and
`scope of the instant invention.
`What is claimed is:
`1. An overvoltage/overcurrent protection system for Eth
`ernet networks using unshielded twisted pair cabling com
`prising:
`(a) a ?rst overvoltage/overcurrent protection circuit
`adapted to be connected in series with ?rst and second
`twisted wires carrying incoming digital signals. the ?rst
`circuit comprising a primary overvoltage protection
`section. a secondary overvoltage protection section and
`an overcurrent protection section.
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`(l) the overcurrent protection section of the ?rst circuit
`comprising ?rst and second fuses. the ?rst side of the
`?rst fuse for being connected to the ?rst twisted wire
`and the ?rst side of the second fuse for being
`connected to the second twisted wire.
`(2) the primary overvoltage protection section of the
`?rst circuit comprising a ?rst three-electrode gas
`discharge tube. the ?rst electrode being connected to
`the second side of the ?rst fuse. the second electrode
`being connected to the second side of the second
`fuse. and the third electrode for being connected to
`ground.
`(3) ?rst and second resistors. the ?rst side of the ?rst
`resistor being connected to the second side of the
`?rst fuse and the ?rst electrode of the ?rst gas
`discharge tube. and the ?rst side of the second
`resistor being connected to the second side of the
`second fuse and the second electrode of the ?rst gas
`discharge tube.
`(4) the secondary overvoltage protection section of the
`?rst circuit comprising a diode bridge comprising
`?rst. second. third. fourth. ?fth and sixth diodes and
`a ?rst avalanche diode. the anode of the ?rst diode
`being connected to the cathode of the second diode
`and to the second side of the ?rst resistor. the cathode
`of the third diode being connected to the anode of the
`fourth diode and to the second.side of the second
`resistor. the cathode of the ?rst diode being con
`nected to the cathode of the fourth diode. to the ?rst
`side of the ?rst avalanche diode and to the cathode of
`the ?fth diode. the anode of the second diode being
`connected to the anode of the third diode. the second
`side of the ?rst avalanche diode and the anode of the
`sixth diode. the anode of the ?fth diode and the
`cathode of the sixth diode for being connected to
`ground; and
`(b) a second overvoltage/overcurrent protection circuit
`adapted to be connected in series with third and fourth
`twisted wires carrying outgoing digital signals. the
`second circuit comprising a primary overvoltage pro
`tection section. a secondary overvoltage protection
`section and an overcurrent protection section.
`(1) the overcurrent protection section of the second
`circuit comprising third and fourth fuses. the ?rst
`side of the third fuse for being connected to the third
`twisted wire and the ?rst side of the fourth fuse for
`being connected to the fourth twisted wire.
`(2) the primary overvoltage protection section of the
`second circuit comprising a second three-electrode
`gas discharge tube, the ?rst electrode being con
`nected to the second side of the third fuse. the second
`electrode being connected to the second side of the
`fourth fuse and the third electrode for being con
`nected to ground.
`(3) third and fourth resistors. the ?rst side of the third
`resistor being connected to the second side of the
`third fuse and to the ?rst electrode of the second gas
`discharge tube. and the ?rst side of the fourth resistor
`being connected to the second side of the fourth fuse
`and to the second electrode of the second gas dis
`charge tube.
`(4) the secondary overvoltage protection section of the
`second circuit comprising a diode bridge comprising
`seventh. eighth. ninth. tenth. eleventh and twelfth
`diodes and a second avalanche diode. the anode of
`the seventh diode being connected to the cathode of
`the eighth diode and to the second side of the third
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`resistor. the cathode of the ninth diode being con
`nected to the anode of the tenth diode and to the
`second side of the fourth resistor. the cathode of the
`seventh diode being connected to the cathode of the
`tenth diode. to the ?rst side of the second avalanche
`diode and to the cathode of the eleventh diode. the
`anode of the eighth diode being connected to the
`anode of the ninth diode. the second side of the
`second avalanche diode and the anode of the twelfth
`diode. the anode of the eleventh diode. and the
`cathode of the twelfth diode for being connected to
`ground.
`2. An overvoltagelovercurrent protection system for Eth
`ernet networks using unshielded twisted pair cabling com
`prising:
`(a) a ?rst overvoltagelovercurrent protection circuit
`adapted to be connected in series with ?rst and second
`twisted wires carrying incoming digital signals. the ?rst
`circuit comprising a primary overvoltage protection
`section. a secondary overvoltage protection section and
`an overcurrent protection section.
`(1) the primary overvoltage protection section of the
`?rst circuit comprising a ?rst three-electrode gas
`discharge tube. the ?rst electrode for being con
`nected to the ?rst twisted wire. the second electrode
`for being connected to the second twisted wire and
`the third electrode for being connected to ground.
`(2) the overcurrent protection section of the ?rst circuit
`comprising ?rst and second positive temperature
`coe?icient resistors (PTCRs). the ?rst side of the ?rst
`PTCR being connected to the ?rst electrode of the
`?rst gas discharge tube and the ?rst side of the
`second PI‘CR being connected to the second elec
`trode of the ?rst gas discharge tube.
`(3) the secondary overvoltage protection section of the
`?rst circuit comprising a diode bridge comprising
`?rst. second. third. fourth. ?fth and sixth diodes and
`a ?rst avalanche diode. the anode of the ?rst diode
`being connected to the cathode of the second diode
`and to the second side of the ?rst P'I‘CR. the cathode
`of the third diode being connected to the anode of the
`fourth diode and to the second side of the second
`PI‘CR. the cathode of the ?rst diode being connected
`to the cathode of the fourth diode. to the ?rst side of
`the ?rst avalanche diode and to the cathode of the
`?fth diode. the anode of the second diode being
`connected to the anode of the third diode. the second
`side of the ?rst avalanche diode and the anode of the
`sixth diode. the anode of the ?fth diode and the
`cathode of the sixth diode for being connected to
`ground; and
`(b) a second overvoltage/overcurrent protection circuit
`adapted to be connected in series with third and fourth
`
`20
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`25
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`35
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`45
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`50
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`5 ,790.363
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`8
`twisted wires carrying outgoing digital signals. the
`second circuit comprising a primary overvoltage pro
`tection section. a secondary over-voltage protection
`section and an overcurrent protection section.
`(1) the primary overvoltage protection section of the
`second circuit comprising a second three-electrode
`gas discharge tube. the ?rst electrode for being
`connected to the third twisted wire. the second
`electrode for being connected to the fourth twisted
`Wire and the third electrode for being connected to
`ground.
`(2) the overcurrent protection section of the second
`circuit comprising third and fourth P'TCRs. the ?rst
`side of the third P'I‘CR being connected to the ?rst
`electrode of the second gas discharge tube and the
`?rst side of the fourth P'I'CR being connected to the
`second electrode of the second gas discharge tube.
`(3) the secondary overvoltage protection section of the
`second circuit comprising a diode bridge comprising
`seventh. eighth. ninth. tenth. eleventh and twelfth
`diodes and a second avalanche diode. the anode of
`the seventh diode being connected to the cathode of
`the eighth diode and to the second side of the third
`PCI'R. the cathode of the ninth diode being con
`nected to the anode of the tenth diode and to the
`second side of the fourth P'I‘CR. the cathode of the
`seventh diode being connected to the cathode of the
`tenth diode. to the ?rst side of the second avalanche
`diode and to the cathode of the eleventh diode, the
`anode of the eighth diode being connected to the
`anode of the ninth diode. the second side of the
`second avalanche diode and the anode of the twelfth
`diode. the anode of the eleventh diode and the
`cathode of the twelfth diode for being connected to
`ground.
`3. An overvoltage/overcurrent protection system accord
`ing to claim 1 wherein the ?rst. second. third and fourth
`fuses me less than one ampere.
`4. An overvoltage/overcurrent protection system accord
`ing to claim 1 wherein at least one of the ?rst. second. third
`and fourth resistors is between about 5 and about 6 ohms.
`5. An overvoltage/overcurrent protection system accord
`ing to claim 1 or claim 2 wherein at least one of the ?rst and
`second gas discharge tubes has a breakdown voltage
`between 150 and 300 VDC.
`6. An overvoltage/overcurrent protection system accord
`ing to claim 1 or claim 2 wherein the secondary overvoltage
`protection circuit clamps voltages in excess of about 15
`volts.
`7. An overvoltage protection system according to claim 1
`or claim 2 wherein the secondary overvoltage protection
`circuit clamps voltages in excess of about 35 volts.
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`D-Link-1009
`Page 7 (of 7)