_
`
`United States Patent [191
`Bulan et al.
`
`IllllllllllllllllIlllllllllllllllllllIlllllllllllllllllllIlllllllllllllllll
`[11] Patent Number:
`5,089,927
`[45] Date of Patent:
`Feb. 18, 1992
`
`USOO5089927A
`
`[54] POWER FEED CIRCUIT FOR DIGITAL
`COMMUNICATIONS TERMINAL
`EQUIPMENT
`[75] Inventors: Sergiu Bulan, Willowdale; Johannes
`,
`L. Holt, Brampton, both of Canada
`[73] Assignec: Northern Teleeom Limited, Montreal,
`Canada
`[21] Appl. No.: 420,487
`[22] Filed:
`Oct. 12, 1989
`
`[51] Int. vCl.5 ........................ ., ................... .. HOZH 3/08
`[52] US. Cl. ......................................... .. 361/87
`[58] Field of Search ................... .. 361/87, 93, 100, 78,
`361/66, 111; 379/2, 32, 93, 94, 412
`References Cited
`U.S. PATENT DOCUMENTS
`
`[56]
`
`3,858,089 12/1974 Poindexter .................... .. 361/120 X
`4,983,955 l/l99l Ham, Jr. et a]. .
`4,987,512 l/l99l Mulshine ............................. .. 361/93
`Primary Examiner-Steven L. Stephan
`Assistant Examin er-E. To
`
`Attorney, Agent, or Firm-J. E. Moorhouse
`[57]
`ABSTRACT
`Integrated Services Digital Network (ISDN) terminal
`equipments (T Es) are remotely powered from a central
`source via line interface circuits. One function of the
`line interface circuit is that of conducting energizing
`direct current for the associated telecommunications
`terminal equipment while providing effective overcur
`rent protection in spite of widely variable load current
`requirements which occasionally may mimic a faulty
`over current condition. The line interface circuit in
`cludes ?rst and second power terminals for connection
`to a source of power, ?rst and second line terminals for
`supplying said energizing direct current, and a current
`control means, being connected between a one of the
`?rst and second power terminals and a respective one of
`the ?rst and second line terminals, for temporarily
`switching off an inrush current in excess of a dynamic
`limit, and permanently switching off a load current in
`excess of a static limit until a virtual open circuit condi
`tion occurs across the ?rst and second line terminals.
`
`4 Claims, 3 Drawing Sheets
`
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`D-Link-1009
`Page 1 (of 8)
`
`

`
`US. Patent
`
`Feb. 18, 1992
`
`Sheet 1 of 3
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`5,089,927
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`D-Link-1009
`Page 2 (of 8)
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`

`
`US. Patent
`
`Feb. 18, 1992
`
`Sheet 2 of 3
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`5,089,927
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`D-Link-1009
`Page 3 (of 8)
`
`

`
`US. Patent
`
`Feb. 18,1992
`
`Sheet 3 of 3
`
`5,089,927
`
`‘lll'llllllllll
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`TIME
`FIG. 7
`
`D-Link-1009
`Page 4 (of 8)
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`

`
`1
`
`5,089,927
`
`POWER FEED CIRCUIT FOR DIGITAL
`COMMUNICATIONS TERMINAL EQUIPMENT
`
`25
`
`FIELD OF THE INVENTION
`The invention is in the ?eld of power feed apparatus
`and methods for supplying transmission line connected
`terminals with energizing current. More particularly
`the invention relates to apparatus and methods for pow
`ering a digital signal telecommunications terminal
`equipment wherein an initial power-up current in-rush
`may exceed a normal load current by many times.
`‘BACKGROUND OF THE INVENTION
`Traditionally a telephone terminal apparatus, for
`example telephones and the like, is coupled through an
`associated telephone line with a central power source
`via a line interface circuit. The line interface circuit
`includes circuitry, usually of a resistive nature, for feed
`ing energizing direct current from the central power
`source to the telephone terminal apparatus. Recently,
`various active line interface circuits have been devel
`oped wherein the function of feeding the energizing
`direct current is performed by active elements which
`may optimize the coupling of the telephone with an
`associated telephone system. Such active line circuits
`may include over current protection circuitry which
`responds to unintended operational faults, for example
`power line crosses, short circuits or ground faults, by
`somewhat limiting current flow in the interest of pre
`venting catastrophic failure of line interface circuit.
`Recently telephone terminal apparatus of a digital
`nature have been developed to take advantage of the
`recommended Integrated Services Digital Network
`(ISDN) standard. In ISDN jargon a terminal apparatus
`is usually referred to as a terminal equipment (TE), a
`line interface circuit as a network termination (NTl),
`and a line for connection between a NT] and a TE as a
`terminal (T) interface, hereafter referred to as a T bus.
`The TEs are characterized by digital circuitry requiring
`an operating voltage or voltages not conventionally
`available from an associated telephone facility. How
`ever the traditional reliability of telephony service is
`never the less preferred. Hence one arrangement is
`provided wherein a convenient physical location for a
`45
`group of NTls is also provided with a line power
`source, which is intended to be more reliable than the
`supply service expected from a local electrical utility.
`Each of the NTls is provided with power from the line
`power source, at a potential of about 50 volts, so that
`energizing direct current is made available to each asso
`ciated TE via the wires of the interconnecting T bus. A
`typical TE includes a direct current to direct current
`(DC to DC converter which utilizes between about 40
`to 60 milliamperes of current from the T bus to provide
`55
`the required voltage or voltages for normal operation.
`However initiation of operation of a TE, such as when
`it is ?rst plugged into a T bus or when power is initially
`applied at the NTl, typically draws a momentary surge
`of current. The surge of current is that which is re
`quired to initiate operation of the typical DC to DC
`converter and associated ?lter capacitors. In a normal
`power up event in a TE, the DC to DC converter usu~
`ally draws a current peak or current inrush, which may
`exceed an ampere for as much as ten milliseconds. In
`65
`such circumstances the typical current limiting circuit
`intended to protect the typical line interface circuit is
`inappropriate for operation throughout the whole cur
`
`40
`
`50
`
`2
`rent load regime. For example, if a current limit of
`twice the normal operating current is set, there will be
`insufficient current for start up of the DC to DC con
`verter and on the other hand if a current limit suffi
`ciently great to accommodate start up is set, a fault may
`be permitted to draw current for a period of time suffi
`cient to seriously jeopardize the operations of the NH
`physically adjacent, and the line power source circuits.
`It is an object of the invention to supply operating
`current from a central line power source via a line inter
`face circuit, to a terminal equipment having a DC to
`DC converter, while providing an over current protec
`tion feature which is effective across the entire load
`current regime of the terminal equipment.
`
`SUMMARY OF THE INVENTION
`In accordance with the invention, a current control
`apparatus is provided for supplying an energizing direct
`current flow from a source of power via a transmission
`line to a telecommunications terminal apparatus being
`continuously operable while drawing a load current
`which is exceeded by an inrush current being greater
`than the load current at a moment of power up. The
`current control apparatus is for connection in series
`between the power source and the transmission line and
`comprises: means for generating a magnitude signal
`being representative of an amount of said energizing
`direct current flow; means for generating a static con
`trol signal for de?ning a maximum limit of load current;
`means for generating a momentary dynamic control
`signal for de?ning a maximum limit of the inrush cur
`rent in response to the magnitude signal increasing from
`a level representative of less than the maximum limit of
`load current to a level representative of more than the
`maximum limit of load current; and switch means re
`sponsive to the magnitude signal and the static and
`dynamic control signals, to be set in an ON condition
`for conducting said current flow, when either one of
`said maximum limits is greater than the energizing di
`rect current as is instantly represented by the magnitude
`signal, otherwise to be reset in an OFF condition, and
`while in the OFF condition being responsive to an ap
`parent open circuit condition of the transmission line to
`become set in the ON condition.
`In one example, a line interface circuit couples ener
`gizing direct current, from a line power supply to a
`communications line, for operation of a telecommunica
`tions terminal apparatus. The line interface circuit com
`prises: ?rst and second power terminals for connection
`to the line power supply; ?rst and second line terminals
`for connection to the communication line; and a current
`control means, being connected between a one of the
`?rst and second power terminals and a respective one of
`the ?rst and second line terminals, for conducting the
`energizing direct current there between, for temporar
`ily isolating the power terminal from the line terminal to
`stop an inrush current in excess of a dynamic limit, and
`permanently isolating the power terminal from the line
`terminal to stop a load current in excess of a static limit,
`until a virtual open circuit condition occurs across the
`?rst and second line terminals.
`Also in accordance with the invention a method is
`provided for supplying an energizing direct current
`?ow, from a source of power via a transmission line to
`a telecommunications terminal apparatus, said terminal
`apparatus being continuously operable while drawing a
`load current which is exceeded by an inrush current
`
`D-Link-1009
`Page 5 (of 8)
`
`

`
`3
`being greater than the load current at a moment of
`power up. The method comprising the steps of:
`a) generating a magnitude signal being representative
`of an amount of said energizing direct current flow;
`b) generating a static control signal for de?ning a
`maximum limit of load current;
`c) generating a momentary dynamic control signal
`for de?ning a maximum limit of the inrush current in
`response to the magnitude signal increasing from a level
`representative of less than the maximum limit of load
`current to a level representative of more than the maxi
`mum limit of load current;
`d) in response to the magnitude signal and the static
`and dynamic control signals, providing a path with an
`impedance of less than a ?rst value suitable for conduct
`ing the energizing current flow, when either one of said
`maximum limits is greater than the energizing direct
`current as is instantly represented by the magnitude
`signal, otherwise increasing the impedance of said path
`to a second value greater than the ?rst value and unsuit
`able for conducting the energizing current flow; and
`e) while said path is of at least the second impedance
`value and in an event where the transmission line ap
`pears to be an open circuit, reducing the impedance of
`the path to less than the ?rst value.
`25
`
`- 0
`
`20
`
`5,089,927
`4
`via positive and negative power terminals 22 and 21
`respectively. Each of the NTls is connected via a group
`of leads 10 as illustrated with respect to the terminal
`equipment 13. This is usually a transformer connection
`as is illustrated by transformer windings 11 with sending
`terminals 81 and S3 and receiving terminals S2 and S4
`connected to a 4 wire T bus 14, as shown. Power termi
`nals at P91: and P917 are connected to centre taps 16 and
`17 of the transformer windings 11 in a well known
`phantom power feed arrangement. In like manner the
`TE 130 is connected via a T bus 140. In an alternate
`arrangement, not shown, an additional two wires may
`be provided in the T bus to separately power the TE via
`the terminals P90 and P9b. In the alternate arrangement
`the transformer 11 is spared the chore of having to
`conduct dc current via its winding portions.
`Each of the NTls includes a line interface circuit for
`coupling current from the power source 20, hereinafter
`referred to as the line power supply 20, to its associated
`T bus. The line interface circuit is generally illustrated
`in FIG. 2. In FIG. 2, the positive power terminal 21 is
`connected directly to the line terminal P90. However
`the negative power terminal 22 is connected via a cur
`rent sensor 26, a current path 9b, and a current path
`switch 25 to the line terminal P9b. A static reference
`generator 23 provides a negative voltage reference on a
`lead 70 for use by a dynamic reference generator 24.
`- The static reference generator 23 also provides a stable
`voltage supply on a lead 61 for use by the dynamic
`reference generator 24 and the .current path switch 25.
`The dynamic reference generator 24 is provided with a
`sense signal on a lead 50 connected from the current
`sensor 26. The dynamic reference generator uses the
`signals on the lead 70 and 50 to generate a control signal
`on a lead 60 for use by the current path switch 25. The
`current path switch is required to provide a current
`path which at any one time is of a very low impedance,
`or alternately is of a much higher impedance, in accor
`dance with dynamics of the ongoing operation of the
`MI] and any TE connected thereto. Operation of the
`line interface circuit is discussed in more detail with
`reference to FIGS. 3, 4 and 5.
`Referring to FIG. 3, a differential ampli?er 31 in
`cludes an open collector output connected as shown
`with a resistor 32, a capacitor 35 and a zener diode 36 to
`control conduction of energizing current for an associ
`ated TE via an enhancement mode ?eld effect transistor
`(FET) 41. The FET 41 is connected as part of the cur
`rent path 9b in series between a diode 46 and a current
`sensing device in this case a resistor 40, as shown. An
`initializing circuit path 90 includes a FET 42 connected
`as shown with resistors 43 and 45 and a capacitor 44. An
`inverting input of the differential ampli?er 31 is con
`nected with a capacitor 33, a resistor 34 and the FET
`42. A non-inverting input of the differential ampli?er 31
`is connected to receive a control signal. While the FET
`42 is ON, the current path 90 in combination with the
`resistor 34 establishes a positive feedback path from a
`junction of the resistor 40 and the FET 41, to the invert
`ing input of the differential ampli?er 31. A secondary
`current path 9d is provided by a resistor 48 connected in
`series with a diode 49 and the diode 46. When the FET
`41 is switched OFF as in response to an over current
`condition, the secondary current path 9d provides a
`small trickle of currentwhich ?ows via the terminals
`P90 and P912 when these terminals are other than effec
`tively open circuited. In an event wherein the trickle
`current ceases to flow the Fet 41 may be returned to the
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`An example embodiment is described with reference
`to the accompanying drawings in which:
`FIG. 1 is a block schematic diagram of a telecommu
`nications facility and a network termination equipment
`group for coupling terminal equipment thereto;
`FIG. 2 is a block schematic diagram of a line interface
`circuit, used in the network termination equipment
`group shown in FIG. 1 in accordance with the inven
`tion;
`FIG. 3 is a schematic diagram which illustrates a
`detailed example of a current switch used in the line
`interface circuit illustrated in FIG. 2;
`FIG. 4 is a schematic diagram which illustrates a
`detailed example of a dynamic reference generator used
`in the line interface circuit illustrated in FIG. 2;
`FIG. 5 is a schematic diagram which illustrates a
`detailed example of a static reference generator used in
`the line interface circuit illustrated in FIG. 2; and
`FIGS. 6 and 7 are graphical representations of cur
`rent limiting events which may occur during operation
`in accordance with the invention of the interface circuit
`as illustrated in FIGS. 2 to 5.
`
`35
`
`40
`
`45
`
`DESCRIPTION OF THE EXAMPLE
`EMBODIMENT
`FIG. 1 illustrates a typical connection of several ter
`minal equipments (TE) shown at 13 and 130. Each TE
`has associated with it a DC to DC converter, 15 and 150
`55
`respectively. Each TE is connected to a telecommuni
`cations facility 100 via a U interface 101. The U inter
`face 101 is a digital signals link which typically con
`forms to a telephony standard such as the well known
`TlEl transmission standard. The U interface 101 trans
`60
`ports signals between the telecommunications facility
`100 and a network termination equipment group 8.
`These signals are distributed on a predetermined basis
`across a group of network terminations (NTI) shown at
`80 through 8n. A battery power source 20 supplies
`65
`termination equipment power for operation of each of
`the NTls 8a through 8n, and in this example supplies
`line power for the operations of the TEs 13 through 130
`
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`5,089,927
`5
`6
`ON condition. The circuit, so arranged, is operable to
`the dynamic level switched via the diode 56 to the
`output 60.
`maintain the FET 41 in an ON condition as long as the
`voltage at the inverting input is less positive than the
`Referring to FIG. 3, the current path switch operates
`voltage of the control signal at the non-inverting input
`in response to a sample of the voltage sensed across the
`of the differential amplifier 31.
`resistor 40 and the control signal provided on the lead
`The dynamic reference generator in FIG. 4 includes
`60. In an event where power is ?rst applied across the
`a differential ampli?er 51 having an inverting input and
`power terminals 21 and 22 from the line power supply
`a non-inverting input. The differential ampli?er 51 in
`20 as illustrated in FIG. 1, the FET 42 is initially main
`this example includes an open collector output con
`tained in an OFF condition by voltage at the junction of
`the capacitor 44 and the resistor 43. This voltage rises in
`nected as shown to a series arrangement of resistors 52
`and 54 and a capacitor 53. A junction of the resistor 54
`accordance with the RC time constant of these elements
`and the capacitor 53 is connected to an anode electrode
`toward the potential —V1 to switch the FET 42 into
`the ON condition which persists during the rest of the
`of a diode 56. A cathode electrode of a diode 56 is
`connected to a junction of resistors 57 and 58 arranged
`circuit function, as along as power is not interrupted.
`in series.
`Hence positive feedback, normally coupled via a cur
`The static reference generator, in FIG. 5, is used to
`rent path 9c, is suppressed for the moment shortly after
`provide a stable supply voltage supply —V1 and a refer
`the initial power application at the power terminals 21
`ence voltage V REF». The static reference generator
`and 22, to prevent a premature locking of FET 41 into
`is shown to be connected across the power terminals 22
`a permanent OFF state, that is before all operating static
`voltages have stabilized. The inrush of current gener
`and 21 to receive current across a potential +V, —V
`20
`from the battery power source. A resistor 62 is con
`ates a very much more positive voltage on the sense
`lead 50 which causes the dynamic reference generator
`nected as shown in a shunt combination with a voltage _
`regulator 63 to provide the intermediate voltage —Vl
`to generate a positive going pulse control signal, which
`on a lead 61. In this case, the intermediate voltage —V1
`in turn tends to maintain the FET 41 in the ON condi
`tion. Voltage appearing at the inverting input of the
`is about 10 volts more positive than the potential —V.
`25
`differential ampli?er 31 is developed across the resistor
`Resistors 65, 66 and 67 are connected in series and pro
`vide a controlled potential at a ?rst voltage tap 631 for
`40 and the capacitor 33 and rises in accordance with the
`operation of the voltage regulator 63. A second voltage
`RC time constant of the resistors 34 and 40 and capaci
`tap provides a voltage V REF- which is about a volt
`tor 33, so that the differential ampli?er 31 is prevented
`more positive than the potential —V. Capacitor 68 and
`from responding too quickly to the rising amplitude of
`64 provide ?ltering.
`the sensed current in the current path 9b. This permits
`Operation of the dynamic reference generator 24
`the control signal pulse to be generated before it is
`shown in FIG. 4 is as follows. With reference to the
`compared to the sensed current by the differential am
`differential ampli?er 51, assuming that its inverting
`pli?er 31. If the sensed inrush of current does not ex
`input is more positive than its non-inverting input, its
`ceed the permissible level as set by the control signal,
`output then assumes a potential near that of —V. Alter
`the FET 41 is maintained in the ON state. However, if
`nately in an event where the sense voltage (50) becomes
`the sensed current exceeds the control signal permissi
`greater than the potential of V REF- , the output of the
`ble level, the FET 41 is controlled by the differential
`differential ampli?er 51 becomes a high impedance.
`ampli?er 31 to be in a less conductive state. This causes
`Hence a positive going pulse, with an edge of near 10
`a rise in the voltage level in the path 9c which is im
`volts with respect to —V, is coupled across the capaci
`pressed upon the inverting input of the differential am
`tor 53. The amplitude of the pulse edge is determined by
`pli?er 31 and thereby causes the FET 41 to be locked
`ohmic values of the resistors 52 and 57 in a voltage
`OFF. While the may be FET 41 locked OFF the trickle
`divider arrangement. Following the pulse edge, a fall
`current is conducted via the current path 9a’. This con
`ing or decay portion is characterized by the RC value of
`dition is maintained until an effective external open
`45
`the capacitor 53 and the resistors 57 and 52. The pulse is
`circuit condition is established across the terminals P9a
`transmitted via the diode 56 until its amplitude ap
`and P9b. The open, indicated by an absence of the
`proaches to within the forward voltage drop, of the
`trickle current, circuit condition is characteristic of the
`diode 56, with respect the normal voltage at a junction
`impedance presented at the power feed terminals of a
`60!. As time passes, the potential of the pulses further
`typical DC to DC converter in the event it has failed to
`50
`reduced and the diode 56 becomes nonconductive. Of
`function due to insuf?cient voltage supply. The diode
`course, if at any time during the decay portion the sense
`49 may be of a light emitting type to give a visual indica
`signal becomes less than the potential of V REF—, the
`tion of an occurrence of the trickle current. In the event
`output of the differential ampli?er 51 swings toward the
`that the open circuit condition is established, the trickle
`-V potential, thereby terminating the pulse. While the
`current fails to ?ow on the path 9d and hence the volt
`pulse is terminating, a capacitor 59 extends and smooths
`age in the path 9c tends toward the —V level. In this
`the terminating portion of the pulse. If however the
`case the differential ampli?er 31 again switches the
`‘sense signal again rises in a few moments, the pulse is
`FET 41 ON, into the conduction state.
`reinitiated with an amplitude reduced by an amount
`FIGS. 6 and 7 show the extremes of wave shapes
`generally as characterized by said RC value. The resis
`which the control signal in the lead 60 may assume.
`tor 54 discharges the capacitor 53 when the pulse is
`These wave forms are presented in terms of the dy
`tenninated such that after about ?ve or more time con
`namic current characteristics of the circuit as these
`'stants a full amplitude pulse will be generated. This is
`would be sensed across the resistor 40, in FIG. 3. Refer
`illustrated in FIG. 6 and FIG. 7 wherein FIG. 6 shows
`ring to FIG. 6, current is shown on a vertical axis and
`a typical continuous pulse, and FIG. 7, shows an inter
`time is represented on the horizontal axis. Assuming an
`rupted pulse. The control signal at the output 60 is
`inrush of current at the power terminals P9a and P9b,
`the maximum permissible limit rises abruptly to a peak
`therefore a static level as determined by the potential at
`the junction 60!, unless the potential is over-ridden by
`whereafter the limit is reduced exponentially in accor
`
`40
`
`30
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`35
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`7
`dance with the RC time constant previously discussed
`in relation to FIG. 4. If however the current demand at
`the terminals P9a and P9b falls to less than the static
`threshold as determined by the V REF- potential on
`lead 70, then as shown in FIG. 7, the maximum permit
`ted current returns to the normal operating current
`level. If however the apparatus during start up requires
`several inrushes, the maximum permitted current will
`return to a high point of slightly more than the current
`which was permitted just before the envelope returned
`to the normal load current level. This may happen sev
`eral times, as may be peculiar to the particular terminal
`equipment being connected to the line.
`As before mentioned, FIGS. 6 and 7 show the ex
`tremes of operation, however there are various scenar
`ios which are not illustrated here as these will be ob
`servable by a person having constructed and used the
`invention.
`The illustrated embodiment is achieved by a combi
`nation of analog circuit elements. However, it will be
`apparent to persons of typical skill in the electronic arts
`that the functionality of the invention may also be
`achieved by means of a suitably interfaced and pro
`grammed digital controller or microcomputer, a binary
`output of which may be used to control the FET 41 in
`place of the ampli?er 31 and the circuits illustrated in
`the FIGS. 4 and 5.
`We claim:
`1. A line interface circuit, for coupling energizing
`direct current, for operation of a telecommunications
`terminal apparatus, from a line power supply to a com
`munications line, the line interface circuit comprising:
`?rst and second power terminals for connection to
`the line power supply;
`?rst and second line terminals for connection to the
`communication line; and
`a current control means, being connected between a
`one of the ?rst and second power terminals and a
`respective one of the ?rst and second line terminals
`for conducting the energizing direct current there
`between, for temporarily isolating the power ter
`minal from the line terminal to stop an inrush cur
`rent in excess of a dynamic limit, and permanently
`isolating the power terminal from the line terminal
`to stop a load current having been in excess of a
`static limit, until a virtual open circuit condition
`occurs across the ?rst and second line terminals.
`2. A line interface circuit as de?ned in claim 1
`wherein the current control means comprises:
`current sensing means for generating a magnitude
`signal being representative of an amount of said
`energizing direct current being conducted via one
`of the ?rst and second line terminals;
`?rst means for generating a static control signal for
`de?ning the static limit of load current;
`second means for momentarily generating a dynamic
`control signal for momentarily de?ning the dy
`namic limit of the inrush current in response to the
`magnitude signal abruptly increasing to a level
`which exceeds the static limit; and
`switch means being connected in series between the
`power terminal and the line terminal, and being
`responsive to the magnitude signal and the static
`and dynamic control signals,
`to be switched ON to conduct the energizing current
`when either one of said static and dynamic limits is
`
`5,089,927
`8
`greater than an instant value of the magnitude sig
`nal,
`and otherwise to become switched OFF, and in this
`event becoming switched ON, solely in response to
`an apparent open circuit condition at the ?rst and
`second terminals.
`3. A current control means for supplying an energiz
`ing direct current flow, from a source of power via a
`transmission line to a telecommunications terminal ap
`paratus, said terminal apparatus being continuously
`operable while drawing a load current which is ex
`ceeded by an inrush current being greater than the load
`current at a moment of power up, the current control
`apparatus being for connection in series between the
`power source and the transmission line and comprising:
`means for generating a magnitude signal being repre
`sentative of an amount of said energizing direct
`current ?ow;
`means for generating a static control signal for de?n
`ing a maximum limit of load current;
`means for generating a momentary dynamic control
`signal for de?ning a maximum limit of the inrush
`current in response to the magnitude signal increas
`ing from a level representative of less than the
`maximum limit of load current to a level represen
`tative of more than the maximum limit of load
`current;
`switch means responsive to the magnitude signal and
`the static and dynamic control signals, to be set in
`an ON condition for conducting said current flow,
`when either one of said maximum limits is greater
`than the energizing direct current as is instantly
`represented by the magnitude signal, otherwise to
`be reset in an OFF condition, and while in the OFF
`condition being responsive to an apparent open
`circuit condition of the transmission line to become
`set in the ON condition.
`4. A method for supplying an energizing direct cur
`rent flow, from a source of power via a transmission line
`to a terminal apparatus, said terminal apparatus being
`continuously operable while drawing a load current
`which is exceeded by an inrush current being greater
`than the load current at a moment of power up of said
`terminal apparatus, the method comprising the steps of:
`a) generating a magnitude signal being representative
`of an amount of said energizing direct current ?ow;
`b) generating a static control signal for de?ning a
`maximum limit of the load current;
`0) generating a momentary dynamic control signal
`for de?ning a maximum limit of the inrush current
`in response to the magnitude signal increasing from
`a level representative of less than the maximum
`limit of load current to a level representative of
`more than the maximum limit of load current;
`d) in response to the magnitude signal and the static
`and dynamic control signals, providing a path with
`an impedance of less than a ?rst value suitable for
`conducting the energizing current ?ow, when ei
`ther one of said maximum limits is greater than the
`energizing direct current as is instantly represented
`by the magnitude signal, otherwise increasing the
`impedance of said path to a second value greater
`than the ?rst value and unsuitable for conducting
`the energizing current flow; and
`e) while said path is of at least the second impedance
`value and in an event where the transmission line
`appears to be an open circuit, reducing the impe
`dance of the path to less than the ?rst value.
`t I‘
`i i t
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