(19) Japan Patent Office (JP)
`
`(12) Japanese Unexamined Patent
`Application Publication (A)
`
`
`
`
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`(11) Japanese Unexamined Patent
`Application Publication Number
`H10-13576
`(43) Publication date January 16, 1998
`
`Technical indications
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` FI
`H 04 M 19/08
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`Identification codes
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`JPO file numbers
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`(51) Int. Cl.6
`H 04 M 19/08
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`(21) Application number
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`(22) Date of application
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`Request for examination Not yet requested Number of claims 9 OL (Total of 11 pages)
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`Japanese Patent
`Application
`H08-159323
`June 20, 1996
`
`
`
`(71) Applicant
`
`(72) Inventor
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`
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`(74) Agent
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`000005223
`Fujitsu Ltd.
`4-1-1 Kamikodanaka, Nakabara-ku, Kawasaki-shi,
`Kanagawa-ken
`
`Hideki MATSUNO
`℅ Fujitsu Ltd.
`4-1-1 Kamikodanaka, Nakabara-ku, Kawasaki-shi,
`Kanagawa-ken
`Patent Attorney Shoji KASHIWAYA (and 2 others)
`
`
`Explanatory diagram describing the
`principle of the present invention.
`
`Power supply
`voltage
`switching part
`
`Loop
`detection
`part
`
`U point
`
`S/T point
`
`Power stop-
`page detection
`
`part
`
`Power
`supply
`voltage
`switching
`part
`
`Loop
`detection
`
`part
`
`
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`
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`(54) (TITLE OF THE INVENTION) Power Supply Circuit
`
`(57) (ABSTRACT)
`(PROBLEM) To provide a power supply circuit that switches power
`supply voltage and supplies the desired power while ensuring safety.
`(MEANS FOR SOLVING) A loop detection part 4 is provided that
`detects a DC loop based on current that flows to a constant-current circuit,
`the voltage at two terminals of the constant-current circuit, or the line
`voltage of the digital subscriber line, when local power supply to a
`network terminal device 2 to which a subscriber terminal 3 is connected
`is stopped, a contact breaker point 8 is turned ON by a stoppage detection
`part 10, and a DC loop is formed via a phantom power supply part 9
`on the side of the power supply circuit 1; and a power supply voltage
`switching part 5 is provided that supplies power to a digital subscriber line
`12 at a low voltage V2 of −48 V during local power supply to the network
`terminal device 2 and switches to a high voltage V1 of −120 V which is
`supplied to the digital subscriber line 12 when the DC loop formed due
`to stoppage of local power supply is detected by the loop detection part 4.
`
`SH-1004
`
`

`
`
`(SCOPE OF PATENT CLAIMS)
`(CLAIM 1) A power supply circuit whereby station power sup-
`ply is carried out via a digital subscriber line when local power
`supply is stopped to a network terminal device to which a sub-
`scriber terminal is connected;
`said power supply circuit characterized in that it is provided
`with a loop detection part that supplies low-voltage power
`supply to said digital subscriber line and detects a direct cur-
`rent loop formed by stoppage of local power supply to the
`network terminal device; and
`a power supply voltage switching part that switches to high
`voltage power supply when a direct current loop of said net-
`work terminal device is detected by said loop detection part.
`(CLAIM 2) The power supply circuit according to Claim 1,
`characterized in that said loop detection part is constituted by a
`current detection part that performs low voltage power supply
`to said digital subscriber line and detects current running in a
`direct current loop formed upon stoppage of the local power
`supply of said network terminal device.
`(CLAIM 3) The power supply circuit according to Claim 1,
`wherein said loop detection part has a configuration whereby
`low-voltage power is supplied via a constant-current circuit to
`said digital subscriber line that is connected to said network
`terminal device and detects a direct current loop formed by
`stoppage of the local power supply of said network terminal
`device due to the current flowing in said constant-current cir-
`cuit.
`(CLAIM 4) The power supply circuit according to Claim 1,
`characterized in that said loop detection part has a configura-
`tion whereby low-voltage power is supplied via a constant-
`current circuit to said digital subscriber line that is connected
`to said network terminal device, and detects a direct current
`loop formed by stoppage of the local power supply of said
`network terminal device due to the voltage between the two
`terminals of said constant-current circuit.
`(CLAIM 5) The power supply circuit according to Claim 1,
`characterized in that said loop detection part has a configura-
`tion whereby low-voltage power is supplied via a constant-
`current circuit to said digital subscriber line that is connected
`to said network terminal device, and detects a direct current
`loop formed by stoppage of the local power supply of said
`network terminal device by detecting a drop in line voltage of
`said digital subscriber line.
`(CLAIM 6) The power supply circuit according to any of
`Claims 1 to 5, characterized by having first and second current
`detection parts that detect current flowing respectively in the
`tip line and ring line of said digital subscriber line and a power
`supply voltage switching part that detects the difference in
`currents detected by said first and second current detection
`parts, determines that a ground or contact fault has occurred
`when said difference exceeds a set value, and switches the
`power supply voltage that supplies power to said digital sub-
`scriber line to a low voltage.
`(CLAIM 7) The power supply circuit according to any of
`Claims 1 to 6, characterized by having first and second current
`detection parts that detect current flowing respectively in the
`tip line and ring line of said digital subscriber line and a power
`supply voltage switching part that detects the difference in
`currents detected by said first and second current detection
`parts, switches the power supply voltage supplied to said digi-
`tal subscriber line to low voltage when said difference exceeds
`a first set value, and blocks the power supply voltage to said
`digital subscriber line when the difference in currents detected
`
`
`
`Japanese Unexamined Patent Application Publication H10-13576
`(2)
`
`by said first and second current detection parts exceeds a sec-
`ond set value when switched to said low voltage.
`(CLAIM 8) The power supply circuit according to any of
`Claims 2 to 5, characterized by having first and second con-
`stant-current circuits that supply constant current respectively
`to the tip line and ring line of said digital subscriber line, first
`and second voltage detection parts that detect the voltages at
`the two terminals of said first and second constant-current
`circuits, and a power supply voltage switching part that detects
`the difference in detected voltages from said first and second
`voltage detection parts, determines that a ground or contact
`fault has occurred when said difference exceeds a set value,
`and switches the power supply voltage supplied to said digital
`subscriber line to a low voltage.
`(CLAIM 9) The power supply circuit according to any of
`Claims 2 to 5 or 8, characterized by having first and second
`constant-current circuits that supply constant current respec-
`tively to the tip line and ring line of said digital subscriber line,
`first and second voltage detection parts that detect the voltages
`at the two terminals of said first and second constant-current
`circuits, and a supply voltage switching part that detects the
`difference in detected voltages from said first and second volt-
`age detection parts, switches the voltage supplied to said digi-
`tal subscriber line to a low voltage when said difference ex-
`ceeds a first set value, and blocks the voltage supplied to said
`digital subscriber line when the difference in the voltages de-
`tected by said first and second voltage detection parts exceeds
`a second set value when switched to said low voltage.
`(DETAILED DESCRIPTION OF THE INVENTION)
`(0001)
`(Technological Field of the Invention) The present invention
`relates to a power supply circuit that switches the power supply
`voltage. Subscriber terminals that are connected to digital sub-
`scriber lines receive power supply from commercial power
`sources in the homes of subscribers, and systems are known in
`which station power supply is carried out to allow minimal
`restricted communication during power outages. With these
`types of power supply systems, it is necessary to ensure safety.
`(0002)
`(Prior Art) Fig. 11 is an explanatory diagram of a conventional
`example, wherein 101 is a power supply circuit for a switching
`station, 102 is a network terminal device (NT1) (or digital
`service unit (DSU)), 103 is a subscriber terminal (DTE), 104 is
`a digital subscriber line consisting of a tip (TIP) line and ring
`(RING) line, 105 is a power supply source that allows constant
`current power supply, 106 is a transformer, 107 is a capacitor,
`108 is a transformer, 109 is a capacitor, 110 is a full wave
`rectification circuit, 111 is a commercial alternating current
`power source (AC100 V), and 112 is a phantom power supply
`part. The U and S/T points represent reference points in the
`ISDN.
`(0003) The transformers 106 and 108 are transformers used for
`communication, and the capacitors 107 and 109 that are linked
`between the coils are used for blocking direct current. The
`power supply circuit 101 of each switching station has a power
`supply power source 105 and supplies power to the network
`terminal device 102 via the digital subscriber line 104. The
`power supply voltage mediated by the coil of the transformer
`108 and the digital subscriber line 104 is added to the phantom
`power supply part 112 via the full wave rectification circuit
`110, and the power supply from the power supply circuit 101
`serves as the station power supply that is supplied from the
`phantom power supply part 112 to the subscriber terminal 103
`when the commercial AC power source 111 is shut down.
`
`
`
`

`
`(0004) When the commercial AC power source 111 is func-
`tioning normally, for example, an AC current of 100 V is recti-
`fied in the phantom power supply part 112 and is converted to
`a prescribed voltage, for example, a DC voltage of 40 V, for
`use as the local power supply that is supplied to the subscriber
`terminal 103. Switching to the aforementioned station power
`supply occurs with shutdown of the commercial AC power
`supply, and power sufficient to allow minimal communication
`on the digital subscriber terminal 103 is thus supplied.
`(0005)
`(Problems to be Solved by the Invention) The voltage of sta-
`tion power supply for analog subscriber lines is generally −48
`V. However, in regard to the voltage for a station power supply
`for a digital subscriber line 104, in order to provide the pre-
`scribed power to the subscriber terminal 103, for example, the
`line voltage is taken to be about 120 V for the power supply
`power source 105 of the power supply circuit 101. In addition,
`because the digital subscriber line 104 runs into the home of
`the consumer, it is desirable to ensure safety by decreasing the
`line voltage of the digital subscriber line 104 in the home of
`the subscriber.
`(0006) During station power supply, the line impedance of the
`digital subscriber line 104 in the network terminal device 102
`becomes small, and the line voltage is sufficiently reduced.
`However, during local power supply, the line impedance of the
`digital subscriber line 104 is large, and thus the line voltage is,
`for example, 85 to 105 V. This type of voltage has been prob-
`lematic in terms of safety when applied as the line voltage for
`the digital subscriber line 104 that runs into the homes of sub-
`scribers. An object of the present invention is to supply a pre-
`scribed power level while maintaining safety by applying a low
`voltage during local power supply and a high voltage during
`station power supply.
`(0007)
`(Means for Solving the Problems) The power supply circuit of
`the present invention is described in reference to Fig. 1. (1)
`When the local power supply to the network terminal device 2
`to which the digital subscriber line 3 is connected is shut down,
`low-voltage power is supplied to the digital subscriber line 12
`in the power supply circuit whereby station power supply oc-
`curs via the digital subscriber line 12, and a loop detection part
`4 is provided that detects the DC loop formed by stoppage of
`local power supply for the network terminal device, and a
`power supply voltage switching part 5 is provided that
`switches to a high-voltage power supply when the DC loop of
`the network terminal device 2 is detected by the loop detection
`part 4. For example, the power supply voltage switching part 5
`is switched to V2 = −48 V and power is supplied during local
`power supply, whereas switching to V1 = −120 V occurs and
`power is supplied during stoppage of the commercial AC
`power source.
`(0008) (2) A configuration may be used in which the loop
`detection part 4 supplies low-voltage power to the digital sub-
`scriber line 12, and is constituted by a current detection part
`that detects current flowing to the DC loop formed as a result
`of stoppage of the local power supply of the network terminal
`device 2. The network terminal device 2 detects stoppage of
`power from the commercial AC power supply 11 via the power
`stoppage detection part 10, the contact breaker point 8 is turned
`ON, and a DC loop is formed via the phantom power supply
`part 9. Consequently, the DC loop is detected by the loop de-
`tection part 4 of the power supply circuit 1 that supplies low-
`voltage power to the digital subscriber line 12, and the supply
`is switched to high-voltage power supply by the power supply
`voltage switching part 5.
`
`
`
`Japanese Unexamined Patent Application Publication H10-13576
`(3)
`
`(0009) (3) A configuration may also be used in which the loop
`detection part 4 supplies low-voltage power via the constant-
`current circuit to the digital subscriber line 12 connected to the
`network terminal device 2, and the DC loop formed due to
`stoppage of local power supply of the network terminal device
`2 is detected based on the current flowing to the constant-
`current circuit.
`(0010) (4) A configuration may also be used in which the loop
`detection part 4 supplies low-voltage power via a constant-
`current circuit to the digital subscriber line 12 connected to the
`network terminal device 2, and the DC loop formed due to
`stoppage of the local power supply of the network terminal
`device 2 is detected based on the voltage at the two terminals
`of the constant-current circuit.
`(0011) (5) A configuration may also be used in which the loop
`detection part 4 supplies low-voltage power via a constant-
`current circuit to the digital subscriber line 12 connected to the
`network terminal device 2, and the DC loop formed by stop-
`page of the local power supply of the network terminal device
`2 is detected based on the decrease in line voltage of the digital
`subscriber line 12.
`(0012) (6) First and second current detection parts may be
`provided that detect respective currents flowing in the RING
`line and TIP line of the digital subscriber line 12, and a power
`supply voltage switching part 5 may be provided that detects
`the difference in detected current in the first and second current
`detection parts, determines that there has been a ground or
`contact fault when this differential exceeds a set value, and
`switches the power supply voltage supplied to the digital sub-
`scriber line 12 to a low voltage.
`(0013) (7) First and second current detection parts may be
`provided that detect respective currents flowing in the RING
`line and TIP line of the digital subscriber line 12, and a power
`supply voltage switching part 5 may be provided that detects
`the difference in detected current in the first and second current
`detection parts, switches the power supply voltage supplied to
`the digital subscriber line 12 to low voltage when the differ-
`ence exceeds a first set value, and blocks the power supply
`voltage of the digital subscriber line 12 when the difference
`between the first and second current detection parts exceeds a
`second set value when switched to this low voltage.
`(0014) (8) First and second constant-current circuits may be
`provided that supply respective constant currents to the RING
`line and TIP line of the digital subscriber line 12; first and
`second voltage detection parts may be provided that detect
`voltages at the two terminals of the first and second constant-
`current circuits; and a power supply voltage switching part
`may be provided that detects the difference in detected voltage
`in the first and second voltage detection parts, determines that
`a ground or contact fault has occurred when this difference
`exceeds a set value, and switches the power supply supplied to
`the digital subscriber line 12 to a low voltage.
`(0015) (9) First and second constant-current circuits may be
`provided that supply constant current respectively to the TIP
`line and RING line of the digital subscriber line 12; first and
`second voltage detection parts may be provided that detect the
`voltages at the two terminals of said first and second constant-
`current circuits; and a supply voltage switching part may be
`provided that detects the difference in detected voltages from
`said first and second voltage detection parts, switches the volt-
`age supplied to said digital subscriber line 12 to a low voltage
`when said difference exceeds a first set value, and blocks the
`voltage supplied to said digital subscriber line 12 when the
`difference in the voltages detected by said first and second
`voltage detection parts exceeds a second set value when
`switched to the low voltage.
`
`
`
`

`
`(0016)
`(Embodiments of the Invention) Fig. 1 is an explanatory dia-
`gram that describes the principle of the present invention. 1
`denotes a power supply circuit for a switching station, 2 de-
`notes a network terminal device (NT1), 3 denotes a subscriber
`terminal (DTE), 4 denotes a loop detection part, 5 denotes a
`power supply voltage switching part, 6 denotes a communica-
`tions transformer, 7 denotes a capacitor for DC blocking, 8
`denotes a contact breaker point for forming a DC loop, 9 de-
`notes a phantom power supply part, 10 denotes a power stop-
`page detection part, 11 denotes a commercial AC power sour-
`ce, 12 denotes a digital subscriber line, and 13 denotes a zener
`diode used for protection.
`(0017) The network terminal device 2 contains transformers
`and the like for communication, as with conventional exam-
`ples, but these are not included in the diagram. In addition,
`power is supplied to the subscriber terminal 3 from a phantom
`power supply part 9. In addition, with the contact breaker point
`8 ON, a DC loop is formed with the power supply circuit 1 via
`the inner part of the phantom power supply part 9 when power
`stoppage of the commercial AC power source 11 has been
`detected by the power stoppage detection part 10. A DC circuit
`is shown based on resistance and inductance for the interior of
`the phantom power supply part 9, and the zener diode 12 is
`used for protection from over-voltage.
`(0018) With the power supply circuit 1, a power supply voltage
`switching part 5 that switches the high-voltage V1 of −120 V
`and low voltage V2 of −48 V and a loop detection part 4 that
`detects the DC loop of the network terminal device 2 are pro-
`vided, and the power supply voltage switching part 5 supplies
`power to the digital subscriber line 12 with the low voltage V2
`supplied at −48 V when no DC loop is detected from the loop
`detection part 4, specifically, when local power supply is oc-
`curring. Consequently, when leak current or the like is negligi-
`ble in the digital subscriber line 12 that runs into the home of
`the subscriber, a 48 V line voltage or voltage to ground is pro-
`duced, and thus safety can be ensured.
`(0019) When the contact breaker point 8 is turned ON due to
`detection of stoppage of the commercial AC power supply 11,
`specifically, due to stoppage of the local power supply to the
`network terminal device 2, a DC loop is formed, and current is
`supplied to the digital subscriber line 12 that is being supplied
`with low-voltage power V2 of −48 V. This is detected by the
`loop detection part 4, and the power supply voltage switching
`part 5 is controlled and is switched to the high voltage power
`supply V1 of −120 V. By this means, it is possible to supply
`the desired current from the power supply circuit 1 of the
`switching station to the network terminal device 2.
`(0020) Consequently, when local power supply is occurring,
`low voltage is supplied to the digital subscriber line 12, and
`thus safety can be improved. In addition, when local power
`supply is stopped, high-voltage is supplied to the digital sub-
`scriber line 12, thereby allowing the desired power to be sup-
`plied from the station. In addition, the voltage to ground or line
`voltage of the digital subscriber line 12 that leads into the
`home of the subscriber in this case drops to less than 80 V due
`to the decrease in voltage of the digital subscriber line 12 re-
`sulting from the power supply current.
`(0021) Fig. 2 is an essential explanatory view of a first em-
`bodiment of the present invention. The same symbols as in Fig.
`1 denote the same parts. 21a and 21b denote constant-current
`circuits, 22a and 22b denote current detection parts, and 23a
`and 23b denote contact breaker points. Diagrams of the power
`
`
`Japanese Unexamined Patent Application Publication H10-13576
`(4)
`
`stoppage detection part or the communication trunk of the
`network terminal device 2 and other such components are not
`presented. A situation similar to that shown in Fig. 1 occurs at
`the point when a DC loop is formed with respect to the power
`supply circuit 1, with the contact breaker point 8 ON resulting
`from detection of a local power supply stoppage due to stop-
`page of the commercial AC power supply 11.
`(0022) The constant-current circuits 21a and 21b of the power
`supply circuit 1 have a configuration whereby control occurs
`so that a constant current is supplied during station power
`supply to the network terminal device 2 via the digital sub-
`scriber line 12. The constant current supply can utilize various
`configurations that are already known. The current detection
`parts 22a and 22b supply low voltage power of V2 = −48 V to
`the digital subscriber line 12, and the loop current flowing at
`the time of DC loop formation of the network terminal device
`2 is detected. The contact breaker points 23a and 23b turn ON,
`and the voltage is switched from low voltage power supply to
`high voltage power supply.
`(0023) When the commercial AC power supply 11 is restored,
`local power supply is restarted, the contact breaker point 8 is
`turned ON, and current flows to the constant-current circuits
`21a, 21b of the power supply circuit 1. Consequently, the con-
`tact breaker points 23a, 23b turn OFF, the high voltage power
`supply is switched to low voltage power supply, and the cur-
`rent detection parts 22a and 22b return to their initial states.
`Specifically, the current detection parts 22a, 22b correspond to
`the loop detection part 4 in Fig. 1, and the constant-current
`circuits 21a, 21b and the contact breaker points 23a, 23b corre-
`spond to the power supply voltage switching part 5 of Fig. 1.
`The contact breaker points 8, 23a and 23b can be configured
`with FETs and other transistors, relays, photocouplers, and the
`like.
`(0024) Fig. 3 is a circuit diagram showing the essential parts of
`the first embodiment of the present invention, and shows con-
`stant-current circuits 21a, 21b, the current detection parts 22a,
`22b and the contact breaker points 23a, 23b of Fig. 2 as cir-
`cuits. 24a and 24b denote transistors, 25a and 25b denote op-
`erational amplifiers, 26a and 26b denote photocouplers, 27a
`and 27b denote phototransistors, 28a and 28b denote light-
`emitting diodes, and R1 to R7 denote resistors.
`(0025) The constant-current circuits 21a and 21b in Fig. 2 are
`composed of transistors 24a, 24b and operational amplifiers
`25a, 25b. The current detection parts 22a, 22b are composed of
`resistors R2, R6 and light-emitting diodes 28a, 28b. The con-
`tact breaker points 23a, 23b are composed of phototransistors
`27a, 27b of photocouplers 26a, 26b.
`(0026) When the commercial AC power supply 11 is operating
`normally, and the local power supply is being supplied to the
`network terminal device 2, the contact breaker point 8 is OFF,
`and low voltage V2 of −48 V is applied to the digital subscriber
`line 12 via the resistors R2, R6 and the light-emitting diodes
`28a, 28b of the photocouplers 26a, 26b. However, no current
`flows because a DC loop is not formed in the network terminal
`device 2, and the light-emitting diodes 28a and 28b do not
`light. Consequently, the phototransistors 27a and 27b are in an
`OFF state. Specifically, contact breaker points 23a and 23b in
`Fig. 2 are in an OFF state, and low voltage is supplied to the
`digital subscriber line 12. The voltage to ground or the line
`voltage of the digital subscriber line 12 that runs into the home
`of the subscriber is thus at approximately 48 V, allowing safety
`to be ensured.
`
`
`
`

`
`(0027) When the commercial AC power source 11 is stopped
`and the local power supply is stopped, the contact breaker
`point 8 turns ON and a DC loop is formed via the phantom
`power supply part 9. Current thus flows to the light-emitting
`diodes 28a and 28b of the photocouplers 26a and 26b, the
`diodes light, and the phototransistors 27a and 27b are thus
`turned ON. Consequently, high voltage V1 of −120 V is sup-
`plied to the digital subscriber line 12, and the current of the
`station power supply in this case is controlled at a constant
`current by the constant-current circuit consisting of the transis-
`tors 24a, 24b and the operational amplifiers 25a, 25b. In this
`case, the voltage to ground or the line voltage of the digital
`subscriber line 12 that runs into the home of the subscriber can
`be decreased so that it is less than 80 V, including the voltage
`drop of the digital subscriber line 12 and the voltage drop of
`the transistors 24a, 24b (constant-current circuit).
` (0028) In addition, a series circuit is formed with the resistor
`R2 and light-emitting diode 28a parallel to the series circuit of
`the transistor 24a, the resistor R1 and the phototransistor 27a.
`Similarly, a series circuit formed from the resistor R6 and the
`light-emitting diode 28b is connected in parallel with the series
`circuit of the transistor 24b, the resistor R5 and the phototran-
`sistor 27b. Thus, when a DC loop is formed during supply of
`high-voltage power, a configuration can be produced where
`current flows continually to the light-emitting diodes 28a, 28b,
`so that the ON condition of the phototransistors 27a, 27b is
`maintained.
`(0029) When stoppage of the commercial AC power supply 11
`is restored and local power supply is reinitiated, the contact
`breaker point 8 turns OFF and current to the digital subscriber
`line 12 decreases to zero. Consequently, emission of the light-
`emitting diodes 28a, 28b of the photocouplers 26a, 26b stops,
`and the phototransistors 27a, 27b assume an OFF condition.
`Switching is thus carried out to change the power supply volt-
`age from a high voltage V1 of −120 V to a low voltage V2 of
`−48 V.
`(0030) Fig. 4 is an explanatory diagram of the essential com-
`ponents of a second embodiment of the present invention. In
`the embodiment of Fig. 2, a case is presented that involves a
`configuration in which loop current flowing to both the TIP
`line and RING line of the digital subscriber line 12 is detected,
`and the DC loop of the network terminal device 2 can be de-
`tected based on detection of current flowing only to the RING
`line.
`(0031) Consequently, this corresponds to a configuration for
`the second embodiment in which the current detection part 22b
`and the contact breaker point 23b are omitted in the first em-
`bodiment. During local power supply to the network terminal
`device 2, the contact breaker point 23a is switched to a low
`voltage V2 of −48 V, and low voltage is supplied to the digital
`subscriber line 12 via the constant-current circuits 21a, 21b,
`but the contact breaker point 8 of the network terminal device
`2 is OFF, so current does not flow to the constant-current cir-
`cuits 21a, 21b.
`(0032) The current detection part 22a is controlled so that the
`contact breaker point 23a is switched to the high voltage V1 of
`
`
`Japanese Unexamined Patent Application Publication H10-13576
`(5)
`
`−120 V when current flowing to the constant-current circuit
`21a is detected, and current does not flow to the digital sub-
`scriber line 12 during local current supply, so a low voltage
`power supply condition is produced. Next, upon shut-down of
`local power supply, the contact breaker point 8 is turned ON,
`and a DC loop is formed, so that current flows to the digital
`subscriber line 12 via the constant-current circuits 21a, 21b.
`When this current is detected by the current detection part 22a,
`the contact breaker point 23a is switched to the high voltage V1
`of −120 V. High voltage is thus applied to the digital sub-
`scriber line 12, and the desired power is supplied to the net-
`work terminal device 2.
`(0033) Fig. 5 is an explanatory diagram of the essential com-
`ponents of a third embodiment of the present invention. The
`same symbols in Fig. 2 and Fig. 4 denote the same parts. 31a
`and 31b are voltage detectors and 32 is a contact breaker point.
`In this embodiment, the voltages at both terminals of the con-
`stant-current circuits 21a and 21b are detected by the voltage
`detection parts 31a and 31b, the DC loop of the network termi-
`nal device 2 is detected, and thus the contact breaker point 8 is
`OFF during local power supply of the network terminal device
`2. Thus, a DC loop is not formed from the side of the power
`supply circuit 1, and current does not flow to the digital sub-
`scriber line 12 via the constant-current circuits 21a, 21b. The
`voltages at both terminals of the constant-current circuits 21a,
`21b are thus 0 or values close to zero.
`(0034) Consequently, in this state, the contact breaker point 32
`is switched to the low voltage V2 of −48 V, and low voltage
`power supply is supplied to the digital subscriber line 12. The
`contact breaker point 8 is thus turned ON as a result of shut-
`down of local power supply to the network terminal device 2,
`and a DC loop is formed from the standpoint of the power
`supply circuit 1. Consequently, current flows to the digital
`subscriber line 12 via the constant-current circuit 21a, 21b, and
`a voltage is seen on both terminals of the constant-current
`circuits 21a, 21b.
`(0035) When the voltage is at or above a set value, the voltage
`detection parts 31a, 31b detect loops, the contact breaker point
`32 is switched to the high voltage V1 of −120 V, and supply of
`high voltage to the digital subscriber line 12 is performed. As a
`result, the power supply voltage of the digital subscriber line
`12 during local power supply is low, which improves safety,
`whereas the power supply voltage is high during local power
`supply stoppage, which allows the desired power to be sup-
`plied.
`(0036) The contact breaker point 8 turns OFF as a result of
`restarting of the local power supply, and, after the current that
`flows to the digital subscriber line 12 goes to zero, the voltage
`at the two terminals of the constant-current circuits 21a, 21b
`becomes zero or a value close to zero. Consequently, the volt-
`age detection parts 31a, 31b control the contact breaker point
`32 so that it is switched from the high voltage V1 of −120 V to
`the low voltage V2 of −48 V.
`(0037) Fig. 6 is an explanatory diagram of the essential com-
`ponents of a fourth embodiment of the present invention. The
`same symbols as in Fig. 5 denote the same parts. 34 denotes a
`
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`

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`voltage detection part. In this embodiment, the line voltage
`between the RING line and the TIP line of the digital sub-
`scriber line is detected by the voltage detection part 34, the
`contact breaker point 8 turns ON when the local power supply
`of the network terminal device 2 is stopped, and the resulting
`DC loop is detected. The voltage detection part 34 switches
`between the voltage detection standard during low voltage
`power supply V2 of −48 V and the voltage detection standard
`during high voltage power supply V1 of −120 V. A configura-
`tion is thus produced in which the presence or absence of the
`DC loop of the network terminal device 2 is detected when low
`voltage is supplied and when high voltage is supplied.
`(0038) The contact