JPH05260537A - Overvoltage protecting circuit for two-wire subscriber circuit - Google Patents

Abstract

PURPOSE:To prevent a TIP side feeding resistor and a ring side feeding resistor from being broken down, even if an overvoltage is applied in case a commercial power source line touches a subscriber's line, etc. CONSTITUTION:This two-wire subscriber's circuit is provided with photocouplers 19, 20 inserted between an overvoltage protective varistor 9 and a ground, a relay control circuit 22 for receiving outputs of these photocouplers, and protective relays 17, 18 for receiving a control signal from the control circuit 22 and cutting off the circuit at the time when overvoltage is applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-wire subscriber circuit, and more particularly to an overvoltage protection circuit for a two-wire subscriber circuit, which can protect the subscriber circuit from application of overvoltage due to contact of commercial power lines.

[0002]

2. Description of the Related Art FIG. 2 shows a conventionally known two-wire subscriber circuit provided on the exchange side. In this figure, two subscriber lines consist of a tip side line (TIP line) on the ground side and a ring line (RING line) on the power feeding side, and the subscriber terminal 1 connects to the tip line and the ring line via the subscriber line. It is connected.
The tip side ringer relay 2 is connected in series to the tip line, and the ring side ringer relay 3 is connected in series to the ring line. 4 is a ring trip circuit. A series circuit of an NPN transistor 5 and a tip side power feeding resistor 6 is connected between the ground and the tip side ringer relay 2.
A series circuit of a ring side power feeding resistor 7 and a PNP transistor 8 is connected between the ring side ringer relay 3 and the DC power supply V _BB (-48V). Also, tip side ringer relay 2
One end of an overvoltage protection varistor 9 is connected between and the tip side power feeding resistor 6, and the other end of the varistor 9 is grounded.
Further, one end of an overvoltage protection varistor 9 is connected between the ring side ringer relay 3 and the ring side power feeding resistor 7, and the other end of this varistor 9 is grounded. When the connection point between the transistor 5 and the tip side power supply resistor 6 is P1, the overvoltage protection diodes 10 and 10 are connected between the DC power supply V _{BB and} this connection point P1 and between this connection point P1 and the ground. .. When the connection point between the ring side power supply resistor 7 and the transistor 8 is P2, the overvoltage protection diodes 10 and 10 are connected between the DC power supply V _{BB and} this connection point P2 and between this connection point P2 and the ground. To be done. 11
Is a power supply circuit section, and 12 is a subscriber status monitoring section.
The 2-wire / 4-wire conversion unit 13 includes a codec unit (CODEC).
Part) 14 and is connected to the exchange side.

In the two-wire subscriber circuit configured as described above, power is supplied from the power supply circuit section 11 to the subscriber terminal 1 by raising the handset. At this time, the feeding current (loop current) is from the ground to the transistor 5, the tip side feeding resistor 6, the tip side ring relay 2, the tip side line resistor 15, the subscriber terminal 1, the ring side line resistor 16, the ring side ringer relay 3, the ring. It flows through the loop of the side power feeding resistor 7 and the transistor 8. The call signal transmitted from the subscriber terminal 1 is sent to the 2-line / 4-line conversion unit 13 via the 2-line subscriber line, and the conversion unit 13 converts the transmission / reception signals from the 2-line signal. The transmission signal and the reception signal are converted into a separated four-line signal. This 4 which is an analog signal
The line signal is converted into a digital signal by the codec unit 14 and then sent to the exchange side.

If the tip line contacts the commercial power supply line in this subscriber circuit, the contact voltage generated on the tip side is applied to the overvoltage protection varistor 9 via the tip side ringer relay 2, and this varistor 9 is used. It is applied to the internal circuit after being voltage clamped by. The contact voltage clamped by the varistor 9 is clamped to a lower voltage by the overvoltage protection diode 10.

At this time, the tip side power feeding resistor 6 (resistance value is R
_The current I _p flowing in ( _bft ) is given by the following equation, where the clamp voltage by the varistor 9 is V _p1 and the clamp voltage by the diode 10 is V _p2 . I _p = (V _p1 −V _p2 ) / R _bft Here, when the clamp voltage V _p1 of the overvoltage protection varistor 9 is, for example, ₁₄₀ V, the maximum value of the current I _p is V _p1 = 140 V, V _p2 = 0. When the voltage is 0.7V, 2.8
It will be about A. As a result, the electric power withstanding capability of the resistor 6 required is about 400 W. However, it is not economical to mount the resistor 6 that satisfies the power withstanding capability in the subscriber circuit,
Moreover, since a situation in which an overvoltage is applied does not always occur, a resistance of about 2 to 3 W is used in an actual circuit. This is the same for the ring side,
The power supply resistor 7 on the ring side is also of the order of 2 to 3 W.

[0006]

As described above, in the conventional two-wire subscriber circuit, the tip-side power feeding resistor 6 and the ring-side power feeding resistor 7 do not use resistors having a power withstanding capability in consideration of application of overvoltage. , For example, if a situation occurs in which the commercial power supply line is touched and an overvoltage is erroneously applied for a long period of time,
There is a problem that the resistors 6 and 7 are damaged by heat generation.

The present invention has been proposed in order to solve the problems of the prior art as described above. Even if an overvoltage is applied due to contact of a commercial power line with a subscriber line, the tip type An object of the present invention is to provide an overvoltage protection circuit for a two-wire subscriber circuit in which the side power feeding resistor and the ring side power feeding resistor are not thermally destroyed.

[0008]

In order to achieve this object, the present invention provides a two-wire subscriber circuit which is provided on the exchange side and whose subscriber terminals are connected by two subscriber lines. An overvoltage detection circuit for detecting an overvoltage applied to the subscriber line, line disconnection means connected in series to each of the two subscriber lines, and capable of disconnecting the line when the overvoltage is applied, and the overvoltage detection circuit. And a control circuit for controlling the line disconnecting means so as to disconnect the two subscriber lines from the two-line subscriber circuit in response to the overvoltage detection signal of 1. It consists of an overvoltage protection element and a photocoupler that are inserted in series between the two subscriber lines and the ground potential, and the overvoltage detection signal is taken out from this photocoupler.

[0009]

According to the above-mentioned structure, when an overvoltage is applied to a two-line subscriber line by contact with a commercial power supply line, for example, the two-line subscriber line can be disconnected from the two-line type subscriber circuit. Therefore, the power feeding resistance mounted in the subscriber circuit is not destroyed by heat generation due to the application of the overvoltage.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A specific embodiment of an overvoltage protection circuit for a two-wire subscriber circuit according to the present invention will be described in detail below with reference to the drawings. In the description, the same parts as those in the conventional example are designated by the same reference numerals, and part of the description of the overlapping parts will be omitted. The circuit diagram of FIG. 1 shows an embodiment of an overvoltage protection circuit for this two-wire subscriber circuit. In this figure, tip side ringer relay 2
The tip side mixed contact protection relay 17 is inserted in series between the ring side ringer relay 3 and the ring side feed resistance 7, and the ring side mixed contact protection relay 18 is inserted in series between the ring side ringer relay 3 and the ring side feed resistance 7. ing. Here, the contact protection relays 17 and 18 which are respectively inserted in series with the two subscriber lines constitute a line disconnecting means, and are turned off when an overvoltage is applied so that the subscriber line can be disconnected. Is becoming

Further, one end of an overvoltage protection varistor 9 which is an overvoltage protection element is connected between the tip side ringer relay 2 and the contact protection relay 17, and the other end of the varistor 9 is turned on by a bidirectional conduction current. Light-emitting diode 1 of photocouplers 19 and 19 connected in parallel to each other in opposite directions
It is grounded via 9a and 19a. Further, one end of an overvoltage protection varistor 9 is connected between the ring side ringer relay 3 and the contact protection relay 18, and the other end of this varistor 9 is similarly connected in parallel so as to be turned on by a bidirectional conduction current. Light emitting diodes 20a, 2 of the coupler 20, 20
It is grounded through 0a. Here, the varistor 9 and the photocoupler 19 and the varistor 9 and the photocoupler 20 which are respectively inserted in series between the subscriber line and the ground potential constitute an overvoltage detection circuit.

A DC power supply _Vcc is connected to the collectors of the phototransistors 19b and 20b of the photocouplers 19 and 20 through an output resistor 21, and the emitters of the photocouplers 19b and 20b are grounded. Output resistor 21 and phototransistor 19b,
The connection points of 20b are respectively connected to the control input terminals of the relay control circuit 22. The control signal output from the relay control circuit 22 drives the mixed contact protection relays 17 and 18.

In this two-wire subscriber circuit, power is supplied from the power supply circuit section 11 to the subscriber terminal 1 by raising the receiver. At this time, the power supply current (loop current) is from the ground to the transistor 5, the tip side power supply resistor 6, the tip side contact protection relay 17, the tip side ring relay 2, the tip side line resistor 15, the subscriber terminal 1, the ring side line resistor 16. , The ring side ringer relay 3, the ring side mixed contact protection relay 18, the ring side power feeding resistor 7, and the transistor 8 flow. The call signal transmitted from the subscriber terminal 1 passes through the two-line subscriber line and the two-line / four-line conversion unit 13
Sent to. The 2-wire / 4-wire converter 13 converts a 2-wire signal in which a transmission / reception signal is mixed into a 4-wire signal in which a transmission signal and a reception signal are separated. This 4-wire analog signal is converted to a digital signal by the codec unit 14 and then sent to the exchange side.

Next, the operation of the overvoltage protection circuit for the two-wire subscriber circuit configured as described above will be described. If the tip line contacts the commercial power supply line, the contact voltage generated on the tip side is applied to the overvoltage protection varistor 9 via the tip side ringer relay 2. When this contact voltage exceeds the clamp voltage of the varistor 9, the photocoupler 19 (one or both) is turned on by the protection current, and the overvoltage protection information (overvoltage detection signal) is sent to the relay control circuit 22 via the output resistor 21. .. As a result, the control circuit 22
The tip-side mixed contact protection relay 17 is turned off by the control signal from and the line is disconnected, so that thermal destruction of the tip-side power feeding resistor 6 due to the application of overvoltage can be prevented.

Also, in the case where the commercial power supply line touches the ring line, the overvoltage protection varistor 9 turns on when the touching voltage exceeds the clamp voltage, and the photocoupler 20 turns on by the protection current flowing through the varistor 9. .. As a result, the overvoltage protection information is input to the relay control circuit 22 via the output resistor 21, and the ring-side mixed contact protection relay 18 is turned off by the control signal from the control circuit 22. Since the line is disconnected by this operation, it is possible to prevent thermal destruction of the ring side power supply resistor 7 due to the application of the overvoltage.

When the contact of the commercial power supply line is restored and the application of the overvoltage disappears, the overvoltage protection varistor 9 is turned off and the transmission of the overvoltage protection information to the relay control circuit 22 is released, so that the contact protection relays 17 and 18 are released. Is reconnected and the subscriber line is automatically restored.

The present invention is not limited to the above-mentioned embodiments, but various modifications can be made within the scope of the invention.

[0018]

As described above, according to the present invention, when an overvoltage is applied to the subscriber line due to the contact of the commercial power source line, the subscriber line is provided on the exchange side. Therefore, even if an overvoltage is applied to the subscriber line for a long period of time, there is no problem that the power feeding resistor mounted in the subscriber circuit is destroyed by heat generation or the like. As a result, there is an advantage that it is not necessary to use a power feeding resistor having an electric power withstanding capability in consideration of application of an overvoltage, and a power feeding resistor having a normal capacity can be mounted in a subscriber circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an overvoltage protection circuit for a two-wire subscriber circuit according to the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional two-wire subscriber circuit.

[Explanation of symbols] 1 Subscriber terminal 2 Tip side ringer relay 3 Ring side ringer relay 4 Ring trip circuit 5,8 Transistor 6 Tip side power supply resistor 7 Ring side power supply resistor 9 Overvoltage protection varistor 10 Overvoltage protection diode 11 Power supply circuit section 12 Join Person status monitoring unit 13 2-wire / 4-wire conversion unit 14 Codec unit 15 Tip side line resistance 16 Ring side line resistance 17 Tip side mixed contact protection relay 18 Ring side mixed contact protection relay 19, 20 Photo coupler 21 Output resistance 22 Relay control circuit

Claims (2)

[Claims] 1. A two-wire subscriber circuit, which is provided on the exchange side and has subscriber terminals connected by two subscriber lines, detects overvoltage applied to the two subscriber lines. The circuit and the line disconnecting means that are respectively connected in series to the two-line subscriber line and can disconnect the line when an overvoltage is applied, and the two-line subscriber line are connected to the two-line subscriber line in response to the overvoltage detection signal from the overvoltage detection circuit. An overvoltage protection circuit for a two-wire subscriber circuit, comprising: a control circuit for controlling the line disconnecting means so as to be disconnected from the two-wire subscriber circuit. 2. The overvoltage detection circuit comprises an overvoltage protection element and a photocoupler inserted in series between two subscriber lines and a ground potential, and an overvoltage detection signal is taken out from the photocoupler. 2 according to claim 1.
Overvoltage protection circuit for wire subscriber circuits.