JPS63164137A - Digital subscriber's line protecting circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Summary] The present invention provides a digital subscriber line transmission device in which the subscriber An external DC current detection circuit and a corresponding switch are inserted on the line to prevent external DC current from entering the signal transmission/reception circuit, and to automatically return to the normal state when the external DC power supply is disconnected. be.

[Industrial application field]

The present invention relates to a subscriber line protection circuit, and more particularly to a two-wire balanced digital subscriber line protection circuit for bipolar signals supplied to a signal transmitting/receiving circuit.

Subscriber lines include conventional analog telephone lines and digital lines. In the former case, a DC voltage is always applied, but in the latter case, a bipolar line is used to eliminate signal deterioration due to DC components. In other words, alternating current digital signals are used, and these lines are housed within the same cable.

In such a situation, there may be cases where lines are connected incorrectly during subscriber line connection work, or external DC voltage is incorrectly applied. In such a state, it is necessary to protect the elements in the signal transmitting/receiving circuit without destroying them. Further, the subscriber line is a two-wire balanced signal line, and if it is to be protected by a circuit, it must be protected in a manner insulated from ground potential.

[Prior Art] A schematic diagram of a conventionally known subscriber line protection circuit is shown in FIG. A subscriber line protector 32 is inserted and connected between the subscriber line 30 and the subscriber line protector 32 . This protector 32 includes an arrester 33 and a fuse 3 that suppress voltage.
It consists of 4. In this case, since the protector 32 generally functions as a protection device against lightning surges on the subscriber line, the voltage applied to the arrester 33 is relatively high.

Furthermore, the fuse 34 also has a high current capacity.

On the other hand, on the terminal device 31 side, a varistor 36 is generally provided to protect the signal transmission/reception circuit 35 within the device 31 from voltages that cannot be suppressed by a protector.

Furthermore, with respect to current flowing from an external power source (not shown), the device is protected by blowing out the fuse 34 of the protector 32. Therefore, for some reason fuse 3
When fuse 4 blows, this is dealt with by replacing the fuse 34 with a new one.

[Problem that the invention seeks to solve]

In such a conventional subscriber line protection circuit, the subscriber line output in the terminal device 31 and the subscriber line protector are usually connected by wiring.

Therefore, for example, if there is an incorrect connection with another wire during the connection work at point A shown in the figure, the varistor 36 installed in the terminal device 31 will operate, but this varistor 36 will not respond to the instantaneous voltage. Since it operates to suppress the voltage, if the abnormal voltage is constantly applied, an excessive current will flow, which may cause thermal breakdown. This is because the varistor 36 is selected on the condition that the fuse 34 of the protector will melt under such constant voltage application.

Also, if a voltage lower than the voltage at which the varistor 36 operates (if the external power supply is a constant current power supply) is applied, the varistor 36
6 does not operate, and current flows into the terminal device 31 side.

The same can be said about point B. However, in this case, since the fuse 34 is blown, protection can be achieved by designing the terminal device 31 in consideration of the capacity of the fuse 34. However, in this case, it becomes necessary to replace the fuse 34 each time.

In order to reliably protect the terminal device from such problems, it is sufficient to use circuits and elements that can withstand the operating voltage and current of the protector, but in this case,
The circuit or component becomes large-scale.

Therefore, an object of the present invention is to realize a circuit that protects a signal transmitting/receiving circuit from unnecessary direct current flowing through a subscriber line without having to replace the used elements depending on any situation.

[Means for solving problems]

Figure 1 shows a signal transmitting/receiving circuit l to achieve the above purpose.
This figure shows a conceptual diagram of a protection circuit according to the present invention for a 2's balanced digital subscriber line 30 for bipolar signals supplied to the subscriber line 30. 1 switch 3 is inserted in series, and a second switch 4 and a second DC current detection circuit 5 are inserted between the connection point between the first DC current detection circuit 2 and the first switch 3 and the other subscriber line 30. The first DC current detection circuit 2 closes the second switch 4 only when it detects a DC current, and the second DC current detection circuit 5 closes the first switch only when it detects a current. 3 is configured to open.

[For production]

In FIG. 1, when an external DC power source is accidentally connected to the digital subscriber line 30 for some reason and a DC current flows, the first DC current detection circuit 2 closes the second switch 4 in response to the DC current. When the second switch 4 is closed, the direct current flows through the series circuit of the second switch 4 and the second direct current detection circuit 5. The second DC current detection circuit 5 opens the first switch 3 due to the current flowing through the second DC current detection circuit 5 .

This state is maintained while the DC current is flowing through the first DC current detection circuit 2, but when the DC current state is released, the DC current detection circuit 2 returns the second switch 4 to its original open state. Since the state is restored, no current flows through the second DC current detection circuit 5, and therefore, the first switch 3 closes again and normal AC current flows through the signal transmitting/receiving circuit 1.

〔Example〕

Embodiments of the digital subscriber line protection circuit according to the present invention will be described below.

FIG. 2 shows an embodiment of the digital subscriber line protection circuit of the present invention conceptually shown in FIG. is a circuit that allows only a direct current of a predetermined value or less to flow through a coil 11, and this coil 11 and a normally open contact forming the second switch 4 form a relay. The coil 11 has a signal transmitting/receiving circuit 1
A bypass capacitor 12 is connected in parallel to allow only normal alternating current signals transmitted and received on the side to pass through, and series circuits of a Zener diode 13 and a diode 14, and a Zener diode 15 and a diode 16 are also connected in parallel. , overcurrent current 1+a and I exceeding a predetermined value
lm is still bypassed.

Further, the second DC current detection circuit 5 is a relay coil, and the normally closed first switch 3 constitutes a contact point of the relay coil 5. Furthermore, a resistor 1 is added to the switch 4 and the coil 5.
7 is connected.

Next, the operation of the embodiment shown in FIG. 2 will be explained.

Under normal conditions, switch 3 is closed and switch 4 is open.

Now, if we assume that a DC voltage with the polarity shown in the figure is applied due to contact with the external power supply 20, the current flows like 1+ (dotted chain line), and as a result, the DC current flows only in the coil 11, which acts as a relay contact. Switch 4 closes and a current of 1 g (dashed line) flows.

As a result, a direct current flows through the coil 5, and the switch 3, which is the relay contact, opens.

As a result, the DC current flowing to the signal transmitting/receiving circuit 1 side is
, and the inside of the circuit 1 can be protected.

On the other hand, when the external TFI2G is released in this state, the coil 11 and then the coil 5 are deenergized, the switch 4 is opened, and the switch 3 is closed, automatically returning to the normal state.

The above operation is similarly executed normally even if the polarity of the external ta is reversed.

In the above embodiment, a relay configuration including a coil and a contact is used, but it is needless to say that the present invention is not limited to this, and a well-known circuit that performs a switching operation based on a detection signal when direct current is detected can be used.

〔Effect of the invention〕

As described above, according to the digital subscriber line protection circuit of the present invention, only when the external DC power supply is incorrectly connected, the DC current is detected and prevented from being applied to the signal transmission/reception circuit of the terminal. It is possible to automatically protect and restore the power line insulated from the ground without requiring replacement of elements, and has the effect of realizing an extremely simple configuration and low-cost circuit.

[Brief explanation of the drawing]

Fig. 1 is a principle block diagram of a digital subscriber line protection circuit according to the present invention, Fig. 2 is a block diagram showing an embodiment of the digital subscriber line protection circuit shown in Fig. 1, and Fig. 3 is a block diagram of a conventional digital subscriber line protection circuit. 1 is a schematic diagram of a subscriber line protection circuit. In FIGS. 1 and 2, l indicates a signal transmitting and receiving circuit, 2 and 5 indicate first and second DC current detection circuits, respectively, 3 and 4 indicate first and second switches, respectively, and 11 indicates a coil, respectively. In the drawings, the same reference numerals indicate the same or corresponding parts. Patent applicant Tsuji Fuji Agent Patent Attorney Co., Ltd.
Mori 1) Hiroshi (1 other person) Figure 1 A diagram showing an example of the present invention

Claims (3)

[Claims] (1) A protection circuit for a two-wire balanced digital subscriber line (30) for bipolar signals supplied to the signal transmission/reception circuit (I), which is inserted in series in one of the subscriber lines (30). A first DC current detection circuit (2) and a first switch (3); The first DC current detection circuit (2) and the first switch (3)
a second switch (4) and a second DC current detection circuit (5) inserted in series between the connection point with the subscriber line (30) and the other subscriber line (30); Circuit (2
) is the second switch (4) only when DC current is detected.
A digital subscriber line protection circuit characterized in that the second DC current detection circuit (5) opens the first switch (3) only when a current is detected. (2) The first DC current detection circuit (2) is a circuit that allows only a DC current of a predetermined value or less to flow through the coil (11), and the second switch (4) and the coil constitute a relay. The digital subscriber line protection circuit according to claim 1, which is a normally open contact. (3) Claim 1 or 2, wherein the second DC current detection circuit (5) is a coil of a relay, and the first switch (3) is a normally closed contact of the relay. digital subscriber line protection circuit.