JPS6086995A - External current protecting system - Google Patents

Abstract

PURPOSE:To protect a subscriber line against an external current impressed continuously by providing a detecting circuit of an external current on the subscriber line, and opening a channel by an output of a timing circuit for discriminating a fact that a detecting output exists for a prescribed time. CONSTITUTION:An external current detecting circuit 23 is provided on a subscriber's circuit 2 of a telephone exchange, and the circuit 23 detects an external current impressed between a line A and a line B, and between the line A or the line B and the ground by a detecting circuit of a high impedance. A timing circuit 24 discriminates a fact that a detecting output of the circuit 23 has been continued exceeding a prescribed time, and drives a relay C only in case said output has been continued, for instance, about two seconds or more. The relay C cuts off a line wire side channel and a current supplying circuit, and a channel of a supervisory circuit 22 side. In this way, it is possible to prevent a breakdown of a subscriber's circuit caused by an external current impressed continuously to the channel.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to an extraneous current protection system in a subscriber circuit of a telephone exchange comprised of electronic circuits.

[Prior art]

An example of a conventional external current protection system will be explained with reference to the circuit diagram shown in FIG. - The secondary protection circuit 1 is composed of an escape protection circuit that combines a discharge type escape tube 11, a resistor 12, and a varistor 13, and a varistor 21 is generally included in the subscriber circuit 2 as a secondary protection circuit. It is set in.

Here, the voltage starting voltage of the discharge type escape tube and tube 11 used in the secondary maintenance circuit 1 is generally several hundred volts, and The conduction voltage of the varistor 21, so-called varistor voltage, is lower than the discharge starting voltage of the discharge type shunt tube 11. However, among the signals flowing through the communication path, there is a ringing signal for ringing the bell of the subscriber's telephone, and therefore a varistor is used that has a varistor voltage higher than the voltage of this ringing signal.

However, the sampling circuit described above is mainly used for the purpose of protecting against surges that are instantaneously applied to the line, such as lightning, and is used to protect the line from continuously applied foreign current, such as interference from commercial power lines, etc. No effective protection was provided. ), in the event of crosstalk in the low-voltage commercial power supply, the discharge type evacuation tube 11
did not discharge, and external current continuously flowed through the varistor 21 in the subscriber circuit 2 and the current supply/monitoring circuit 22, causing a failure in which not only the current supply/monitoring circuit 22 but also the varistor 21 was destroyed. .

On the other hand, as protection against continuously applied external current,
As mentioned above, the conventional protection system has the disadvantage that complete protection cannot be obtained against continuously applied external current.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an external current protection system that eliminates such conventional drawbacks.

[Structure of the invention]

The external current protection system of the present invention includes, in a subscriber circuit, an external current detection circuit, a timing circuit that identifies that the detection output of the detection circuit exists for a certain period of time, and an external current protection system that uses the output of the timing circuit to detect external current. The present invention is characterized in that means is provided for disconnecting the circuit-side communication path that joins the i-side and a-side communication paths.

[Embodiments of the invention]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 2 is a circuit diagram showing an embodiment of the external current protection method according to the present invention. In Figure 2, the external current detection circuit (
DBT) 23 is a high input impedance detection circuit.
Detects the current applied between the A line and the B line, or between the A mound or the B line and the ground.

The timing circuit (TiM) 24 is a circuit for identifying when the detection output of the external electric power detection circuit (DET) 23 has continued for a certain period of time or more, and is provided to ensure that the communication path is on. The timing times % (TIM) 24 is determined by taking into account the sending time of the normal ringing signal and the external current detection circuit (D
The timing is determined so that the relay C is driven only when the output of the ET) 23 continues for about 2 seconds or more.

The relay C driven by the output of the timing circuit (TIM) 24 is a relay for disconnecting the communication path on the outside line side and the communication path on the current supply and monitoring circuit 22 side through its contacts.

These external current detection circuits (DET) 23, timing 1
A protection circuit, such as a TIM 24 and a relay C, has been added to the subscriber circuit 2 in FIG.

In the above configuration, if multiple continuous external currents are applied to the communication line due to crosstalk between the subscriber line and the commercial power line,
First, the external electric current is detected by the external electric current detection circuit (NET) 23. and timing circuit (TIM)
) 24, the relay C is activated by the output of the timing circuit (TIM) 24.
is driven, and its contact disconnects the communication path on the outside line side and the communication path on the current supply and monitoring circuit 22 side. This state continues until the external current disappears. When the external current disappears, the external current detection circuit (DBT) 23 stops its output, and subsequently the output of the timing circuit (1'IM) 24 also stops, and as a result, the relay C is restored, so the subscriber circuit 2 is in a steady state. available as a state.

In addition, when a calling signal is sent to the communication path.

Since the calling signal is discontinuous, the timing circuit (T
Due to the action of IM) 24, relay C remains inoperative and normal calling operation is performed.

〔Effect of the invention〕

As explained above, the present invention adds the above-mentioned protection circuit to the conventional subscriber circuit, thereby preventing damage to the subscriber circuit caused by external current continuously applied to the communication path. It has a preventive effect.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing an example of a conventional external current protection system.
FIG. 2 is a circuit diagram of one embodiment of the present invention.

Claims (1)

[Claims] A subscriber circuit of a telephone exchange is provided with an external current detection circuit, a timing circuit for identifying that a detection output of the detection circuit exists for a certain period of time, and means for opening a communication path based on the output of the timing circuit. An external current protection method characterized by: