JPH0654356A - Subscriber interface protective circuit - Google Patents

Abstract

PURPOSE:To provide a subscriber interface protective circuit continuously usable without necessitating the exchange with a new one by limiting an excess current intruding through an arrester with a positive characteristic thermistor. CONSTITUTION:When an AC voltage intrudes into a subscriber line 12 by the mixed contact of commercial power lines, the positive characteristic thermistor (posistor) 32 is turned to high resistance and limits the excess current intruding through the arrester 21 into a subscriber interface 13. Thus, a protective function can be repeatedly displayed to the excess current without being exchanged with the new one and the continuously usable subscriber interface protective circuit can be obtained.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a subscriber interface protection circuit. Generally, an exchange and a plurality of subscribers are connected one-to-one by individual subscriber lines. Such an individual connection form causes no problem when the exchange and a plurality of subordinate subscribers accommodated in the exchange are relatively short distances. If they are scattered several tens of kilometers or more away from the exchanges, such an individual connection becomes uneconomical.

Therefore, the exchange and the vicinity of a group of subscribers scattered far away are connected by one transmission line (pair cable or the like), and each subscriber is connected through a subscriber interface installed in the vicinity thereof. A connection form of connecting to the transmission line is adopted. In addition, for example, a PC is provided on the one transmission line.
M multiplexed signals are transferred. The subscriber interface mainly houses a so-called codec or the like that converts an analog signal of each subscriber into a PCM digital signal and vice versa. These are constituted by ordinary IC parts or LSI. To be done. Therefore, the subscriber interface itself is quite vulnerable to electrical shock.

However, the subscriber interface is directly connected to the subscriber through each subscriber line made of a conductor such as a pair cable, and is placed in an environment where it is extremely susceptible to the above electrical shock. This is because, in order to reduce the cost, each subscriber line is usually laid on a general utility pole together with a commercial power line, and there is a possibility that contact with the commercial power line may occur first. Because. Secondly, the subscriber line laid in the air often receives induced lightning due to lightning strikes in the vicinity.

The present invention describes a protection circuit for protecting the subscriber interface from the above-mentioned touch and induced lightning.

[0005]

2. Description of the Related Art FIG. 7 is a diagram schematically showing the positioning of the present invention. In the figure, 11 is a central office including an exchange (not shown), and 14 is a plurality of subscribers (subscriber terminals) accommodated under the exchange. A subscriber interface 13 is first installed between the subscriber 14 and the exchange in the central office 11, and a subscriber line 12 is laid between the subscriber interface 13 and each of the subscribers. . The subscriber line 12 is usually installed on each utility pole 16 together with the commercial power line 17.

[0006] The present invention relates to a protection circuit for protecting the subscriber interface 13. The protection target of this protection circuit is, firstly, the above-mentioned touch contact (represented by F in the figure), and secondly, induced on the subscriber line 12 by a lightning strike (represented by L in the figure) generated in the vicinity. It is the above-mentioned guided lightning. The effects of these touches and induced lightning enter the subscriber interface 13 as F'and L'in the figure and destroy them. It should be noted that although the effect of such contact and induced lightning on each subscriber terminal is dealt with for each subscriber terminal, protection of the subscriber terminal is not mentioned in the present invention.

FIG. 8 is a diagram showing a conventional subscriber interface protection circuit. In this figure, a subscriber interface protection circuit 20 is installed on the subscriber line 12 side of the subscriber interface 13. According to the conventional configuration, the subscriber interface protection circuit 20 is provided in the subscriber line 12
In order from the side, an arrester 21 inserted between the pair of subscriber lines (T: Tip, R: Ring) and the ground G, and a resistor 22 with a built-in fuse function inserted in each pair,
Surge absorbing element 2 inserted between the pair and the ground G
3 and 3. Here, a triode is generally used as the arrester 21, and a varistor is generally used as the surge absorbing element 23. Incidentally, 24 is a rectifying bridge.

[0008]

The subscriber interface protection circuit 20 having the above-mentioned conventional structure has a problem. First, regarding the resistor 22 with a built-in fuse function, this resistor 22 is a large resistor that flows to the subscriber interface main body 15 especially when the above-mentioned cross-contact occurs (a voltage of several 100V intrudes from the commercial power line 17 to the subscriber line 12). Although it is convenient to cut off the electric current, on the other hand, there is a problem that once the fuse is blown, the communication network is completely cut off until the maintenance person replaces it with a new one.

Next, regarding the surge absorbing element (varistor) 23, this varistor 23 has its peak at the first stage arrester (triode) 21 especially when the above-mentioned induced lightning enters the subscriber interface 13. It serves to release the residual voltage (which may reach 500V or more) remaining after being suppressed to the ground. In this case, the current absorption characteristics are relatively good (even if a large current flows, it is difficult to break).
On the other hand, the voltage step-down characteristic is not so good. Therefore, there is a problem that the above-mentioned IC circuit or LSI may be easily damaged.

Therefore, in view of the above problems, the present invention does not require replacement with a new product, can be used continuously, and can suppress the residual voltage after passing through the arrester to almost the ground potential in a short time. An object is to provide an interface protection circuit.

[0011]

The present invention employs a positive temperature coefficient thermistor (so-called posistor) instead of the resistor 22 having a built-in fuse function. In addition, the above surge absorber 2
As 3, a silicon symmetrical switch (so-called Sidac) is adopted.

The arrester 21 remains unchanged.

[0013]

The positive characteristic thermistor (possistor) particularly counters the contact, and has a high resistance when an overcurrent flows, and has a function equivalent to the resistance of the built-in fuse function described above. The silicon symmetrical switch (Sydac) particularly counters the induced lightning and serves to suppress the residual voltage that has passed through the arrester 21 to almost the ground potential.

The use of the above positive temperature coefficient thermistor (hereinafter referred to as "positor") is indispensable in an environment where there is a high possibility of contact, and in the environment in which there is a high possibility of generation of inductive lightning, the silicon symmetrical switch described above is used. The use of (Sydac) is essential. In an environment where both contact and induced lightning are likely to occur, it is preferable to use the posistor and sidac at the same time.

[0015]

1 is a circuit diagram showing an embodiment of the present invention.
The subscriber interface protection circuit 20 according to the present invention comprises:
As a first form, a positive temperature coefficient thermistor (positor) 32 for limiting an overcurrent that enters the subscriber interface 13 via the arrester 21 is provided.

As a second form, a silicon symmetrical switch (sideac) 33 is provided for suppressing the residual voltage that enters the subscriber interface 13 via the arrester 21 to almost the ground potential. Further, as a third embodiment, as shown in the figure, a positive characteristic thermistor (positor) 32 for limiting an overcurrent that enters the subscriber interface 13 via the arrester 21 and a positive characteristic thermistor (positor) 32 are passed. It has a silicon symmetrical switch (33) for suppressing the residual voltage intruding into the subscriber interface 13 to almost the ground potential.

First, the first mode will be examined. When the AC voltage (50 Hz or 60 Hz) enters the subscriber line 12 due to the contact (F) of the commercial power line, the overcurrent associated therewith is limited by the posistor 32. FIG. 2 is a diagram showing how overcurrent is generated. In this figure, AC is contact (F)
Represents an AC voltage waveform of, for example, 600 V _rms , 50 Hz. With this AC voltage, the subscriber interface body 1
A current flows through the internal impedance Z of 5. This is the above-mentioned overcurrent.

FIG. 3 is a graph showing the general characteristics of the posistor. When the above-mentioned overcurrent (I) flows into the posistor 32 (internal resistance R), so-called Joule heat (I ² R) is generated, and the posistor 32 generates heat and its temperature rises. At this time, as shown in the graph of FIG. 3, the resistance value of the posistor 32 increases as the temperature rises. After all, the larger the overcurrent, the larger the internal resistance value of the posistor 32, and the function of limiting the overcurrent is exhibited.

When the contact is removed, the internal resistance of the posistor 32 automatically returns to the initial value (for example, 10Ω at room temperature),
It is equivalent to the conventional resistor 22 with a built-in fuse function. However, it is a non-fuse. Thus, it can be used repeatedly without replacement with a new one. FIG. 4 is a graph showing a residual voltage waveform after passing through the arrester. Rise of this waveform (time from 10% to 90% of peak level)
Is, for example, 10 μS, and the half-width (50% of the peak at the time of rising to 50% of the peak at the time of falling)
%) Is, for example, 1000 μS, which is a steep pulse waveform (surge voltage waveform). Since the peak is after the discharge by the arrester 21, it is stepped down to, for example, about 1000 V or less.

The posistor 32 hardly responds to the surge voltage waveform as described above, and therefore the internal resistance of the posistor 32 maintains the initial value (for example, 10Ω) even when the surge voltage waveform is applied. This 10Ω is
When the sidac 33 is provided in the next stage (the second embodiment described above), it serves to limit the current flowing through the sidac 33 when it is turned on.

Next, the above-mentioned second form will be examined.
The second mode is characterized in that the sidac 33 is provided between the sydac 33 and the ground G. FIG. 5 is a graph showing the residual voltage waveform after passing through the Sidac. Conventional varistor 23
The surge absorption characteristic due to V has a trapezoidal shape as indicated by V ₂₃ shown by the dotted line in the figure, and therefore the voltage applied to the subscriber interface main body 15 slowly drops as indicated by V ₁₅ shown by the dotted line in the figure. This will give a considerable shock to the subscriber interface body 15.

On the other hand, the surge absorption characteristic of the sidac 33 according to the present invention has a triangular shape as seen at the trailing edge of the double hatched portion in the figure, and therefore the voltage applied to the subscriber interface body 15 is as shown in the figure. Solid line V ₁₅
As shown in, it descends sharply. Therefore, the time during which the high voltage is applied to the subscriber interface body 15 is extremely short, and the effective impact is very small.

FIG. 6 is a graph showing the VI characteristic of the Sidac, where the horizontal axis is the voltage V and the vertical axis is the current I. Further, V _BO is a breakover voltage, V _F is a forward voltage,
I _H is a holding current. When the surge voltage exceeds V _BO of the sidac 33, the sidac 33 turns on and discharges to the ground G. At this time, the forward voltage V _F (V _F1 , V _F2 ) is about 3 V, and the surge voltage can be reduced to a level almost close to the ground potential.

It should be noted that the induced lightning may occur with a positive polarity.
Although it may occur in a negative polarity, it can be applied to either polarity as can be seen from the characteristics shown in FIG. In order to turn off the sidac 33 that has been turned on once, the current flowing therethrough may be made smaller than the holding current I _H (I _H1 , I _H2 ), but the conduction current of the sidac 33 is the resistance of the posistor 32 (10Ω). In addition, since it disappears as the surge voltage disappears, the sidac 33 will automatically turn off after the surge voltage has passed.

The present invention can naturally be applied to the central office (11) side of the subscriber interface body 15.

[0026]

As described above, according to the present invention, the protection function is repeatedly exhibited against an overcurrent without replacement with a new product, and the surge voltage is suppressed to almost the ground potential in a short time. can do.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing how an overcurrent is generated.

FIG. 3 is a graph showing general characteristics of a posistor.

FIG. 4 is a graph showing a residual voltage waveform after passing through an arrester.

FIG. 5 is a graph showing a residual voltage waveform after passing through a sidac.

FIG. 6 is a graph showing VI characteristic of Sidac.

FIG. 7 is a diagram schematically showing the positioning of the present invention.

FIG. 8 is a diagram showing a conventional subscriber interface protection circuit.

[Explanation of symbols]

11 ... Central office including exchanges 12 ... Subscriber line 13 ... Subscriber interface 14 ... Subscriber 15 ... Subscriber interface main body 16 ... Utility pole 17 ... Commercial power line 20 ... Subscriber interface protection circuit (triode) 21 ... Arrestor 22 … Built-in fuse function resistor (varistor) 23… Surge absorption element 24… Rectifier bridge 32… Positive characteristic thermistor (positor) 33… Silicon symmetrical switch (sydac)

Claims (3)

[Claims] 1. Protecting a subscriber interface (13), which is connected on the one hand to a plurality of subscribers (14) via each subscriber line (12) and on the other hand to a switch in a central office (11). In order to do so, in the subscriber interface protection circuit (20) including an arrester (21) on the subscriber line side, a positive temperature coefficient thermistor for limiting an overcurrent that enters the subscriber interface through the arrester. A subscriber interface protection circuit having (32). 2. Protecting a subscriber interface (13) which, on the one hand, is connected via a respective subscriber line (12) to a plurality of subscribers (14) and, on the other hand, to an exchange in a central office (11). In order to achieve this, in the subscriber interface protection circuit (20) including an arrester (21) on the subscriber line side, the residual voltage that enters the subscriber interface via the arrester is suppressed to approximately the ground potential. A subscriber interface protection circuit having a silicon symmetric switch (33) that operates. 3. Protecting a subscriber interface (13) which, on the one hand, is connected via a respective subscriber line (12) to a plurality of subscribers (14) and, on the other hand, to a switch in a central office (11). In order to do so, in the subscriber interface protection circuit (20) including an arrester (21) on the subscriber line side, a positive temperature coefficient thermistor for limiting an overcurrent that enters the subscriber interface through the arrester. (32) and a silicon symmetrical switch (33) for suppressing a residual voltage that passes through the arrester and the positive temperature coefficient thermistor and enters the subscriber interface to approximately a ground potential. User interface protection circuit.