JPH0724447B2 - Ground fault protection method in subscriber circuit for public telephone - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ground fault protection method in a public telephone subscriber circuit of a digital exchange.

[Background of the Invention]

The subscriber circuit of a digital exchange is being electronically adapted for downsizing and high reliability, and so is the subscriber circuit for public telephones. A public subscriber circuit developed for such a purpose is, for example, "Consideration of fully electronic subscriber circuit for public telephone" (Ishizawa et al.
It is shown in Fig. 07) and its theoretical equivalent circuit is shown in Fig. 1. In the figure, a circuit 101 surrounded by a dotted line is a power supply circuit during normal power supply (referred to as a normal side), and a circuit 102 surrounded by a dotted line is a power supply circuit during current reversal (referred to as a revers side). is there. The public subscriber circuit requires two power supply circuits, one for normal use and the other for revers, so that a billing signal (reversal) is sent to the public telephone during a call. Both are basically the same, and are connected so that the output polarities are opposite. In Figure 1,
Only the parts necessary for explanation are shown. Terminals A and B are communication lines, and terminals such as telephones are usually connected between these terminals. Resistance 2,
The resistance 9 is equivalently a power supply resistance of several 100Ω (for example, 220Ω, which is usually realized by configuring a feedback circuit with an electronic circuit), and the inductances 3 and 10 are for AC blocking, The inductance is equivalently several H (for example, 10 H). These and the power supply 7 realize a constant resistance power supply circuit. The current sources 5, 6, 12, 13 are constant current sources having the same current value, and the currents they flow are called mutual currents. To make this mutual current flow, it is necessary to send a billing signal during a call in a public subscriber circuit, and a DC bias for operating an electronic circuit is required so that this can be sent by an electronic circuit switch. Because there is.

For such a public subscriber circuit, protection against the ground fault of the communication line is necessary, and switches 4 and 8 and resistors 1 and 11 are provided for this purpose in the equivalent circuit of FIG. (Details of this ground fault protection method are disclosed in Japanese Patent Laid-Open No. 56-81038). Line connected to A terminal side (A line)
Is shorted to ground with a ground fault resistor 15 of several hundred Ω or less, and the current flows above a certain value (for example, 80 mA), the power supply circuit monitors the voltage across resistor 9 to limit the current (protection mode). Transition. That is, when the switches 4 and 8 fall to the contact on the opposite side to the one shown in the figure, the communication line side (A, B
The left side of the terminal is separated from the constant resistance power supply circuit (excluding the power source 7) including the resistors 2 and 9 and the inductances 3 and 10, and is then connected to the resistors 1 and 11 newly. The resistors 1 and 11 have a resistance value of 10 compared to the resistors 2 and 9.
The resistance is about 0 times higher (for example, 40 KΩ), and as a result, the ground fault current is significantly limited and the power supply 7 is protected (this can be easily realized by disconnecting the feedback loop in an electronic circuit).

However, the above conventional techniques have the following problems. Considering the relationship of the current when a ground fault occurs in FIG.
In the symbols shown in the figure, I _NB = I _RA and I _G + I _RB = I _NA . On the other hand, the characteristics of the feed _{circuit, I RB = I RA = I} M, I NB
= I _NA = I _M. When these equations are satisfied, the ground fault current I _G = 0 is inevitably obtained (the current flowing through the high resistances 1 and 11 is very small and can be ignored). Therefore, the protection mode is released, and the power feeding circuit tries to perform normal power feeding. However, the fault condition continues, and the ground fault current I _G
Will immediately switch to protection mode. When the protection mode is entered, I _G = 0 is established again and the protection mode is released. This state is repeated at a cycle determined by the inductances 3 and 10, and the circuit enters an oscillating state.
That is, although a circuit configuration that requires mutual currents for all-electronization is adopted, this is incompatible with the above ground fault protection method.

[Object of the Invention]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems of the prior art and provide a ground fault protection method suitable for a power supply circuit of a public subscriber circuit that requires mutual current.

[Outline of Invention]

According to the present invention, another current source is provided in one of the power supply circuit on the normal side or the lever side, and when the ground fault of the communication line is detected and the protection mode is entered, the other current source is turned on. The feature is that the ground fault current I _G is allowed to flow until the fault is removed.

Example of Invention

Hereinafter, the present invention will be described in detail with reference to examples. FIG. 2 shows a theoretical equivalent circuit of a subscriber circuit using the method of the present invention, in which constant current sources 18 and 19 and switches 16 and 17 are added to the equivalent circuit of FIG. . As a result, the switches 16 and 17 are closed in the protection mode, and I _NB = I
_{Since NA} = I _M + I _D , from the above relational expression, I _G = I
It becomes _D. Therefore, by properly selecting the current I _D of the constant current sources 18 and 19, the ground fault current I from these constant current sources 18 and 19 is selected.
Oscillation as in the prior art is avoided because _G can be provided so that the protected mode is not released.

FIG. 3 shows an embodiment of the present invention, which is the normal side of FIG.
3 illustrates a specific configuration example of a power supply circuit 101A. Terminal
If a load is connected between A and B, high resistance 29,30 (eg
Current flows to 40 KΩ). This current is the current mirror 3
It is folded back at 2 and 34, and further current mirrors 33, 35, 36, 31
After that, it is fed back to the output operational amplifiers 27 and 28. Opean
The outputs of the transistors 27 and 28 drive the output transistors 25 and 26,
A current is passed through the emitter resistors 21 and 22. This is the output current
Then, the current flows from the terminal B to A through the load. This return
The gain of the loop is the mirror ratio of each current mirror, the resistor 23 and
Determined by the ratio of 21 and 24 and 22. The above is the equivalent circuit of FIG.
It is the constant resistance feeding part indicated by resistors 2 and 9 in Fig. Again
With the resistor 39 and the capacitor 40 in Fig. 3,
By blocking the return loop, the
Dactance 3, 10 has been achieved. In addition, a current source 45 is installed.
Current through the current mirror 37 to the feedback loop.
In addition, the functions of the constant current sources 5 and 6 in FIG. 2 are realized.
It Transistors 41 and 42 form a current switch
Disconnect the above feedback loop between the current mirrors 35 and 36.
Have a function that That is, the output Q of the ground fault detection control circuit 47 is
When a current is applied, the transistor 42 turns on and the current mirror
-35 output current no longer flows to the current mirror 36,
As a result, the feedback loop is disconnected and the constant resistance feeding characteristic is lost.
Be done. This corresponds to the function of switches 4 and 8 in Fig. 2.
Hit As a result, in Fig. 3, the communication line side (A, B terminals
Only the high resistance of resistors 29 and 30 can be seen from the left side).
Corresponds to the resistors 1 and 11 shown in FIG. Next
And the switch of FIG. 2 provided for realizing the present invention.
16 and 17, and the realization of the functions of the current sources 18 and 19 are explained.
It This function is performed by the current source 46, transistors 43, 44, and
And the current mirror 38. Transis
The switch 44 detects the ground fault non-detection by the inverted output of the detection control circuit.
It is always on when it leaves. Therefore, the current source 46
Does not work. On the other hand, when the ground fault is detected, the current of the current source 46 is
Output via current mirrors 38, 31 and operational amplifiers 27, 28
Since the transistors 25 and 26 are driven, constant current feeding characteristics can be obtained.
To be Therefore, the gain characteristic of this circuit is set appropriately and
Power supply current I_DIf you decide, I_G= I
_DGround fault current flows, resulting in oscillation in the subscriber circuit.
Tearing is prevented. In addition, the detection of the ground fault is shown in FIG.
As shown, the potential of the emitter resistors 21 and 22 is detected and controlled.
It can be done by monitoring at 7. Also detection control
V of circuit 47 Adjusts the operation of the current switch 43 appropriately.
It is a reference voltage for

FIG. 4 shows another embodiment of the present invention. The basic structure is the same as that of FIG. 3, but filters 48 and 49 for removing power supply noise are inserted in the power supply units of the current mirrors 32 and 34. Has been done. In this case, it is necessary to provide the current source 50 to increase the output current of the current mirror 35 in order to compensate for the voltage drop in this filter. Conventionally, this current was also removed from the feedback loop via the transistor 42 during a ground fault, but the current switches 51 and 52 bypassed the switch 42 during a ground fault so that this current could flow to the current mirror 36. In the present embodiment, the output of I _G = I _D is obtained by 50. The outputs Q ₁ and Q ₂ of the detection control circuit 47 both turn on the transistors 42 and 52 when a ground fault is detected. The resistor 33 is a pull-up resistor.

When it is not necessary to set the injection position of the current source 50 to the collector side of the switch 42, the output of the current source 50 may be directly connected to the collector side of the transistor 41.

〔The invention's effect〕

As is apparent from the above embodiments, according to the present invention, there is an effect that a ground fault protection function of a public subscriber circuit that requires mutual current can be realized without causing oscillation.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of a conventional public subscriber circuit, FIG. 2 is an equivalent circuit diagram of a public subscriber circuit using a ground fault protection method according to the present invention, and FIGS. 3 and 4 show the present invention. 2 is a circuit configuration diagram showing an embodiment of FIG. 1,2,9,11 …… resistance, 15 …… ground fault resistance, 3,10 …… equivalent inductance, 5,6,12,13 …… current source, 4,8 …… switch,
101 …… Feed circuit (normal side), 102 …… Feed circuit (revers side), 16,17 …… Switch, 18,19 …… Current source, 31
~ 38 …… Current mirror, 47 …… Detection control circuit, 45,46
...... Current source, 48,49 …… Filter, 27,28 …… Op Amp

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Hayashi 3-1, Morinosato Wakamiya, Atsugi City, Kanagawa Prefecture, Atsugi Telecommunications Research Laboratories, Nippon Telegraph and Telephone Public Corporation (72) Inventor Tadatsu Kimura 3-1, Wakamiya Morinosato Wakamiya, Atsugi City, Kanagawa Prefecture Japan Telegraph Telephone Corporation Atsugi Electro-Communications Research Laboratory (56) References JP-A-56-81038 (JP, A)

Claims (1)

[Claims] 1. A call line to a public telephone is used as a load, and from the normal side current supply circuit to the call line, to a constant resistance power supply circuit included in the normal side current supply circuit and to the constant resistance power supply circuit. A current flows from each of the first constant current circuits provided in parallel in the same direction, and a constant current having the same current value as the constant current from the first constant current circuit is supplied from the lever side current supply circuit. When the ground fault in the communication line is detected by the constant resistance power supply circuit in the normal side current supply circuit while the current is flowing in the reverse direction, the current limiting high voltage of the constant resistance power supply circuit based on the detection. At the time of shifting to the protection mode by switching replacement with the resistor, the second constant current circuit provided in parallel with the first constant current circuit is allowed until the ground fault is removed. Is the current direction First and has been a constant current is to be flowed, ground fault protection method in a subscriber circuit for a public telephone.