JPH0662445A - Overvoltage protecting circuit - Google Patents

Abstract

PURPOSE:To provide an overvoltage protecting circuit which is applied to a subscriber line and can suppress the fluctuation of the power voltage VBB. CONSTITUTION:An overvoltage protecting circuit is connected between both lines A and B of a subscriber circuit and consists of the thermistor resistances RF and RF which are connected in series to the lines A and B respectively and a diode bridge circuit 20 which is connected to the ground potential at one of both ends and to the power voltage VBB at the other end. Then the high voltage resistances RM and RM are connected in series at a part where the circuit 20 is connected to both lines A and B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overvoltage protection circuit used in a subscriber circuit.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing a conventional configuration example of a subscriber circuit. In the figure, reference numeral 1 is a subscriber telephone connected between the A and B lines. Reference numeral 2 is a power supply circuit for supplying a direct current to the telephone 1, and 3 is a power supply circuit for absorbing the power supply current from the high-power circuit 2. Power supply circuit 2 is ground G
And the power supply circuit 3 is connected to the power supply voltage VBB. For example, −48V is used as the power supply voltage VBB.

4 is a ringer, 5 is a ringer between the lines A and B 4
Is a ringer connection circuit for connecting to the telephone and detecting an off-hook of the subscriber's telephone. From the ringer connection circuit 5, a signal RTP indicating that the subscriber's telephone is off-hook
Is sent. 6 is an overcurrent protection circuit configured by connecting a thermistor resistance in series with the A and B lines, and 7 is the A and B lines.
It is an overvoltage protection circuit connected between the lines. A ground G and a power supply voltage VBB are connected to the overvoltage protection circuit 7.

Reference numeral 8 is a 2-wire / 4-wire converting circuit for converting 2-wire and 4-wire, and for example, a hybrid transformer or the like is used. Reference numeral 9 is a codec for converting an analog signal into a digital signal, the output of which is sent to the highway HW. Reference numeral 10 is a monitoring circuit that receives signals from the power supply circuits 2 and 3 and monitors the on-hook and off-hook states of the telephone 1. The output of the monitoring circuit 10 is sent to the outside as an SCN signal.

In the device constructed as described above, when an incoming call is received from the outside, the ringer connection circuit 5 connects the A line and the B line.
Connect the ringer 4 to the wire. As a result, the telephone 1 in the on-hook state rings. When the subscriber off-hooks the telephone 1, the ringer connection circuit 5 turns off the connection of the ringer 4 to the lines A and B.

In the off-hook state, a direct current is supplied from the power supply circuit 2 to the telephone set 1, a current flows in a loop of the power supply circuit 2 → B line → telephone 1 → A line → power supply circuit 3, and the subscriber is an external subscriber. It becomes possible to talk with. External highway H
The voice signal from W is converted into an analog signal by the codec 9, converted into a 2-line signal by the 2-line / 4-line conversion circuit 8, and flows into the telephone 1. On the other hand, the voice signal from the telephone 1 is 2
After being converted into a 4-wire type signal by the line / 4-line converting circuit 8, the codec 9 converts it into digital data and sends it to the highway HW.

Here, a commercial power source (for example:
When a contact such as AC100V occurs, an overcurrent flows in the subscriber circuit. When this overcurrent flows, the overcurrent protection circuit 6
The thermistor has a high resistance due to overcurrent and limits the current flowing to the subscriber circuit. On the other hand, the overvoltage protection circuit 7
The voltage applied to the A and B lines is the ground G and the power supply voltage VBB.
Clamp within.

FIG. 6 is a diagram showing a conventional circuit example of the overvoltage protection circuit 7. RF is an overcurrent protection resistor connected in series to each of the A line and the B line, and for example, a thermistor that has a high resistance due to an overcurrent is used. Reference numeral 11 denotes a diode bridge circuit connected between the A line and the B line, which includes diodes D1 to D4.
It is composed of. The line A is connected to the connection point of D3 and D4, and the line B is connected to the connection point of D1 and D2.
The ground G is connected to the common connection point (cathode) of D1 and D3, and the power supply voltage VBB is connected to the connection point (anode) of D2 and D4.

In the circuit thus constructed, for example, when a positive overvoltage of the ground potential or more is applied to the A line, the diode D3 is turned on and the potential of the A line becomes the ground potential. When a negative overvoltage of VBB or less is applied to the A line, the diode D4 is turned on and the potential of the A line is VBB.
It becomes a potential. The above operation is performed with a positive overvoltage or VBB on the B line.
The same applies when the following negative overvoltage is applied. In this way, the potentials of the A and B lines are always the ground potential and VB.
It is designed to be held during the B potential.

[0010]

However, the resistance value of the thermistor resistance RF is about 10Ω, and as shown in FIG. 7, for example, the negative peak voltage of about 1 KV from the noise generator 12 of the signal source resistance RS is the B line. (Or line A). The instantaneous current I flowing at this time is I = 1
000 / (RF + RS), both RF and RS are 10
If it is Ω, the peak current I flowing is 50 A.

The power supply voltage VBB normally operates so that a current flows therein. However, when a negative overvoltage as shown in FIG. 7 is applied to the A line or the B line, the current starts to flow in the loop shown in the figure. . Depending on the type and capacity of the VBB power supply, when the overcurrent flows, the output impedance included in the overcurrent causes a change in the power supply voltage VBB. Then, there is a problem that the subscriber circuit is destroyed by the fluctuation of the VBB power supply.

Further, if the resistance value of the protective element is increased in order to suppress the overcurrent, it becomes difficult to obtain accuracy, the variation of the AB line of the subscriber circuit becomes large, and the characteristics such as the ground unbalance attenuation amount are deteriorated. Will cause.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an overvoltage protection circuit capable of suppressing fluctuations in the power supply voltage VBB.

[0014]

FIG. 1 is a circuit diagram of the principle of the present invention. The same parts as those in FIG. 6 are designated by the same reference numerals. In the figure, RF is a thermistor resistor connected in series to the A and B lines of the subscriber circuit, and 20 is a diode bridge circuit whose one end is connected to the ground potential and the other end is connected to the power supply voltage VBB. The diode bridge circuit 20
Is D1 and D of the diode bridge circuit 11 shown in FIG.
The high breakdown voltage resistor RM is connected to the portion where 2 is connected and the portion where D3 and D4 are connected. Moreover, these high breakdown voltage resistors RM are configured to be connected in series with the A line and the B line. Here, the reason why the high breakdown voltage resistor is used is that the resistor is not broken down even when an overvoltage is applied.

[0015]

FIG. 2 is an explanatory view of the operation of the present invention. For example, when a positive overvoltage is applied to the B line, the overcurrent flows to the ground G through the thermistor resistance RF in the loop shown in FIG. 2A, and when a negative overvoltage is applied to the B line, the overcurrent is applied to the B line. In the loop shown in (b), the current flows to the B line side through the high breakdown voltage resistor RM. Here, the current flowing out from VBB can be reduced due to the high withstand voltage resistor RM, and the power supply voltage VB can be reduced.
The voltage fluctuation of B can be suppressed.

[0016]

The operation of the embodiment of the present invention will be described below.

FIG. 3 is a diagram showing the operation when a positive overvoltage is applied. It is assumed that a positive overvoltage is applied to the B line from the noise generator 12. At this time, the current I is the signal source resistance RS
→ RF → Flows through D1. The overvoltage at this time is VS,
Assuming the signal source resistance RS, the flowing overcurrent I is expressed by I = VS / (RS + RF) (1).

FIG. 4 is a diagram showing the operation when a negative overvoltage is applied. When a negative overvoltage is applied to the B line from the noise generator 12, the power source voltage VBB causes the current I to flow to the noise generator 12.
Flows. That is, power supply voltage VBB → diode D2 →
High voltage resistance RM → Thermistor resistance RF → Signal source resistance RS
→ Current flows in the loop to the noise generator 12. The overcurrent I at this time is expressed by I = VS / (RF + RS + RM) (2). As is clear from the equation (2), the high withstand voltage resistor RM reduces the overcurrent I flowing from the power supply voltage VBB, and can suppress the voltage fluctuation of the power supply voltage VBB.

In the above embodiments, the case where the thermistor is used as the overcurrent protection resistor RF is taken as an example, but the present invention is not limited to this, and the resistance value increases when an overcurrent flows. If it exists, it doesn't matter what kind.

[0020]

As described above in detail, according to the present invention, a high breakdown voltage resistor is provided in the diode bridge circuit in order to suppress the outflow current from the power supply voltage VBB when a negative overvoltage is applied. By reducing the overcurrent flowing out from the power supply voltage VBB,
The fluctuation of BB can be suppressed.

[Brief description of drawings]

FIG. 1 is a principle circuit diagram of the present invention.

FIG. 2 is an explanatory view of the operation of the present invention.

FIG. 3 is a diagram showing an operation when a positive overvoltage is applied.

FIG. 4 is a diagram showing an operation when a negative overvoltage is applied.

FIG. 5 is a block diagram showing a conventional configuration example of a subscriber circuit.

FIG. 6 is a diagram showing a conventional configuration example of an overvoltage protection circuit.

FIG. 7 is a diagram showing a flow of an overcurrent loop when an overvoltage is applied.

[Explanation of symbols]

 20 Diode bridge circuit D1 to D4 Diode RF Thermistor resistance RM High voltage resistance

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Shinichi Ito 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor, Kiyoshi Taguchi, 3-9-18 Shin-Yokohama, Kohoku-ku, Yokohama City, Kanagawa Prefecture, Fujitsu Communication・ Inside Systems Co., Ltd.

Claims (1)

[Claims] 1. A circuit connected between a line A and a line B of a subscriber circuit, wherein the thermistor resistors (RF) and (RF) are connected in series to the lines A and B, and one end thereof is grounded. And a diode bridge circuit (20) whose other end is connected to the power supply voltage VBB, and the diode bridge circuit (20) is connected in series to the portions connected to the A line and the B line. A high voltage resistance (RM), (RM) is connected to such an overvoltage protection circuit.