JPH0193245A - Line interface circuit - Google Patents

Abstract

PURPOSE:To realize a line interface circuit with superior branching characteristic which enables a call to be placed even when the voltage of a line is decreased by connecting a device for forming a bypass with high impedance in AC way in parallel with a circuit for forming a line loop current passage. CONSTITUTION:A constant current diode 11 is connected in a forward direction in parallel with a transistor 2 on the rear stage side of a switch 10 on the line on the primary connection side of an AC coupling transformer 7 in a diode bridge 1. By branch-connecting another telephone set between the lines T and R, the voltage between the lines T and R decreases, and a current of around 1-2mA possible to be permitted to flow by the constant current diode 11 even when the voltage obtained by voltage division by resistors 4 and 5 drops to <=1-1.4V flows through the diode, thereby, a line loop current can be held.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a line interface circuit for a terminal device connected to a line in a telephone network.

(Prior Art) In a telephone network, lines and terminal devices are connected via line interface circuits built into the terminal devices. A conventional example of this line interface circuit is shown in FIG.

In the figure, T and R are lines (for example, a subscriber line of a telephone office or an extension line of a private branch exchange) to which direct current is applied. Lines T and R are connected to the input side of diode bridge 1,
In addition, there is an NP on the positive output side of this diode bridge 1.
The collector side of the N-type transistor 2 and the emitter side of the transistor 2 are connected to the negative output side of the diode bridge 1 through a series circuit of a resistor 9 and a switch 10. Also, the base side of transistor 2 is NPN.
The emitter side of the transistor 3 is connected to the collector side of the transistor 3, and the collector side of the transistor 3 is connected to the collector side of the transistor 2, and these transistors 2 and 3 are connected in a Darlington connection. 4.5 is the resistance, and this resistance 4,
5 is connected in series between the positive and negative lines after the switch 10 on the output side of the diode bridge 1, and is configured to apply a divided voltage at the midpoint to the base of the transistor 3, thereby biasing the transistor 3. It is something. In addition, a capacitor 8 is connected in parallel to a resistor 5 whose one end is connected to the negative line side on the output side of the diode bridge 1, and a capacitor 8 is connected to the output side of the diode bridge 1 for coupling audio components. The primary side of the AC coupling transformer 7 is connected.

Further, a capacitor 6 for cutting off DC current is connected in series to the output positive line of the diode bridge 1 on the primary side of the AC coupling transformer 7 .

In such a configuration, the telephone has an AC coupling transformer 7
connected to the secondary side of the Then, when the call state is established, the switch 10 is closed. By closing this switch 10,
The direct current supplied from the lines T and R follows the route of passing through the diode bridge 1, the collector/emitter of the transistor 2, the emitter resistor 9, the switch 101, the diode bridge 1, and returning to the lines T and H. It will flow. Here, resistance 4.5 is transistor 2°3
capacitor 8 is a filter capacitor for the base of transistor 3,
A DC voltage from the diode bridge 1 filtered by the capacitor 8 is applied to the base of the transistor 3, and the emitter output of the transistor 3 outputs this, making the transistor 2 conductive and forming a flow path for the line loop current. Ru. Since the AC coupling transformer 7 has a capacitor 6 connected in series to cut the DC component, a line loop current flows through the collector and emitter of the transistor 2, and the AC coupling transformer 7 has a capacitor 6 connected in series to cut off the DC component. This means that the AC component of the line can be transmitted via the line. As a result of the transistor 3 being driven using the line output smoothed by the filter 8 as a base signal, the collector-emitter of the transistor 2 normally exhibits a high impedance to alternating current. Therefore, the audio signal (AC signal) from the line passes through the diode bridge 1, passes through the capacitor 6, and reaches the AC coupling transformer 7. The audio component is then provided to the telephone via the AC coupling transformer 7. Thereby, it is possible to extract the voice component by flowing current from the line TR, and conversely, it is also possible to apply the voice signal component from the telephone to the line via the AC coupling transformer 7.

Here, when another telephone is branch-connected between the line TR in FIG. 2, the voltage between the lines TR and R is lowered. As a result, when the voltage obtained by dividing by the resistor 4.5 becomes 1 to 1.4 V or less, the transistor 2 is cut off, and the DC loop circuit from the line is therefore cut off. Then, since no forward current flows through the diode bridge 1, the diode bridge 1 is also cut off, and the audio signal is also cut off. Therefore, in the above circuit, if a branch of another telephone at the far end or a telephone with low DC impedance is branched as seen from the central office, a phenomenon occurs in which the call is disconnected for the above reason.

(Problems to be Solved by the Invention) As mentioned above, the telephone network line and the telephone, which is a terminal device, are connected through a line interface circuit built into the telephone.
A diode bridge is connected to i1T-R, an AC coupling transformer is connected to this diode bridge via a capacitor, and a telephone is connected to the secondary side of this AC coupling transformer. Then, a Darlington-connected transistor is connected between the primary side connection lines of the AC coupling transformer to the diode bridge, forming a line loop current sink circuit, and during a call, the line loop current returns DC current from the line to the line. I'm trying to form a path. This makes it possible to extract audio components by applying current from the line. Moreover, conversely, the voice signal component from the telephone can be applied to the line via the coupling transformer.

However, as mentioned above, the DC loop current sink circuit is formed by Darlington-connected transistors, so in order to operate the DC loop current sink circuit, a bias voltage is required to operate the Darlington-connected transistors. Must.

Therefore, when a telephone is branch-connected to such a line interface circuit, the voltage between the lines T- and R will drop, so the voltage between the lines T-R will drop, and this reduced voltage will be further divided by the resistor 4.5. As a result of applying pressure, this resistance 4
, 5 becomes less than 1 to 1.4 V, the transistor 2 is cut off,
The DC loop circuit from the line is cut off. Then, since no forward current flows through the diode bridge 1, the diode bridge 1 is also cut off, and the audio signal is also cut off.

Therefore, in the line interface circuit, if you branch another telephone at the far end from the point of view of the central office or private branch exchange, or branch a telephone with low DC impedance, a phenomenon occurs in which the call is disconnected for the above reason and becomes unusable. happens.

Therefore, an object of the present invention is to obtain only a bias voltage that is less than the threshold voltage of the Darlington-connected transistors that form a line loop current sink circuit when a terminal device such as a telephone is branch-connected to a line. Therefore, it is an object of the present invention to provide a line interface circuit with good branch characteristics that enables telephone calls even when the line voltage drops.

[Structure of the invention]

(Means for solving problems) In order to achieve the above object, the present invention is configured as follows.

That is, a line loop current flow path that is built in a terminal device connected to a line in a telephone network, receives voltage from the line via a rectifying diode bridge, and is connected in parallel to this diode bridge and has a high impedance in terms of alternating current. In a circuit interface circuit, a line loop current is passed through a forming circuit, and an AC component is transmitted to the line by an AC coupling transformer connected in parallel to a diode bridge via a capacitor that blocks a DC component. The flow path forming circuit is configured by connecting a bypass forming element having high AC impedance in parallel.

(Function) In such a configuration, the voltage from the line is received via the rectifying diode bridge, and the line loop current is rectified by a circuit for forming a line loop current flow path with high impedance in terms of alternating current, which is connected in parallel to this diode bridge. flowing,
Transmission of AC components to the line is performed by an audio coupling transformer connected in parallel to a diode bridge via a capacitor that blocks DC components. Normally, the line loop current flows through a circuit for forming a line loop current flow path, but since the circuit for forming a line loop current flow path uses a Darlington connected transistor circuit with high AC impedance, for example, the threshold The level is high, so
When a telephone is branch-connected to a line at the far end from the point of view of a central office or private branch exchange, when the voltage on the line drops and the bias of the transistor becomes low enough to maintain this threshold level, a loop current path is formed. The circuit used for this becomes inoperable. However, in this case, a bypass forming element with high AC impedance, such as a constant current diode, is connected in parallel to the circuit for forming the loop current flow path, so that the line loop current flow path is connected through this device. can be secured. Therefore, transmission of alternating current components to the line is possible, and telephone calls can be secured.

Therefore, when branch-connecting a terminal device such as a telephone to a line, the voltage of the line drops to such an extent that only the bias voltage that can be obtained is less than the threshold voltage of the Darlington-connected transistors that form the sink circuit for the line loop current. However, it is possible to provide a line interface circuit with good branch characteristics that enables telephone calls.

(Example) Hereinafter, an example of the present invention will be described with reference to FIG.

FIG. 1 is a block diagram showing the configuration of this device. In the figure, T and R are telephone network lines to which direct current is applied. The circuits 1iiT and R are connected to the input side of the diode bridge 1, and the collector side of the NPN transistor 2 is connected to the positive output side of the diode bridge 1, and the resistor 9 and switch 10 are connected to the emitter side of the transistor 2.
It is connected to the negative output side of the diode bridge 1 through a series circuit. Further, the emitter side of an NPN transistor 3 is connected to the base side of the transistor 2, and the collector side of the transistor 3 is connected to the collector side of the transistor 2, and these transistors 2 and 3 are connected in a Darlington connection.

4.5 is a resistor, and this resistor 4.5 is connected in series between the positive and negative electrode lines at the rear stage of the switch 10 on the output side of the diode bridge 1, and applies a divided voltage at the midpoint to the base of the transistor 3. This configuration provides a bias to the transistor 3. Further, a capacitor 8 is connected in parallel to a resistor 5 whose one end is connected to the negative line side on the output side of the diode bridge 1, and a primary AC coupling transformer 7 is connected to the output side of the diode bridge 1. The sides are connected. Further, a capacitor 6 for cutting off DC current is connected in series to the output positive line of the diode bridge 1 on the primary side of the AC coupling transformer 7 .

The above configuration is the same as the conventional example, but in this device, a constant current diode 11 is further installed in parallel with the transistor 2 on the downstream side of the switch 10 in the line connecting the secondary side of the AC coupling transformer 1 of the diode bridge 1. Directional connection.

In such a configuration, the telephone has an AC coupling transformer 7
connected to the secondary side of the Then, when the call state is established, the switch 10 is closed. By closing this switch 10,
The direct current supplied from the lines T and R follows the route of passing through the diode bridge 1, the collector/emitter of the transistor 2, the emitter resistor 9, the switch 10, the diode bridge 1, and returning to the lines T and R. It will flow. Here, resistance 4.5 is transistor 2°3
capacitor 8 is a filter capacitor for the base of transistor 3,
A DC voltage from the diode bridge 1 filtered by the capacitor 8 is applied to the base of the transistor 3, and the emitter output of the transistor 3 outputs this, making the transistor 2 conductive and forming a flow path for the line loop current. Ru. Since the AC coupling transformer 7 has a capacitor 6 connected in series to cut the DC component, a line loop current flows through the collector and emitter of the transistor 2, and the AC coupling transformer 7 has a capacitor 6 connected in series to cut off the DC component. This means that the AC component of the line can be transmitted via the line. As a result of the transistor 3 being driven using the line output smoothed by the filter 8 as a base signal, the collector-emitter of the transistor 2 normally exhibits a high impedance to alternating current. Therefore, the audio signal (AC signal) from the line passes through the diode bridge 1, passes through the capacitor 6, and reaches the AC coupling transformer 7. The audio component is then provided to the telephone via the AC coupling transformer 7. Thereby, the voice component can be extracted by passing a current from the line TR, and conversely, the voice signal component from the telephone can be applied to the line via the AC coupling transformer 7.

Here, it is assumed that another telephone is branch-connected between line T-R in FIG. And with this, it's time 1! T-
Suppose that the 118 voltage decreases and the voltage obtained by dividing the voltage by the resistor 4.5 becomes 1 to 1.4V or less. In this case, in a conventional circuit, transistor 2 would be in a cutoff state, the DC loop circuit from the line would be cut off, and forward current would no longer flow through diode bridge 1, resulting in a cutoff state and the audio signal would also be cut off. Put it away.

However, in this device, since there is a constant current diode 11 connected in the forward direction between the positive and negative electrode lines of the diode bridge 1, a current of about 1 to 2 mA that can be passed by this constant current diode 11 flows through this, causing a line loop. Can hold current. Therefore, it is possible to prevent the current of about 1 to 2 mA from continuing to flow through the diode bridge 1 and causing the diode bridge 1 to be cut off. Therefore,
The voice signal from the line TR passes through the diode bridge 1, passes through the capacitor 6, and reaches the coupling transformer 7, making it possible to maintain the call. Therefore, even if a branch of another telephone at the far end seen from the telephone office or a branch of a telephone with low DC impedance is connected, a call can be secured.

The present invention is not limited to the embodiments described above and shown in the drawings, and can be implemented with appropriate modifications without changing the gist of the invention.The constant current diode may be a choke coil with high AC impedance. Similar effects can be obtained by replacing .

〔Effect of the invention〕

As described in detail above, according to the present invention, when a terminal device such as a telephone is branch-connected to a line, the bias voltage must be lower than the threshold voltage of the Darlington-connected transistor forming the line loop current sink circuit. It is possible to provide a line interface circuit with good branch characteristics that allows telephone calls even if the voltage of the line drops to such an extent that it is unobtainable.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the configuration of the present invention, and FIG. 2 is a circuit diagram showing a conventional example. T, R...Line, 1...Diode bridge, 2゜3...Transistor, 4.5.9...Resistance, 7...
・AC coupling transformer, 8... capacitor, 10...
Switch, 11... Constant current diode. Applicant's agent Patent attorney Takehiko Suzue

Claims (3)

[Claims] (1) A line loop current flow with high AC impedance that is built into a terminal device connected to a line in a telephone network, receives voltage from the line via a rectifying diode bridge, and is connected in parallel to this diode bridge. In a line interface circuit, a line loop current is passed through a circuit for forming a line, and an AC component is transmitted to the line by an AC coupling transformer connected in parallel to a diode bridge via a capacitor that blocks a DC component. A line interface circuit characterized in that a circuit for forming a current flow path is configured by connecting an element for forming a bypass with high AC impedance in parallel. (2) The line interface circuit according to claim 1, characterized in that a constant current diode is used as a bypass forming element. (3) The line interface circuit according to claim 1, characterized in that a choke coil is used as the bypass forming element.