JPH0736587B2 - Current loop forming circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] A switch circuit composed of a transistor and a diode and a current limiting resistor are inserted in series between the communication paths of a subscriber circuit of a digital exchange so that the switch circuit is turned on. By controlling with a set of constant current drive circuit,
This promotes downsizing and economy of subscriber circuits.

[Industrial application field]

The present invention relates to an improvement of a current loop forming circuit in a subscriber circuit of a digital exchange.

In the subscriber circuit of the digital exchange, the power feeding circuit and other various components are all becoming electronic.

In an electronic subscriber circuit, placing a direct current of a specified value (for example, 10 to 20 milliamperes) in the communication path from the power supply circuit activates the constituent elements and enables transmission of the required signals. Needed for.

In the subscriber circuit that accommodates the telephone, the telephone that is in a call is supplied with the call current of the predetermined value or more from the subscriber circuit, which is effective for activating the above-mentioned components. However, in a subscriber circuit that accommodates a terminal device that transmits and receives a predetermined signal without supplying a direct current, such as a facsimile terminal device, in order to activate the above-mentioned constituent elements, It is necessary to form a loop circuit capable of flowing the DC current of the predetermined value in the communication path of.

This type of current loop forming circuit has a performance that can sufficiently withstand a high voltage induced on a subscriber line, has a simple structure, and promotes downsizing and economy of the subscriber circuit. Is strongly desired.

[Conventional technology]

FIG. 4 is a diagram showing an example of a conventional current loop forming circuit in a subscriber circuit of this type.

In the conventional subscriber circuit, the line A on the communication path is supplied with the negative potential of the exchange power supply from the power supply circuit, and the line B on the communication path is supplied with the positive potential of the exchange power supply from the power supply circuit (normal ground potential). Shall be supplied.

In FIG. 4, the current loop forming circuit has a resistance value (for example, 3) suitable for passing a direct current of 10 mA to 20 mA between the line A and the line B of the communication path of the subscriber circuit.
A current limiting resistor 1 having a current limiting resistance of 1 to 5 kΩ, a PNPN switch 2, transistors 3 to 5, diodes 6 to 8, a capacitor 9, and resistors 10 to 12, and a constant current drive circuit 14 and 15. Composed of.

When the control signal sn is input to the input terminal of the constant current drive circuit 14 while the PNPN switch 2 in the switch circuit 13 is shut off, the constant current drive circuit 14 causes the PNPN switch 2 via the diode 6.
A short-time conduction signal is supplied to the gate terminal of the switch 2. As a result, the PNPN switch 2 becomes conductive, and the conductive state is maintained thereafter.

As described above, a direct current loop circuit is constructed between the line B and the line A in the communication path of the subscriber circuit via the current limiting resistor 1, the PNPN switch 2 and the diode 8, and 10 to 20 milliamperes from the power feeding circuit described above. DC current flows.

In such a state, the control signal is input to the input terminal of the constant current drive circuit 15.
When sf is input, the constant current drive circuit 15 energizes the transistor 3 and then the transistor 4 via the diode 7.

The energized transistor 4 short-circuits the gate terminal and the cathode terminal of the PNPN switch 2 via the diode 8.
As a result, the gate terminal of the PNPN switch 2 has a lower voltage than the cathode terminal by the forward voltage drop of the diode 8,
The PNPN switch 2 is cut off, and the cut-off state is maintained thereafter.

The transistor 5 and the capacitor 9 are B in the communication path.
PN when a high external voltage is applied between the line A and line A
A short circuit between the gate terminal and the cathode terminal of the PN switch 2 constitutes a false ignition prevention circuit for preventing the PNPN switch 2 from being erroneously turned on.

[Problems to be solved by the invention]

As is clear from the above description, in the conventional current loop forming circuit, since the PNPN switch 2 having the function of holding the conductive state and the cutoff state by itself is adopted, the PNPN switch is used.
Constant current drive circuit 14 to set switch 2 to conduction
In addition, it is necessary to provide a constant current drive circuit 15 for setting the PNPN switch 2 in a cutoff state, and also a false ignition prevention circuit for preventing malfunction due to an instantaneous external voltage. However, there is a drawback that the structure is complicated and unsuitable for high integration, and the control is complicated.

[Means for solving problems]

The present invention solves the above-mentioned problems by taking measures as shown in FIG.

That is, in FIG. 1, the current loop circuit comprises a switch circuit 22 composed of first and second diodes 16, 18 and first to third transistors 19, 20, 21; a constant current drive circuit 23; It is composed of a current limiting resistor 1. Then, the output signal of the constant current drive circuit 23 driven by the control signal is passed through the first diode 16 of the switch circuit 22 to the first
Of the first transistor 19 is connected to the base of the second transistor 20, and the emitter of the first transistor 19 and the collector of the second transistor 20 are connected to the base of the third transistor 21. It is connected to the base, the cathode of the second diode 18 is connected to the emitter of the second transistor 20 and the collector of the third transistor 21, and the anode of the second diode 18 is connected to the communication path via the current limiting resistor 1. Is connected to the line B on one side, and the emitter of the third transistor 21 is connected to the line A on the other side of the communication path.
Connect to.

The output terminal of the constant current drive circuit 23 and the second diode 18
It is also considered to insert a third diode 17 between the connection point and the connection point of the third transistor 21 and apply a predetermined bias.

[Action]

That is, according to the present invention, since the switch circuit is composed of the high breakdown voltage transistor and the diode which do not have the function of holding the conductive state and the cutoff state, the logical level of the control signal input to the input terminal of the constant current drive circuit is changed. Since only the constant current drive circuit that sets the switch circuit to the conductive state and the cutoff state is provided only by changing the configuration, the configuration is simplified and suitable for high integration, and the control is simple.
Promote downsizing and economy of the subscriber circuit.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 is a diagram showing a current loop forming circuit according to an embodiment of the present invention. The same reference numerals denote the same objects throughout the drawings.

Also in the subscriber circuit according to the embodiment of the present invention, as in the conventional subscriber circuit, the line A of the communication path is supplied with the negative potential of the exchange power supply from the power supply circuit, and the line B of the communication path is supplied. It is assumed that the power supply circuit supplies the positive potential (normal ground potential) of the exchange power supply.

In FIG. 2, diodes 16 to 18 and transistor 19 are shown.
To 21 and resistors 24 and 25 form a switch circuit 22.

When the control signal s input to the input terminal of the constant current drive circuit 23 is set to the logic “1” level, the constant current drive circuit 23 activates the transistors 19, 20 and 21 via the diode 16.

In such a state, a DC loop circuit passing through the current limiting resistor 1, the diode 18 and the transistor 21 is formed between the line B and the line A on the communication path of the subscriber circuit, and 10 to 10 of the above-mentioned power feeding circuits are formed. A direct current of 20 mA flows.

Since the output terminal of the constant current drive circuit 23 and the emitter of the transistor 20 are connected by the diode 17, the emitter potential of the transistor 20 is held in a predetermined relationship with the base potential and the collector potential, and the switch circuit 22 Prevents abnormal characteristics. If the abnormal characteristics of the switch circuit 22 have no effect, it may be considered to remove the diode 17 as shown in FIG.

In this state, when the control signal s input to the input terminal of the constant current drive circuit 23 is set to the logic "0" level, the transistors 19, 20 and 21 are not activated.

As a result, the DC loop circuit formed between the line B and the line A in the communication path of the subscriber circuit is cut off, and the DC current flowing from the power supply circuit is cut off.

As is clear from the above description, according to the present embodiment, the switch circuit 22 is composed of the high breakdown voltage transistors 19, 20, 21 and the diodes 16, 17, 18 and the like, so that high integration is easy. Further, since it does not have the function of holding the conduction state and the interruption state, the conduction and the interruption control only need to change the logic level of the control signal s input to one constant current drive circuit 23, and the configuration is simple. , Control is also simple.

〔The invention's effect〕

As described above, according to the present invention, a current loop forming circuit having a simple circuit configuration, easy integration, and simple control can be realized, and miniaturization and economicization of the subscriber circuit can be promoted.

[Brief description of drawings]

FIG. 1 is a block diagram showing the principle of the present invention, FIG. 2 is a diagram showing a current loop forming circuit according to an embodiment of the present invention, and FIG. 3 is a current loop forming circuit according to another embodiment of the present invention. FIG. 4 is a diagram showing an example of a conventional current loop forming circuit. In the figure, 1 is a current limiting resistor, 2 is a PNPN switch, 3
To 5 and 19 to 21 are transistors, 6 to 8 and
16 to 18 are diodes, 9 is a capacitor, 10, 11, 24 and 25 are resistors, and s, sn and sf are control signals.

Front page continued (72) Inventor Yasunori Inabe 3-1, Morinosato Wakamiya, Atsugi City, Kanagawa, Japan Atsugi Telecommunications Research Laboratories, Nippon Telegraph and Telephone Corporation (72) Masaaki Tanabe, 3-9-11 Midoricho, Musashino City, Tokyo Japan Telegraph Telephone Co., Ltd., Communication Network Research Laboratories

Claims (2)

[Claims] 1. A subscriber circuit of a digital exchange, wherein a switch circuit (22) comprising first and second diodes (16, 18) and first to third transistors (19, 20, 21). A constant current drive circuit (23) and a current limiting resistor (1), and the output signal of the constant current drive circuit (23) driven by the control signal is the first diode of the switch circuit (22). (1
6) and input to the first transistor (19) to drive the second transistor (20) by the output signal of the first transistor (19), and the output of the second transistor (20). Depending on the signal, the current limiting resistor (1) and the second diode (1
A current loop forming circuit comprising a current loop circuit comprising 8) and the third transistor (21). 2. A third diode (17) is provided between the output terminal of the constant current drive circuit (23) and the connection point of the second diode (18) and the third transistor (21). The current loop forming circuit according to claim 1, wherein the current loop forming circuit is inserted and applied with a predetermined bias.