JPH0681204B2 - Call current supply circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a call current supply circuit used in a subscriber circuit or an incoming trunk of a telephone exchange, and in particular, a low withstand voltage transistor is used as a call current supply current amplification transistor. The present invention relates to a call current supply circuit suitable for use.

[Background of the Invention]

The subscriber circuits and incoming trunks of modern telephone exchanges are becoming increasingly electronic. In particular, the call current supply circuit is becoming electronic by using a transistor or the like as described in Japanese Patent Publication No. 57-14061. However, since the subscriber circuit and the incoming trunk are, by their nature, in a position where they are directly susceptible to external surges such as lightning surges and electric waves via the external cable, the electronic components used are
There is no choice but to use parts with high pressure resistance.

However, increasing the withstand voltage of electronic components has a limit in terms of price, and therefore, it is necessary to reduce the withstand voltage to a level at which there is no practical disadvantage.

Therefore, a high-performance lightning protection circuit is provided in the preceding stage of the subscriber circuit and the incoming trunk to suppress an inflowing external surge, thereby lowering the withstand voltage of electronic components constituting the subscriber circuit and the incoming trunk.

However, in the conventional call current supply circuit, as shown in FIG. 4, the diode D is inserted in series with the transistor TR. Therefore, when the polarity of the external surge is opposite to that of the transistor TR, a high voltage is applied to the transistor TR. Therefore, the withstand voltage of the transistor TR to be used needs to have a withstand voltage higher than the voltage suppressed by the lightning protection circuit provided in the preceding stage.

[Object of the Invention]

An object of the present invention is to provide a speech current supply circuit in which an abnormally high voltage is not applied to a transistor regardless of the polarity of an external surge.

[Outline of Invention]

To achieve this object, in the present invention, a diode is short-circuited between the emitter and collector of the current amplification transistor of the speech current supply circuit so that a reverse voltage is not applied between the emitter and collector of the transistor. Then, when an external surge is applied, the fuse in the lightning protection circuit is positively blown to prevent abnormal high voltage from being applied to the speech current supply circuit for a long time.

Example of Invention

An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a block diagram of a main part of a call current supply circuit according to this embodiment, and FIG. 2 is a block diagram of a subscriber circuit or an incoming trunk. FIG. 3 is a block diagram of the lightning protection circuit provided in the preceding stage of the call current supply circuit.

First, explaining the subscriber circuit (or incoming trunk) of FIG. 2, the A0 and B0 terminals are connected to a communication terminal or incoming line via a lightning protection circuit described later shown in FIG.
The A1 and B1 terminals are connected to the switching unit in the telephone exchange. The call current is supplied by connecting the call current supply circuit 1 and the resistor R0 to the A0 terminal to, for example, a −48V power source, and connecting the B0 terminal to the call current supply circuit 1 and the ground G via the resistor R1. Be done. When the communication terminal or the incoming line is activated, a current flows through the resistors R0 and R1, and the call detection unit 2
The call is detected at.

In the electronic high performance lightning protection circuit shown in Fig. 3, A0 and
An external cable is connected to the B0 terminal, and the subscriber circuit or incoming trunk shown in FIG. 2 is connected to the A1 and B1 terminals via the intra-station cable. In the figure, 3 is a primary arrester 4
Is a secondary arrester. Fuses ₀ and ₁ are inserted between the arresters 3 and 4.

The call current supply circuit 1 of FIG. 1 provided at the subsequent stage of such a lightning protection circuit is provided with a call current supply current amplification transistor TR that has a high impedance with respect to a voice signal.
The emitter and collector of the transistor TR are short-circuited by the diode D.

Then, when an extraneous surge is input and an abnormal high voltage is applied to the collector of the transistor, a current flows through the transistor TR or the diode D regardless of the polarity of the abnormal high voltage. That is, the fuse ₀ in FIG.
_A current flows through the fuse ₁ , and the fuses ₀ and ₁ are blown.
Therefore, the transistor TR does not need to have high withstand voltage, and moreover, the breakage of the call current supply circuit is prevented.

〔The invention's effect〕

According to the present invention, the fuse in the lightning protection circuit provided in the preceding stage of the call current supply unit is positively blown when the abnormal voltage is applied, and the breakage of the call current supply circuit is prevented. Therefore, the current amplification transistor and high withstand voltage are unnecessary.

[Brief description of drawings]

FIG. 1 is a block diagram of a main part of a call current supply circuit according to an embodiment of the present invention, FIG. 2 is a block diagram of a subscriber circuit or an incoming trunk, FIG. 3 is a block diagram of a lightning protection circuit, and FIG. It is a principal part block diagram of the conventional call current supply circuit. DESCRIPTION OF SYMBOLS 1 ... Call current supply circuit, 2 ... Call detection part 3 ... Primary arrester, 4 ... Secondary arrester TR ... Transistor, D ... Diode ₀ , ₁ ... Fuse

Claims (1)

[Claims] 1. A call current supply circuit provided in the call current supply circuit, which is used in a subscriber circuit or an incoming trunk of a telephone exchange, and in which a lightning protection circuit having a built-in fuse is arranged in a preceding stage. Speech current supply, characterized in that the emitter and collector of the amplification transistor are short-circuited with a diode for preventing reverse breakdown and the fuse is blown by making the transistor or the diode conductive when an external surge is applied. circuit.