FR2535928A1 - Loop current detector in a two-wire telephone line. - Google Patents

Abstract

The invention relates to a loop current detector in a two-wire telephone line. This detector comprises two identical detection circuits connected head-to-tail and inserted in series in the line between the terminals 1 and 2. Each detection circuit includes a first branch, formed by the photoemissive diode of an optoelectronic coupler C1, C2 and by two resistors R1-R3, R2-R4 in series, and a second branch, formed by the collector-emitter circuit of a bypass transistor T1, T2 and a diode D1, D2 in the emitter circuit of the transistor. The base of the transistor is connected to the common point of the two resistors. The invention applies to the detection of the loop current of a subscriber line.

Description

 The present invention relates to a loop current detector in a two-wire telephone line.

 The detection of the current, more particularly of the direct current, circulating in a telephone line to detect the opening and closing of the loop of a subscriber line, makes it possible, as is well known, to detect the dropout, the numbering and hanging up of the subscriber at a telephone exchange or a concentrator to which he is connected.

 A circuit performing this function is described, for example, in French patent no. 74 16389 (2 279 285), filed on May 13, 1974 by the applicant. Such a circuit uses an optoelectronic coupler which has the advantage of offering a high isolation voltage protecting the circuits downstream from overvoltages which may appear on the line.

 However, a circuit of this type does not make it possible to satisfactorily resolve the problem which arises in the case of the wider use of telephone line links. Indeed, it is more and more common that a telephone connection line no longer serves only a subscriber station, but several independent terminals. When a terminal is in service, you must avoid trying to put another into service in order not to cause disturbances in the operation of the first. For that; it is necessary to know the state of the line (free or busy) and to visualize this state on the terminals. However, we do not have, at the level of the terminals, the common supply voltage (-48 V) as reference, contrary to what is happening at the central level. Thus, a detection circuit of the type described in the patent cited above cannot be used, since a device operating only with current inserted in series in the telephone line is required. Furthermore, this device must not introduce an unacceptable weakening of the signals.

 The subject of the invention is therefore a loop current detector making it possible to solve these problems.

 According to the invention, a loop current detector is provided in a two-wire telephone line, using at least one shot of their opto-electronics, the photo-emissive diode of which is crossed by at least part of the current flowing in the line. , characterized in that it comprises two identical detection circuits, connected in series in a wire of the telephone line, and in that each detection circuit, comprising an opto-electronic coupler, comprises a transistor parallel bypass on the photoemissive diode of the coupler and in series in the telephone line wire, to keep the current in the photoemissive diode lower than a predetermined value whatever the current in the line.

 Such a detector, being symmetrical, has the advantage of being able to be inserted into the line without worrying about the polarity. By keeping the current in the hot-emitting diode below a limit value, whatever the current in the line which can vary considerably from one line to another (for example between ten to fifteen milliamps and one hundred to one hundred fifty milliamps), rapid degradation of the performance of the opto-electronic coupler due to a strong current flow is avoided.

 Furthermore, since the detector according to the invention is sensitive to line currents flowing in both directions, it is possible to envisage other applications than the detection of direct current and, for example, the detection of alternating ringing signals, the discrimination of alternating and direct current or line telemetry.

The invention will be better understood and other characteristics will appear with the aid of the description below and of the attached drawings in which - FIG. 1 represents the diagram of the current detector according to
the invention; and - Figure 2 shows characteristic curves of the operation of the
detector of figure 1.

 FIG. 1 shows the diagram of a loop current detector according to the invention having two access terminals 2 and 2 and which can be inserted, via these terminals, in series in. a subscriber line wire. The detector consists of two identical detection circuits connected head to tail. The first detection circuit comprises an opto-electronic coupler C1, the photo-emissive diode of which is connected, in a first branch, in series with a first resistance Ri and a second resistance R3, and a shunt transistor T1, the circuit of which collector-emitter is connected, in a second branch, in series with a diode D1 in the emitter circuit of this transistor. The base of the transitor T1 is connected to the common point between the first and the second resistors R1 and R2.

The other detection circuit includes, identically, the coupler C2, the resistors R2 and R4, the transistor T2 and the diode D2.

 As can be seen, such a detector does not require any reference supply voltage and can therefore be used both on the subscriber side and on the central side of the telephone line.

The operation is as follows, assuming that the line current flows from terminal 2 to terminal 1, the second detection circuit then being blocked.

 The line current flows, at least in part, through the resistors R3, R1 and the photo-emissive diode of the coupler Cl. If the current in the line is such that the voltage drop in the photo-emissive diode and in the resistor R1 is less than the threshold value (for example 1.3 V), sum of the threshold voltage of the diode D1 and the voltage VBE of the transistor T1, the transistor T1 remains blocked and all the current flows in the photo diode -emissive. As soon as the current in the line exceeds a limit value, the transistor TI conducts and drifts the excess current so as to keep the current in the photo-emissive diode lower than a predetermined value. This value can be adjusted by the resistor R1 .

 Furthermore, the resistance R3 directly influences the voltage drop between terminals 1 and 2.

FIG. 2 shows the variations of the current ID in the light-emitting diode and of the voltage drop VF at the terminals of the detector as a function of the current line IL, in an exemplary embodiment where
R1 = 10 Q
R3 = 110 It
As we can see, even with an IL line current of 150 iii &, the ID current in the photo-emissive diode remains less than 4.5 mA and the voltage drop VF remains less than 1.8 V. As we 'already mentioned above, the symmetrical structure of this current detector allows easy use without precautions as to the polarity; In addition, it allows its use for alternating currents by using separately the information produced by the output transistors of couplers C1 and C2. It is thus possible to detect a ringing signal, possibly increasing the sensitivity of the detector.

 This detector can also be used for line telemetry, central side, by sending an alternative tone that does not trigger the ringing of the subscriber unit (very short, or too weak or at 400 Hz for example). We can thus test the continuity of the line without the subscriber having to pick up.

 Of course, the embodiment described is in no way limitative of the invention.

Claims (2)

 1. Loop current detector in a two-wire telephone line, using at least one opto-electronic coupler whose photo-emissive diode is crossed by at least a portion of current flowing in the line, characterized in that it comprises two identical detection circuits, connected head to tail in series in a wire of the telephone line, and in that each detection circuit, comprising a stroke their optoelectronics (Cl, C2), comprises a bypass transistor (T1 , T2) in parallel on the photo-emissive diode of the coupler and in series in the telephone line wire, to keep the current in the photo-emissive diode lower than a predetermined value whatever the current in the line.  2. Current detector according to claim 1, characterized in that each detection circuit comprises a first branch, comprising in series the photo-emissive diode of the optoelectronic coupler (Cl, C2), a first resistor (R1, R2) and a second resistor (R3, R4), and a second branch comprising the collector-emitter circuit of the shunt transistor (tri, T2) and a diode (D1, D2) in series in the emitter circuit of this transistor, in that the two branches are connected in parallel, the free terminal of the second resistor being connected to the collector of the transistor, and in that the base of the bypass transistor (Tî, T2) is connected to the common point of the first and second resistors (R1- R3, R2-R4).