GB2035753A

GB2035753A - Monitoring telephone line current

Info

Publication number: GB2035753A
Application number: GB7936847A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-10-25
Filing date: 1979-10-24
Publication date: 1980-06-18
Also published as: FR2439998A1; SE7908436L; CH635216A5; GB2035753B; DE2942440A1; FR2439998B1; DE2942440C2

Abstract

A device for monitoring the line current in a telephone line has a series measuring resistance (3) connected into a wire (4) of a telephone line, and connected to the measuring inputs of two comparators (7, 8). The reference potentials of the comparators (7, 8) are tapped off a voltage divider (12) in such a manner that one comparator (7) responds to positive and the other one (8) symmetrically to negative line currents. The outputs of both comparators (7, 8) are together connected to an optical coupler (24) in order to deliver a measuring signal decoupled, with respect to direct current, to a processing device. For supplying the comparators (7, 8), the voltage divider (12) and the light-emitting diode (23) of the optical coupler (24) with an AC-decoupled direct voltage, a mini-transformer (33) with separate primary and secondary windings (34, 35) is supplied with a high-frequency signal which is rectified by means of a rectifier arrangement (40, 41) connected to the secondary winding (35). <IMAGE>

Description

SPECIFICATION Monitoring telephone line current The invention relates to a device for monitoring the line current in a telephone line by means of a series measuring resistance connected into the telephone line.

It is often desirable to be able to monitor the line current in a telephone line at the telephone set of a subscriber, that is, for example, between the exchange line connection and the telephone set of the subscriber, namely to obtain information on whether or not the receiver has been taken off the telephone set and the DC connection has been established with the exchange line. This criterion is used to an increasing degree for accessory units of a subscriber connection, for example for electronic telephone number dialling units and automatic telephone answering devices.

It is known to connect a measuring resistance in series into a wire of a telephone line for the purpose of monitoring the line current, and to use the voltage drop at this measuring resistance, caused by the line current, as a monitoring signal. However, several conditions in the operation of a telephone network act against such a measure. On the one hand, the series measuring resistance must have a very small value which, according to more recent regulations, must be 10 ohms at the most. Furthermore, regulations require a high dielectric strength, a high insulation resistance and a slight capacitive coupling between the measuring element introduced into the telephone line and a processing element, connected to the former, for the monitoring signal obtained at the measuring resistance.In addition, consideration must be given to the fact that the line current can flow in both directions. It has also been proposed already to monitor the line current only for short periods and then to bridge the series measuring resistance, for example by means of a relay contact.

This has the disadvantage, however, that a reduction in the line current to a low value, for example to zero, which means the interruption of the telephone connection, cannot be detected.

It is the object of the invention to provide a device which permits the measuring of the telephone line current, with negligible permanent interference with the telephone line, without switch-over for line currents flowing in both directions, which has a specified working point for the measuring of the line current, is decoupled with respect to direct current and has a high dielectric strength and a high insulation resistance between the connections for the telephone line and the connections to the telephone line without interfering with the telephone set.

According to the invention, the device is characte rised in that it is provided with two comparators which can be connected to the measuring resistance and the reference potentials of which are obtained from a voltage divider, connected to a direct supply voltage, and which are connected for the separate measurement of positive and negative line currents, that the outputs of the two comparators are connected to an optical coupler for the isolated transmission of the measurement signal to a processing device, and that for generating the direct supply voltage a transformer is available which must be fed with an alternating high-frequency voltage and the secondary winding of which which is decoupled from the primary winding with respect to direct current, is connected to a rectifier arrangement.

In the text which follows, an illustrative embodiment of the device according to the invention is explained in greater detail with the aid of a circuit diagram represented in the sole figure.

The input terminals 1 and 2 of the device are connected to a measuring resistance 3 which is connected into a wire 4 of a two-wire telephonesline of a subscriber connection. The measuring resistance 3 has a resistance value of 1 to 6 ohms so that, after the direct-current connection between the telephone set and the telephone line has been established, it causes only a negligible drop in direct voltage and also does not damp the speech currents.

In the device, the input terminals 1 and 2 are connected to a protection circuit for excess voltages which consists of two diodes 5 connected in parallel with opposite polarities and of a parallel-connected capacitor 6. The input terminals 1, 2 are also connected to the inputs of two voltage comparators 7 and 8, one of which is connected for picking up positive line currents and the other one of which is connected for picking up negative line currents in the wire 4 of the telephone line, For this the input terminal 1 is connected via a series resistance 9 to the negative input of the comparator 7 and via a series resistance 10 to the positive input of the comparator 8.The input terminal 2 is connected to the lower end 11 of a voltage divider 12 which comprises four series-connected resistances 13, 14, 15, 16 and is connected to a positive and negative supply voltage line 17 and 18, respectively. The positive input of the comparator 7 and the negative input of the comparator 8 are each connected via a series-resistance 19 and 20, respectively, to further taps on the voltage divider 12 in order to provide these comparator inputs with a reference potential.

The comparators 7, 8, constructed as integrated circuits, are fed via the supply lines 17 and 18. The outputs of the two comparators 7,8 are connected to each other and are also connected via feedback lines, each of which contains a highly resistive resistance 21 and 22, respectively, to the respective positive input of the comparators. This produces a hysteretic behaviour in the comparators.

The outputs of the comparators 7,8 are connected to a light-emitting diode 23 of an optical coupler 24, this light-emitting diode also being connected via a resistance 25to the positive supply line 17. The emitter and collector of a phototransistor 26 of the optical coupler 24 are connected to two output terminals 27 and 28 of the device, the output terminal 27 being intended to be connected to earth and the output terminal 28 being intended to deliver a monitoring signal TEL of the line current in the wire 4 of the telephone line. The direct-current supply of the phototransistor 26 is provided via a resistance 29 which, on the one hand, is connected to a further terminal 30 of the device, to which terminal a direct supply voltage is applied, and, on the other hand, is connected to the collector of the phototransistor 26.

A Miller-effect capacitor 31, which connects the collector of the phototransistor 26 to its base, effects a delay in the switching time of the phototransistor 26, that is it produces sloping edges in the output signal, when it is driven via the light-emitting diode 23.

A direct supply voltage of approximately 5 V for the voltage divider 12, the comparators 7,8 and the light-emitting diode 23 of the optical coupler 24 is generated by means of a high-frequency signal HF applied to a further terminal 32 of the device. For the AC-decoupled transfer of the high-frequency signal of a mini-transformer 33 is provided which has separate primary and secondarywindings 34 and 35, respectively. The high-frequency signal is supplied to the primary winding 34 from the terminal 32 via an isolating and amplifying transistor 36, the collector of which is connected via the primary winding 34 to the terminal 30 of the positive direct supply voltage.A capacitor 37 is used for decoupling the high-frequency voltage from the terminal 30, a further capacitor 38 for a highly-damped resonance tuning of the output circuit of the transistor 36, and series resistances 39 for current limiting. The secon darywinding 35 is connected to a rectifier bridge 40, the direct-voltage connections of which are connected to the supply lines 17, 18 and to a charging and filtering capacitor 41.

The high-frequency signal HT fed to the terminal 32 can have a frequency of several MHz, for example approximately 3.5 MHz, and can in many cases be derived directly from the processing device which is fed with the monitoring signal TEL appearing at the terminal 28, that is for example, from a telephone dialling device, since such accessory units usually have a clock oscillator with a frequency in the abovementioned high-frequency range. Instead of that, a free-running oscillator, which can be produced at little cost, can also be provided. The direct supply voltage for the phototransistor 26 and the isolating and amplifying transistor 36, which is to be applied to the terminals 30 and 27, can also be taken from this processing device.

For the abovementioned supply frequency of the high-frequency signal the transformer 33 can have a small toroidal ferrite core which carries a primary winding 34 and a secondary winding 35, each with less than 10 turns, for example 4 to 6 turns. Because of the low number ofturns, such a transformer is not only small and can be produced at low cost, but it also makes a high degree of isolation possible between the primary and secondary winding, without special effort. Because of its low required capacity the charging capacitor 41, of course, can also have small dimensions.

In the operation of the device described, according to the direction of current flow in the wire 4 of the telephone line the voltage across the measuring resistance 3 is compared either by the comparator 7 or the comparator 8 with the reference voltage applied to the respective comparator and determined by the tap at the voltage divider 12. If, accordingly, the voltage at the negative input of the comparator 7 rises above a certain positive value, or at the positive input of the comparator 8 below a certain negative value with respect to the lower end 11 of the divider, the comparator 7 or 8 delivers a constant currentwhich flows through the lightemitting diode 23 of the optical coupler 24.If the voltage at the negative input of the comparator 7 again drops below a different certain value, or at the positive input of the comparator 8 rises above a different certain value, the output current of the particular comparator disappears. By appropriate selection of the resistances 13, 14, 15, 16 of the voltage divider 12 a fully symmetrical behaviour of both comparators 7, 8 can be achieved and the respective signal feedback via the resistance 21 and 22 is the determining factor for the abovementioned hysteretic behaviour. An example for this is that the comparator 7 switches on with a voltage of +24 mV at its negative input and the comparator 8 with a voltage of -24 mV at its positive input, and that the comparators switch off with a voltage of +12 mV and -12 mV, respectively.The hysteretic behaviour is of advantage because this avoids that in the case of a drop in the line current, which is not caused by an interruption, such an interruption in the connection is indicated.

Thus the optical coupler 24 transmits a digital output signal which is generated independently of the direction of the line current for a specified value of the line current, which signal is constantly available even in the case of fluctuations in the line current and only resumes the value 0 if the line current has dropped below an also specified lower limiting value.

The switching time of the phototransistor 26 of the optical coupler 24 is delayed by means of the Miller-effect capacitor 31 so that, when the lightemitting diode 23 switches off, a linear voltage drop with a time constant determined by the value of the capacity of the capacitor 31 is produced at the collector of the phototransistor 26. Such a characteristic of the monitoring signal TELtakes account of the fact that the line current in the wire 4 of the telephone line can be interrupted for short periods without signifying an interruption in the connection.

In certain telephone networks the exchange, for example, changes the polarity of the line current at certain time intervals which results in an interruption of the line current for about one second.

The capacitor 31 also produces a delayed rise in the monitoring signal TEL during the picking-up of a line current in the wire 4 of the telephone line. In this way short-period interference currents in the telephone line can be differentiated from the line current proper.

The optical coupler 24, too, provides the possibility of a high dielectric strength of the measuring element proper, with the comparators 7, 8 with respect to the supply and processing element with the connecting terminals 27, 28,30,32,so that overall along the dividing line 42, drawn in the figure, a total dielectric strength of typically 2,500 V, if necessary up to 5,000 V, an insulation resistance of more than 100 M-ohms, and a coupling capacity of less than 10 pF can be achieved effortlessly.

Whilst speech signals present on the telephone line 4 produce such a small voltage drop at the measuring resistance 3 that at the output of the comparators 7,8, which in themselves work quickly, no signal appears which imitates a line current, this is not always the case for the timing pulses of a frequency of approximately 10to 12 KHztransmitted in many telephone networks, particularly because for the speech signals an input impedance of a few hundred Ohms, but for the timing pulses a considerably smaller impedance, of approximately 20 to 50 Ohms, is present and the timing pulses have relatively high amplitudes. A capacitor in parallel to the measuring resistance 3, like the capacitor 6 shown, is unsuitable because of the low value of the measuring resistance 3. Filtering between the output of the comparators 7, 8 and the optical coupler 23 also fails because of the required effortfor discriminating against line current signals. Surprisingly, it has now been shown to be extremely effective to arrange a filter between the reference voltage inputs and the measuring signal inputs of the two comparators 7,8.

As shown in the figure, this filter can consist in a simple manner of one capacitor 21' and 22' each, the capacity of which, for example, is 0.1 microfarad and has thus no disadvantageously high value. The capacitors 21 22' effectively suppress the timing pulse signals occurring at the measuring resistance 3 without disadvantagous effects on the operation of the device shown.

Claims

1. Device for monitoring the line current in a telephone line by means of a series measuring resistance connected into the telephone line, comprising two comparators which can be connected to the measuring resistance, and the reference potentials of which are obtained from a voltage divider connected to a direct supply voltage, the comparators being connected for the separate measurement of positive and negative line currents, the outputs of the two comparators being connected to an optical coupler for the isolated transmission of the measurement signal to a processing device, and for generating the direct supply voltage, the transformer being fed with an alternating high-frequency voltage, and the secondary winding thereof which is decoupled from the primary winding with respect to direct current, being connected to a rectifier arrangement.

2. Device according to claim 1, wherein each comparator is provided with circuit means for achieving a switch-off voltage which is different from the switch-on voltage of the comparator.

3. Device according to claim 2, wherein the circuit means is a feedback resistance.

4. Device according to claim 1,2 or 3 wherein a filter is connected between the input connected to the measuring resistance and the input provided with a reference potential of each comparator, whereby pulses, for example timing pulses, occuring at the measuring resistance, are suppressed.

5. Device according to claim 4, wherein the filter is a capacitor.

6. Device according to any preceding claim, wherein a delay means is provided in order to delay the switching time of a phototransistor of the optical coupler.

7. Device according to claim 6, wherein the delay means is a Miller effect capacitor.

8. Device according to any preceding claim, wherein the transformer has a toroidal core, and the number of turns of the primary and secondary windings mounted on the toroidal core is less than ten.

9. Device according to claim 8, wherein the core is a ferrite core.

10. Device for monitoring the line current in a telephone line, substantially as described with reference to the drawing.