GB1570373A - Supply regulation and dc signalling systems - Google Patents

Description

(54) SUPPLY REGULATION AND DC SIGNALLING SYSTEMS (71) We, INTERNATIONAL STANDARD ELECTRIC CORPORATION, a Corporation organised and existing under the Laws of the State of Delaware, United States of America. of 320 Park Avenue, New York 22, State of New York, United States of America. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a system for supplying a regulated electric variable to a load: via a line, the system including an electric power source coupled to the load via switching means and the line, a detector coupled to the line and able to measure the value of the electric variable on the line and to provide a corresponding output signal, and control means having an input coupled to the output of the detector circuit and having an output coupled to the switching means to so control the operation thereof that the detector output signal and hence the electric variable both are regulated.

Such a system is known from "Choose switching regulators" by E.R. Hnatek, published in Electronic Design 6, March 15, 1975, pp 54-59. This known system is more particularly able to regulate the voltage supplied to the load.

An object of this invention is to provide a supply regulation system of the above type but which can supply a regulated voltage to a floating load, e.g. a subscriber telex equipment, and which is not liable to damage, independently of the load being connected across the line or not, when either or both of the line conductors are coupled to a fixed potention point via a spurious impedance including direct short-circuits, or when such a spurious impedance is coupled across these two conductors.

According to the invention there is provided a supply regulation system for supplying a regulated electric variable to a load via a line, including an electric power source coupled to the load via switching means and said line, a detector circuit coupled to the line and able to measure the value of the electric variable on the line and to provide a corresponding output signal, and control means having an input coupled with the output of the detector circuit and an output coupled with said switching means to control the operation thereof such that the detector output signal and hence the electric variable are both regulated, wherein the output of the detector circuit is coupled to said control means via a rectifier circuit including an operational amplifier with a feedback first resistor, wherein inverting and non-inverting inputs of the operational amplifier are coupled to the output of said detector circuit via respective ones of two oppositely poled diodes and a second resistor, said non-inverting input being grounded via a third resistor, wherein the detector circuit is coupled to each of two conductors forming the line and coupling said switching means to the load and can measure the values of said electric variable on both said conductors, the detector output signal being a function of both of these values.

Because the detector output signal is a function of the values of the electric variable measured on both conductors of the line and is regulated, each of these values is automatically limited. For instance, when the detector is a current detector normally providing a regulated output signal which is a function of the sum of the normally equal currents flowing in the two conductors, it is clear that when the load is disconnected from the line and when due to spurious conditions one of the two conductors thereof is directly grounded, the value of the current flowing in the latter conductor is limited to this sum value because no current flows in the other conductor.

The invention is applicable to a DC signalling system wherein a DC source is coupled via a line with a load which is associated with means for opening and closing said line, thus disconnecting said load from said line and connecting said load to said line respectively. Such a system is known, e.g. a telex system. Because a spurious impedance formed by a spurious capacitance shunted by a spurious resistance is generally present across the conductors of the line it is not possible to detect characters formed by opening and closing the line solely by a line current detector. Indeed, on a line opening disconnecting the load from the line the line current continues to flow until the spurious capacitance is charged to the voltage of the DC source. Note that when the DC source is a current source this current even remains constant until the capacitor has been fully charged. However, when the line is closed and when the load for instance has a small resistance, e.g. 300 ohms for a telex equipment, the capacitance rapidly discharges through the load so that the line current is very rapidly able to flow. Hence using only a line current detector circuit it is only possible to detect line closures in a relatively accurate way.

An embodiment of the invention will now be described in conjunction with the accompanying drawings in which Fig. 1 represents a DC signalling system and a supply regulation system embodying the invention, and Fig. 2 shows signals appearing in the system of Fig. 1.

This DC signalling system forms part of a telex system and is used to transmit data characters towards a subscriber telex equipment 1, connected in series with a subscriber-cdntrolled switch 2 via floating conductors 3 and 4 of a line. The characters are sent by selectively connecting the conductors 3 and 4 to a respective voltage of +60V and -60V (a mark sign), -60V and -60V (a space sign) or -60V and +60V (a special signal) and by thus supplying a constant current, no current or a current of opposite sign to the line respectively. The system can also reproduce at its output 75 data characters transmitted over the line 3, 4 and formed by the subscriber telex equipment 1 by closing or opening this line, with the help of the switch 2.

The supply regulation system is integrated with the DC signalling system so it can supply a constant current to the subscriber telex equipment 1 independently of the length of the line 3, 4 and of the electric load constituted by this equipment. It is also protected from damage when either of the conductors 3, 4 or both is or are coupled to a fixed potential point via a spurious impedance, including direct short-circuits, or when such a spurious impedance is coupled across these two conductors.

It must be pointed out that the supply regulating system may be used separately.

As mentioned above the conductors 3, 4 can be selectively connected to +60V, -60V; -60V, +60V; and -60V, -60V. However in the drawing only the connection with +60V, -60V is shown. More particularly, +60V is coupled to line 3 via the emitter-collector connection of PNP transistor 5, protection diode 6, resistor 7, lowpass filter 8, formed by coil 9 and capacitor 10, and resistor 11, -60V being further connected to the junction point of the components 7 and 9 via the so-called catching diode 12. In a similar way line 4 is coupled to -60V via resistor 13, lowpass filter 14, formed by coil 15 and capacitor 16, resistor 17, protection diode 18, the collector-emitter connection of NPN transistor 19, +60V being further connected to the junction point of the components 15, 17 via the catching diode 20.

The corresponding terminals of the resistors 11 and 13 are interconnected by resistors 21, 22 and 23, 24, and the junction points of the resistors 21, 22 and 23, 24 are grounded via resistors 25 and 26 respectively and connected to the inverting and noninverting inputs of operational amplifier 27 via resistors 28 and 29 respectively. The amplifier 27 has a feedback resistor 30, and forms with resistors 11, 13 and 21 to 29, a differential amplifier used as a current detector to detect current flow in the resistors 11 and 13.

The output of the amplifier 27 is coupled via a parallel circuit comprising resistor 31 and the series connection of resistor 78 and capacitor 79 to the inverting and noninverting inputs of an operational amplifier 32 via the oppositely-poled diodes 33 and 34 respectively. The non-inverting input of the amplifier 32 is grounded via resistor 35, whilst the inverting input is connected to its output via feedback resistor 36. The amplifier 32 and associated components 33 to 36 form a rectifier circuit. Ground is connected to the output of the amplifier 32 via bias diode 37, which output is also connected to the non-inverting input of the operational amplifier 38. The inverting input of this amplifier 38 is connected via resistor 39 to a 100kc oscillator 40 and to an integrator circuit comprising resistor 41 connected to + SV, grounded resistor 42 and grounded capacitor 43. The amplifier 38 whose output is connected to +5V via resistor 44 is a comparator.

It must be noted that the circuit including components 38 to 44 may be considered as a pulse-width modulator.

The output of the amplifier 38 is also coupled to the base of PNP transistor 45 and to the base of NPN transistor 46 via inverter 47. The base of PNP transistor 45 is also connected to +5V via resistor 48, the emitter being connected to +5V via the parallel connection of resistor 49 and speed-up capacitor 50. The collector of transistor 45 is connected to -60V via the resistors 51 and 52 in series, the junction point of these resistors being connected to the base of transistor 19. Similarly, the base of NPN transistor 46 is also connected to +5V via resistor 53; the emitter of this transistor 46 is grounded via resistor 54 and the collector thereof is connected to t-60V via the resistors 55 and 56 in series. The junction point of the latter resistors is connected to the base of transistor 5.

The junction point of the resistor 13 and butter 14 is also connected to the noninverting input of an operational amplifier 57 via capacitor 58 and resistors 76 and 59 in series, the junction point of the components 76, 59 being connected to the tapping point of a potentiometer circuit formed by the resistors 60 and 61 connected between 5V and ground. In a similar way the junction point of the resistors 11, 21 is also connected to the inverting input of the amplifier 57 via capacitor 62 and resistors 77 and 63 in series, the junction point of the components 77, 63 being grounded through resistor 64. The components 58, 76, 60, 61 and 62, 77, 64 form differentiator circuits, whilst the operational amplifier 57 is a comparator.

The output of the amplifier 32 is also connected to the non-inverting input of the operational amplifier 65, whose inverting input is connected to the tapping point of a potentiometer formed by the resistors 66 and 67 connected between 5V and ground.

The amplifier 65 and associated circuitry also is a comparator.

The outputs of the amplifiers 57 and 65 are interconnected and connected to 5V via resistor 68 and to the inverting input of operational amplifier 69 via an integrator circuit constituted by resistor 70 and grounded capacitor 71. The non-inverting input of the amplifier 69 is connected to the tapping point of a potentiometer formed by resistors 72 and 73 and connected between ground and +5V. The output 75 of the amplifier 69 which is a comparator is connected to 5V via resistor 74.

The operation of the above described systems is described hereinafter.

First the transmission of a data character to the telex equipment 1 by connecting +60V and -60V and supplying a constant current Io to the line 3, 4 is considered. As the output of the amplifier 38 is de-activated (oV), as will be explained, both the transistors 45 and 46 are operative. By the current thus flowing from +60V to ground via resistors 56 and 55, the collector-emitter connection of transistor 46 and resistor 54 conducts as does transistor 5, while due to the current flowing from +5V to -60V via resistor 49 and capacitor 50 in parallel, the emitter-collector connection of transistor 45 and the resistors 51 and 52 conducts, as does transistor 19.

Because the transistors 5 and 19 are both conductive a current I flows in the following circuit: +60V, emitter-collector path of transistor 5, diode 6, resistor 7, lowpass filter 8, resistor 11, conductor 3, telex equipment 1, subscriber switch 2, conductor 4, resistor 13, lowpass filter 14, resistor 17, diode 18, collector-emitter path of transistor 19, -60V.

The currents I11 and I13, both equal to I, flowing through the resistors 11 and 13 are measured by the differential amplifier 27 which provides an output voltage: V27 = (R11.I11 + R13.I13) . R30 if the relations: R21 = R25 + R30 R23 R26.R23 R25.R30 R21 = R22 and R23 = R24 are satisfied, neglecting R28 and R29. Rx is the resistance of resistor x e.g. R11 is the resistance of resistor 11.

If the line conductors 3 and 4 are coupled to -60V and +60V respectively, the above voltage V27 is negative. Thus to obtain a control voltage which is independent of the flow directions of the current I (or I11 or I13), the above output voltage V27 is applied to the rectifier circuitry including amplifier 32. When V27 Is negative, only diode 33 conducts so that the output voltage V32 of the operational amplifier 32 is equal to: V32 = (V27 - V33) . R36 wherein V33 is the voltage drop across diode 33. When R31 V27 is positive only diode 34 conducts so that the output voltage V32 of the amplifier 32 is then equal to: R35 V32 = (V27 - V34) . R31 + R35 wherein V34 is the voltage drop across diode 34.

In order that V32 should be equal in both cases, the relation R36 R35 iS to be satisfied.

R31 R31 + R35 Note that in the above calculations the resistor 78 and capacitor 79 have been neglected, as they only have an effect on transients, that the resistance values have been so chosen that V32 is equal to about V27 and that the diode 37 prevents the output voltage V32 from decreasing below ground potential.

The output voltage V32 of the amplifier 32 is now applied to the non-inverting input of the comparator, including the amplifier 38, to be compared with a reference voltage waveform Vref applied to the inverting input of the latter amplifier 38. This reference voltage waveform Vref is triangular and varies between about 1 6V and about 2V because the oscillator 40 alternately applies, via resistor 39 and at a rhythm of lOOkc, a voltage of about 4.5V and about 0.5V to the output of the integrator circuit formed by the elements 41, 42, 43 so that the capacitor 43 alternately charges for 5 micro-seconds to 2V and discharges for 5 micro-seconds towards 1.6V when: R39: 2.2 kilo-ohms; R41 : 6.8 kilo-ohms; R42: 3.0 kilo-ohms; C43: 10 nano-Farad.

The comparator 38 provides at its output a control pulse waveform constituted by OV and 5V pulses. Thus a change of the level from 5V to OV occurs when V32 becomes smaller than Vref, whereas a change of the level from OV to 5V takes place when V32 becomes larger than Vref. When V32 has its preferred value, which is equal to 1.8 Volts (in between 1.6V and 2V) corresponding with a preferred line current of Io = 40 mA, the 5V and OV pulses of the control pulse waveform V38 are of equal length. However, when this output voltage V32 is larger than its preferred value the duration of the 5V pulses is longer than that of the OV pulses, and the reverse is true when the output voltage V32 is smaller than the preferred value. This means that in the rest position of the system when V32 is zero the output voltage V38 is OV.

The control pulse waveform V38 is applied to the base of transistor 45 and via the inverter 47 to the base of transistor 46. Hence the time during which the latter transistors 45, 46 and therefore also the associated transistors 19, 5 conduct is a function of the duration or width of the OV pulses of the control pulse waveform V38. Thus the voltage V27 and the current supplied to the line 3, 4 and to the load are automatically adjusted to their preferred values equal to 1.8 Volts and Io - 40 mA respectively.

The resistor 78 and the capacitor 79 shunting the resistor 31 serve to prevent the line current from increasing rapidly from 0 to 40 mA and vice-versa when characters are transmitted on the line 3, 4 such rapid line curren chwnges being able to produce crosstalk with neighbouring lines. The effect of the components 78, 79 is as follows: when the switching means 5, 19 which for instance apply -60V to both the conductors 3 and 4 of the line suddenly apply +60V, -60V thereon to start the transmission of a positive mark the line current starts increasing from 0 towards 40 mA so that also the output voltage V27 gradually increases. Because in the first instance the capacitor 79 may be consi dered as a short-circuit the amplification factor by which the voltage transient V27 is amplified is not only determined by the resistance R31 of resistor 31, as described above, but also by the resistance R78 of resistor 78 so that the amplification factor is much larger. Thus the voltage V32 now more rapidly increases beyond 1.8V, which causes the switches 5 and 19 to open to slightly decrease the line current and thus also the voltage V32. When the latter lS decreased to 1.8V, switches 5, 19 are again opened to slightly increase the line current and the voltage V32 etc. Thus the line current is gradually and slowly increased and decreased towards its nominal value of 40 mA, so that there is no danger of producing cross-talk in neighbouring lines.

The above system prevents excessively high currents from being generated, which prevents damage. Indeed, when a spurious impedance such as 80, 81 is connected between the line conductors 3, 4 or when one or both of these conductors, are connected to a fixed potential point, e.g. ground, via a spurious impedance such as 82 and 83, or directly, the currents I11 and I13 in these conductors are always limited since they satisfy the above given relation: V27 = (R1 1111 + R13I13) . R30 wherein V27 is maintained substantially constant in the way described above. R23 When the switching means 5, 19 apply + 60V, -60V to the conductors 3, 4 and also when -60V, +60V is applied to these conductors the worst case occurs when the sub scriber switch 2 is open and e.g. conductor 3 is grounded, because the current I13 is then zero so that the current I11 is then maximum. However, by a suitable choice of the resistances this current may be limited to any desirable value. For instance, when Rll = R13 and when R23 = R30 the maximum value of I11 is twice its normal value; normally I11 =I13 =Io.

When -60V is applied to both the conductors 3, 4 the worst case occurs when both these conductors are grounded, because the currents I11 and I13 are then equal and of opposite sign so that V27 is then zero if R11 = R13. Hence these currents are then not limited. For this reason one chooses Rll different from R13 e.g. R11 = 2R13, so that V27 becomes different from zero and the currents I11 and I13 are again limited.

The receipt of characters formed by operating the subscriber switch 2 to interrupt the line current Io normally equal to 40 mA is described hereinafter by making reference to Figs 1 and 2.

In the same way as described above the current flowing in the line is detected and transformed into a corresponding voltage V27 by the current detector including amplifier 27, whereafter the output voltage V27 is rectified and transformed into an output control voltage V32. To be sure that a current really flows or not in the line the latter output voltage V32 is compared, in the comparator including amplifier 65, with a threshold voltage provided by the potentiometer 66, 67 and corresponding by with a threshold current It. This means that to be recognized as a non-zero current the latter should have a value larger than It. As 5V is connected to the output of the amplifier 65, if the latter output would not have been connected to that of the amplifier 57, the output voltage of the amplifier 65 would be at 5V as long as the detected current exceeds It and at OV when the detected current becomes smaller than It.

When the telex subscriber opens the line 3, 4, the constant current Io flowing thereinand shown in Fig. 2 does not instantaneously drop to zero but continues to flow through the spurious impedance formed by capacitance 80 and resistance 81 which is usually present across the two conductors 3 and 4. This capacitor 80 is thus charged between +60V and -60V. As the charge current is contant and the spurious resistance 81 is normally high, the upper plate of capacitor 80 charges substantially linearly towards +60V, see curve 81 in Fig. 2, whilst the lower plate of capacitor 8 discharges substantially linearly towards -60V, see curve 82 in the same figure. Only when these values are reached does the line current drop from IO to zero. This means that the line opening is in fact only detected by the current detector including amplifier 27 some delay time T after this opening. Consequently also the output voltage V65 of the amplifier only becomes OV after this delay has elapsed. This delay would be not harmful if a subscriber line closure would produce an equal delay. Unfortunately this is not so because upon such line closure the charged spurious capacitance is very rapidly discharged because the load has a small resistance e.g. 200 ohms. Thus the output voltage waveform V65 of the comparator 65, if not connected to the amplifier 57, is not an exact replica of the characters formed by line openings and closures.

To correct this output voltage use is made of the differentiator circuits 58, 76; 60, 61 and 62, 77, 64 and of a comparator formed by the operational amplifier 57. Due to the bias voltage provided by the potentiometer circuit 60, 61 and applied to the noninverting input of this operational amplifier 57 the output voltage thereof is normally at 5V. As mentioned above, upon the line 3, 4 being opened the voltage on conductor 3 moves to +60V whilst that on conductor 4 moves to -60V. Thus the voltages at the left hand plates of the capacitors 58 and 62 decrease and increase respectively, so that, as the non-inverting input is positively biased, the differentiated voltage applied to the inverting input of the comparator 57 substantially immediately increases above that applied to the non-inverting input thereof. Thus the output voltage V57 of the comparator 57 very rapidly changes from 5V to OV, so that substantially from the moment the line is opened a negatively directed voltage pulse V'57 (Fig. 2) is generated at the output of the amplifier 57.

If the output of the amplifier 65 were not connected to that of the amplifier 57 the latter negatively directed pulse would finish at the moment the voltages on the conductors 3 and 4 would have reached +60V and -60V respectively (see V'57 on Fig. 2).

However, as the outputs of both the amplifiers 57 and 65 are interconnected and coupled to 5V via common resistor 68, the common output voltage V57, 65 (Fig. 2) of these amplifiers remains at OV as long as one of the individual outputs of these amplifiers is at OV i.e. as long as the line is open.

Upon the line being closed the capacitor 80 rapidly discharges through the load 1, and as soon as the voltages on the upper and lower plates thereof differ somewhat from +60V and -60V respectively a constant current Io starts flowing due to which the voltage pulse V57, 65 again becomes 5V.

Thus it follows that the duration of the pulses of the pulse waveform V57, 65 exactly correspond to the line openings and closures.

To prevent spurious signals from appearing at the output 75 even when no characters are formed by the subscriber, use is made of the integrator circuit 70, 71 the aim of which is to dampen such spurious signals. Simultaneously, the circuit 70, 71 delays the output voltage of the amplifiers 57, 65 and applies it to the comparator including the amplifier 69 which provides at its output 75 a substantially exact replica of the characters formed by the subscriber telex equipment 1.

Instead of using two differentiator circuits 58-61, 76, 62-64, 77 it is also possible to use only one of them in combination with a simple detecting transistor, e.g. providing a negative output pulse upon detecting a voltage change. Moreover, even when using two such differentiator circuits, for security reasons, it is still possible instead of using the operational amplifier 57 to employ two detecting transistors and means for OR-gating their output pulses.

It should also be pointed out that the above described supply regulation system for regulating the current in the line 3, 4 may be modified in such a way that it is able to regulate the voltage applied to the load.

WHAT WE CLAIM IS: 1. A supply regulation system for supplying a regulated electric variable to a load via a line, including an electric power source coupled to the load via switching means and said line, a detector circuit coupled to the line and able to measure the value of the electric variable on the line and to provide a corresponding output signal, and control means having an input coupled with the output of the detector circuit and an output coupled with said switching means to control the operation thereof such that the detector output signal and hence the electric variable are both regulated, wherein the output of the detector circuit is coupled to said control means via a rectifier circuit including an operational amplifier with a feedback first resistor, wherein inverting and non-inverting inputs of the operational amplifier are coupled to the output of said detector circuit via respective ones of two oppositely poled diodes and a second resistor, said non-inverting input being grounded via a third resistor, wherein the detector circuit is coupled to each of two conductors forming the line and coupling said switching means to the load and can measure the values of said electric variable on both said conductors, the detector output signal being a function of both of these values.

2. A system according to claim 1, wherein said control means include a pulse-width modulator having an input coupled to the output of the detector circuit and including a reference signal source the modulator being able to generate at its said output a control pulse waveform the pulses of which have widths which are a function of both the detector output signal and the reference signal.

3. A system according to claim 1, wherein said electric variable is a current and said detector circuit is a current detector circuit.

4. A system according to claim 1, wherein said detector circuit includes a detector and a multi-port network comprising a 3-port bridge circuit with six resistances connected in a loop the junction points of which constitute terminals of the port, the ports being coupled to said power source, to said load and to said detector respectively.

5. A system according to claim 4, wherein the detector is a differential amphfier.

6. A system according to claim 1, wherein that said second resistor is in parallel with the series connection of a fourth resistor and a capacitor.

7. A system as claimed in claim 1 when used in a DC signalling system, wherein said source is a DC source and wherein the system includes means co-operating with said current detector circuit to transform said output signal into an output pulse waveform the durations of the pulses of which correspond to line openings and closures.

8. A system according to claim 7, wherein said means cooperating with said current detector circuit include at least one second detector circuit at least one differentiator circuit having an input coupled with one of said conductors and having an output coupled with said second detector circuit wherein a said line opening causes the charge from said DC source (+60V; -60V) of a spurious capacitance across said line and also a corresponding variation of the voltage on said conductors wherein said variation is differentiated by said differentiator circuit to trigger said second detector circuit which produces a second output signal indicative of said line opening and lasting as long as the voltage on said conductors varies, and wherein gating means couples the outputs of said current and second detector circuits and providing said output pulse waveform.

9. A system according to claim 7, wherein said DC source is a current source.

10. A system according to claim 8, and which includes a pair of said second detector circuits and a pair of said differentiator circuits the inputs of which are c

Claims (19)

**WARNING** start of CLMS field may overlap end of DESC **. negative output pulse upon detecting a voltage change. Moreover, even when using two such differentiator circuits, for security reasons, it is still possible instead of using the operational amplifier 57 to employ two detecting transistors and means for OR-gating their output pulses. It should also be pointed out that the above described supply regulation system for regulating the current in the line 3, 4 may be modified in such a way that it is able to regulate the voltage applied to the load. WHAT WE CLAIM IS:

1. A supply regulation system for supplying a regulated electric variable to a load via a line, including an electric power source coupled to the load via switching means and said line, a detector circuit coupled to the line and able to measure the value of the electric variable on the line and to provide a corresponding output signal, and control means having an input coupled with the output of the detector circuit and an output coupled with said switching means to control the operation thereof such that the detector output signal and hence the electric variable are both regulated, wherein the output of the detector circuit is coupled to said control means via a rectifier circuit including an operational amplifier with a feedback first resistor, wherein inverting and non-inverting inputs of the operational amplifier are coupled to the output of said detector circuit via respective ones of two oppositely poled diodes and a second resistor, said non-inverting input being grounded via a third resistor, wherein the detector circuit is coupled to each of two conductors forming the line and coupling said switching means to the load and can measure the values of said electric variable on both said conductors, the detector output signal being a function of both of these values.

2. A system according to claim 1, wherein said control means include a pulse-width modulator having an input coupled to the output of the detector circuit and including a reference signal source the modulator being able to generate at its said output a control pulse waveform the pulses of which have widths which are a function of both the detector output signal and the reference signal.

3. A system according to claim 1, wherein said electric variable is a current and said detector circuit is a current detector circuit.

4. A system according to claim 1, wherein said detector circuit includes a detector and a multi-port network comprising a 3-port bridge circuit with six resistances connected in a loop the junction points of which constitute terminals of the port, the ports being coupled to said power source, to said load and to said detector respectively.

5. A system according to claim 4, wherein the detector is a differential amphfier.

6. A system according to claim 1, wherein that said second resistor is in parallel with the series connection of a fourth resistor and a capacitor.

7. A system as claimed in claim 1 when used in a DC signalling system, wherein said source is a DC source and wherein the system includes means co-operating with said current detector circuit to transform said output signal into an output pulse waveform the durations of the pulses of which correspond to line openings and closures.

8. A system according to claim 7, wherein said means cooperating with said current detector circuit include at least one second detector circuit at least one differentiator circuit having an input coupled with one of said conductors and having an output coupled with said second detector circuit wherein a said line opening causes the charge from said DC source (+60V; -60V) of a spurious capacitance across said line and also a corresponding variation of the voltage on said conductors wherein said variation is differentiated by said differentiator circuit to trigger said second detector circuit which produces a second output signal indicative of said line opening and lasting as long as the voltage on said conductors varies, and wherein gating means couples the outputs of said current and second detector circuits and providing said output pulse waveform.

9. A system according to claim 7, wherein said DC source is a current source.

10. A system according to claim 8, and which includes a pair of said second detector circuits and a pair of said differentiator circuits the inputs of which are coupled with distinct ones of said conductors and the outputs of which are coupled with distinct ones of said pair of second detector circuits.

11. A system according to claim 7, which includes a second detector circuit constituted by an operational amplifier connected in a comparator configuration and having its inverting and non-inverting inputs coupled with the positive and negative poles of said DC source via a respective one of a pair of differentiator circuits and a respective one of said two conductors.

12. A system according to claim 11, and wherein said inputs of said operational amplifier are biased at different voltage values.

13. A system according to claim 7, wherein said DC source is coupled with said load u switching means and said line and wherein the output of said current detector circuit

is coupled with the input of control means having an output coupled with said switching means to control the operation thereof in such a manner that said detector output signal and therefore the current in said line are regulated.

14. A system according to claim 13, wherein said control means are a pulse-width modulator having an input coupled with the output of said detector circuit and including a reference signal source said modulator being able to generate at its said output a control pulse waveform the control pulses of which have widths which are function of both said detector output signal and said reference signal.

15. A system according to claim 7, wherein said current detector circuit is coupled with each of two conductors forming said line and coupling said switching means with said load and can measure the values of said electric variable on both said conductors, said detector output signal being function of both said values.

16. A system according to claim 7, wherein said detector circuit includes a detector and a multi-port network comprising a 3-port bridge circuit with six resistances connected in a loop and the junction points of which constitute terminals of said port, said ports being coupled to said electric power source, to said load and to said detector respectively, said detector being constituted by a differential amplifier.

17. A system according to claim 13, wherein said second resistor is connected in parallel with the series connection of a fourth resistor and a capacitor.

18. A supply regulating system substantially as described with reference to the accompanying drawings.

19. A DC signalling system substantially as described with reference to the accompanying drawings.