TECHNICAL FIELD ι

The present invention relates to a method for supplying current to subscriber telephones from a telephone exchange.

BACKGROUND ART

In telephone installations subscriber t elephones are connected to an exchange via so-called subscriber lines which are connected in the exchange to an adaption circuit, a so-called subscriber line interface circuit (SLIC). DC supply, inter alia, to the associated subscriber telephones is carried out by such a circuit.

A constant direct voltage is utilized for this power supply, according to the prior art. The current flowing through the subscriber line will thus be a function of the internal resistance of the subscriber telephone or terminal, but naturally also of the line length. The internal resistance in apparatus can be kept within narrow limits but the line length and line resistance can naturally vary considerably. If there is a constant direct voltage, this must be put at such a value that sufficient direct current can also be fed out to long lines. This results in that a greater current than necessary flows through the shorter lines and larger losses than necessary have to be reckonned with in the line, as well as in the adaption circuit, above all in the feed resistance in this circuit.

Heat due to the losses puts a limit on the packing density of the circuits in the exchange, and it is accordingly a primary object to reduce heat generation.

It is also known to utilize a voltage source with a plurality of outputs for different voltages, the selection of voltage thus being enabled with regard to, inter alia, line length such that heat due to losses is reduced.

As a further development of such a system, a solution could be considered which utilizes a continuously variable voltage source with low self power

consumption, which is controlled by the conditions on the line such that the subscriber apparatus is satisfactorily supplied with current, while the loss due to heat is minimized. A known realization according to this principle utilizes a symmetric arrangement of two so-called chopper, each feeding one of the subscriber line wires.

From the aspect of saving in components, an arrangement with only one chopper would be preferable, in spite of the unsymmetry which would be thus introduced. The Swedish Patent Application SE 78.02565-7 illustrates such an unsymmetric arrangement, in which the current feed resistance is eliminated with the exception of a small protective resistance. The magnitude of the power gain achieved is considerable, and is dependent on the relationship between the values of the simulated resistance of the apparatus and the protective resistance.

DISCLOSURE OF INVENTION

The technical problem solved by the present invention is associated with maintaining, in unsymmetric DC current feed of the kind described above, the potential of one of the two terminals of the subscriber line circuit at a constant mean value, simultaneously as the potential of the other terminal is given a mean value which is dependent on the load, in this case functionally dependent on the mean current taken out between the terminals. After possible pole reversal of the current feed, the condition for one pole must apply to the other and vice versa.

The solution to this problem is characterized as disclosed in the following patent claim.

BRIEF DESCRIPTION OF DRAWING

The invention will be described in the following in more detail with reference to the accompanying drawing, on which Figure 1 illustrates an embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of an apparatus in accordance with the invention is illustrated in Figure 1. The subscriber telephone fed with current is symbolized by a resistor R. . The telephone is connected to the associated line circuit via a subscriber line having an a wire and a J wire. The DC feed is illustrated by a voltage source 11 and a resistor 10 symbolizing the internal resistance. A pole reversal device is illustrated as being realised with the aid of a relay 9, which is controlled by unillustrated control circuits in the exchange. Reversal can naturally be provided in other ways as well. In a telephony application there are speech signals superposed on the direct voltage in both directions on the subscriber line. The corresponding alternating voltage can be assumed generat¬ ed in the AC generator 12 in this connection.

The voltage between terminal a and terminal 1J on the output of the line circuit towards the line is sensed with the aid of a differential amplifier 1. Transversal signal components e g from speech signals on the line, are filtered off from the output voltage from the amplifier 1 with the aid of a lowpass filter 2. A reference voltage U _RFF - is then added to the filtered voltage in an adding circuit 3. The output signal from the adding circuit is made to constitute the reference value in the regulating system which constitutes the invention.

A corresponding actual value is formed with the aid of a resistive voltage divider comprising two resistors 4, 5 connected between the terminals a and b_.

A longitudinal amplifier 8 with two current outputs is connected such that it can tap off current in equal phase from the a wire and the b wire against a constant potential, e g ground potential. The resistors 6 and 7 accordingly represent the internal resistance of the two current generators.

A steady state in the circuit may be as follows. The terminal a has the voltage -5V and the terminal b has the voltage -30V. The gain in the differential amplifier 1 is put at +0,5, the output voltage from this amplifier will then be -12,5V. The reference voltage U _R _- _F is put at -5V, the output voltage from the adding circuit 3 assuming the value of -17,5V, said voltage thus constituting the system reference value. This voltage is connected to the negative input of the

longitudinal amplifier 8. The resistors 4 and 5 are equal, and the system actual value will then be -17,5V. The centre point of the voltage divider is connected to the positive input of the longitudinal amplifier 8. In the conditions given above there is thus no control signal to the current generators.

Suppose now that a similar subscriber telephone, alternatively an apparatus of the same kind but having a different line length, is connected to this line circuit. The load on the line circuit will then change. Suppose now that the new load resistance becomes higher. In order to feed out sufficient current to the line the transversal voltage from the voltage source 11 must be increased. This voltage source is controlled in a manner not illustrated in the figure.

Assume that the new transversal voltage is 30V. The system then finds that the new steady state is defined by the voltage -5V on terminal a and the voltage -35V on terminal b. With the gain of +0,5 in the differential amplifier 1 there is obtained an output voltage from it of -15V. The reference value to the amplifier 8 will thus be -20V. Since the actual value from the voltage divider 4,5 will also be -20V, no control signal to the current generators is obtained.

We note thus that the condition

^U D ^U M = T ^{+ U} REF ^Where

Ua , is the arithmetical mean of the transversal voltage, U_ is the differential voltage between the terminals and b^ U _R -.-. is the reference voltage fed to the adding circuit

is met the whole time. In other words, the transversal voltage average value is all the time equal to half the difference voltage plus a constant reference voltage, irrespective of the load on the line circuit. This means that the terminal a on the line circuit is kept at a potential equal to the reference potential fed to the adding circuit.

In signalling between a public telephone exchange and a subscriber exchange, a so-called PABX (private automatic exchange) or a coin apparatus, it may be required to carry this out with the aid of pole reversal of the current feed.

OMPI

After pole reversal, with retained reference voltage and reversed sign on the amplification of the amplifier 1, a steady state is satisfied in the circuit, i e without control voltage to the current generators, of the voltages -30V on the terminal a and -5V on terminal b. The same reasoning as applied above naturally also applies to change in the load on the line circuit.

The inventive concept also includes variations of the above-described embodi¬ ment, e g with other values for the gain on the amplifier 1 and other division conditions in the resistive voltage divider 4, 5.

OMPI t

"V/IPO ^"