Method for current supply to a subscriber telephone from a telep

Details Method for current supply to a subscriber telephone from a telep Method for current supply to a subscriber telephone from a telep

1984-03-06

1986-10-14

Rubinson, Gene Z.

179 77, H04M 1900

Patent

active

046174265

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

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


BACKGROUND ART

In telephone installations subscriber telephones 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. Thus a greater current than necessary flows through the shorter lines and larger losses than necessary have to be reckoned with in the line, as well as in tha adaption circuit, and 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 choppers, 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 an 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 polarity reversal of the current feed, the condition for one pole must apply to the other and vice versa.


BRIEF DESCRIPTION OF DRAWING

The invention will be described in the following in more detail with reference to the accompanying drawing whose sole FIGURE 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 the FIGURE. The subscriber telephone fed with current is symbolized by a resistor R.sub.L. The telephone is connected to the associated line circuit via a subscriber line having an a wire and a b 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 w

REFERENCES:
patent: 4315106 (1982-02-01), Chea, Jr.
patent: 4385336 (1983-05-01), Takeshita et al.
patent: 4419542 (1983-12-01), Embree et al.

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