US 4,398,066 AGranted

Automatic power denial circuit for a subscriber line interface circuit

AssigneeSiemens Aktiengesellschaft

InventorHoward Sinberg

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Abstract

A subscriber line interface circuit is disclosed for a 2-wire telecommunication subscriber line. The circuit includes a DC voltage source for supplying power to the subscriber line and an automatic power denial circuit for interrupting the power if an overload condition exists. The power denial circuit includes two resistors connected in series in one wire of the subscriber line and a transistor connected to respond to the voltage drop across these two resistors, thereby to sense an overload condition. A second transistor, controlled by the first transistor, is connected across one of the two resistors and is biased to be normally conducting when no overload condition exists. When excessive current flows through the two resistors, indicating an overload condition, the second transistor is cut off to deny power to the subscriber line.

Claims15

Dependent claims

A subscriber line interface circuit for a two-wire telecommunication subscriber line, comprising in combination:

(a) means for supplying DC power to said subscriber line including a DC voltage source connected across the two wires thereof;
(b) an automatic power denial circuit, including:
(1) a relatively low value, first resistor connected in one wire of said subscriber line;
(2) a relatively high value, second resistor connected in series with said first resistor;
(3) a first transistor connected to respond to the voltage drop across said first and said second resistors, thereby to sense an overload condition;
and
(4) a second transistor, connected across said second resistor and controlled by said first transistor, said second transistor being biased to be normally conducting when no overload condition exists, thereby providing a low resistance circuit path around said second resistor, and biased to be cut off when an overload condition exists, thereby to interrupt said circuit path around said second resistor and deny power to said subscriber line.

4 more elements

Dependencies are inferred from the claim text.