Northern Telecom Limited

Donald S McGinn

Stanley D Rosenbaum

Brian A Sutherland

Francis Y Hung

A line interface circuit for coupling signals between a telephone line and a hybrid circuit includes a loop driver circuit for supplying energizing current to the telephone line and for driving the telephone line with alternating current signals. A loop current detector connected in series between the telephone line and the loop driver circuit, generates a voltage signal in response to current flow in the telephone line. Amplifier circuits are arranged to be responsive to differential signals on the telephone line, and to signals from the hybrid circuit, for generating a composite signal. The composite signal and the voltage signal are combined in a network having an output connected to control the operation of the loop driver circuit so that the telephone line is terminated with a preferred impedance.

Line interface circuit

High voltage converters are disclosed. The converters include at least a pair of transistor switches provided on the secondary windings of a flyback transformer. The transistor switches are controlled to be conducting, variably conducting and non-conducting so that the power flow between the transformer and a load can be controlled. Various configurations of a connection switch network are also disclosed.

High voltage converter

A circuit for protecting sensitive telephone equipment from voltage or current overloads on the TIP or RING line includes a resistor serially positioned in the line and a triac triggered by a voltage differential across the resistor. A pair of fast diodes is positioned on the line adjacent to the telephone equipment for momentarily clamping the voltage on the line at that point between ground and the voltage supply of the equipment. The mementary clamping of the voltage creates the voltage differential across the resistor which triggers the triac. The fast diodes are capable of passing current for a time sufficient to allow the triac to begin shunting current to ground. An enhanced form of the protection circuit includes a second pair of diodes extending in parallel with the fast pair of diodes and positioned on the line between the pair of fast diodes and the triac. Inclusion of the second pair of diodes allows the circuit to utilize a triac with a slower turn-on time and a higher on-state voltage.

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Almonte, Canada

Donald S McGinn