Voltage and protection arrangement for a telephone subscriber line interface circuit

A protection arrangement for a line circuit comprising:

• current sensing means for sensing current flowing through the telephone subscriber line;
• line circuit isolation means for selectively coupling the line circuit to the telephone subscriber line;
• power supply isolation means for selectively coupling a power supply to the line circuit;
• and
• control means for operating the power supply isolation means to decouple the power supply from the line circuit in response to a current sensed by the current sensing means exceeding a current threshold, and to recouple the power supply to the line circuit responsive to a predetermined time interval having passed;
• wherein the power supply isolation means comprises a FET having a source for connecting to the power supply, a drain for connecting to the line circuit, and a gate;
• and an interface circuit connected to the source and drain of the FET, having an input connected to the control means, and an output connected to the gate of the FET, the interface circuit for operating the FET in saturation mode to couple the power supply to the line circuit and for turning off the FET to decouple the power supply from line circuit;
• and
• wherein the interface circuit comprises a voltage divider having first and second resistors, the first resistor connected to the source of the FET at one end and to the gate of the FET at the other end, and the second resistor connected to the gate of the FET at one end;
• a pnp transistor having a base connected to ground, an emitter coupled to the controller means, and a collector connected to the other end of the second resistor, and a zener diode having an anode connected to the source of the FET and a cathode connected to the gate of the FET.

A method of protecting a lien circuit connected to a power supply and to a telephone subscriber line from an over-current condition, the over-current condition being defined as when current flowing through the telephone subscriber line exceeds a predetermined current threshold value, comprising the steps of:

• a) checking for a presence of the over-current condition;
• b) starting, responsive to the over-current condition being present, a timer of predetermined duration;
• b)(1) checking, responsive to starting the timer and to the over-current condition being present, for a presence of an over-voltage condition, the over-voltage condition being defined as when voltage on the telephone subscriber line exceeds a predetermined voltage threshold value;
• b)(2) stopping, responsive to the over-voltage condition being present, the timer and continuing the method from the step (a) of checking;
• c) disconnecting, responsive to the timer having expired, the line circuit from the power supply;
• d) waiting a predetermined amount of time;
• and
• e) reconnecting the line circuit to the power supply.

A method of protecting a line circuit connected to a telephone subscriber line and to a separate power supply from an over-voltage condition, the over-voltage condition being defined as when voltage on the telephone subscriber line exceeds a predetermined voltage threshold value, comprising the steps of:

• a) checking for a presence of the over-voltage condition;
• b) starting, responsive to the over-voltage condition being present, a first timer of predetermined duration;
• c) disconnecting, responsive to the timer having expired and to the over-voltage condition being present, the line circuit from the power supply and from the telephone subscriber line;
• d) waiting a predetermined amount of time;
• e) reconnecting the line circuit to the power supply and to the telephone subscriber line;
• f) checking for the presence of the over-voltage condition;
• g) restarting, responsive to the over-voltage condition being present, the first timer;
• and
• h) disconnecting, responsive to the first timer having expired after being restarted and to the over-voltage condition being present, the line circuit from the telephone subscriber line.

A method of protecting a lien circuit connected to a telephone subscriber line and to a separate power supply from positive and negative over-voltage conditions, the positive over-voltage condition being defined as when voltage on the telephone subscriber line exceeds a predetermined positive voltage threshold value and the negative over-voltage condition being defined as when voltage on the telephone subscriber line exceeds a predetermined negative voltage threshold value, comprising the steps of:

• a) checking for a presence of the positive over-voltage condition;
• b) checking, responsive to the positive over-voltage condition not being present, for a presence of the negative over-voltage condition;
• c) starting, responsive to the negative over-voltage condition being present, a first timer of predetermined duration;
• d) disconnecting, responsive to the timer having expired and to the negative over-voltage condition being present, the line circuit from the power supply and from the telephone subscriber line;
• e) waiting a predetermined amount of time;
• f) reconnecting the line circuit to the power supply and to the telephone subscriber line;
• g) checking for a presence of the positive over-voltage condition;
• h) checking, responsive to the positive over-voltage condition not being present, for a presence of the negative over-voltage condition;
• i) restarting, responsive to the negative over-voltage condition being present, the first timer;
• and
• j) disconnecting, responsive to the first timer having expired after being restarted and to the negative over-voltage condition being present, the line circuit from the telephone subscriber line.