Over-voltage protection circuit for use with a telephone appliance
AssigneeActiontec Electronics, Inc.
InventorsMinsiu Huang, Jingwei Lin
Life of this patent
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Abstract
Over-voltage protection circuitry for use with a PCMCIA modem card or a mobile modem is disclosed. The protection circuitry detects excessive voltage between the tip and ring lines of a telephone subscriber loop when the modem is “off-hook”, and immediately places the modem “on-hook”, protecting the line interface circuitry, when the over-voltage condition is detected. The over-voltage protection circuitry of the present invention may share common components and signal lines with the modem's ring-detection circuitry, and may be implemented using a minimal number of additional components.
Claims17
Protection circuitry for use in a telephone appliance having line interface circuitry adapted to be coupled to a telephone line and ring-detection circuitry, the protection circuitry comprising:
- a switch, connected between the interface circuitry and the telephone line, the switch selectively having an open state and a closed state wherein the line interface circuitry is coupled to the telephone line;
- detection circuitry comprising a pair of diodes and portions of the ring-detection circuitry, connected to the telephone line in parallel with the line interface circuitry, to detect an over-voltage condition on the telephone line when the switch is closed, the diodes having a breakdown voltage above which current may flow through the diodes in a reverse direction, the diodes disposed with opposing directions of forward current flow at voltage levels below the breakdown voltage, the portions of the ring-detection circuitry generating a signal indicative of the over-voltage condition when the switch is closed and the voltage on the telephone line exceeds a predetermined voltage level determined by the breakdown voltage of the pair of diodes, wherein the ring-detection circuitry and the detection circuitry share a signal line connecting the ring-detection circuitry and the detection circuitry to the controller;
- and
- a controller coupled to the detection circuitry and the switch, the controller responsive to the signal causing the switch to assume the open state.
Protection circuitry for use in a modem, the modem electrically coupled to an external telephone subscriber line having a tip line and a ring line, the modem including ring-detection circuitry and line interface circuitry adapted to be coupled between the tip line and the ring line, the protection circuitry comprising:
- a switch, connected in series in the tip line between the line interface circuitry and the external telephone subscriber line, the switch having an open state and a closed state, wherein the line interface circuitry is coupled to the tip and ring lines;
- detection circuitry comprising a resistor and a pair of diodes coupled to the tip line, and an opto-coupler that also comprises a portion of the ring-detection circuitry, the detection circuitry being connected in parallel with the line interface circuitry between the switch and the line interface circuitry to detect an over-voltage condition on the tip and ring lines when the switch is closed, the diodes having a breakdown voltage above which current flows through the diodes in a reverse direction, the diodes disposed with opposing directions of forward current flow at voltage levels below the breakdown voltage, wherein the detection circuitry generates a signal indicative of the over-voltage condition when the switch is closed and the voltage between the tip and ring lines exceeds a predetermined voltage level determined by the breakdown voltage of the pair of diodes;
- and
- a controller coupled to the detection circuitry and the switch, the controller responsive to the signal to cause the switch to assume the open state.
A method of protecting the line interface circuitry of a modem connected to a telephone line and ring-detection circuitry, the method comprising steps of:
- using a switch to selectively open or close an electrical connection between the telephone line and the line interface circuitry;
- detecting an over-voltage condition when the voltage on the telephone line exceeds a predetermined level using diodes having a breakdown voltage above which current may flow through the diodes in a reverse direction, the diodes disposed with opposing directions of forward current flow at voltage levels below the breakdown voltage, portions of the ring-detection circuitry being coupled to the diodes and generating a signal indicative of the over-voltage condition when the switch is closed and the voltage on the telephone line exceeds a predetermined voltage level determined by the breakdown voltage of the pair of diodes, wherein the ring-detection circuitry and the detection circuitry share a signal line connecting the ring-detection circuitry and the detection circuitry to the controller;
- and
- opening the electrical connection between the telephone line and the line interface circuitry when the over-voltage condition is detected and the electrical connection between the telephone line and the line interface circuitry is closed.
Dependencies are inferred from the claim text.