Low-voltage transistor circuit for suppressing high-voltage surges in a telephone line interface circuit
AssigneePCTEL, Inc.
InventorsRafi Rahamim, Brian Stroud
Life of this patent
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Abstract
A relay-less telephone line interface circuit is disclosed which incorporates a low-voltage transistor to suppress high-voltage, short-term voltage surges. Specifically, a low-voltage transistor such as a Central Semiconductor C2TA44, a Motorola MPSA42, or similar such transistors actually can withstand a high-voltage spike exceeding the manufacturers specified parameters. The present circuit exploits this undocumented feature by employing such a transistor in combination with a metal oxide varistor in order to provide adequate voltage surge protection for the interface circuit. Thus, a zenor diode is not necessary (avoiding on-hook problems) and the relay-less circuit still passes the FCC Part 68 test.
Claims17
A telephone line interface circuit capable of suppressing high-voltage surges, the circuit connecting to a tip line and ring line of a telephone system and consisting of:
- a metal oxide varistor (MOV) connected between the tip line and the ring line;
- a diode bridge having a first node connected to the tip line and a second node connected to the ring line;
- and
- a low-voltage loop current transistor connected to the diode bridge, wherein the low-voltage loop current transistor suppresses short-term high-voltage surges across the tip line and the ring line.
The circuit of claim 4, wherein the first resistor is 27Ω and the second resistor is 22 kΩ.
The circuit of claim 4, wherein a second transistor has a base connected to the collector of the transistor, a collector connected to the AC signal line, and an emitter connected to the fourth node of the diode bridge.
The circuit of claim 4, wherein the circuit meets the requirements of the FCC Part 68 pulse test.
A method for suppressing high-voltage surges in a relay-less telephone interface circuit comprising the steps of:
- connecting a metal oxide varistor (MOV) between a tip line and a ring line;
- and
- connecting a first node of a diode bridge to the tip line and connecting a second node of the diode bridge to the ring line;
- connecting a low-voltage loop current transistor to a third node of the diode bridge, wherein the low-voltage loop current transistor suppresses short-term high-voltage surges across the tip line and the ring line.
A telephone line interface circuit capable of suppressing high-voltage surges, the circuit connecting to a tip line and ring line of a telephone system and comprising:
- a metal oxide varistor (MOV) connected between the tip line and the ring line;
- a diode bridge having a first node connected to the tip line and a second node connected to the ring line;
- a low-voltage loop current transistor connected across the diode bridge;
- a first resistor connected between an emitter of the transistor and a third node of the diode bridge;
- a second resistor connected between a base of the transistor and a node formed by the first resistor and the third node of the diode bridge;
- an off-hook signal input line connected to a base of the transistor;
- and
- an AC signal line connected to a collector of the transistor and a fourth node of the diode bridge;
- wherein the low-voltage loop current transistor suppresses short-term high-voltage surges across the tip line and the ring line.
Dependencies are inferred from the claim text.