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Protection devices and arrangements for telephone lines
| Details |
Inventors: Borkowicz, Jerzy;
Assignee: Northern Telecom Limited (Montreal, CA)
Primary Examiner: Jennings; Derek S.
Assistant Examiner:
Attorney, Agent or Firm: Haley; R. John
A two-wire telephone subscriber line protection arrangement consists of two protection devices each comprising a pair of complementary thyristors connected in reverse parallel with one another between a respective wire and a reference point, with their gates connected together and directly to a control terminal which has a connection to the wire via a resistance which determines an overcurrent threshold level for the device. The reference point is ground for the ring wire protection device, and is desirably the ring wire for the tip wire protection device, whereby a directional sensitivity of the protection devices is taken advantage of and the overcurrent threshold level can be reduced. Zener diodes connected between complementary thyristors provide for convenient overvoltage protection. |
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DETAILED DESCRIPTION According to one aspect this invention provides a protection device comprising: first and second terminals, for connection to a wire of a telephone subscriber line and to ground respectively, and a third terminal; a first SCR (semiconductor controlled rectifier) having a cathode and an anode connected to the first and second terminals respectively, and a gate adjacent to the cathode coupled directly to the third terminal; a second SCR, complementary to the first SCR, having an anode and a cathode connected to the first and second terminals respectively, and a gate adjacent to the anode coupled directly to the third terminal; and resistive means connected between the first and third terminals, whereby a predetermined current flowing via the resistive means triggers one of the SCRs to conduct current between the first and second terminals.
In order to facilitate providing an overvoltage protection function in such a protection device, the device preferably includes zener diode means having an anode connected to an electrode of the second SCR between the gate and cathode thereof, and having a cathode connected to an electrode of the first SCR between the gate and anode thereof, the zener diode means having a reverse breakdown voltage which is less than reverse breakdown voltages of junctions of each SCR adjacent said electrode thereof.
According to another aspect this invention provides a protection device for a wire of a two-wire telephone subscriber line, comprising: first and second complementary SCRs (semiconductor controlled rectifiers) having controlled paths connected in parallel with one another between first and second terminals of the device for connection to the wire and ground respectively, and having complementary gates connected together and directly to a third terminal of the device for connection to a central office termination for said wire; and zener diode means, having a reverse breakdown voltage which is less than reverse breakdown voltages of central junctions of the SCRs, connected in parallel with the central junctions of the SCRs
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