Protection of active telephone line interface circuits

Details Protection of active telephone line interface circuits Protection of active telephone line interface circuits

1994-10-06

1996-07-23

Hofsass, Jeffery

Telephonic communications

Subscriber line or transmission line interface

Protective circuit

379377, 379378, 379385, 379399, 361 18, 361118, 361119, H04M 1700

Patent

active

055398200

ABSTRACT:
A telephone line interface circuit with d.c. amplifiers coupled to tip and ring paths is protected against transient voltages by a crowbar-type protection device coupled via a diode bridge to the tip and ring paths, positive and negative terminals of the diode bridge being connected respectively to ground and via a diode to a negative supply voltage line for the amplifiers. The diode prevents discharge of a capacitor, coupled between the negative supply voltage line and ground, through the protection device when it conducts. A locked up state, in which the amplifiers maintain holding current to the protection device after a transient voltage, is avoided by further diodes coupled from the negative terminal of the diode bridge to inputs of the amplifters, or by controlling a current limiter to interrupt current to the capacitor when the diode is reverse biased, so that the capacitor discharges. The current limiter includes a resistance and inductance in series for sensing current, whereby initial charging current for the capacitor is reduced on closing of relay contacts in series with the current limiter, and relay contact deterioration is avoided.

REFERENCES:
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patent: 4440980 (1984-04-01), Bakker
patent: 4661878 (1987-04-01), Brown et al.
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Application Note--"SLICs Protection Circuits" (AN297/01/89) , W. Rossi et al., SGS-Thomson Microelectronics Line Card ICS Databook--1st Edition, Oct. 89, pp. 849-853, para.4.
"Telephone Exchange Primary Protection Development towards Silicon Structures", R. Pezzani et al., L'Onde Electrique, vol. 73, No. 5, Sep. 93, pp. 55-60, XP000396444.

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