US 5,539,820 A · Protection of active telephone line interface circuits

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.

Claims14

Dependent claims

A telephone line interface circuit comprising:

two amplifiers having outputs coupled via tip and ring paths to tip and ring terminals, respectively, for connection to a two-wire telephone line;
a diode bridge having two a.c. terminals coupled to the tip and ring paths, respectively, a positive d.c. terminal coupled to ground, and a negative d.c. terminal;
a crowbar-type protection device coupled between the negative d.c. terminal of the diode bridge and ground, responsive to a negative transient voltage at the negative d.c. terminal of the diode bridge exceeding a threshold level to short the transient voltage to ground;
a negative supply voltage line for the amplifiers;
a capacitor coupled between the negative supply voltage line and ground;
a diode coupled between the negative supply voltage line and the negative d.c. terminal of the diode bridge with a polarity such that the diode is reverse biased when the protection device shorts said negative transient voltage to ground;
and
circuitry responsive to the short of the negative transient voltage to ground for reducing current supplied by the amplifiers to below a holding current for maintaining the short by the protection device.

5 more elements

A telephone line interface circuit comprising a capacitor for smoothing a supply voltage, relay contacts via which the capacitor is charged to the supply voltage, and a current limiting circuit connected in series with the relay contacts for limiting charging current to the capacitor on closing of the relay contacts, the current limiting circuit comprising:

a current sensor and a controlled path of a transistor connected in series with the relay contacts for supplying charging current to the capacitor;
and
a control circuit responsive to voltage dropped across the current sensor for controlling the transistor to limit current through the controlled path of the transistor;
wherein the current sensor comprises a resistance and an inductance in series.

1 more element

Dependencies are inferred from the claim text.

Protection of active telephone line interface circuits

Life of this patent

Protection of active telephone line interface circuits

Life of this patent

Abstract

Claims14