| 摘要 |
A telephone line interface circuit with d.c. amplifiers (12) coupled to tip and ring paths is protected against transient voltages by a crowbar-type protection device (52) coupled via a diode bridge (28, 30) to the tip and ring paths, positive and negative terminals of the diode bridge being connected respectively to ground and via a diode (54) to a negative supply voltage line for the amplifiers. The diode prevents discharge of a capacitor (26), 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 (56, 58) coupled from the negative terminal of the diode bridge to inputs of the amplifiers, or by controlling a current limiter (34) to interrupt current to the capacitor when the diode is reverse biased, so that the capacitor discharges. The current limiter includes a resistance (38) and inductance (40) in series for sensing current, whereby initial charging current for the capacitor is reduced on closing of relay contacts (K1, K2) in series with the current limiter, and relay contact deterioration is avoided.
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