Circuit arrangement for protecting electronic interface circuit of subscriber line circuits

DETAILED DESCRIPTION A task of the invention is to provide a circuit arrangement which assures certain protection of the subscriber lines also under the circumstances described.
In accordance with an aspect of the invention, a circuit arrangement for protects an electronic interface circuit for subscriber line circuits of a digital time multiplex telecommunications network.
The subscriber line circuits are coupled group-wise to a supply battery for providing subscriber current supply, using thyristors as protective elments for providing a low impedance shunt path for over-voltages on conductors of a subscriber line compled to a subscriber line circuit.
The over-voltages exceed a supply voltage provided by the supply battery.
The circuit arrangement comprises a plurality of thyristors, each having a main conduction path coupled between a respective one of said conductors of said subscriber line and a circuit point referenced to earth potential.
Respective semiconductor switches are coupled between one main electrode and a control electrode of respective ones of the plurality of thyristors and have the supply voltage provided by the supply battery to respective control electrodes thereof.
The respective semiconductor switches require less control current than the thyristors.
The task is solved by a circuit arrangement of the aforementioned type which, in accordance with the invention, is characterized in that a semiconductor switch having a control electrode to which the supply battery voltage is applied and having a lower current requirement than the thyristors, is coupled between a main electrode and the control electrode of respective thyristors having a main conduction path coupled between a subscriber line conductor and grounded circuit point.
In the protective circuit in accordance with the invention, the supply battery need only provide the substantially lower trigger currents for the semiconductor switches, whereas the trigger current for the actual protective elements, that is, the thyristors, is branched off from the lightning current flowing towards earth potential and thereby does not load the supply battery