Circuit arrangement for protecting electronic interface circuit of subscriber line circuits

Claim 1

Circuit arrangement for protecting an electronic interface circuit for subscriber line circuits of a digital time multiplex telecommunications network, said subscriber line circuits being coupled group-wise to a supply battery for providing subscriber current supply, and using thyristors as protective elements for providing a low impedance shunt path for over-voltages on conductors of a subscriber line coupled to a subscriber line circuit, said over-voltage exceeding a supply voltage provided by said supply battery, comprising: a plurality of protective thyristors each having a main conduction path coupled between conductors of a respective subscriber line and a circuit point referenced to earth potential, and each having a respective control electrode responsive for turn-on to a first control electrode current magnitude; and a plurlaity of respective semiconductor switch means, each having main electrodes and a respective control electrode responsive for turn-on to a second control electrode current magnitude which is substantially less than said first control electrode current magnitude, said plurality of semiconductor switch means having their main electrodes coupled between one main electrode and a control electrode of respective ones of said plurality of protective thyristors, and having their control electrodes connected in common to said supply battery, whereby in the event of lightning or another over-voltage event, said supply battery need only provide the substantially lower trigger currents for the plurality of semiconductor switch means, and the control electrode currents of said protective thyristors are drawn from the shunted lightning or over-voltage currents via the main conduction paths of said semiconductor switching means, in order to render said protective thyristors conductive so that shunting of the lightning or other over-voltage currents to earth potential can occur and so that said supply battery does not otherwise become loaded down by the larger control electrode currents of the protective thyristors.