829,249. Automatic exchange systems. STANDARD TELEPHONES & CABLES Ltd. Oct. 25, 1957 [Oct. 31, 1956], No. 33388/57. Class 40 (4). In a circuit arrangement for the supervision or identification of lines, or for the marking of connecting elements allotted to the lines, the voltage supplied to the switching means which effects the supervision, identification, or marking changes abruptly when the current drawn by the switching means reaches a predetermined value. A starting and marking circuit MK for the finder switches AS, which may be cross-bar switches, is connected to individual subscriber's lines, such as T1n11, via semiconductor diodes, such as D11a, D11b and Dz11, Dell, which are backed off in known manner by the potential changes which take place on the a, b and c wires when a connection is set up through the finder stage AS. In the idle condition a current determined by the insulation resistance RJ flows in each subscriber's line via resistors such as R11a, R11b If U1 is - 60 v., U2 is - 30 v. and U3/U4 is - 47/- 40 v., the point P1 will have a potential of - 30 v. to - 39 v. for insulation resistances down to 20,000 ohms and the diodes Dz11 and De11 are backed off. If the insulation resistance of a line, say Tln11, decreases further, the potential at P1 becomes more negative until finally Dz11 and Dell become conductive, thus allowing the transistors Tz1 and Te1 in the circuit MK to conduct also. Collector current thereupon operates the tens relay Z1 and the units relay E1 relevant to the subscriber 11. This current is provided by the potential U3 (- 47 v.) supplied to point P2 by the supply device SPQ. If this potential were to remain constant, then only one marking relay, say Z1, might operate, so that the finder would not be started and the circuit MK would be out of service to other lines. To avoid this, the circuit SPQ is so designed that the potential supplied to P2 rises to - 40 v. (U4) when the current is of such a value that the most sensitive of the marking relays has just not responded, and remains at - 40 v. during further increases of current. Lock-out arrangements are provided to ensure that only one line can affect the circuit MK at any one time. Current supply equipment SPQ.-This consists of a voltage divider comprising resistors Wi1 to Wi4 connected in series between ground and a source of - 60 v. With no load current and for load currents lower than the predetermined value, P2 is at - 47 v. When the load current reaches that value the voltage drop across resistor Wi5 is so high that transistor Tsp1 conducts and its emitter-collector path shunts resistor Wi3 so that the potential at P2 rises to - 40 v.
展开▼
