GB1251150A - - Google Patents

Abstract

1,251,150. Telephone exchange systems. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. (Philips Gloeilampenfabrieken N.V.). 8 July, 1970, No. 33174/70. Heading H4K. A subscriber's line circuit requires more current to operate a relay, such as B 1 , B 2 , than it is necessary to send round the loop to the substation circuit indicated by impedance RL, a symmetrical four-pole network, 1, 2, 3, 4, being put in the line to shunt a proportion of the line current in the exchange end of the loop and to do this regardless of loop polarity. With an onhook condition both transistors T 1 and T 2 are cut off. With an off-hook condition, and with the polarity of battery in the line circuit in one sense, transistor T 1 remains cut off but is shunted by diodes DZ 1 and D 1 , T 2 conducting to set up the shunt path in accordance with bias produced from back conducting Zener DZ 2 in the subscriber's loop. The proportion of current shunted varies in accordance with the loop impedance. With battery supplied in the opposite sense the circuit acts to the same effect but with T 1 conducting and T 2 cut off. The shunt path provides a high impedance to voice frequencies on the line, the presence of Zener diodes in the loop providing small impedance.