Multiple extension exchange line hold circuit - uses diode bridge and switchable thyristor with LED indication to hold exchange line loop

摘要

A diode bridge, with thyristor, and LED indication circuits holds a telephone exchange connection whilst the telephone speech circuit is disconnected to talk to a further extension prior to transfer of the call to that extension. The diode bridge makes the controlling thyristor, which holds the exchange loop, independent of the voltage polarity on the exchange line. The diode bridge (D1-D4) is connected across the exchange line (L1, L2) at a point on the exchange side of the switch hook springset in the telephone and ensures the correct polarity on a Zener diode (Z2) and thyristor (T1) in the holding circuit. In normal condition the thyristor (T1) is non-conducting and no current is drawn from the exchange line. The thyristor (T1) is turned on by operating a key (Re to short the gate and anode. An LED may be connected in series with the thyristor to indicate a held line.