Telephone subscriber current supply circuit - uses controlled constant current sources for call current pulses and loop current

摘要

The current supply circuit for a telephone subscriber uses a power supply (S1a, S1b) controlled by the call clock pulses connected to each leg (a, b) of the line for supplying call current pulses (IS) of the same polarity. The operating mode of each current source (S1a, S1b) alters in response to the subscriber handset being lifted to provide the modulated loop current. The circuit uses no coils, relay contacts or transformers and it can be fully integrated with the 2-leg/4-wire branch circuit within the exchange.