Telephone line circuit with differential loop current sensing and compensation

Abstract From the Canadian patent

Abstract of the Disclosure The invention provides a telephone line circuit in which the likelihood of exposing circuit devices in an associated switching facility to potentially damaging currents or voltages is greatly reduced. A repeat coil includes a primary winding for connection to a subscriber loop circuit. Direct loop current is maintained in the primary winding and the resulting d-c flux is compensated for by a regulator including a balancing winding. The core material in the repeat coil is saturable and limited in bulk to that sufficient to transmit normal a-c voice signals across the line circuit. Signals of a magnitude which cause the core material to saturate are thereby impeded …