Abstract: A ringer isolator is used in conjunction with a pair of telephone lines to isolate ringer coils coupled to one of the telephone lines from a reference terminal except when a ringer signal is applied to the telephone lines. The ringer isolator includes an isolator such as a neon lamp/photo resistor coupler or a transformer partly coupled between the telephone lines and partly coupled to a thyristor such as a silicon controlled rectifier or a triac. The thyristor is coupled in series relationship between the ringer coils and the reference terminal. Normally, the thyristor is maintained nonconductive so that the ringer coils cannot be energized by potentials on the telephone lines. In response to the ringer signal being applied to the telephone lines, the isolator enables the thyristor to be rendered conductive so that the ringer coils are energized by the ringer signal.

Abstract: An induction cancellation circuit is used to minimize the effects of AC induced signals appearing on a pair of telephone lines on the activation of a current sensor by a DC loop current flowing on the telephone lines. An input of the current sensor is coupled to one of the telephone lines by a coupling resistor so as to receive energizing potentials from both the AC induced signals and the DC loop current. On the other hand, the having an cancellation circuit includes an amplifier having an amplifier input coupled to the other telephone line by a DC blocking capacitor. The amplifier is biased at unity gain and provides an output signal equal to but 180.degree. out of phase with the AC induced signals received at its input.