A Solid-State Ringer Isolator for Balance Improvement of Party Lines

Recent surveys of noise on subscriber loops indicate that a significant number of party lines fail to meet Bell System objectives. Investigations have shown that connection of the ringer to ground seriously degrades the balance of the wire facility and, as a result, lines having longitudinally induced voltages from power systems are quite likely to have high metallic noise. In the past, ringers have been isolated from ground, except during ringing, by using cold cathode gaseous discharge tubes. However, the tubes are not compatible with present automatic number identification schemes and, therefore, are not applicable on a large percentage of two-party lines. A new solid-state ringer isolator has been developed to solve these problems. The isolator, which is connected in series with the ringer and ground, consists of two thyristors. The circuit is designed to isolate the ringer from ground for longitudinal voltages as high as 150 volts. The large ac difference in potential across the line, which exists only during ringing, is the source of base current to the thyristor, causing it to switch to a low impedance state and the isolator has little effect on the sensitivity of the ringer.