Telephone line ring signal and dc power generator

Abstract

A ring generator circuit provides both a ring signal and talk battery to a subscriber telephone line. The ring generator recognizes whether talk battery voltage or a ring signal voltage is required and supplies the right voltage without the use of external relays. A switching network is coupled to the primary of a transformer to generate AC power at the transformer secondary. The switches intermittently conduct current from a DC power source through the primary of the transformer at a frequency substantially greater than the ring signal. A rectifier circuit converts the generated AC power into DC power to be provided to the telephone line. A sampling circuit samples the AC power, and the sampled power is additively combined with the DC power from the rectifier to form a ring signal component whose frequency is equal to the difference between the sampling frequency and the AC power frequency. When a ring signal is to be provided to the line, the AC power is sampled at a frequency that differs from the AC power frequency by the desired ring signal frequency. When talk battery is to be provided to the line, the AC power is sampled at the same frequency as the frequency of the AC power, so that the resulting AC component of the telephone line voltage is zero.