| 摘要 |
1,195,341. Automatic exchange systems. SUPERIOR CONTINENTAL CORP. 15 Feb., 1968 [2 Aug., 1967], No. 7469/68. Addition to 1,185,678. Heading H4K. [Also in Division H2] The Specification describes two separate improvements in the invention claimed in the parent Application. In one improvement each additional subscriber equipment is adapted to use a conventional A.C. bell ringer while in the other improvement the battery is connected to the line pair so that the direction of the D.C. charging current in the line pair is irrelevant. The parent describes a telephone system in which line pairs leading to ordinary subscriber sets may be additionally used by added subscriber sets employing a carrier frequency and driven by local batteries charged from the exchange. The added sets employ D.C. bell ringers switched on by a transistor circuit on the reception of a carrier frequency. In one improvement (Fig. 5) this transistor circuit 234 switches on an oscillator 418 which generates an alternating (20-oycle) ringing voltage suitable for a conventional A.C. ringer. The oscillator is switched on by a positive potential on a line 236. The potential is divided by a resistor 430 and a diode 428, and biases both of a pair of transistors 424 and 426 towards conduction. One transistor commences to conduct first inducing a current in one half of a centre-tapped primary winding 420 of a saturable transformer. While the transformer is saturating currents induced in each half of a further centre-tapped winding 422 control the bases of the transistors to turn the conducting transistor hard on and switch the other off. When the transformer saturates the current in the winding 420 reduces and the currents in the winding 422 reverse causing the conducting transistor to switch off and vice versa. This periodic reversal (at a frequency determined by the magnetic time constants of the transformer) induce the required alternating current in the secondary winding 440 leading to an A.C. ringer 238a. In one form of the other improvement the battery is connected to the line pair via a full-wave rectifier with a by-pass capacitor for carrier frequency across each end. In another form (Fig. 3) a full-wave rectifier 355 and a carrier frequency by-pass capacitor 392 are connected to the line pair 112 and the output of the rectifier enables a modified Hartley oscillator comprising a transistor 360, an LC circuit comprising a first portion of a primary winding 364 of a transformer 362, and a feedback path comprising the remainder of this winding and an RC network 374, 376. The output of the oscillator as picked up in the secondary winding of the transformer, is again full wave rectified by diodes 380, 382 and a capacitor 386 to give the charging current. |
