GB699941A - Improvements in or relating to apparatus for supplying ringing current in automatic telephone-exchange and like systems - Google Patents

Abstract

699,941. Automatic exchange systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Nov. 29, 1951 [Dec. 2, 1950], No. 28023/51. Class 40 (iv). [Also in Group XL (c)] In an automatic telephone system of the kind in which the talking connection to a called line is established by a selector comprising a discharge tube of which an output electrode is coupled with the line and means is provided to render a discharge path to that electrode conductive when the line is called, ringing current is supplied to a called line through a discharge path of an additional discharge tube which is controlled in accordance with the voltages of the output electrode and of a conductor of the line so that the discharge path of the additional tube is conductive for part at least of each period of the ringing current if the discharge path of the first tube is conductive and the line loop is not closed. In the simplest form (Fig. 1), the additional tube BB is normally extinguished but when the path OB becomes conductive (line L selected) the potential of point Q drops and the potential difference between the anode a and cathode k exceeds the starting voltage of tube BB for part of the ringing cycle of generator WG. Tube BB may extinguish during the negative half-cycles but will fire again for the positive. On the reply of the called party the potential of Q increases and tube BB is extinguished. In the embodiment of Fig. 2 the discharge path OB is coupled to the line L1, L2 by a transformer and the tube BB fires when the voltage difference between the starting electrode f and the cathode k exceeds a critical value. When the line is selected current flow through R4 raises the potential at Q and tube BB fires, but on the reply of the called party the potential of k increases and that of f decreases so that tube BB is extinguished. In a modification (Fig. 3 not shown), the tube BB has two starting electrodes, one connected to Q and the other to L1. The cathode k is connected to L2 and the connection from L2 via R6 to Q is omitted. Generation of ringing current, Fig. 4. The tube BB which is a gas-filled triode has its anode and cathode connected to L1 and L2 respectively, and its grid to point Q. When the line is selected the potential of Q rises, BB conducts and discharges C1 rapidly whereupon BB becomes non-conductive. C1 is recharged slowly through R2 and R3 and the tube BB again conducts and so on. On the reply of the called party the potential across BB is reduced so that BB remains cut off.