DIRECT CURRENT COMPENSATION CIRCUIT FOR TRANSFORMER COUPLINGS

1,226,846. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. 30 April, 1968 [2 May, 1967], No. 20370/68. Heading H4K. Net magnetomotive force in line transformers, particularly in systems having PNPN crosspoint networks having a holding current in the talking path, is reduced with consequent reduction in transformer cost by providing an optical coupling between each line loop and corresponding exchange loop so that the resistance of an exchange loop is automatically adjusted about a mean value to provide a current balancing the current in the line loop. As shown in Fig. 1 photon-emissive gallium arsenide diodes 120, 121, in the line loop control respective photonsensitive resistors 133, 134 in the exchange loops. Variations in length of line loop result in loop currents between 10 and 50 milliamperes. In an alternative arrangement, Fig. 2, not shown, a single photon-sensitive resistor between a centre-tap on the transformer secondary and earth is employed, the resistor being controlled by photon-emissive diodes in both legs of the line loop.