Abstract: In a transformer having a primary winding which is connectable to an a.c. voltage source via switch means which opens and closes at a controllable frequency, there is connected in parallel with the primary or secondary winding of the transformer a circuit which for at least a part of the time during which the switch means opens maintains magnetization of the transformer in a direction opposite to the direction of the magnetization caused by the subsequent closing of the switch means.

Abstract: A better filtering effect for disturbing harmonic currents in a high-tension direct current transmission installation is described. A balancing winding is provided between a primary winding as an outer winding and a secondary winding in a 3-winding transformer with a transformer core. The distances between the balancing winding and the primary winding and the secondary winding, respectively, are selected such that the equivalent reactance of this balancing winding is equal to or less than zero. A second balancing winding can be provided as the inner winding in a series with the first balancing winding. In the case of a 4-winding transformer, two winding arrangements are provided with two parallel-connected primary windings as the outer windings, with a separate secondary winding for each and a balancing winding for each between the primary and secondary windings, the two balancing windings being connected in series.

Abstract: A two-part separable inductive coupler is provided in one part with a current-limiting reactor whose control winding is energized by control means in response to the formation of a magnetic circuit between primary and secondary windings incorporated in said one part and the other said part respectively. Said control means may comprise a rectifier connected across said primary winding and is arranged to saturate the reactor when the coupler parts are connected and leave the reactor unsaturated when the parts are separated, so as to limit the primary current.

Abstract: A compensated circuit for use in interfacing a transmission device to a cable facility. The circuit includes a transformer one winding of which is connected to the cable facility and the other winding of which is connected through an impedance generating device to the transmission device. The circuit provides a predetermined one of a number of selectable impedances to the cable facility. The impedance generating device allows the desired impedance to be elected and includes a negative resistance generating circuit which responds to the current flowing in the loop formed by the winding and the impedance generating device.

Abstract: A telephone line circuit comprises a transformer that has a primary (13) and a secondary (14) winding. A low resistance (15) is connected in series with the primary winding. An amplifier circuit (17) is coupled across the resistance and serves to develop a predetermined dc control voltage. This control voltage is used to establish a dc current flow in the secondary which is n times that of any dc current flow in the primary, where n is the transformer turns ratio. The dc current flow established in the secondary is in a direction opposite that of the dc current flow in the primary. A second amplifier circuit (31, 36, 23) is coupled across the primary winding and serves to generate an ac control signal which is also delivered to the secondary winding to create a predetermined ac termination impedance.

Abstract: A circuit for increasing the inductance of a transformer as used in a telephone line circuit. An operational amplifier with appropriate feedback is connected across a separate winding of the transformer to provide the reactive inductance. To compensate for changes in resistance of the winding, a field effect transistor is controlled by the same amplifier output to provide a positive resistance in the feedback path and thus maintain balance. An operational amplifier and diode are arranged to cooperate with the negative inductor to prevent lock-up.

Abstract: A circuit for increasing the inductance of a transformer as used in a telephone line circuit. An operational amplifier with appropriate feedback is connected across a separate winding of the transformer to provide the reactive inductance. To compensate for changes in resistance of the winding, a field effect transistor is controlled by the same amplifier output to provide a positive resistance in the feedback path and thus maintain balance.

Abstract: A trunk interface circuit useful for remotely terminating a two-wire central office trunk at a remote telephone facility includes a current compensated miniature transformer. The transformer includes windings for coupling voice band signals between the trunk and the remote telephone facility and a compensating winding for reducing d.c. flux associated with supervisory d.c. current applied to the trunk at the central office. A circuit connected with first and second portions of a first transformer winding provides signal indication of magnitude and either of two possible directions of current flow in the first winding. A bidirectional current regulation circuit conducts a compensating current in either of two directions in the compensating winding. The compensating current is regulated in accordance with the signal indication from the circuit connected with the first winding to reduce the d.c. flux in the transformer.