Transmission network including flux compensation

摘要

Magnetic flux generated in a core of a transformer employed in a transmission network by subscriber loop direct current is compensated by controllably generating a current to induce a magnetic flux equal and oppositely phased to that induced by the loop current. This is achieved without need for an additional transformer winding other than the normal coupling windings by employing a current sensing element in circuit with the normal primary transformer winding which is effectively electrically isolated from a controllable current source connected in circuit with the normal secondary winding. The magnitude of the compensation current supplied to the secondary winding is controlled via the primary current sensing element. Negative feedback enhances the stability and linearity over a desired current range. Isolation between the primary and secondary circuits is realized by employing optical isolators, each including a light emitting diode and associated phototransistor.