A circuit with a magnetically variable transformer having primary and secondary windings wound on a magnetic core wherein the transformer core with associated windings is placed in close proximity to an independently wound control coil. The transformer core is placed within the bore of the control coil and an optional focusing armature concentrates the magnetic field at the poles. Application of a control current forms poles at the control coil extremities and causes a change in permeability of the transformer core thereby altering the power output of the transformer inversely to the magnitude of the control current. The control current from the output of the secondary coil of a current transformer in series with the load and conditioned by a feedback conditioning circuit modulates the level of the control current. The magnetically variable transformer controls a D.C. to A.C. inverter circuit, which is useful in supplying power to a fluorescent lamp and other A.C. receptive loads. Additionally, a microprocessor optionally modulates the feedback from the secondary of the current transformer while receiving inputs from manual and automatic environmental controls.