Abstract: The output voltage of a transformer is regulated by controlling the flow of magnetic flux through a shunt core. The transformer includes a main core fabricated from U-I laminations and is arranged to form a substantially rectangular structure. A primary and a secondary coil are seated on the main core. A shunt core with shunt winding having lamination size and structure substantially analogous to that of the main core is positioned in spaced alignment to said main core. A flux interconnecting path, called the bottleneck, connects the main core and the shunt core to form a sturdy unified structure. The shunt winding is activated by control circuitry by which the flux flow through the shunt core is limited and hence the output voltage is regulated.