Abstract: A transformer has a primary and a secondary. The primary is supplied by a source and is divided into permanent and selectable turns. Switch circuitry is connected by taps to the segments to selectively energize or de-energize them. The secondary is connected by a rectifier and filter to a voltage regulator which supplies a constant voltage to a load. Circuitry senses the voltage drop across the voltage regulator and comparator circuitry compares the series regulator voltage drop to reference values. The comparator generates error signals whenever the voltage drop falls outside of a desired range above the drop-out voltage of the regulator. The error signal is transmitted to an up/down counter which correspondingly increases or decreases its count. The count of the up/down counter is decoded by switch control circuitry to open and close the appropriate switches S.sub.1 -S.sub.6 to energize the desired turns of the primary to bring the series regulator voltage drop back within the desired range.