Abstract: First and second branches, each containing two thyristors in series, all four thyristors of same polarity, are connected across positive and negative supply lines, in turn connected to positive and negative terminals of D.C. voltage source. D.C. motor connected between junction of two thyristors in one branch and junction of two thyristors in other branch. One pair of diagonally opposite thyristors, or other pair of diagonally opposite thyristors, rendered conductive, for flow of motor current in respective first or second direction. First and second transistors respectively connected in positive and negative supply lines to permit and interrupt thyristor current flow. Selection of a thyristor pair for conduction occurs based on sense of motor-current error. The transistors and selected two thyristors rendered conductive and non-conductive by pulse-width-modulated actuating signal derived from value of motor-current error.