In order to energize a load with an output current generally conforming to a control voltage, trains of firing pulses are alternately applied during respective conduction intervals of a recurrence period to a pair of main thyristors connected to opposite terminals of a balanced power supply which may be a DC source. The main thyristors are quenched, at the ends of their respective conduction intervals, by ancillary thyristors which lie in series with a resonant circuit including a commutating capacitor and alternately receive firing pulses in continuous succession in order to keep that capacitor operatively connected at all times to one or the other terminal of the power supply. The transmission of firing pulses to the several thyristors is timed by a logic circuit under the control of a comparator to which the control voltage is fed together with a sawtooth voltage establishing the aforementioned recurrence period.