Abstract: A power unit including a self-oscillating series resonance converter which is intended to supply a load, for instance an electric bulb, a battery charger and/or electronic equipment. The series resonance converter includes two mutually connected transistors, each of which is activated by a respective control transformer, and further includes an inductor and a capacitor which are connected in series. Each transistor and an associated series-connected capacitive voltage divider and/or a further transistor are connected in parallel with the inductor, capacitor and control transformer. The load is connected across the capacitor. The magnetic flux through the control transformer can be influenced by a magnetic flux applied externally via an electromagnet to change the oscillation frequency of the transistors and therewith the voltage output of the power unit.