Abstract: A circuit for energizing a load (10) with different input voltages comprises a transformer (6), having a primary winding (n.sub.1) in series with a switching transistor (T.sub.1) the load (10), and a current sensing resistor (R.sub.2) between the input terminals. The secondary winding (n.sub.2), in series with a diode (D.sub.1) connected is in parallel with the load. A starting resistor (R.sub.1) starts the circuit oscillating via a positive-feedback loop comprising, in series, a first resistor (R.sub.4), a capacitor (C.sub.1) and a second resistor (R.sub.5). This feedback loop is coupled between one end of the secondary winding and the base of the switching transistor. A control transistor (T.sub.2), whose base is connected to the current-sensing resistor (R.sub.2), turns off the switching transistor as a function of the primary current. The energy stored in the transformer is then delivered to the load by an output current in the secondary winding.