A power supply for delivering voltage for use in an internal-combustion engine includes an onboard d.c. power source; an alternator connected to the d.c. power source and having an output for supplying a normal, onboard-network voltage during engine run; a step-up transformer having a primary winding and a secondary winding; and an inverter having an input connected to the d.c. power source and an output. There is further provided a change-over switch that has a first input connected to the output of the alternator, a second input connected to the output of the inverter and an output connected to the primary winding of the transformer. The change-over switch has a first switching state connecting the output of the alternator with the primary winding and a second switching state connecting the inverter with the primary winding. An rpm-responsive arrangement sets the change-over switch into the second switching state from the first switching state when the engine-rpm falls below a predetermined magnitude.