A power supply (V1) for an electrical device comprising a transformer (X1) having primary and secondary windings. The primary winding is connectable to an AC voltage supply and circuitry on the secondary side is arranged to provide a DC output voltage for the electrical device. The power supply also comprises a switch (111—transistors Q1 & Q2) between the primary winding of the transformer and the AC supply, and a rectifier (Diode D6) for rectifying the AC voltage. The switch is arranged to switch on at some point as the rectified AC voltage increases, once it has reached a non-zero value, thereby providing a current flow through the primary winding and hence through the secondary winding when the switch (111—transistors Q1 & Q2) is switched off. The switch is further arranged to switch off before the rectified AC voltage starts to increase again.