This invention relates to a supply device including: the input lines of a three-phase supply N, L1, L2 and L3, output line of a single-phase supply n, 1, a voltage processor, main relays J1, J2, J3, J4, J5, J6 asnd J7 and voltage mutual-inductors V1, V2, V3, V4, V5 and V6. This simple structure can provide most suitable supply to single-phase supply applicances, which either guarantees their normal use in the region net or automatically balances the three-phase supply net and provides the ideal supply condition for single-phase supply applicances to guarantee power devices and supply applicances to operate efficiently and selects three-phase voltages to be used for single supply when the net is at the super-low voltage.