Disclosed is an electrical power supply apparatus, comprising a switch circuit, an output circuit, single-direction pass circuit and a control circuit. The output circuit is used for outputting a supply of main power source. The standby circuit is used for providing a supply of standby power source. The control circuit is for being capable of controlling the switch circuit according to a state of the external power source, wherein when the external power source is on, the switch circuit is in a cutoff state for allowing a first period or a second period of a power signal of the external power source transferred to the standby circuit through the single-direction pass circuit.