Power supply circuit maintaining electrical isolation

A power supply circuit for converting an AC voltage of an input AC power into a desired DC output voltage, comprising:

• a first rectifying smoother for generating a DC voltage from said AC power supplied thereto;
• a standby power source supplied with said DC voltage from said first rectifying smoother and having a plurality of DC outputs;
• a second rectifying smoother for converting said AC voltage into a DC voltage;
• a main power source for converting the DC voltage of said second rectifying smoother into said desired DC output voltage;
• a main switch for actuating said main power source, electrically coupled with said primary side of said main power source and said standby power source;
• a relay controller disposed on the primary side of said main power source and said standby power source and, in response to actuation of said main switch, said relay controller is actuated by the first of the plural DC outputs obtained from said standby power source;
• and
• a relay switch disposed on the primary side of said main power source and said standby power source and, upon actuation of said relay controller, said relay switch is driven to connect said second rectifying smoother to said AC power source.