A power supply apparatus

A power supply apparatus includes a switching element for relaying electrical current to be supplied to a transformer and a controlling circuit for controlling an operation of said switching element. The controlling circuit has a first oscillator which works to continuously drive the switching element and a second oscillator which works to intermittently operate the switching element. An amount of electrical current to be supplied to the transformer is controlled by changing a duty cycle or an oscillating period of the second oscillator in accordance with a variation of the amount of load current. Further, an initiating resistance for initiating the controlling circuit and a "cut-off" circuit cutting the resistance off: it is arranged such that the resistance is "cut- off" from the controlling circuit after initiating the controlling circuit in order to avoid that a significant amount of current is consumed at the resistance after initiating the circuit.