A power supply includes a rectifier circuit to rectify power, a smoothing circuit unit to smoothen a voltage of the rectified power, and a voltage drop circuit unit to drop the smoothened voltage. The voltage drop circuit unit includes an electric charge storage circuit to output the voltage-dropped voltage, a resonance circuit to receive first current from the smoothing circuit unit and to supply second current to the electric charge storage circuit, and a current interruption circuit to control the first current and the second current using zero current switching for performing switching when current does not flow using a resonance circuit.