Power source system

During an intermittent operation mode, a switch is normally opened and a capacitor with a large capacitance is isolated from a circuit. Under this condition, a power source voltage is intermittently supplied to a driven device. Since a charge/discharge current of the capacitor during the intermittent operation mode is limited to the charge/discharge current of the capacitor with a small capacitance, the power consumption can be lowered. In addition, since no switch exists in the current path from a power source voltage conversion circuit to the driven device, there is no drop, due to a switch, in the voltage supplied from the power source voltage conversion circuit to the driven device. On the other hand, during a continuous operation mode in which power source voltage is continuously provided to the driven device, the switch is normally closed and a capacitor with a large capacitance is connected to the power source system. Then the noise level in the supplied power is lowered.