A power supply apparatus includes an output voltage generator, a reference voltage
generator, a voltage divider, and a voltage control circuit. An input voltage is
supplied from a direct current power source. The output voltage generator generates
a constant output voltage based on the input voltage. The reference voltage generator
generates a reference voltage. The voltage divider divides the constant output
voltage into a divided voltage in accordance with a voltage dividing ratio variable
in response to an externally-input control signal. The voltage control circuit
controls the output voltage generator to regulate the constant output voltage such
that the divided voltage from the voltage divider is equalized to the reference voltage.
