说明：  VARIABLE POWER SUPPLY USING A FIXED-VOLTAGE REGULATOR IC..Avoltage regulator (also called a ‘regulator’) with only three terminals appears to be a simple device, but it is in fact a very complex integrated circuit. It converts a varying input voltage into a constant ‘regulated’ output voltage. Voltage regulators are available in a variety of outputs like 5V, 6V, 9V, 12V and 15V