LM 317 is the popular adjustable voltage regulator IC commonly used to make variable power supply. It is a three terminal device with Input, Output and Adjustment pins. Actually, LM317 is an Op Amp with relatively high output current capacity. The Inverting input of the Om Amp is Adjustment pin, while the Non-inverting input is set by an internal band gap voltage reference which produces a stable reference voltage of 1.25 volts.
LM317 can be used to design a variable power supply using just two resistors. Addition of two capacitors can smooth the output voltage. In the figure, the capacitor C1 removes input noise and C2 removes output transients. Output voltage range can be fixed by selecting appropriate resistors.