If you are a hobbyist or a professional electronic engineer who make many electronic experiment, troubleshooting, or testing, providing wide range variable voltage power supply in your lab or workshop will make your life easier. You can build your variable power supply using linear adjustable regulator, but switching technology let you design a very wide voltage adjustment range without loosing the efficiency too much. Switching regulator is used in this variable voltage power supply circuit to preserve the efficiency around 70%.  Here is the schematic diagram of the circuit:

variable switching regulator circuit schematic

The heart of this variable power supply is the LM2575 integrated circuit chip from National Semiconductor. This IC gives you a very simple design, require only few external components.  The optional filter shown in the dashed line box is recommended for cleaner voltage output, give the best performance for lab requirement. A grounded metal box is recommended for this variable power supply casing, since it elliminate the EMI radiation. Using a grounded metal case will make sure that your power supply cicruit won’t violate FCC rules regarding EMI radiation. [Source: National Semiconductor Application Notes]