"F Murtz"
** The data sheet schematic for the Natsemi LM338K will give you 1.2 to 20
volts at up to 5 amps.
However, you will need a large heatsink to dissipate 6 watts or so ( 2.5
times 25 ) at max current and low volts.
http://www.national.com/pf/LM/LM338.html
......... Phil