Here is a simple one for 0-30V @ 3A with current limiting

http://www.electronics-lab.com/proje...003/index.html

If you read thru the forums you'll find a parts list and updated
schematic that expanded this to dual 0-30 @ 5A complete
with PCB's that can be downloaded.

Further reading will show how to add digital V and A meters.

Good luck

Kevin