No power supply in most circuits is pure nothing. The requirement ranges from a few milliamps to several amps dozens. Also one often needs a DC power supply, which can easily be regulated voltage and current.
Here is a small collection of power supplies for all possible applications

Here you will find Schematic Circuit Diagram for DIY and further information on Power supply DC power supply
Sponsored Ads:

Small Symmetrical power supply with voltage regulators 78XX/79XX	www.synrise.de/diy/supply/schaltplan.htm
Strengthening for a standard-78XX controller allows high currents	www.electronics-lab.com/projects/power/009/
Beautiful 0 to 30V power supply with pcb layout	www.electronics-lab.com/projects/power/003/index.html