Here are 4 simple 12V power supply circuits with output voltages around 12V. First power supply circuit is built with BD139, one zener diode and a few passive components. Each of the schematic is very simple to construct and will function without problems if you respect the maximum power supply ratings.
12V BD139 power supply circuit
LM7812 power supply schematic
A very simple PS circuit with the basic 3 Amper version of LM7812 IC.
LM317 variable power supply circuit
2N3055 adjustable power supply schematic
This power supply circuit has a over-current protection and a good stabilized voltage. It can deliver up to 1.6 A.