This is a dc regulator circuit which will give output voltage of 13.8V with current output about 10A. The circuit use 4 pieces of power transistor 2N3055 to boost the electric current, to make the circuit capable to deliver 10A current output. It also use IC regulator 7812, change the IC type will give different output voltage value.
For 13.8VDC output voltage, you need the DC input voltage about 18VDC at least. Don’t forget to mount the heatsink on the power transistor and give good trace/cabling/wiring to the circuit especially for grounding connection.
13.8V / 10A DC regulator circuit source: http://sound.westhost.com/project77.htm