A simple 12 volts voltage regulator that can be used to supply your electronic devices from your car can be constructed using few common components .  This 12 volts car adapter circuit use a 12.7 volts zenner diode but maybe you can’t find a 12,7 volts zenner , so in that case you can use a 12 v or 13 volts zenner diode . If you will use a 12 volts zenner you will need to stick in another diode like 1N4001 (in this case the output voltage will be a little under 12 )  and if you will use a 13 volts zenner the output voltage will be around 12.3 volts .

The 2N3055 transistor must be mounted on a heatsink .