Originally Posted by jeepbluetj
Or use a 7812 and save 2 resistors.
Doesn't matter anyway, in all likelihood there's a regulator of some sort from the wall wart socket to the rest of the stuff in the router - it runs at 3.3v or 5v inside. I'd take it apart to look, but that's me.
If you want 12V out, you can't
use a 317 or 7812-type regulator. These are great linear regulators, but they require that the input voltage be at least 2V greater than the regulated output voltage. If you set one up for 12.0V output and feed it with (say) 13V, the output will be somewhere around 10.8V and will not
be regulated. Feed it with 10V and the output will be less than 8V.
You need a switcher, not a linear regulator. For the router you can probably get by with a good direct battery
connection, and forget the regulator. The battery
gives you a pretty clean voltage, but transient protection is good to have. I would be reluctant to use a bare 15V zener for that though, since the transient current will probably fry the diode. A resistor / inductor / capacitor / zener circuit would be safer.