Hi Mick,
There are somany different methods. You can use a simple variable resistor, connect it across the supply and you can vary the voltage at the variable pin by rotating the screw.
You can generate any voltage from 0 to 12 on the variable pin if you are using a 12V power supply.
I guess you want to know something more, is that so?
Ofcorse it is not very easy to generate 19 volt from a 12V supply.