This is what I was shown by an engineering frind of mine .
And you probaly have one at home right now .
If not , you can build this one for about 20 to 25 dollars .
Dual 12 volt 20 amp rails , 5 volt ,7 volt , can be configured 17 volt and 19 volt with the combined amperage from the 5 volt 12 volt rails , or the 12 and 7 volt rails .
http://www.wikihow.com/Convert-a-Com...b-Power-Supply