Hi David,
A lot of freeview boxes are 12v or less. Mine is 6v & when powered from mains is steped down through a transformer in the mains plug, much the same as a mobile phone charger.
I bought a transformer from Maplins that converts from 24 or 12volt to 9,6,3,volt etc. it comes with several diferent plugs to attach to your equipment.
I think I paid about