Appliances that run from low voltage DC have the input socket marked with the voltage and polarity.
These adaptors come with instructions and a complete set of plugs.
The instructions show you how to fit the right polarity.
Other Power Supplies
AC to DC
DC to DC
AC to AC
Digital Camera AC & 12v
PDA AC & 12v
Mobile Phone AC & 12v
DC to AC Inverter
Mains and 12v Plug adaptors