While AC and DC adaptors are rated using similar terminology and units, the outputs of these adaptors are very different. Due to the nature of its constantly-reversing electrical current, AC electricity can damage some electrical circuits that were designed for DC electricity. Similarly, DC electricity will cause excess heat in some electrical components, such as transformers, thus causing them to be damaged or destroyed.
Because damage can occur if the wrong type of adapter is plugged into a device, it is important to be able to tell the difference between the two. An AC adaptor will list the voltage and electrical current settings it is converting from (such as 120 V AC, 1.5 A, 60 Hz) first. The voltage that the AC adapter is converting to should appear below the “from” voltage (such as 12 V AC, 15 A). AC voltage may also be represented by a line that resembles a tilde (~). An example of this would be “120 V ~ 1.5 A, 60 Hz”.
such as:compaq Presario R3000 AC Adapter
http://www.udtek.com/compaq-presario-r3000-ac-adapter-135w-19v-71a-3prong-p-32680.html
Compaq Presario R3000 AC Adapter 135W 19V 7.1A 3-prong Description:
Input: AC100-240V (worldwide use)
Output: DC19V 7.1A
Power: 135W
Outlet: 3-prong
Connecter size:
Internal Diameter: 2.5mm
External Diameter: 5.5mm
A DC adaptor will also have the voltage and electrical current settings it is converting from listed first (such as 120 V AC, 1.5 A, 60 Hz). However, the voltage that a DC adaptor is converting to will be listed as “DC” (such as 12 V DC, 10 A), or will contain a symbol that has dashed lines below a solid line (see photo).
Which adaptor is correct for my device?
Power requirements of an electrical deviceTypically, your electronic device will tell you which type of electrical current is required. This will also be in the form of “12 V DC, 10 A” or “12 V AC, 10 A, 60 Hz” (see photo).