power source for a laptop

I am asking this as much out of curiosity as need. Besides the plug having to fit into the computer, what are the specs that must match
for a power source to work with a laptop, or a printer, mp3 player or anything else for that matter.
Is there a website that might have that information?
I am not very familiar with electrical products and engineering.
Thanks for the help.
Don't bother using the email, spam forced me to shut it down.
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paul13 expressed precisely :

It needs to be:-
1. The correct type of supply (AC or DC)
2. If DC the correct polarity at the plug
3. The correct voltage and perhaps regulated if DC
4. It needs to have a suitable output current for the device it is to supply
For a laptop, these generally require DC, 18 to 20v, at between 1.5 to 4amps.

The manufacturers web site for the appliance might prove to be helpful.
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