Posted 21 August 2009 - 05:48 PM
The voltage output by a supply is constant. So the 19.5V PSU always supplies 19.5V.
The current rating on a supply is the maximum it can provide and still function correctly. Up to this amount, it will only supply as much current as the appliance wants.
If the adapter voltage is higher than the device requires, the device can be damaged. If the voltage is lower than the device needs, it will probably not operate, and in rare instances could also be damaged.
The adaptor current rating should be a little more than the maximum device requirement. If the device needs more current than the PSU can supply, the PSU should shut off to protect itself from damage.
So from your example, the 19.5V supply will deliver 90W if the device draws 4.62A. If you connect a laptop that only draws 3.1A, only 60W is being supplied. If the supply was plugged into a laptop that needs 14V, the laptop would probably be damaged.
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