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Laptop AC Adapter rating questions...


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#1 SpaCeTraNce

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Posted 21 August 2009 - 01:44 PM

What do the power rating number on laptop power supplies mean???

As an example:
A Dell 90 watt AC Adapter says:
Output: DC 19.5V 4.62 A
From what I remember about Ohms Law Volts*Amps=Watts

Here are my questions:
Are the voltage and amperes ratings MAXIMUM rating or CONSTANT ratings???
Does the ac adapter give these volts and amps all the time or just as needed???
Can I plug aa ac adapter into a device when the ac adapter is rated higher then the device??? Or will this fry the device???
Can I plug an ac adapter into a device when the ac adapter is rated lower than the device??? or will this fry the adapter???


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#2 Platypus

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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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#3 SpaCeTraNce

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Posted 23 August 2009 - 12:32 PM

Cool thanks.... So amps are drawn as needed but the voltage is constant.

I should have known that about the amps because I have frequently tested a ac adaptor for voltage and know that a good one will put out slightly over what the adaptor is rated for.


If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world. -- C.S. Lewis

The more I study science, the more I believe in God. -- Albert Einstein

Mathematics is the language with which God has written the universe. -- Galileo Galilei

I tremble for my country when I reflect that God is just, that His justice cannot sleep forever. -- Thomas Jefferson




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