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My adapter is broken, and I'm currently using another one while I'm waiting for a new adapter.

The input voltage and amps are the same for both adapters, while there are difference in the output specification.

The original adapter is 19V and 3.42A, and the one I'm using now is 20V 3.25A.

How will this affect the laptop? Is it safe to use this adapter for a week?

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While it should work, I would strongly recommend against using it. (also, is the polarity same?) – Akash May 12 '12 at 17:50
YYes, the polarity is the same. – Lars May 12 '12 at 17:53
I know it's not recommended, but It's my only possibility at the moment, and i need to use the laptop for the next days. Is it likely it will affect my computer over a short period, and if so, in which way? – Lars May 12 '12 at 17:57

While the laptop will have circuits to protect against overvoltage and regulate the voltage to the needed levels (any decently-designed laptop should have those), this will result in some overheating which might be harmful in the long run.

Probably nothing bad will happen, however.

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thanks for the answer. So you think I most likely will be able to use this adapter for a short period of time without any crucial damages? – Lars May 12 '12 at 18:00
I'm definitely not an expert in electronics, but just out of curiosity, would it be worse using an adapter with lower output voltage vs. higher output voltage? – Lars May 12 '12 at 18:06
I don't know, sorry. As far as I can tell, using an adapter with lower output might result in erratic operation or no charging. – Renan May 12 '12 at 18:07
20 volts is 5% more than 19v, and within (or right at) the typical tolerance of power supplies and input voltage specifications. But if the substitute power adapter has a high voltage output (& they typically do), then the input voltage could be more than 10% higher and out of spec for the laptop. – sawdust May 12 '12 at 22:57

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