Calculating that power supply won't burn my laptop

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The Finnish Rein Deer
评论： Fangjun Jiang 2020-2-1
These are two laptop power supplies:
(1) Output 20V 4.25A max. (2) Output 16.5V 3.65A max.
Assuming my laptop safely works with (1), I want to know if (2) would be safe to use.
The laptop input impedance is 4.70 Ohms, which means it will draw less than 3.65A from the power supply (2). So I conclude nothing will burn.
I do this by using Ohm's Law.
R=V/I - > 20/4.25 = 4.70 Ohms for power supply (1) V(2)/R(1) = 16.5 / 4.70 > 3.65A
Because the amperage is less than power supply (2), the laptop will consume less than 3.65A from the power supply, so it won't burned. The only drawback which does not constitute a hazard, would be that the laptop would get charged slower.
Could a real engineer (unlike me) confirm or correct me? Thanks!
Adelita
Note: both power supplies are from the same maker for the same line of laptops, only laptops are different models. So other variability should not be a problem.

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回答（1 个）

Fangjun Jiang
Fangjun Jiang 2020-1-30
编辑：Fangjun Jiang 2020-1-30
Modern power supplier/charger are all "smart" devices. The power/volt/max current numbers are not derived directly from the simple Ohm's law. Your logic here is most likely invalid. You could try. The second power supplier may not even be able to charge your laptop at all. 16.5V is much less than the required 20V. It may not be able to "open the gate".
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The Finnish Rein Deer
The Finnish Rein Deer 2020-2-1
Thanks for commenting. Would it be possible that the laptop may request more power than supplied by the power supply (2), and this "trying to draw" more power is damaging the laptop architechture itself? Imagine I am running Matlab on my laptop.
Fangjun Jiang
Fangjun Jiang 2020-2-1
Extreamly unlikely in your case. Not good for your laptop battery though.

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