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I have a laptop that says it needs a 19V 3.42A power supply. I have a Dell 20V 4.51A power supply.

Can I use this power supply with my laptop, or will it destroy the machine?

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As long as the connectors are the same size and the polarity of the connectors are the same then everything should be fine. Some generic laptop power supplies have a rated output of 19-24v so as Tom Strike says, electronic equipment can generally handle some difference - and 19/20v are pretty close.

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Electronic equipment is typically capable of handling voltages of a nearly 20% difference. As for the current, the laptop will only draw what it needs. If the PS was lower I'd worry, as long as it's rated higher than needed by the laptop you're ok.

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Most probably it won't destroy your laptop. Even though, I can't recommend using it, as then someone might say I am responsible, if something goes wrong. If I had same case, I would try Dell power supply.

One volt difference isn't critical, as laptop usually tolerances rather high differences (plus minus several volts). Side effect may be that laptop gets little bit warmer.

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