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Have a 40W power supply - will it power a laptop that needs 65W?

I know that the basic calculation, voltage x amps = required wattage, so 18.5v x 3.5A =65W

I realise that it should only draw the power it needs, but in practice, will it damage anything? Can supplying not enough power cause problems?

Bottom line is, I'm trying to under-power the laptop.....

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It really depends on both the power supply and the machine in question. My rule of thumb is to not load a power supply over about 75% of it's rated max. So, you're pretty far over. I'd like to think the worst case situation is you burn out the PS pretty fast, but if it's a crappy one you could damage your laptop. Unless you like breaking things, I suggest you just find a bigger PS for that. Also, I think you mean 'power brick' as your laptop should have part of a regular power supply within it.. –  Doc Sep 27 '11 at 19:58
    
Thanks for the quick response.... you pretty much told me what i suspected - was clinging to the hope that it would just switch off if underpowered..... Thanks! –  Neil Sep 27 '11 at 20:16
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It should handle under and over current situations like that, but with electronics being as competitiveness as they are, you sadly can't assume the designer was able to afford the necessary parts. It'd probably fail gracefully, but it's rarely worth taking that chance. –  Doc Sep 27 '11 at 20:25
    
It will power the laptop just fine if there is No battery installed, power supplies for notebooks are designed to power the notebook and charge the battery at the same time, so 40 watts will be enough to run the notebook without a battery charging. –  Moab Sep 27 '11 at 21:12

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up vote 3 down vote accepted

Not only would you be underpowering the laptop but you will be overloading the power supply so something's got to give - take anything from:

  • Power supply overheats and dies
  • Power supply trips its internal overload protection and shuts off
  • Laptop partly works but, say, disks do not spin up properly.
  • Everything works but the power supply gets much hotter than is good for it.

Best use the right PSU or get a replacement.

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Thanks.... as i commented above, i pretty much knew it would be a stupid idea.... Luckily for me, the packaging didn't have any seals, so back to the shop to swap it for the correct one! –  Neil Sep 27 '11 at 20:18

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