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Question about power adapters, wattages, voltage and amps

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3 replies to this topic

#1 shashman


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Posted 02 September 2013 - 11:56 AM

I have a laptop that has two ratings: 18.5V/3.5 mA and 19.0V/4.74mA.  This means the watts is 64.75 for the first rating and 90.06 for the second (P=IV)

I have a charger that is rated 19.5V/3.34mA, which means the power supplied is 65.13W

The wattage of the charger is close to that of the first rating of the laptop.  Does that mean the charger will work?  Or is it something to do with the voltage and amp rating?

Also, how is it that the laptop can take both ratings of 64.75W and 90.06W?


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#2 Chris Cosgrove

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Posted 02 September 2013 - 06:51 PM

To take your second point first, I don't know !  But it could be to do with differing specifications for the laptop.


These ratings on your laptop are maximum current draws with everything running flat out - a fairly rare occurrence, as come to that, are the ratings on the charger. I am assuming here that you mean Amps and not milliAmps in your data.


So, you would be quite safe using this charger on the laptop. Yes, its maximum rating doesn't quite match the maximum drain of the computer, but it is rare for the drain to be at maximum, and if it occurred, the battery would supply the difference.


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#3 Platypus


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Posted 02 September 2013 - 07:33 PM

Also, how is it that the laptop can take both ratings of 64.75W and 90.06W?


Where do these ratings appear? What is the brand and model of laptop? I'm thinking like Chris, the laptop may have different possible configurations (CPU, GPU, battery capacity) with the variation being base spec to top spec.


Incidentally the current rating is A (ampere) not mA (milliamp). It's best to avoid over voltage rating, or under current rating. 19.5V is probably not going to be any problem for a laptop specified 18.5 - 19V, but we don't really know how close the 19V is to any internal component limit. For safe operation of the PSU current rating should be at or above the required amperage. However this does adjust to some degree - because the laptop uses switching regulators internally, its current consumption will fall proportionally as the supplied voltage rises. As long as both voltage and current remain within the safe operating range of both laptop and PSU, this retains the same total power transfer.

Edited by Platypus, 02 September 2013 - 07:34 PM.

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#4 rotor123


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Posted 03 September 2013 - 01:09 PM

Is it a Dell? HP? I've seen that sort of thing with Dell laptops. Me I'm conservative I stay with what the maker says will work. My suspicion is that the battery charge time will be longer with the lower current AC adapters. If that isn't a problem.............


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