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- Laptop power supply question
September 28, 2010, 8:28 am
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Many questions, hope you can help with some, but first some background:
I've got an old Sony Vaio laptop which uses, or at least used, a 19.5V
psu. I subsequently purchased a universal laptop power supply given it
was considerably cheaper than a direct replacement. This has a number of
voltage choices including 19V and 20V but no 19.5V. I've run it
reasonably sucessfully on 19V but on 2 worrying occasions the laptop has
just powered down without warning and then not been able to turn it back
on, until I presume the power supply had cooled. I make the assumption it
was the power supply and not the laptop given that the laptop did turn
back on after having left it; the concern about using the power supply at
the 'wrong' voltage; and the cheapness of it's build quality.
Can anyone confirm that this kind of laptop behaviour is symptomatic of
using an undervolted power supply?
Do I run the risk of creating considerable damage (to the laptop) using
this 0.5V wanting power supply? I.e. Will the, presumed, current increase
(given V=IR and P=IV etc) is enough to cause damage?
Does this kind of logic apply to laptops and their power supplies, i.e.
how do they draw power? Constant current? Constant Power?
Re: Laptop power supply question
All these physics-type things can get very complicated and I don't
have any direct experience with problem laptop batteries, but I do
have years of experience with related stuff, including electronics and
You're still talking about running it at 19 volts, right?
So where do you get the idea that current will increase when the
voltage is less than it should be?
Let's start with V=IR, usualy written E=IR where E is electromotive
force. If the voltage is lower and the resistance is the same, the
current is also lower. Not higher.
Similarly, the power used will be lower because power use is a product
of amps time volts and both the amps and volts are lower than with
19.5 volts. Another formula, a combination of your two, is that
Power=IV=I*IR=I-squared * R, which shows in one step the great effect
lowering current has, in that power varies as the square of the
current. If the current is 8/10's what it waslower, power is 8/10 *
8/10's what it was.
It depends on what is going on in the laptop. I don't know if any
have fans that turn on and off, but if a fan turns on, it will take
more current. If the CD is spinning, that will take more current.
IIUC, the CPU will use more current when it is doing more, for
example, maybe it's copying a CD to the harddrive and there are a lot
of files and it never stops copying until it's done. This will also
heat the CPU which will cause a fan to go on if there is a fan that
does so. I know for sure from my desktop that, as a general rule,
the more tabs I have open, the hotter the CPU gets, and if I'm
watching vide in a tab, it gets even hotter. That all means it's using
Until perhaps it would use more current but no more is available. The
output of both the laptop batteries and the power supply is limited.
However until they are old, the batteries should be able to more than
meet the power requirements, and the power supply should alsway be
able to do so. Checke the output amperage rating on your new one with
the original. It should be as large, but if it's not, if it is still
as large as the maximum your laptop can use, you're okay. "It" is not
the number on the label but the actual amount of current it can put
out, but from your pov, it makes no difference, because you don't know
how much your laptop needs or how much the PSU is putting out.
What happens if the PSU is being called on to deliver more than it
can. It's output voltage drops, and electronics can act flakey if the
voltage is too low. If you can find a compatible jack and plug (I
understand Dell laptops use 3 pin plugs that are not available, but my
old Thinkpad uses a standard two-conductor design and size.) You could
make a wire to go between the cord and the computer. Then you could
measure the voltage all the time, and you could also measure current
use. I don't know if it is worth the effort, though. It depends on
how curious you are.
It used to be that a power supply, internal or external, for a simple
purpose, would have only 5 parts or so, and the nature and purpose of
each was clear. Now even a wall-wart might have 25 parts including
an IC with a 100 other parts inside. I don't know what things all
these parts do. I don't know what things your universal PSU does.
Other than regulate voltage, Votltage when a PSU isn't in use will,
on all but maybe the very expensive (100's of dollars) will always
appear higher than when it is in use. The internal resistance of the
power supply lowers the voltage when current is traveling through the
internals and out to a device.
Anyhow, have you tried the 20 volt setting. Don't sue me if I'm wrong
but it's very rare or never that an increase of 1/2 volt, 2.5%, will
do any damage to anything. Whether this will make the PSU overheat
less, I don't know.
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