how to determine PSU wattage

[Udi Finkelstein]

PSU wattage is something that hints on the maximum total power your PSU can
drive. Unless you are talking about specific Lenovo laptop DC bricks, that
somehow encode their ID over the DC bannana jack, a normal PC power supply
is not intelligent and has no communication protocol for describing its
features.

The actual power is a sum of the power dissipated across all DC rails in
practice (+5V, +12V, +3.3V, -12V, etc.).
There is no way to measure the maximum PSU wattage on a powered PC.

The only way to measure a PSU max power  is to connect it to a programmable
DC load that can change its resistance and can withstand the total power
dissipated on it. You then reduce the resistance, causing the current to
increase, until the voltage starts falling below the rated value. After
doing this on all power rails, you sum up the power of each rail to get the
total max power.

You need ofcourse to take some margins because the fact that a PSU can
produce a certain level of current on its rails when it's open on a lab
bench does not mean it can do this over long periods of time (24/7) on a
less ventilated PC case.

And another option is to simply read what is the maximum power, as written
on the power supply itself, but then there are lies, damn lies, and PC
power supply ratings.

Udi


On Wed, Jul 31, 2013 at 7:35 PM, Oleg Goldshmidt <pub at goldshmidt.org> wrote:

>
> Hi,
>
> Is there a way to determine the nominal PSU power without taking the
> computer apart (actually, preferably without powering it down)? Is there
> any vendor information that Linux could in principle read? It's a
> vanilla home desktop, not a brand name server.
>
> I know of dmidecode and lshw, but neither returned anything for
> PSU. There is nothing on the outside of the PSU that I can find (well,
> it does say "220Vac").
>
> Can acpid help? Am I out of luck?
>
> Thanks,
>
> --
> Oleg Goldshmidt | pub at goldshmidt.org
>
> _______________________________________________
> Linux-il mailing list
> Linux-il at cs.huji.ac.il
> http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
>
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