Arthur Hagen

Daniel James <wastebasket@nospam.aaisp.org> wrote:
> In article
> news:<slrnfkluma.q2l.aznomad.2@ip70-176-155-130.ph.ph.cox.net>, AZ
> Nomad wrote:
>
>> power supply: don't use the crap that is included with most cases.
>> A blowout on that $8 power supply can take out your whole system.
>> Spend $50-90 here.
>
> I'll add a rider to that -- spend those dollars on a good quality PSU
> rather than a lower-quality unit that claims a higher power output.
> Work out roughly how much poser your system will actually use, add
> 10-20% as a margin for error, and buy the best PSU you can that
> exceeds that power by as little as possible.

I strongly advise people to choose a PSU that delivers more than 10-20%
above what you will actually use. The main reason is that PSUs deteriorate
over time, and after a year or two, you /will/ run into problems unless you
have a stronger powersupply that can show wear, but still deliver enough.
And no, buying a quality PSU won't stop the deterioration in output over
time. (Buying a very cheap one might accellerate it, though.)

> My Athlon64 4200+ system in the M2NPV-VM (with integrated graphics --
> faster GPUs eat energy) idles at less than 100W (as measured by my
> UPS) and probably uses less than 200W maximum. My 430W PSU is huge
> overkill! (but I got it cheap)

No, it's not. The total wattage is irrelevant. You need to measure the
current polled /per rail/. When a PSU is "430W", that's the total power for
the powersupply on all rails, and what's unused on one rail won't become
available on another. It may be able to deliver 170W on two 12V rails, 75W
shared between the 3.3V and 5V rails, and 15W for the negative currents.
When new and cold. If old and heated up, it will be able to deliver far
less.

Look at how much power is available /per rail/ when buying the PSU, and
estimate your needs based on that. Usually, the manufacturer hides this
information in plain sight, relying on the customers not to be tech savvy
enough to realise that you get the wattage by multiplying the voltage (V) by
the current (A) and divide it by the number of rails for that voltage. As
an example, if a PSU states +12V : 14A and has two 12V rails (typically one
to the motherboard and one to hard drives and other peripherals), you get
12*14/2 W per rail, or max 84 W. While a HD might be rated at 7W, it may
require twice that for spinning up, and with many hard drives in addition to
other external +12V devices, 84W might not be enough.

Also beware of rounded numbers. If the label says 20A, 10A, 5A for three
rails, it's likely to be numbers rounded *up* to the nearest multiple of 5,
and 20A might in reality mean anything from 16-20A, and for 16A, that's a
48W difference for a 12V rail.

In other words, if you estimate that your rig will at max use 200W, it would
be sheer lunacy to think that a 230W power supply would be big enough. You
may be able to run on it for a while, until you do something that takes all
the power on one of the rails (like smartd spinning up all drives at the
same time, whether mounted or not, while you're burning a DVD), or until the
PSU becomes old or hot.

What's likely to occur if the PSU is slightly underrated is that one of the
voltages will drop to the point that some circuitry that uses it will
experience errors. This may be the RAM, a HD, or just about anything, and
you might not even think of the PSU being the culprit.

That said, there's no point in buying a PSU that's much bigger than what you
will /actually/ need on each rail, keeping in mind that the specs are likely
overrated (yes, even with "top quality" PSUs) and that you add a little
extra for reduced efficiency as the PSU gets older and/or hotter, and any
peripherals you might want to add in the future (like an extra HD or a PCI
card).

Regards,
--
*Art