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Date:      Tue, 17 Nov 2009 11:39:31 -0800
From:      Chuck Swiger <>
To:        Bill Moran <>
Cc:        =?ISO-8859-1?Q?D=E1nielisz_L=E1szl=F3?= <>, FreeBSD - <>
Subject:   Re: hdd voltage
Message-ID:  <>
In-Reply-To: <>
References:  <> <> <>

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Hi, all--

On Nov 17, 2009, at 10:27 AM, Bill Moran wrote:
[ ... ]
> Not all power supplies are created equal.  Unfortunately, there's
> no government oversight on power supply ratings, thus a cheap 450W
> power supply might go unstable if it has to supply 200W for very
> long, whereas a good quality 200W power supply might be able to
> put out 450W for short periods reliably.

A very good-quality power supply with a thermally activated circuit  
breaker might tolerate a 250% overload for 20 seconds to a minute, but  
anything with a fuse is likely to blow in some tens of  
milliseconds.  :-)

There are some widely used standards for computer power supplies;  
almost all modern machines want ATX12V which is used by Intel P4s,  
Core, etc and AMD Athlon, Athon64 platforms.  Multicore boxes commonly  
want another extension to the base ATX standard called EPS12V; both  
are well-documented here:

The other major standard in 80-plus certification, which is linked to  
Energy Star ratings; if you discount the branding, they still perform  
functional tests of PSUs at 20%, 50% and 100% of rated load, and  
confirm that the PSU isn't wasting excessive amounts of power.  Saving  
20-30 watts over time justifies the cost of a more expensive PSU, and  
it doesn't hurt that the machine doesn't have to deal with the extra  
thermal load.  For example:

Any new PSU which isn't 80-plus certified is pretty likely to be  
unable to run at 100% of rated load without failing.

> Additionally, are you sure your service power is good?  Even the
> best power supply will fail if you're not getting 120V/60H at the
> outlet (or whatever voltage/freq you're supposed to get in your part
> of the world).

This is also a good point.  If you know what you're doing and have a  
multimeter, you can check your AC line and look for various issues  
like voltage sag under load, current leakage to ground, etc.  Failing  
that, something like this Kill-A-Watt meter is quite handy:

...although, obviously, one would want to obtain a unit intended for  
the local region's electrical standards if you are not in NA.


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