Cheap PCs from Wal-Mart - mini-itx

Jim Lux James.P.Lux at jpl.nasa.gov
Tue Jun 3 09:54:50 EDT 2003


>   >> > Runs pretty cool, even with just the 2 puny little case fans, and
can be
>   >> > driven off a 12v Wall Wart (?tm?; should I trade mark it?  no...)
at
>   >> > less than 5a.  The wart I have is rated for 4.5a, but I've not
measured
>   >> > it yet.
>   >
>   >we did lots of testing on that 12v dc-dc input to the  mini-itx mb
>   >( well, www.mini-box.com folks did all the testing and ordering of the
>   >( custom 12v dc-to-dc atx power supply
>
> Any numbers on the PF of these ac-to-dv (then to dc) floor-brick-with-
> power-cables xformers? I suspect this stuff isnt worth anything til you
> run it off a site-wide 12V infrastructure to get a good PF rating.
>
> /kc

Power factor, or efficiency?
CE marking requires fairly good harmonic control and power factor, even for
small wall warts or floor bricks. It's become fairly straightforward to
design "good" power supplies, since there is huge consumer demand (via
regulatory process) and the semiconductor mfrs have produced inexpensive
chips to make PFC easier.

Efficiency, though, isn't all that hot, there.  Especially considered in the
system context, because you have a linevoltage:12V conversion, and then
another 12V:5 and 12V:3 conversion, each of which is probably 80-85%
efficient.  In a standard PC, you have only one conversion.

However, you wouldn't want 12V distribution either.  IR losses in the
cabling would eat you alive, and maintaining regulation would be very
difficult.  One better solution would be to distribute unregulated 300VDC to
all the mobos, and have them convert that to the required 12,5,and 3 in one
step (essentially splitting the standard PC power supply down the middle).
For safety reasons, you don't see many 300VDC distribution designs, though.
Telco stuff uses 48VDC for distribution... high enough that IR losses aren't
killing you, you can get it with batteries, and it's below the informal "50V
is low voltage" shock hazard cutoff.



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