Another thing about commercial power-supplies is that they (the cheaper o=
nes)=20
don't deliver what they state/imply.  Some seem to be rated by the *input=
*=20
power, rather the sum-total output power.  Others seem to be rated f/tota=
l=20
output power, which I would think is appropriate since they're purchased =
with=20
a certain output-power requirement in mind.

What's worse is that some don't deliver the current they state.  I've tes=
ted 3=20
power supplies (2 new, 1 old/used) recently and found that under a=20
(resistive) load, they can't deliver the output power they state.  In som=
e=20
cases they may require loading of some primary-voltage outputs to deliver=
=20
what was stated, but on a couple, I could not get 5V at the power specifi=
ed.

FWIW, efficiency-wise, they all were in the 65-85% range.

Once you start considering the expensive (allegedly better-quality) comme=
rcial=20
units available, building your own becomes a little more attractive.

Cheers,
-Neil.



On Saturday 07 June 2003 06:34, Olin Lathrop scribbled:
> > I am currious about switching mode power supplies.  How difficult wou=
ld
> > it be to design a SMPS for use in a desktop computer?
>
> A lot more difficult than to buy one or salvage one from an old PC.
>
> > Why would you do such a thing you ask...  In my experience PC power
> > supplies are junk.  They are made with the cheapest parts availible.
>
> Just like most mass produced commodity items, but that doesn't make the=
m
> junk.  It means they are carefully engineered to do what they need to, =
but
> not cost a penny more than they need to.
>
> > I also assume they aren't exactly pictures of efficency.
>
> Why?  These things are optimized for cost, and up to a point efficiency
> saves money.  Dealing with waste heat is expensive.  On the other hand,
> they probably forgo the last few percent that might require more expens=
ive
> ferrite, special caps, etc.  I've never measured a PC power supply, but
> just from the economic issues I would expect them to be "reasonably"
> efficient, but not on the cutting edge.
>
>
> *****************************************************************
> Embed Inc, embedded system specialists in Littleton Massachusetts
> (978) 742-9014, http://www.embedinc.com

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