You will note that some say it will, some say it won't. It really depends on
the particular power supply you are working on. To be able to just bypass
the input stage you need a supply which:

1: Uses a bridge input configuration rather than a voltage doubler input
configuration.

2: Can regulate down to a relatively low AC voltage

You may be able to tell if condition 1 is met by checking the working
voltage on the caps in the input section.

You could tell if condition 2 is met by attaching the supply to a variac,
loading it realistically, and cranking down the voltage. (btw: make sure the
supply will properly start at the lower voltage).

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)


----- Original Message -----
From: M. Adam Davis <adavis@UBASICS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, September 20, 2000 12:09 PM
Subject: Re: [EE]: 125 VDC Input ATX Supply


> The only problem you are lilely to have is associated with the actual DC
voltage
> present at that section just after rectification. 120VAC rectified and
filtered
> with caps produces a nominal 120*sqrt(2) or 170VDC.  Many good PC power
supplies
> will operate off of as low an AC voltage as 90VAC, giving the DC section a
lower
> limit of 127VDC, which MAY mean that you could put 125VDC in and it would
work
> out OK.
>
> I wish you the best of luck, and please post your results here!
>
> -Adam
>
> Mark Peterson wrote:
> >
> > I need to power a PC from a nominal 125 VDC source.  The voltage may
vary
> > from 110 to 140 VDC.  I want a 300 W ATX supply that will be a direct
swap
> > with the existing 120 VAC unit in the PC.  I'm finding that this type of
> > supply is very rare.  A couple vendors in China responded to my email
> > inquiry stating that they would consider modifying their standard units.
> > They lost interest when I told them I only need three or four of them.
> >
> > I've considered modifying a standard supply, which are abundant and
cheap.
> > I searched for schematics but found that they are nearly impossible to
get.
> > It appears that the first thing a standard ATX supply does is rectify
the
> > AC input, filter it a bit, and then present it to the switching part of
the
> > supply.  I'm wondering if I can simply connect the 125 VDC source just
> > ahead of this switching section.  I'm going to do some experimenting but
I
> > wanted to check if anybody knows of a source of such a supply or if they
> > have ever tried what I'm planning to attempt.
> >
> > Thanks.
> >
> > Mark P
> >
> > --
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