# Add 3VSB to PC SMPS

D

#### default user

Jan 1, 1970
0
Hello.

I have need to add 3VSB function to a typical PC ATX12V power supply.
This is to power a proprietary Compaq motherboard. Multiple attempts
at sourcing suitable power supply from Compaq or elsewhere were
unfruitful, so the original power supply has been reverse engineered
to a minor extent and a standard ATX12V power supply has been
refitted with the original PS wiring harness, duplicating needed
function with the exception of this 3VSB. This new PS is also
dimensionally modified to fit a proprietary chassis form-factor, so it
would be impractical to "start over" with another whole/new PS.

Apparently the motherboard uses the 3VSB instead of 5VSB to power the
pull-down circuit for the power supply power-on line, as the new power
supply, system, won't turn on with 3VSB missing. This was
confirmed(?) by removing only 3VSB rail from original PS, also
resulting in failure to turn on PS or system. Part of the problem may
be that I DO NOT HAVE ACCESS TO THE SYSTEM. I was sent the original
power supply, asked to get a new one working remotely, with the person
on the other end, with the system, having only the typical end-user's
abilities, no electrical experience or diagnostic equipment. Also no
specs are obtainable from Compaq and the original power supply appears
to have no markings to hint at true manufacturer. Yes, apparently
I'm a glutton for punishment.

Original PS is spec'd for 3VSB of 3.3V, 2.2A, but I believe this is
partially for PCI power-on events unseen in the system, so practically
the target is 1-2.2A output @ 3.3V +-5%,

How to implement 3VSB? There is very limited free space remaining in
the new power supply, perhaps a couple square inches contiguous, half
a dozen more sq. inches if the fan were mounted externally, as it is
possible but not preferred. It is not necessary to have this 3VSB
circuit internal to the power supply case, only that it be wired to
the motherboard harness, could be a tethered box that plugs into it,
but it is highly desired that this box be small, perhaps under 9 cubic
inches. Whole 3V power supplies (complete units in case or just the
finished PCB) seem to be rare, overly large, and expensive.

I'm hoping you will point out problems with the following ideas and/or
suggest other alternatives:

- Tap into 5VSB rail, 3 diodes in series, voltage drop to ~3.4V. Best
guess is that the 5VSB has enough reserve to do this, but with no
experience or specs on target system I'm left to rely on experience
with standard PC systems,

- Linear regulated 3VSB from new PS 5VSB rail or from external ~5V PS,
a la LM317.

- Remove and reconstruct 3VSB circut from original PS, which may not
fit in new PS casing. This circuit is it's own simple SMPS, tied into
original after it's AC rectified. This seems the least desirable
alternative due to the time it would take to do it, as well as my
limited abilities to hack SMPS.

- Source a small, affordable (cheap) 3.3V PS. The budget is $0, but the time savings is potentially worth something ($). Anyone know
where low-cost 3.3V 1-4A PS might be found?

James

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