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Add 3VSB to PC SMPS

Discussion in 'General Electronics' started by default user, Sep 22, 2003.

  1. default user

    default user Guest

    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?


    Thanks for your input,

    James
     
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