An SFX PSU in a strange land :(

By Dan_Sarandon
Jul 3, 2006
  1. Kind friends:

    I have recently acquired an (admittedly antediluvian) Toshiba 3200 P-III 600MHz system for use as a dedicated ‘control head’ for a PIC programmer and a digital spectrum analyzer (i.e. frequency domain o’scope) (The former connected via an RS232 serial port and the latter a PCI bus slot).

    As received, the system was equipped with a 40X CDR (IDE), a 10GB HDD (IDE) a 1.44 MB FDD and 256MB of SIMM RAM.

    Please note that the following hardware is integral to the system board:

    -IO ports (Parallel, RS232 and USB).
    -Audio (i.e. "sound card") functionality.
    -Video adapter.
    -IDE and FDD controllers.

    Leaving the bus slots (2 PCI and 1 ISA) free.

    To the above I have installed (added):

    -1 new 100GB HD (Glory be to Maxtor’s DDO scheme! :) )
    -A non self-powered USB hub (for use with ‘flash’ drives)
    -The spectrum analyzer interface card.

    Ok here’s the deal:

    In consideration of the (physically) diminutive PSU, I thought it wise to examine the various EMFs for indication of overload (to wit: excessive voltage drops on any line) -- first via BIOS’ sensor readings then via direct measurements made during worst-case demand. – All readings were ‘better’ than nominal except the –5v supply (which floated about -0.07v)

    Upon inspection I discovered that the –5V line (position #18 at the PSU- to-system board connector) was omitted – further investigation indicated that the PSU was clearly manufactured thus.

    Given that the system board’s PSU connector is clearly marked “ATX Power Supply”, the -5v sensor is ‘active’, the unit contains an ISA slot (-5v position floating) it is clear to me that I’ve got hold of an ‘ATX system’ equipped with an SFX PSU.

    Hence my question: So long as I do not use the ISA slot, does the omission of the –5v supply matter???

    So far as I am aware, third and subsequent generation LSIs, VLSI, etc. have no requirement of external substrate bias application – which, to the best of my knowledge, is/was the sole purpose of the –5v supply???

    At ‘first blush’, Owing to the miniscule current requirement (300 mA) it seems a ‘painless fix’ would be to develop the –5v supply from –12v using an LM7905 linear VR. -- Unfortunately -12 is a low-current supply itself and my application requires reliable RS232! :(

    Many, many thanks in advance for any forthcoming insight/info
    Very best regards
    Dan :)
  2. fastco

    fastco TS Booster Posts: 1,122

    I must say my simple mind had a hard time following your intelligent use of the English language...Well done

    I would replace to PSU with a new more powerful ATX model. If you are using the computer for business and you need it to be very stable then don't take a chance. Just my opinion...
  3. korrupt

    korrupt TS Rookie Posts: 716

    Woooshh-right over my head.

    English please? I am not sure what your question is.

    However kind sir, I am deeply dedicated to assist in thou problematic portable computer and if thou can clarify I will do my best:p


  4. Dan_Sarandon

    Dan_Sarandon TS Rookie Topic Starter


    Thank you for your interest! :)

    Sincere apologies for my OP's lack of perspicuity owed my verbosity and (though I burn with shame at the confession) often highly questionable grammar! Also please don't interpret my formal tone as indicative of pomposity, conceit, etc...

    ___My question:___


    Assuming no ISA cards are or will be installed may a system specified for operation from an ATX PSU perform reliably if powered by an SFX PSU exhibiting equal or better maximum rail currents and overall power capacity???

    Note: The sole distinction between the SFX and ATX ‘form factors’ is SFX's omission of the -5v rail...

    So... more succinctly: Does an ATX powered system require -5v for any purpose beyond maintenance of hardware support for certain ISA devices connected via ISA bus slot?

    Very best regards
    Dan :)
  5. howard_hopkinso

    howard_hopkinso TS Rookie Posts: 24,177   +19

    Hello and welcome to Techspot.

    Provided you don`t use any ISA cards, you should be fine.

    Take a look HERE for further info.

    Regards Howard :wave: :wave:
  6. Dan_Sarandon

    Dan_Sarandon TS Rookie Topic Starter

    howard_hopkinso responded:

    Thas' just what I was hoping to learn! Many thanks! [​IMG]

    But 90 watts!? Aye! Aye! Aye! - I can only hope mine is one of the high performance models referenced in the article...

    Again, Thank you!!!

    Very best regards
    Dan S. :)
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