- psu voltages

Discussion in 'General Electronics Chat' started by electronis whiz, Dec 7, 2010.

  1. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    is there any purpose for the -12 and -5 volt on a computer power supply?
    i beleve the - 5 has ben removed but not for sure.
    what is the use of the - 12 volt if we can get rid of the -5 v. it sounds to me like they could bothe be sciped and not be a big deal.
    if any one knows the porpose of at least the -12 v i would like to know.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I don't think they use -voltages anymore nowa days
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    -12V is only 1A, left over for legacy serial ports on ISA buses and other "PC Compatible" items from the '80s

    -5v was for the same purpose, but died with the ATX power supply.

    The new supplies coming out will be only 12V and 5V, the 12V supplying regulators at "point of use", like today's processors which have their own several hundred amp supply just to keep a multi core, multi gigahertz machine running.
     
    Last edited: Dec 8, 2010
  4. tom66

    Senior Member

    May 9, 2009
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    Not true. -12V is required by PCI. I think it's also required by PCI Express. Sound cards and analog capture devices use the -12V line.
     
  5. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    thats what i was thinking. i was wondering because i like to come up with mor eficent ideas. so a wanted to desing a psu with no transformer or fan. it would just use a rectifer and regulators from 120v dc to +5, +12, +3.3v. i just don't know how to get a - from a regulator. other than use a -regulator. but i think this just means it regulates a already made - voltage?
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    It is suggested to use point of use switching supplies where negative voltages are needed. -12V is from the power supply as a legacy only, and will be going away with the next big change in power supply definitions. The same way 3.3V came to be a high current line and -5V vanished. Intel announced in 2007 not to depend on -12V to always be around.
     
  7. tom66

    Senior Member

    May 9, 2009
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    Ambitious idea, but it won't work, because:

    Switching power supplies are often as efficient or more efficient than big transformers so you'll still need cooling.

    For a 300W power supply the transformer would be VERY bulky and VERY big. And you'd need regulation on the secondary side, to compensate for varying mains voltages. Imagine 1000W transformer now.

    The supply needs to be able to be turned off (or the computer will never switch off) so you will either need a TRIAC or relay on the input or a relay or MOSFET on the output.

    You need a fan anyway because the PSU fan forms part of the cooling system of a computer. It does two purposes; it cools the PSU, but it also keeps air circulating.
     
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