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Thread: -12V and -5V rails explained:

  1. #1
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    -12V and -5V rails explained:

    I've seen some postings on this group about use of various rails on an ATX power supply, so I thought that this might be useful...

    12V rail: Used by 3.5" Hardrives and 5.25" CDROMs motors, high-end video cards and often used to step down V(core) for the CPU power --a P4 CPU consumes 2-5A on this rail. VIA C3/C7 processors do not use this rail for V(core). As a matter of fact, the 12V rail us used only for MOSFET gate drive, about 50mA. If you have a system fan, then add another 100mA for the 12V rail.

    5V rail: Used by 3.5" (logic), 2.5" drives (motor), M/B and I/O logic, video, USB, pretty much everything. VIA C3/C7 processors use the 5V rail for V(core), must reserve 2A for processor alone. Attention: older PIII and PII systems use this rail for V(core).

    5VSB: Used by the 'sleep', monitoring circuitry and sometimes by 'always-ON" USB devices (depending on your BIOS settings), 1.5-2A should be more than enough.

    3.3V rail: Core logic, memory and some monitoring circuitry. Usually 2-6A.

    -12V rail: Used *only* by serial devices. If -12V rail is not present, your motherboard will still fire-up. Some modern serial COM ports have an internal -12V chargepump so you might not need -12V native. Cheap motherboard manufactueres will still use -12V serial for COM chipsets, VIA included -- with the exception of EPIA PD. Depending on baud rate, current consumptin on the -12V rail should be anywhere from 5-50mA per port.

    -5V rail: Used only by legacy ISA devices, your motherboard will fire-up without the need of a -5V rail. The -5V was removed from the last two generations of ATX power supply standards.

    If in doubt of the -12V and -5V rail needs, just cut these two wires from your ATX cable harness and then test. Only 12, 5 and 3.3 are really needed along with the PS_ON and PWR_GD signaling.

    Hope this helps,
    Andrei

  2. #2
    Variable Bitrate
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    very helpful

  3. #3
    Maximum Bitrate Zebelkhan's Avatar
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    Thanks Andrew. Good info.
    Compaq Evo D510 e-PC - P4 2.0 - 512M RAM - 120G 3.5 HD - 7" Xenarc 700TSV - DSATX fitted nicely inside the PC case

  4. #4
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    It's always nice to see sellers go out of their way to make sure people understand what they are getting into.

    Kudos for that Andrei!
    Jan Bennett
    FS: VW MKIV Bezel for 8" Lilliput - 95% Finished

    Please post on the forums! Chances are, someone else has or will have the same questions as you!

  5. #5
    Maximum Bitrate Dave One's Avatar
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    5VSB: Used by the 'sleep', monitoring circuitry and sometimes by 'always-ON" USB devices (depending on your BIOS settings), 1.5-2A should be more than enough.
    I'm having problems with the PC starting in 'stand-by' mode every time. I think it may be a 5vsb. Can I prevent this by changing a BIOS setting?

    Cheers,
    Dave
    Vauxhall Astra SXi Car PC installed.

  6. #6
    VENDOR - ITuner/MiniBox
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    I don't fully understand the problem, however, I suggest using "hibernate' instead of standby, unless you are determined to shorten the life of your Car battery.

    Andrei

    Quote Originally Posted by Davefocus
    I'm having problems with the PC starting in 'stand-by' mode every time. I think it may be a 5vsb. Can I prevent this by changing a BIOS setting?

    Cheers,
    Dave

  7. #7
    Maximum Bitrate Dave One's Avatar
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    I have shutdown selected for soft-button press. However, when I use a dc psu the PC starts with the sleep (stand-by) LED illuminated, and consequently does not continue to boot, not even PoST. The PC works fine from the ac psu.
    I'm not sure why it's starting in sleep (stand-by) mode. All I can think is an initial check of the 5vsb line is preventing boot due to my CarNetix power regulator providing the 5v a fraction too late.
    The question is can I prevent the 5vsb being necessary to perform PC start?
    Vauxhall Astra SXi Car PC installed.

  8. #8
    Maximum Bitrate Dave One's Avatar
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    Problem sorted out now. open circuit found.
    Cheers,
    Dave
    Vauxhall Astra SXi Car PC installed.

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