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ATX power supply power switch

Discussion in 'Embedded' started by AllenB, Sep 26, 2004.

  1. AllenB

    AllenB Guest

    I am planning on using an ATX power supply in a kiosk type
    application, and I am trying to figure out what to do with the power
    The ATX power supply has a normal on/off AC switch, which is what I
    like to use. However, this only puts the supply in standby mode.
    The power switch on the front of a PC, pulls the PS_ON line to ground,
    turns on the rest of the supply.
    What I am not sure, is if I can keep the PS_ON line permanently tied
    to ground,
    or is it just a momentary input? Can it do any damage to the supply if
    I keep it hardwired to ground?
    AllenB, Sep 26, 2004
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  2. You can keep it tied to the ground permanently, no problem.

    Piotr Stawicki, Sep 26, 2004
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  3. AllenB

    John Harlow Guest

    Except that it won't necessarilly work. I had a similar situation and was
    appaled to find there was no consistent way to assure an ATX power supply
    will turn on without the momentary ON signal. It is a rather silly
    oversight IMHO.
    John Harlow, Sep 27, 2004
  4. Not from the point of view of the intended usage, I'd say. I don't
    think ATX was ever intended to be used in unattended systems. Nor was
    it ever planned to *have* a primary-side power switch in an ATX system
    --- it's supposed to be always-on on the primary side.

    Anyway, the issue is quite moot since most BIOSes let you configure
    the mainboard to do this for you. "Automatic AC power loss restart"
    is what this feature is often called.
    Hans-Bernhard Broeker, Sep 27, 2004
  5. AllenB

    Rob Turk Guest

    .... Which will work if he is actually planning to use an ATX motherboard of
    some kind and his BIOS has that option.

    If not, then a simple way to guarantee proper startup is power a small
    microcontroller from +5STBY, have it monitor the +5V line or 'Power good'
    and pulse the ON/OFF line if power isn't present after a short time.

    Rob Turk, Sep 27, 2004
  6. AllenB

    John Harlow Guest

    Or a simple 555 timer circuit or similar.
    John Harlow, Sep 27, 2004
  7. You can keep it tied to the ground permanently, no problem.

    Sure, no problem - except that it will probably shut the entire unit
    down after a couple of seconds of powerup. The switch-on signal has to
    be momentary. I suggest a one-shot powered off the +5V standby power
    Lewin A.R.W. Edwards, Sep 27, 2004
  8. AllenB

    Mood Guest

    Now I'm shooting from the hip, but if only a momentary signal is
    required, couldn't you just tie the PS_ON to ground through a
    capacitor? This would pull the signal to ground until the cap
    charges, and toggle the power supply on.

    Mood, Sep 27, 2004
  9. Where does one find this for say Windows XP?
    Interesting: Bootstrapping the bootstrapping the bootstrapping.

    Where is the uC getting the power to operate to ensure
    the power supply has started?
    Everett M. Greene, Sep 27, 2004
  10. If not, then a simple way to guarantee proper startup is power a small
    +5STBY is the "standby voltage", provided at all times while the PSU
    is plugged in, specifically for circuits like this (and WOL and other
    similar frippery).
    Lewin A.R.W. Edwards, Sep 28, 2004
  11. What makes you think Windows would have anyting to do with that? I
    wrote "BIOS" up there, didn't I?
    Hans-Bernhard Broeker, Sep 28, 2004
  12. Yes, you did!
    Gerard Zagema / StackTools, Sep 28, 2004
  13. Let's be nice now. You are dealing with less than expert
    PCers here in this newsgroup. If we were expert PCers,
    we wouldn't have to work for a living.

    Substituting "BIOS" for "Windows XP", how does one
    find the "Automatic AC power loss restart" feature?
    Everett M. Greene, Sep 28, 2004
  14. Am I? Well, then let me put it in a different way: if you are
    building something that isn't a PC, you probably shouldn't be trying
    to put an ATX supply into it --- or be prepared to blame all the side
    effects on yourself, not the ATX designers. If you _are_ building a
    PC, you really should be enough of a PC expert to recognize the term
    "BIOS" without needing further explanations.
    In the Motherboard's internal BIOS setup.
    Hans-Bernhard Broeker, Sep 28, 2004
  15. AllenB

    Neil Kurzman Guest

    That is Not the Solution.
    Look in the BIOS Settings the is an option about what to do after a power
    the usual options are:
    Stay Off
    Turn On
    Resume at last state.

    Try that.
    Neil Kurzman, Sep 29, 2004
  16. Careful with such blanket statements. There BIOSes out there that
    have been reported to lack this option completely. Silly, yes, but
    they exist.
    Hans-Bernhard Broeker, Sep 29, 2004
  17. Herr Broeker is not inclined to answer the question, so
    I'll try again:

    How does one get to the BIOS settings? I vaguely recall
    something about hitting some key during power startup...
    Everett M. Greene, Sep 29, 2004
  18. AllenB

    John Harlow Guest

    It varies based on system board manufacturer. Look it up in the manual
    which came with the board or go to their website.
    John Harlow, Sep 29, 2004
  19. AllenB

    AllenB Guest

    While the computer is booting, before the OS starts loading,
    the screen normally displays a message, something along the
    lines of:"Press Del to enter setup".
    At this point press the delete key.
    I think where there was some confusion, in that you asked
    specifically for Windows XP, whereas the BIOS is completely
    independant of any OS.

    Thanks everyone for the useful replies, fortunately I have
    some custom hardware attached to the PC motherboard. This
    has an "always on" microcontroller on it, so I can use that
    to pulse the power line.
    AllenB, Sep 29, 2004
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