M789 power loss restart - the solution is very easy!

Alexander Amelkin spirit at reactor.ru
Thu Oct 21 06:48:01 CEST 2004

Hi all!

  I think, this can be put onto the FAQ.

  I found the solution that works for M789 and is very easy and cheap.
  It also must be a universal solution, helping to power-up any ATX
  motherboard on power up of the PSU.

  Just take a small capacitor (i used a 2uF one) and put it in
  parallel with the standard power button on your case. Or just
  connect the capacitor to the PWR BTN header on the motherboard.

  This is much easier and cheaper than using any digital solution
  (with a counter IC or anything). The principle behind my solution is
  that upon powering up the PSU, the 5VSB line becomes high on the
  motherboard. That line powers the circuitry needed to react to the
  power button. Additionally, voltage appears on one of the pins of
  the PWR BTN header on the mobo (the other pin is ground). So, when
  there is a capacitor between the pins, it starts charging. That
  looks like a short circuit to the mobo. It boots up. The capacitor
  becomes fully charged and it starts looking like an open circuit to
  the motherboard. The capacitor is never discharged while the mobo is
  powered and is discharged quickly after the power is removed.
  Additionally, the functionality of the power button is preserved -
  you can still push it to turn the PC off.

  In other words, the capacitor is equal to "AC Power Loss restart:
  Always On" setting in the BIOS.

WBR, Alexander                          mailto:spirit at reactor.ru

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