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Thank you for demystifying the ATX start-up process. Would a relay also work?

Hi Gene_,

Thank you for demystifying the ATX start-up process and for sharing on the Audio Asylum forum. You showed the way to a full linear cMP2 power supply !! Thanx you very much.

I have a question about timing the PWR-OK signal and a question about using a relay to switch the PWR-OK signal.

-> Question about timing the PWR_OK signal

The ATX formfactor says:
- the timing has to be: PWR_OK delay 100 ms <- Timing <- 500 ms.

The +5V Power_Okay signal should be switched to pin 8 within 500 milliseconds. How do you do that by hand?

Or…… is it not so critical after all? And can it also be done by hand? Even after 1, 2 or 3 seconds?


-> Switching PWR-OK with help of a relay?
I was thinking of switching the +5V PWR_OK signal to pin 8 (grey wire) with the help of a relay (with some time delay).
This relay acts on pin 14 (green wire)the PWR-UP signal.

- Idea.

* First: normal ATX PSU start-up proceeding
You power pin 9 (purple wire) with +5 volt because the Power Control Circuit (PCC) on the MoBo reacts on pushing the start-up button.

When the PCC is active (+5 volt on pin 9) and the start-up button is pressed, the PCC than will drop the power on pin 14 (green). No power on pin 14 (green wire) signals the ATX PSU to power up.

When the ATX PSU has it’s power up, the ATX PSU than gives +5 volt to pin 8 (Grey wire). With +5V on pin 8, the PCC now starts the processor.


* Giving PWR-OK signal when using all linear PSU’s

Switch on 3.3, 5 and 12 volt to P24.
Which also powers pin 9 (purple wire) with +5v (standby function). This way the Power Control Circuit (PCC) of the MoBo is active and thus also pin 14 (green wire) gets +5Volt from the PCC.

When PCC is active and the start-up button is pushed, the +5V on pin 14 (green wire) is dropped by the PCC. This causes the relay to switch +5V to pin 8 (grey wire) as PWR_OK signal to the PCC.

With PWR-OK signal on pin 8 (+5V on grey wire) the PCC allows processor to start working.

Since the 3.3, 5 and 12 Volt is already switched on as being the first step, the processor will have no problem starting.

I was brought to this idea because some websites also say:
"Some extremely el-cheapo power supplies may "fake" the Power Good signal by just tying it to another +5 V line. Such a system essentially has no Power Good functionality and will cause the motherboard to try to start the system before the power has fully stabilized.”
http://www.pcguide.com/ref/power/sup/funcPowerGood-c.html


Do you (or any other inmates) think this could work?


Edits: 11/23/09
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