ATX power supply power switch

You can keep it tied to the ground permanently, no problem.

Piotr

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----------------------------------------------------------------- G.shdsl Focus 4.6mbps modems:

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.

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.

... 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

Or a simple 555 timer circuit or similar.

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 line.

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.

-Jim

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?

What makes you think Windows would have anyting to do with that? I wrote "BIOS" up there, didn't I?

Yes, you did!

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?

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.

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

Try that.

Careful with such blanket statements. There BIOSes out there that have been reported to lack this option completely. Silly, yes, but they exist.

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...

It varies based on system board manufacturer. Look it up in the manual which came with the board or go to their website.

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. Regards, Allen