Turning on a PSU w/o a motherboard.

[sundevil_1997]

I read the FAQ's, and did some googling, but I want to make sure I understand this.

I have several junker PSU's that I'm not afraid to mess with. Basically, I'd like to have a PSU handy for plugging in fans to test them, without having to hook it to a motherboard so I'd have an on/off switch handy.

Here's what I think I understand:
1) There is a PWR_ON pin in the 20-pin ATX motherboard connector.
2) When you press the on power button on the case, that PWR_ON pin is pulled low by the motherboard logic, and that starts the PSU.
3) Shorting the PWR_ON pin to a ground pin would have this same effect.
4) Manually shorting the pin might very well break the PSU because of the multiple times an on/off connection is made (no TTL/debounce circuitry to make sure it happens just once).
5) The pin can be permanently shorted (PWR_ON to GND), and then the PSU works fine, but must be unplugged to turn it off.

Do I understand this correctly? If so, then I plan to just short the two pins on the 20-pin connector and plug/unplug the PSU. That seems the safest way to go.

Thanks for any help.

[Steve Rosenthal]

I use one of these. It works, is cheap, and relatively safe. Plus, you get some feeback on the condition of your PSU.

--Steve

[sthayashi]

I read the FAQ's, and did some googling, but I want to make sure I understand this.

I have several junker PSU's that I'm not afraid to mess with. Basically, I'd like to have a PSU handy for plugging in fans to test them, without having to hook it to a motherboard so I'd have an on/off switch handy.

Here's what I think I understand:
1) There is a PWR_ON pin in the 20-pin ATX motherboard connector.
2) When you press the on power button on the case, that PWR_ON pin is pulled low by the motherboard logic, and that starts the PSU.
3) Shorting the PWR_ON pin to a ground pin would have this same effect.
4) Manually shorting the pin might very well break the PSU because of the multiple times an on/off connection is made (no TTL/debounce circuitry to make sure it happens just once).
5) The pin can be permanently shorted (PWR_ON to GND), and then the PSU works fine, but must be unplugged to turn it off.

Do I understand this correctly? If so, then I plan to just short the two pins on the 20-pin connector and plug/unplug the PSU. That seems the safest way to go.

Thanks for any help.

It should be a momentary short, not a permanent short. Your computer switch is a momentary switch that makes that connection (indirectly through the motherboard). In case you hadn't figured it out, momentary switch means it is connected only when pressed.

Anyways, this method will turn them on, but some PSUs are funny about not having a load to power, FYI.

[sundevil_1997]

It should be a momentary short, not a permanent short. Your computer switch is a momentary switch that makes that connection (indirectly through the motherboard). In case you hadn't figured it out, momentary switch means it is connected only when pressed.

Anyways, this method will turn them on, but some PSUs are funny about not having a load to power, FYI.

Ya, but I read somewhere (couldn't find it this google) that the momentary normally occurs via the motherboard, which has the circuitry to make sure that momentary only occurs once, whereas if I try a momentary with a paperclip, I'm probably going to make contact a dozen times without even trying. I thought I read that it would be "bad" for the PSU if that occured.

A PSU tester is not out of the question....but honest to God, it seems every time I get some kind of idea for doing something involving computers, fans, or anything, it ALWAYS ends up needing a purchase. I was hoping to finally get away without another $10 purchase.

Worst case scenario, I just pull out an old motherboard and use that. It's got the built-in load, and the switching logic. It's just not as clean/easy as having just the PSU to work with.

No one here uses some easy trick to get their PSU's running all by themselves?? (with no extra equipment)

[sthayashi]

Ya, but I read somewhere (couldn't find it this google) that the momentary normally occurs via the motherboard, which has the circuitry to make sure that momentary only occurs once, whereas if I try a momentary with a paperclip, I'm probably going to make contact a dozen times without even trying. I thought I read that it would be "bad" for the PSU if that occured.

I'm not exactly sure what you think will happen with a dozen or so connections. Turning it off, I believe you have to hold that connection for 4 seconds or so (could be wrong on this one though). But hence a permanent connection wouldn't be the best of ideas.

[alglove]

With the power supply off, use a paper clip to connect "Power On" (green wire) to "Power OK" (grey wire), then turn the power supply on. I have done this myself many times with many different power supplies without any problems.

[sundevil_1997]

I'm not exactly sure what you think will happen with a dozen or so connections. Turning it off, I believe you have to hold that connection for 4 seconds or so (could be wrong on this one though). But hence a permanent connection wouldn't be the best of ideas.

I think we're mixing up the two connections. There's the power switch you press on the case, which is connected to the motherboard, which then controls what happens to the PS_ON pin on the 20-pin connector. So, the press-and-hold for 4 seconds doesn't apply to the PS_ON pin on the connector...it's a function of the motherboard BIOS (which you can set to be 4 seconds, or none, or go to standby, etc). I believe that after the motherboard senses you have held the button for 4 seconds, then it makes the necessary change to the PS_ON pin...whether making a momentary contact to switch the PSU off, or whichever. So, pressing the power switch on the case a dozen times doesn't make any real difference to the psu, because it's the motherboard that is controlling the PS_ON pin and whether it's momentarily closed or not.

however, if I bypass the motherboard and try to momentarily ground that PS_ON pin myself with any kind of a switch (whether a paperclip, or a store-bought momentary switch from radio shack), in reality it's shorting together several times as the pins get microscopically close before they finally make actual contact. (Forgive me if you already knew all this). I believe that's probably bad for the PSU.

Actually, now that I read some more posts from a search of the forums, I'm pretty sure that the PS_ON pin has to remain shorted to ground for the PSU to stay on. Then, when it's no longer shorted, the PSU will turn off. That would be why any kind of intermittent shorting would be bad because it'd be telling the PSU to turn on/off several times in the span of microseconds.

So I think I get it now:

1) Unplug the PSU.
2) Permanently short the PS_ON (green, usually) wire to GND (black wire)
3) Plug in some peripheral to give a load....an old CD ROM works.
4) Plug in PSU.

PSU will turn on as soon as I plug it in because the PS_ON is being held low. To turn off the PSU, I need to either remove the shorting pin (bad to do while it's on because of the intermittent shorts while doing that), or just unplug the PSU.

Do I have it right now, for all those who have posted? Thanks again.

BTW, this thread I just found seems to indicate this:
http://forums.silentpcreview.com/viewtopic.php?t=2073

[sthayashi]

You're right, I'm confusing motherboard bios inner workings with actualy PSU inner workings.

Yes, it seems that permanently pulling it to ground is the best solution.

[sundevil_1997]

You're right, I'm confusing motherboard bios inner workings with actualy PSU inner workings.

Yes, it seems that permanently pulling it to ground is the best solution.

I was kind of confusing myself, too. When people said "just short it with a paper clip", I was picturing trying to short those two pins WHILE the PSU was plugged in, in a momentary fashion. Yikes, make sure ya get those pins right if ya do that!! Instead, I now understand they meant to permanently short them PRIOR to plugging in the PSU. Ah, now that's a bit safer, I think.

Woohoo, something I can try tonight, WITHOUT having to make another purchase!

[sundevil_1997]

Just to clarify, about the "putting a load on the PSU". I take it the reason for doing it is because some PSU's act weird if they don't have any load at all. I'm not interested in measuring voltages, just using the PSU to power things I'm playing with.

So in this case, would a fan controller, or even just a regular fan be enough of a load? It doesn't have to be something hefty, does it? Just basically as long as SOMETHING is plugged into a molex connector that uses power at all?

Thanks!

[moritz]

When people said "just short it with a paper clip", I was picturing trying to short those two pins WHILE the PSU was plugged in, in a momentary fashion. Yikes, make sure ya get those pins right if ya do that!! Instead, I now understand they meant to permanently short them PRIOR to plugging in the PSU. Ah, now that's a bit safer, I think.

Actually that is what I am doing, and so are lots of others. I do make sure I get the pins right, but it's not that difficult. And not at all dangerous either (the usual disclaimers apply, don't blame me) - none of the PSU output rails is really dangerous to touch, it's not like it was mains voltage or something like that. I still insulate myself from the circuit, I use pliers and not a paperclip, but that's just for show. Maybe you can damage your CPU with the wrong combination, though, I'd think you'd have to try pretty hard - but in any event, it's really not that difficult to just get it right.

Very useful for testing a system on the bench and for checking how loud fans (or just the PSU) are without the computer running etc.