Quick ATX PSU question....(stumped..)

xiao_haozi

xiao_haozi

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Just wanted to get some opinions from those much more knowledgeable than I in working with power supplies.

I have a small ATX PSU for a mod that I'm working on (which I will post about when finished), and trying to decide on how I want to wire this up.

I know that a lot of people just wire the power on (PS_ON green) to the switch and ground that to turn on.

However, I know that some (or all?!) should have a small load to remain on.

For example, from an Instructable here (and found in other tutorials on creating up some lab bench psu conversions):

step 5Power Resistor
Most modern ATX power supplies require a small load to stay in the ON mode. I added a 10 ohm, 10 watt resistor between +5V and ground to provide this small load. It is strapped to the back wall of the power supply, where it should get plenty of air flow
FEWHQYTFK75D2J1.MEDIUM.jpg
Click to expand...

Do I need to add this? I would prefer not to if I don't have to....
 
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Murray

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I ran my first SG with an ATX power supply by just grounding power-on and connecting 5V, -5V, and 12V to the appropriate places. I never had a problem with it turning off on me.

If you really want to check on it, just short power-on to ground and test it with a multimeter. If all the voltages are right, you're good to go.
 
xiao_haozi

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Murray said:
I ran my first SG with an ATX power supply by just grounding power-on and connecting 5V, -5V, and 12V to the appropriate places. I never had a problem with it turning off on me.

If you really want to check on it, just short power-on to ground and test it with a multimeter. If all the voltages are right, you're good to go.
Click to expand...

Cool. That was my thinking as well.

Thanks for the input :)

I'll probably give it a go sometime this week and will post back my results then. I think the bigger issue at this point will be deciding if I am getting sufficient airflow in there where I plan to keep the psu. But the unit I sourced it from didn't have much airflow where it was built in and was surrounded pretty close in plastic. Think I may just run it for a while and check the output over time as the heat increases (if it dramatically does) and plot that out and see what I'm getting. But I digress....first things first, I'll try with simply grounding the ps_on and seeing what I'm getting.

Thanks.
 
DewmanSNK

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I used ATX PS for my supergun.
Same deal, just grounded power-on - never turned off on me.

Just wondering why there would not be a load on it to turn if off - wont the SG be connected to a PCB drawing power? And if the SG is not connected to a PCB, then an auto-off might be kinda nice...
 
xiao_haozi

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NeoGeeO said:
I used ATX PS for my supergun.
Same deal, just grounded power-on - never turned off on me.

Just wondering why there would not be a load on it to turn if off - wont the SG be connected to a PCB drawing power? And if the SG is not connected to a PCB, then an auto-off might be kinda nice...
Click to expand...

Well I guess it would be if the voltage draw goes low enough it could turn off. Like if it dips below the threshold of that PSUs minimum draw. I think this would be more an issue with testing rigs like those converted for benchtop testers, but maybe it would not be an issue with a board.

Most power supplies need a minimum load to keep them running, so this sandbar resistor provides a constant minimum load between the +5V rail and Ground. I've heard that newer power supplies also need a load on the 3.3v rail, your mileage may vary.
Click to expand...

So I guess it would maybe be a matter of testing.... i.e. would the board draw drop below that needed to keep it running, it would certainly suck to have the unit shutdown on you.

That's why I was curious as to what everyone had done and their luck with their respective setups.... Seems by what you guys have noted, that the minimum draw is always sufficient to keep things happy and running.

Thanks :)
 
xiao_haozi

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So I did some more reading this morning...

Seems that PSUs with the POW_OK wire work somehting like:
- PW_ON is grounded by the power switch to turn on the psu
- the PSU turns on and sends out current on the 3.3V line to the POW_OK line to tell it that things turned on okay and it's ready to roll
- Once the POW_OK line has stable current things can be used as normal without the PSU shutting off

.....

I guess that isn't an issue as you guys haven't had any problems with this... just odd that if this is how it should work theoretically, why can we just ground PW_ON to turn on and have it stay on without sending any current to the POW_OK line?
 
xiao_haozi

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Alright according to the last post pinout... I finally tried (as per a comment I finally saw on a forum) hooking up the standby +5v to the switch with power_on and when I turn on the switch I get power to my encoder (havne't tried hooking anything into the harness just to be safe.

But the led on the jrok flickers a bit (not on/off but bright to not so bright)... I get measurements that go from 4.6-5.01 on my +5 line on the harness....

not sure....
 
norton9478

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Doesn't the 360 PSU have a really weak +5v?
 
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buaku

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Constant on. I just used a simple slider switch for my supergun.
 
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buaku

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That website makes it sound like there needs to be some other logic in there.

Especially the part that talks about it putting 3.3V on the on the PowerOn line to tell the power supply to start up. To me it sounds like the power supply won't start up unless the PowerOn line is getting 3.3V.

Maybe that has something to do with it. It's late and I can't totally wrap my head around the whole idea :P
 
xiao_haozi

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buaku said:
That website makes it sound like there needs to be some other logic in there.

Especially the part that talks about it putting 3.3V on the on the PowerOn line to tell the power supply to start up. To me it sounds like the power supply won't start up unless the PowerOn line is getting 3.3V.

Maybe that has something to do with it. It's late and I can't totally wrap my head around the whole idea :P
Click to expand...

Yeah so one thing I tried was to put a 10k resistor between ground and power_ok, then I did ground to power_on with the switch and nothing.

I figured that doing that would be like:
power_on grounded puts in standby which puts 3.3V on the power_ok line if grounded with that resistor. But that was late and I'm not sure I was thinking that through either haha. (which was as close to that diagram in my first post as I could come up with...)

Let me know if you can think of anything else to try... think i tried about 10 different setups and no luck yet... so temporarily using another atx psu (well not yet but will this weekend) to test things out. Still want to get this to work though if I can as it is smaller, less heat output, and at 10-15 bucks on ebay and elsewhere, it's a great value.
 
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xiao_haozi said:
Yeah so one thing I tried was to put a 10k resistor between ground and power_ok, then I did ground to power_on with the switch and nothing.

I figured that doing that would be like:
power_on grounded puts in standby which puts 3.3V on the power_ok line if grounded with that resistor. But that was late and I'm not sure I was thinking that through either haha. (which was as close to that diagram in my first post as I could come up with...)

Let me know if you can think of anything else to try... think i tried about 10 different setups and no luck yet... so temporarily using another atx psu (well not yet but will this weekend) to test things out. Still want to get this to work though if I can as it is smaller, less heat output, and at 10-15 bucks on ebay and elsewhere, it's a great value.
Click to expand...

What if standby means it's just plugged into the wall?
So if that makes sense what if you just hook the blue up to the white? It says Blue is 3.3V on standby and the docs seem to imply the PSU wont turn on until PowerOn gets 3.3V. But then it doesn't jive when it says it only puts out 3.3V on the PowerOK when the psu is stable. It might be worth a shot...hopefully it wouldn't blow anything up :P

I guess that'd be one thing to check. When the PSU is just plugged in are there voltages on any of those lines that are listed as having voltages in standby mode?
 
xiao_haozi

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buaku said:
What if standby means it's just plugged into the wall?
So if that makes sense what if you just hook the blue up to the white? It says Blue is 3.3V on standby and the docs seem to imply the PSU wont turn on until PowerOn gets 3.3V. But then it doesn't jive when it says it only puts out 3.3V on the PowerOK when the psu is stable. It might be worth a shot...hopefully it wouldn't blow anything up :P

I guess that'd be one thing to check. When the PSU is just plugged in are there voltages on any of those lines that are listed as having voltages in standby mode?
Click to expand...

Hmm yeah good point. I will have to plug it in and check the lines and see what's what.

I think I rmember trying to short the power_ok to the power_on and it biting back at me....

Heading to bed now but i'll see whats on which lines tomorrow. Thanks.
 
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buaku

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xiao_haozi said:
Hmm yeah good point. I will have to plug it in and check the lines and see what's what.

I think I rmember trying to short the power_ok to the power_on and it biting back at me....

Heading to bed now but i'll see whats on which lines tomorrow. Thanks.
Click to expand...

Cool, I'll check back tomorrow to see what happens next!
 
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buaku

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Something else I just thought of would be to try and trace the power button to a spot on the PSU or something while the machine is unplugged. Maybe just go over it with a continuity test or something.
 
xiao_haozi

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buaku said:
Something else I just thought of would be to try and trace the power button to a spot on the PSU or something while the machine is unplugged. Maybe just go over it with a continuity test or something.
Click to expand...

Hmm you know the thought did cross my mind to see where that little power/eject pcb plugs into the mobo and then see what happens next and if i can kind of reconstitute that.... I'll take a look and let you know... might not be until the weekend but I'm sure the suspense will be tantalizing :)
 
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xiao_haozi said:
Hmm you know the thought did cross my mind to see where that little power/eject pcb plugs into the mobo and then see what happens next and if i can kind of reconstitute that.... I'll take a look and let you know... might not be until the weekend but I'm sure the suspense will be tantalizing :)
Click to expand...

Totally! I must now wait with great anticipation, lol.
 
xiao_haozi

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buaku said:
Totally! I must now wait with great anticipation, lol.
Click to expand...

Okay... so I decided to, for now, use a regular ATX PSU.

I grabbed a slim one from a dell optiplex I had laying around... wired up the green and ground to the switch.... plugged in and no fan and no power.

Tried then connecting up a 5V and ground to my encoder that has an indicator LED and nothing either.
 
norton9478

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xiao_haozi said:
Okay... so I decided to, for now, use a regular ATX PSU.

I grabbed a slim one from a dell optiplex I had laying around... wired up the green and ground to the switch.... plugged in and no fan and no power.

Tried then connecting up a 5V and ground to my encoder that has an indicator LED and nothing either.
Click to expand...

You need more draw than the encoder....

Try snaping an old HD or CD-Rom in there to make sure it works.
 
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