Starting a dual rail ATX PSU

[Simon]

So needed power for my LED controller I got a PC power supply and hooked it up in the usual way. But it will only stay on for a second and then goes off again. I don't understand it, I have put a full 5M roll of LED (5A) on one 12V rail and my MCU running 1-21 LED's on the 5V rail, I also added 500ohm to the 5V just to be sure.

But it starts for a second, then goes off and I have to wait a while for it to "reset" before I can go again. Do i need to load both 12V rails ?

Other question is can I parallel the rails ? I don't need dual rail but it was the only way of getting the power I need.

[Rerouter]

Grab an ohm meter, you will likely find that both rails are actually terminated in the exact same place,

also depending on the ATX spec the 3.3V rail is now commonly the regulated one, with a separate sense line joined at the 24 pin socket, so try loading that,

The other thing to be aware of is that with most cheap supplies, if you add up all the channel ratings it will exceed the supply rating, what its actually rated for is the current on the 3.3V rail and the rest share the remaining power capacity,

[Simon]

Well this will output 500W total, from what the label say's it's any combination of loads on all lines together much not exceed 500W, i hardly need that so no problem but yess sounds like i may need to load the 3.3V rail too.

i do have separate current ratings for the 2 12v rails so they might have done it properly although that could just mean a separate LC filter rather than transformer windings and switch.

[Simon]

Well i found another 12V line that had a 12 ohm resistance with the one i was using so assumed it was the other 12V rail. Connected a 5m roll of LED's to that too (4A) and put a 47 ohm resistor on the 3.3V line but still no show.

[Rerouter]

Try completely unloading in and just turning it on, most power supplies will happily run like this, but some wont, and it could want something else to run,

[Simon]

I have a brown wire too that I could not explain, having looked at some google images it would seem that this is another 3.3V line

[Rerouter]

Brown is that 3.3V feedback line, if thats free floating it would explain the issue, tie it to your loaded down 3.3V rail and see how it goes, if its already tied in with a 3.3V wire than put your load there,

[Simon]

Oh it's an actual feedback line ?, wow the ATX specs must have been updated, i guess they are aiming for voltage regulation at the load rather than from the PSU with potential voltage drops down the line, sweet.

[Rerouter]

You can also tweak that sense lines voltage by about 400mV each way if you want to shift the 5 or 12V rails up or down a little,

[Simon]

Success, yes good point, why 400mV, are there limits ?

[Rerouter]

its what a 2006 era spec i read over covered, that it was to correct for voltage drops up to 400mV in the wiring from current draw, i would imagine you could leave that window safely, but 400mV over 3.3 (with all other rails being ratio-metric) gives a fair bit of room to move, (12%)

[varesa]

Are you pulling the (usually) green wire low? That tells the supply to start, like in a computer when you press the power button

There is the pinout: http://en.wikipedia.org/wiki/ATX#Power_supply

[Monkeh]

I have a brown wire too that I could not explain, having looked at some google images it would seem that this is another 3.3V line

http://en.wikipedia.org/wiki/ATX#Power_supply
http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf

etc

ffs..

[Simon]

Yes it works, I have never come across one of these before as I've been dabling with old supplies.