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PSU labelled as dual rail, but is it actually single?

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February 14, 2013 3:55:08 AM

Hi gang,

I was wondering if you could help me with figuring out if my power supply is dual rail as it's meant to be because I've done two tests which both suggest to me that it's actually single rail.

The power supply I've got is a Cooler Power GX700 from ebay, there are loads of them for sale on there. The sticker on the unit says that it has dual 12v rails with 24A each rail.

I'm planning on using the unit as a power supply for my battery charger (for my remote control planes). I already converted one power supply over in the usual manner described in numerous threads on the subject, but the unit didn't have the overload protection it was supposed to and it died when I drew too much current from it. This unit (the GX700) is a replacement which is supposed to have plenty of power for my needs and proper over current protection.

Anyway, I went to test the PSU to see if if actually does have the dual 12v rails, as I have the suspicion that this ebay seller may be selling power supplies that don't meet their own advertised specifications. My understanding is that with a dual rail power supply, each rail has it's voltage regulated separately. This means that one rail can be placed under load, causing its voltage to drop a bit, while the other rail will remain unaffected and still put out it's full voltage.

To test if this was the case, I cut one of the yellow and one of the black wires, and connected both to a 55w headlight bulb in order to place one of the rails under load. This caused the voltage on that line to drop from 11.95 volts to 11.25 volts (I didn't have the 5v under load which I expect caused the voltage to drop under load more than it normally would have).

I then went to test the voltage of all the different yellow wires, expecting some to be at 11.25volts (due to being on the same rail as the light bulb) and others to still be at 11.95volts (as they would be on a separate rail which was not under load so should still read the full 11.95). What I found was that every single yellow wire read 11.25 volts. This strongly suggests to me that the unit is a single rail power supply, not a dual rail.

I then did some reading online, and found this article: http://www.techpowerup.com/forums/archive/index.php/t-1... which says that you can test if the PSU has multiple rails with a multimeter by testing the connectivity between all the yellow wires. If all the yellow wires are electrically connected to each other, then this apparently suggests that they are all sharing the one rail. If it is a dual rail power supply, then apparently one set of yellow wires should be connected and a second set should also be connected, with no connection between the two sets. I did the test with my multimeter as described, and found that all the yellow wires were in fact electrically connected to one another.

So, I've found that when one 12v wire is put under load the voltage on all the yellow wires drops, and I've found that all the yellow wires share a common connection. Does this suggest that my PSU is actually single rail and not dual rail, or is there another explanation for my observations?

If this is the case, do you think it would be a single rail of 24A or a single rail of 48A? If the latter, I think this would pose a serious safety hazard as it would allow a single wire to carry 48A which is probably enough to cause it to catch fire.

Thanks for your help!
February 14, 2013 4:55:01 AM

I repeated the test with the 5v under load, just for comparison.
(when using a computer power supply as a bench power supply, you need to place a 5v line under load of at least an amp to allow the power supply to put out maximum power down the 12v line)

I used an indicator bulb that drew about 1.8a, and with this running off a 5v line it bumped up the 12v unloaded output to 12.7v. I hooked up the headlight bulb, as I did before, and again found that each and every yellow wire read the same voltage (12.17)

It's seeming more and more likely that it's a single rail that's been badged as a dual rail. I'm worrying about what else on the label is wrong :( 
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February 14, 2013 5:01:28 AM

Sometimes PSU advertise as quad 12V rails are simply 1 regulated rails splitted in 4 with 4 different fuses/breakers... They do that as some want multiple rail, other single so they have the same internals but only the final part of the PSU change so they save cash when ordering more unit at once to do 2-3-20 models with the same internal but not the same options on it...
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February 14, 2013 5:22:58 AM

Similar but different model Somorizio, the one I got is here http://www.ebay.com.au/itm/GX-700W-Cooler-Power-Xtreme-...

Dual 12v rails with 24A per rail



Thanks for the tip Spawnkiller. So both 'rails' could share the same single power supply, but it's split into two, and each of the two has it's own 24a over current protection circuit?

That would explain why they all share a common connection, and why a voltage sag is seen at all of the lines at once. It still seems like a bit of half-measure though, as I thought one of the main benefits of a multi-rail PSU was that voltage sag on one rail doesn't affect the rail that runs the CPU.

Oh well, I'll just connect most of the yellow wires together, most of the black wires together and use that for my power supply. I was hoping to figure out which wires belonged to which rail so I could just use three wires from each rail but it just seems too hard now... Unless I open the PSU and look at the board myself- which would of course void any warranty I may have on the unit.
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