[QuinnMallory]

My buddy has a lower end PSU from his older computer he's trying to run an HD 5770 with. It's an Alienware 420W with the specs saying +12V1 has 15A and +12V2 with 18A. Do those two combine to give him his total or how does it work? The power requirements for the HD 5770 is "450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended". I read somewhere that you need at least 40amps so would this work? He's getting low FPS and I don't know if this is the problem. Thank you for your help ahead of time.

 

[Phaedrus2129]

Unfortunately, though normally a 420W would be plenty, and normally one rail vs. two rails does not matter at all, I can tell just by the amperage ratings that you'll run into trouble.

1.) Alienware PSUs are rebranded junk PSUs. If you're really lucky it's a Delta. If you're just sort of lucky, it's an FSP. Likely it's a HIPRO or HEC or Bestec or some other crap.

2.) When you have a PSU with two unequal +12V rails that's a good indication that it was one of the really early ATX 2.0 designs, the ones that gave multiple +12V rails a bad name. One of those rails is probably dedicated solely to the CPU. But the other rail is dedicated to your graphics card, motherboard, hard drives, fans, optical drives, PCIe devices, everything that uses +12V power but the CPU. And often, in situations like that, it was the smaller rail that was running everything.

Now first off, a "rail" in a power supply is probably not what you think it is. A "rail" has no physical manifestation. The power all comes from the same +12V transistor. A rail is a group of wires coming off the PSU's PCB that is monitored by an OCP circuit. In English: it's just a protections thing, that shuts the power supply off if it detects too much current on one rail, to stop the power supply from burning if you put too much load on it.

In most modern power supplies, that doesn't matter at all because the rails are divided logically. But with an old unit like yours, you have 80% of the load on one monitoring circuit... And thus it's quite easy to go over the OCP's trip point and cause the PSU to shut off.

If he has *just* the 5770 and a couple of hard drives and doesn't overclock the graphics card, he should be OK, assuming that the PSU isn't some garbage HEC that's derated so it's a 200W power supply. You can even overclock the CPU as much as you want, since no CPU will use more than ~14A. But be careful adding additional hard drives or overclocking the graphics card, because you could trip the OCP under load.

Frankly, I recommend a new power supply. The Corsair CX400W is $40 after rebate at Newegg.com, and it would be a good choice.

 

[Brian_]

nice explanation

 

[QuinnMallory]

Wow thanks I appreciate all the info +rep!

 

[Phaedrus2129]

One final note, if you want to find out who really made that power supply (because Alienware just rebrands things).

Somewhere on the PSU's label there will be an "RU" symbol, and right under that symbol will be a number, starting with an e. That's the UL number, and it takes the form "e******".

Go here:
http://database.ul.com/cgi-bin/XYV/t.../gfilenbr.html
And plug in that number and see what company comes up. If it's Alienware or Dell then they must have paid to get it under their own UL number. But if you're lucky it will tell you the real manufacturer.

SeaSonic - Very good, but not used often
Delta - Usually good
CWT - Usually fairly good, they did have a few lemons
FSP - Anywhere from ok to really bad
Bestec - Not good
Lite-On - Varying levels of crap
HIPRO - Run for your life
HEC - Kill it with fire
Other - ask me or Shinji2k or ask on jonnyGURU.com forums

 

[QuinnMallory]

Well I appreciate that +rep. My buddy knows it's a old, no good PSU but he just wanted to know if those two rails amp's combined to give him what his total is to meet the 5770's power requirements. So he should get a PSU that has either a combined total of 40a on the 12v Rails or one 12v with at least 40a correct?

 

[Phaedrus2129]

He should be looking for a PSU with a single rail of at least 25A, recommended 35A+.

With multi-rail units, you can't add up the rails, since that's just what the OCP tip point is for each rail, not the total power. You're looking for a minimum combined +12V power rating of 300W, preferred 420W+.

The combined +12V output should be on the PSU's label.

The most important thing is to get a reputable brand. I recommend Corsair, SeaSonic, Enermax, or Antec (excluding the Basiq series).

 

[QuinnMallory]

Quote:

Originally Posted by Phaedrus2129 He should be looking for a PSU with a single rail of at least 25A, recommended 35A+. With multi-rail units, you can't add up the rails, since that's just what the OCP tip point is for each rail, not the total power. You're looking for a minimum combined +12V power rating of 300W, preferred 420W+. The combined +12V output should be on the PSU's label. The most important thing is to get a reputable brand. I recommend Corsair, SeaSonic, Enermax, or Antec (excluding the Basiq series).

Thanks, +rep. Well if the power requirements listed on the internet say 40a on the 12v shouldn't that be what he's looking for and not 25-35A?

 

[DuckieHo]

Quote:

Originally Posted by Phaedrus2129 Now first off, a "rail" in a power supply is probably not what you think it is. A "rail" has no physical manifestation. The power all comes from the same +12V transistor.

What if the PSU has two +12v transformers? Huh!?

 

[Phaedrus2129]

Quote:

Originally Posted by DuckieHo What if the PSU has two +12v transformers? Huh!?

I think I hate you.

 

[DuckieHo]

Quote:

Originally Posted by Phaedrus2129 I think I hate you.

Probably shouldn't have brought those up cause it'll blow the minds of many....