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12 volt rail - someone explain please

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Last response: in Components
June 12, 2012 3:49:58 AM

Ok. I guess I'm still considered a noob by most regulars here cause I'm clueless about this [even though I do have two complete builds under my belt, from a few years ago].

I have a question about PSUs and their 12 volt rails- and what it all really means if anything. And I'm having a real hard time getting a straight answer about this. In fact its about as clear as mud at this point, an I've been researching it all day.

OK- here's the deal in a nutshell. I recently bought a pre-built rig (I can't build anymore for a personal reason- so it was a pre-built or nothing). And after a month or so I am actually very happy with what I got. It is a HP Pavilion HPE h9-1120t Phoenix. Mine has the following specs:

• Windows 7 Home Premium 64-bit
• 3rd Gen Intel Core i7-3770 quad-core processor ***was a i7 2600- but I upgraded
• Premium HP keyboard and optical mouse
• 10GB DDR3-1333MHz SDRAM ***upgraded to 16GB Crucial Ballistix DDR3-1600MHz SDRAM
• 160GB Intel SSD
• 1TB 7200 rpm Hitachi SATA HDD
• Microsoft(R) Office reduced-functionality Word & Excel
• FREE Norton Internet Security subscription 15-month FULL VERSION
• 1GB DDR5 NVIDIA GeForce GTX 550 Ti ***changed out to a XFX overclocked Radeon HD 6950 "XXX" version 2GB
• HP Liquid Cooling by Asetek
• HP Blu-ray player & SuperMulti DVD burner
• Premium Wireless-N LAN card
• 15-in-1 memory card reader, 4 x USB 2.0 (front), 2 x USB 3.0 (top)
• 600 watt PSU
• Beats Audio (tm) -- integrated studio quality sound
• 2-year standard HP warranty

This rig is really awesome for a pre-built HP. Seriously. I used to really not like HP or Dell that much, but this system is well made, looks good and is screaming fast. WOW. I'm not kidding about how fast. I was blown away. I can't say enough good things about it (and it has a 2 year warranty to boot). HP has come a long way and I am very happy with it and it was right at my $1000 budget.

But this isn't a love-fest with the HP Phoenix, its a question about PSUs.

My question came up this week because of a friend of mine (who is actually an IT guy, but works in software and isn't really a big hardware guy at all- but still knows WAY more than me).

He looked over my rig, and gave it the two thumbs up (and put it through quite a few paces too). He said it was really a sweet rig which surprised me- cause he made a crappy face when I first told him about it at work.

However- then came the problem.... he told me that the HP 600w PSU, although made by Delta probably has a very weak 12 volt rail. For a card as good as mine, I should use a better PSU, with a strong 12 volt rail for my card or I will hamstring it. Is he correct?

I told him I have a pretty new Corsair TX650w PSU still in the box - that was for a rig I never built. He told me to use it instead of the Delta and I would see BIG gains with the 6950.

I just started getting back into computers about a year ago, and I've been out of it for four or five years before that- and he didn't really explain it to me clearly why the 12v rail is such a big factor (at least I wasn't getting it), but I kind of think he was essentially saying my 6950 card will not perform at its best with a weak 12 volt rail PSU. Am I wrong?

Since I have a Corsair TX650 PSU still in the box, and not being used- should I just switch it out or not?

Here are the specs for my Corsair PSU:

Does this mean the TX650 has a 53 amp +12 volt rail? That sounds crazy high doesn't it?

The stock Delta "HP" PSU has the following specs on the side of it and seem WAY lower than the Corsair's:

PART NUMBER 633186-001


Sorry for such a long post, but I wanted to thoroughly explain myself and this situation. Right now with my rig and the stock HP PSU, I can max out every single setting on BF3. I mean ALL of them on Ultra with everything else turned all the way up- EVERYTHING! I get 40-50 FPS easily with spikes well into the high 60s (@1920x1080). If I turn a few things down (x2 or x4 MSAA) and AA Post (to Med) and Ambient Occlusion to HBAO or off - I can get 70-80+ FPS no prob and spikes into triple digits FPS!!! (and it still looks fantastic at 1920x1080). In other words, I can run the game pretty darn good right now.

I didn't have a chance to test the same game/same server, etc... with the Corsair TX 650 installed- but should I even bother? Its a standard ATX form factor and will fit right in- I just kind of didn't want to mess with it unless it s a big gain like he said.

Or is all this 12v rail stuff B.S.?

Advice please.

So all that to ask this question---

Should I change out the PSU or not? If it doesn't matter that much then I'll leave it be. But if it gives me a nice bump- say 20 FPS or more, I'll do it. That PSU is just setting in my closet not being used anyway, and I can change it out myself and it will fit.

I appreciate any help/advice/etc. Thanks

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a b ) Power supply
June 12, 2012 4:01:55 AM

Well, for starters, unless you're planning on doing heavy OC on the 6950, there's no way you'll see better performance out of the video card just because you can feed it more Amps in the 12V rail. Oh, and a video card is rated to some number of "watts" of power. The 6 pin connector receives 75W and the 8pin receives 150W; the PCIe 2.0 feeds the card 65W or so. And, the short explanation: watt = volt x amp.

Now... There's just one scenario where the Video card (and the whole rig for that matter) would benefit directly from the other PSU. The APCF (Active power correction) and the over current protection circuits (for the most part). There are even more aspects to the PSU as well, but those 2 are major players for me.

Here's a full article from Tom's on the issue. It will really enlighten you and give you a lot more depth than this little I just did:

And another great article:

a c 75 ) Power supply
June 12, 2012 4:09:02 AM

Wow, that was honestly a tl;dr post for me, but I think you're basically asking if that Dell PSU will be enough, and it is.

Don't let the multiple rails of the Dell PSU fool you. They just have the available wattage/amps on the 12V rail(s) split between them. That doesn't mean that they total up to the total power the PSU can provide, though.

The 53 Amps on the Corsair unit = a max of 636W available from the single 12V rail on that PSU, so that sounds about right.

In any case, using the Dell PSU shouldn't be hindering the 6950's performance any. If the card was drawing more wattage than the PSU could provide, it would just shut down, or cause crashes.

I suppose that since you already have the Corsair PSU, you might as well use it, but it will just be for stability and possible safety reasons, if anything, not performance.
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a c 311 ) Power supply
June 12, 2012 4:13:33 AM

Changing the PSU won't get you any extra performance, a 600W PSU has more than enough power for a system with a single 6950 in it, the ECOs report for that exact model shows them testing it with 41.1A on its 12 V rail, given its rail distribution and overall rating I would suspect that it has an overall capacity of about 46 A which is more than enough for a 6950.

In general if the GPU isn't able to get enough power it will either cause the system to shut down(be it the GPU or the PSU that triggers it) or will refuse to move into higher power modes and provide incredibly poor performance. Since you seem to be getting good performance your power supply isn't holding you back. HP Phoenix line up is their new gaming machines so they are much better built than your average home HP or Compaq desktop which still use very old PSUs in them, but similar to the older Dell XPS units, for the big systems which they expect will encounter decent power draw they put in a good PSU since they don't want to be replacing a ton of them.

If it were me, i would just leave the current unit in there, swapping out a PSU can be somewhat of a pain, especially since that system is a pre-built they probably actually did some cable management in there which is good.
a b ) Power supply
June 12, 2012 4:43:35 AM

essentially the 12v rail decides how much power can be given to the most essential parts of the computer. if the rails are split, then not all the amps can be used at once. for instance for your 600w psu, its not as high amperage as say the corsair you pointed out, but its efficiency are on two different levels. because of the fact that you are using a single card build, it works. but if you had a efficient psu, you could easily have done the same work with a 500w psu, and have the psu waste less energy from the ac current through the wall, which turns into excess heat and deteriorates psu quality over time. If the current psu works for you, you are free to stay with it, but the conception that people think a 650w psu for a single card build is very unnecessary. you can easily run one with a 430w psu of good quality, whereas a 430w bad quality psu running say your 6950 would probably shut down.
a c 143 ) Power supply
June 12, 2012 5:35:02 AM

hapkiman said:
Or is all this 12v rail stuff B.S.?

To some extent, yes, since you will end up bridging the various rails through the motherboard and PCIe connectors anyhow so all the different rails indirectly merge into one with wiring resistance acting as the load balancer.
June 13, 2012 2:49:18 AM

Best answer selected by hapkiman.