johnnyq1233

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I've checked out the bfg site on my 7950gt/oc 512..
They say I should have 22+ amps on the +12v rail.
My current supply is only giving 17A but my system runs stable.
Now, I tried to bump my system a few percentiles up from stalk but the system just reboots and I get the 3 sec
beep and no post!
Should I invest in a better psu?
The psu I'm using is an ANTEC SMARTPOWER 450....

ps. I think this might be causing some frame rate dropage as well...
 

cleeve

Illustrious
Alot of system instability problems are caused by insufficient PSUs.

I'd definitely consider a PSU upgrade if your system is unstable. But if you're overclocking other things can hold you back as well, like memory timings...
 

johnnyq1233

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My sig at bottom of my post tells what I've got...just messes up if I try to push my clocks,,etc
I should mention that I've got 2 +12 rails...1 is @15 amps and the second is @17 amps......
Does the second take from the first to make up for the lack of amps on the pci-e buss?
 

4745454b

Titan
Moderator
I also have a SP 450W. Yes, you have two rails, one at 15A and one at 17A. If my memory serves, they together can output about 28A. This is why you can run a card that requires 22A.

I'd love to be able to answer your question, but I have no idea what "I tried to bump my system a few percentiles up" means. What part of your system? FSB? Memory? Video core or memory? All of these? By how many is a few percentiles? What where the temps before and after you did this? Give us a few details other then it doesn't work if you want some help.
 

johnnyq1233

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I tried setting the bios from auto to performance gamer and the system rebooted with a 3 sec. beep and then another 3sec. beep...continuously till I reset the bios.

Sorry, by pecentile I mean 20% over stock..if that helps..
 

cleeve

Illustrious
Remember, if a card has a 22A requirement it's assuming it's sharing a single 22a rail with the rest of the system. The 7950 GT only needs a portion of that.

If you have two 12v rails, even a dedicated 15a should be fine for a 7950GT.
 

cleeve

Illustrious
Try setting it to manual.... find out your memory's stock latencies and increase it a bit, then up the FSB manually.

Better yet... read on overclocking guide on the net before doing anything... your PSU sounds like it should be able to sustain a bit of overclocking. :)
 

4745454b

Titan
Moderator
I've heard of bios's that let you "auto overclock", but how they accomplish this probably varies depending on the motherboard. 20% might also be to much for your parts. Either try a lower percentage, or I also suggest learning how to "really do it". You might just need a small voltage bump to make that 20% stable. Or a 19.5% overclock might be work. If you know what you're doing, manually overclocking is better.
 

kpo6969

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I would have to dis-agree. Combined 22a on the 12v rails for a 7950GT.
EXAMPLE
my previous 305w psu
18a + 18a = 264w maximum combined wattage on the 12v rails (the 2 12v rails)
divided by 12 = 22a maximum combined amps on the 12v
 

cleeve

Illustrious
What does that prove?

The card does not draw all of the 12v power available, kpo6969.
12v is needed to run your system, even if you didn't have a videocard.

The manufacturers of the 7950 GT know this, and their requirement assumes a single 12v rail for the whole system. The 7950 GT does not need all those amps by itself.

That's why the single card requirement will be 22 amps, and two cards in SLI will require something like 29 amps...

...otherwise you'd need a 44 amp 12v rail to run 7950 GTs in SLI. Not likely.
 

kpo6969

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What else runs off the 12v (just asking?)
cpu- watts divided by 12 = ?
ram- xxxxxxxxxxxxxxxxxxx=?

I thought those have to added to the card's total?
Sorry if I'm incorrect.
 

4745454b

Titan
Moderator


I think your post needs another edit, as it doesn't make any sense to me.

Combined 22a on the 12v rails for a 7950GT.

Huh? What we're trying to say is that you don't need a single 12V rail that has 22A. If you have two rails, 15A and 18A, or 18A and 20A, then they probably combined can output enough amperage to drive the 7950GT/card in question.

18a + 18a = 264w maximum combined wattage on the 12v rails (the 2 12v rails)

Huh? 18A + 18A = 432W. (assuming we are still talking about the 12V line. 18 + 18 = 36A. 36A * 12V = 432W, not 264W. If your 305W Dell PSU could only do 264W, then work backwards. 264W / 12V = 22A. Assuming that 264W figure is the total that the PSU can output, this 22A might be one a single line, or spread across two rails. What is likely going on is that you had two rails, each rail can output a max of 18A, but the two rails together can only do 22A.
 

cleeve

Illustrious
Just to clarify, what I'm saying here is that the 7950 GT does not, by itself, require 22 amps on the 12v rail.

The manufacturer recommends a 22 amp power supply because that 12v power is shared between the graphic card and other components like the CPU, hard drives, and fans. This is taken into account when they make the recommendation.

I believe (although I'm not 100% on this part) that power supplies with two 12v rails dedicate one to the motherboard (for CPU, fans, etc.) and one to the graphics card PCIe power connector.

Even though neither rail is 22 amps, the OP was worried that the largest of his two 12v rails is only 17 amps.
However, this rail would easily supply enough juice for the graphics card if it wasn't shared with the rest of the system.
 

kpo6969

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I'm sorry for even posting.
What I'm trying to say having one of your 12v rails @15a doesn't mean you can run the card.

4745454b
sorry for your confusion

I know the specs of the psu I had and what it can do.

Sorry for expressing an opinion, I will avoid getting jumped on for doing such in the future.
 

kpo6969

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Your psu has 22amps which is boardline.
http://forums.guru3d.com/showthread.php?t=205763
Antec SmartPower 2.0 SP-450 22amps
 

kpo6969

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I know how to add
 

cleeve

Illustrious
No need to apologize, nobody's calling you any names. We're just trying to clear this up for you folks.

we can summarize the point thusly:

1. If you have one 12v rail under 22 amps, you might have trouble running the card

2. If you have more than one 12v rail - each under 22 amps - you probably WILL be able to run the card.

 

4745454b

Titan
Moderator


Are you sure about that? Is 18A + 18A = 36A? Does multiplying that 36A by 12V get you to 264?

I also chuckled a bit about the site you linked to. He's close, but not exact for the SP 450. My box says 440W for all rails, and 150W for the 5 and 3.3V rails. This leaves (at least) 290W for the 12V rails. (I said at least because not maxing out the 3.3 and 5V rails MIGHT allow for more power to be sent to the 12V rail, read the bottom of the first post.) 290W / 12V = 24.16A, a bit more then the 22A figure you and he quote.

Don't be sorry for even posting, thats not mine or cleeve's goal. The OP posted a question, and as much as you might deny it, there is some confusion about how a PSU works. If you are only going to get pissy and shut down, then you won't learn anything. If my math is wrong, feel free to show me where. Just don't expect me to believe you if you say, "some guy in some forum says it can do only 22A, so there!"
 

kpo6969

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I thought I was done with this but just to show you you are incorrect.
You said
"Are you sure about that? Is 18A + 18A = 36A? Does multiplying that 36A by 12V get you to 264?"
No it doesn't
But
My 2-18a 12v rails have a maximum power wattage capacity of 264watts (printed on the psu label)
divide that by 12 = 22amps
Please reply with documentation to show I am incorrect.
I did not submit totals for that link so that may be wrong but I can read the label of my own psu.
 

cleeve

Illustrious
In most cases, separate 12v rails can't handle peak draw on both at the same time.

So yes, Two 18A 12v rails might only be able to provide less than 36 amps total between both when under load.
 

kpo6969

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4745454b,
In response to your statement:
"If my math is wrong, feel free to show me where. Just don't expect me to believe you if you say, "some guy in some forum says it can do only 22A, so there!"

1.That statement is patronizing, do you think if you don't have a set # of posts your a "noob"
2.I never post unless I know somewhat the subject being discussed
3.I usually have documentation to back-up my facts

Thanks
 

4745454b

Titan
Moderator
No math to back up your statements huh? I didn't say you are a noob, just someone who is confused about things. I don't know why you are acting this way, but unless you can accept the fact that you might possibly be wrong, you aren't going to learn how/why you are wrong. If you're willing, read the rest of this post. If you are so sure you're right, either back up your claim with evidence, or show me where I made a mistake with my math. (seeing as you haven't done that yet, I'm assuming my math is right.)

Lets review. Each rail can output a certain amount of power. In the case of the SP 450W, the first 12V rail can output a MAXIMUM of 15A, while the second can do a MAXIMUM of 17A. Some people mistakenly believe that you can add these two number together to get the total amount of power for the 12V rails. (or in your case, 18A + 18A) The problem with this is that those are the individual rails, not the total amount of power for the PSU overall. The SP 450W can't output 15A + 17A on the 12V rail at the same time, but in fact 24A. Lets look at a good example. Click on this link, then the third picture.

http://www.newegg.com/Product/Product.aspx?Item=N82E16817371009

This PSU has four 12V rails that individually can output a max of 18A. If you follow the mistaken logic, you would think this PSU could output 864W combined for the 12V rail. (18A * 4 rails = 72A. 72A * 12V = 864W. See how I proved this with math?) Antec doesn't claim that this psu can output a COMBINED wattage that is that high. They say that the combined output for ALL 12V lines is 768W. 768W / 12V = 64A. This means that the circuitry inside the PSU can't supply enough power to feed the maximum that each rail is capable of. If divided equally, each 12V rail could have 16A. (in actuality each rail would probably be different. The motherboard rail might be 10A, the CPU rail might be 15A, the GPU rail might be 18A, while the fourth rail might be at 8A.)

Some PSU manufacturers don't make it easy to figure out how much power the PSU can output. They might list peak power instead of sustained. They might just list each rail and the amperage, with no total figures. These are the power supplies you should avoid.

I'm not claiming that you misread your label. I don't doubt that it said 18A under both rails. But to write what you did, "18A + 18a = 264w maximum combined wattage on the 12v rails" is wrong. 18A + 18A = 36A. 264W / 12V does equal 22A. 18A maximum for each line, but combined, it could only do 22A. (either 18A on one with 4A on the other, or 11A on both, or any number of possible configurations.)

I hope this makes sense.