Another "Power Supply vs Videocard" question...with a twist

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BernardP

Golden Member
Jan 10, 2006
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First, I want to say that I have searched on this subject. I have found some information, but nothing precise enough to eliminate, in the end, what I would call ?wishful guessing? about total power requirement.

I have an Antec SmartPower 2.0 Power Supply rated at 350 watts. Specs are HERE. I am not going to replace this PSU. I want to add a video card, and this card will be selected according to the power the PSU can provide, not the other way around.

This PSU has 10A on the first +12V rail and +15A on the second +12V rail. My system description is in my sig below. System currently runs on NVidia 6150 integrated graphics and comprises:

--- 3700+ A64 Processor
--- 2 sticks of memory
--- 3 HD's
--- 1 DVD burner
--- 1 diskette

What I would like to do is compute the power I have available for an add-on graphics card. It has to be NVidia (same drivers as integrated graphics), and I am thinking about 7600GT or 7900GS. I have seen in recent reviews that the 7900GS consumes a peak of 82 watts according to NVidia . So if I divide this by 12 volts, I get a requirement of just under 7 amps...Right?

I have used the Extreme PSU Calculator, using the 7900GT as an example, as the 7900GS is not yet listed, but should have similar power requirements. The result is a CPU recommendation of 293 Watts. Although my total power seems sufficient, the Calculator leaves questions unanswered, concerning the fact that my PSU has relatively low power on each of its +12V rails.

So I might be alright, but the dual +12V rails confuse me. How can I know what is already feeding off one rail or the other? Looking at the PSU, I don?t see any ?rails?. All I see is a bunch of power connectors coming out of the PSU, the 24-pin mobo connector, the 4-pin mobo connector for the processor and a PCI-E connector. It is my understanding that the PCI-E slot (75 Watts) feeds off the mobo. I have no way to find out what is feeding off one ?rail? or the other. I don?t even know if I have to worry or not about overloading a rail.

Also, what components are using 5V and 3.3V ?

I would like to be able to compute the total +12V power draw of my components, and compare it with total +12V power available from my PSU. I have almost done this with the Calculator, but uncertainties remain: I would like to know if I have to worry about the split between the two +12V rails of this power requirement?And if I do, how do I make sure that I stay within each rail?s power capability.

If my PSU is too weak for a 7900GS, I will get a 7600GT. And if the 7600GT is bordeline, I will get a 7600GS. I am only a casual gamer, but would still like to get as good a video card as my PSU will allow.

Thanks for your comments.

 
jonnyGURU

jonnyGURU

Moderator <BR> Power Supplies
Moderator
Oct 30, 1999
11,815
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If you have a DFI board, you will need aa 1.5kW PSU with at least 15A on the +5VSB.
 
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sdy284

Member
Apr 11, 2006
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i'd go with the 7600GT

if you went with the 7900 you wouldn't leave your PSU much headroom & operating your PSU @ or near its max capacity is a great way to kill a PSU
 
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Bobthelost

Diamond Member
Dec 1, 2005
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You should be fine. It's possible to work out power draw from various sources on the web, HDs for example draw 10-15W each, your RAM + Mobo might draw 20W total, the CPU 60W (But that's from rail 0 not rail 1).
 
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BernardP

Golden Member
Jan 10, 2006
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Yes, I have worked out the power requirements thanks to the Extreme PSU Calculator. What I am trying to understand better are the limiting factors that might exist because the total +25A available power is split on 2 rails (10A + 15 A).

My first post tries to explain this in more details.

In a way, things would look simpler if I had +25A on a single +12V rail.

Take the 7600GT. EVGA says that I need 18A on the +12V rail. Does it mean I can't use it with my PSU because each of the rails has less than 18A? Or should I consider the available total of 25A?
 
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dBTelos

Golden Member
Apr 17, 2006
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I actually Loled at Jonnyguru's post :) I don't usually do that. On topic, I feel that you can run a 7900GT on that PSU without lockups or problems.
 
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Bobthelost

Diamond Member
Dec 1, 2005
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1) The PSU calc is not accurate

2) The parts are going to be reasonably well divided with your setup

EVGA and all the other manufacturers assume you've got a POS generic one that can't supply more than 80% of it's rated power without exploding. The 7900GS won't draw more than 8W, it doesn't care if it's on an 8W rail or a 100W rail.
 
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BernardP

Golden Member
Jan 10, 2006
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Originally posted by: Bobthelost
1) The PSU calc is not accurate

2) The parts are going to be reasonably well divided with your setup

EVGA and all the other manufacturers assume you've got a POS generic one that can't supply more than 80% of it's rated power without exploding. The 7900GS won't draw more than 8W, it doesn't care if it's on an 8W rail or a 100W rail.
Click to expand...

I would appreciate it if you could elaborate on your first 2 points. That is exactly the kind of info I am lookin for.

1): You mean the calc on Extreme PSU Calculator is nor accurate?

2): I would like to know more about which component feeds off which rail. This rail thing remains myterious, as all I see, physically, is one big bunch of power cables coming out of the PSU... So where are the two +12v rails? :disgust:

As for the 7900 GS' power requirements, I guess you meant 8 amps, not watts.
 
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Arcanedeath

Platinum Member
Jan 29, 2000
2,822
1
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w/ 25 amps on the 12V rails total you should have no problem powering a 7900GT or simlar card listed in your system. Just ignore the fact they are seperate rails and look at total 12V power w/ that PSU the 10A goes towards the CPU/MB and the 15A goes to power the rest of the system.
 
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alimoalem

Diamond Member
Sep 22, 2005
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Originally posted by: BernardP
Originally posted by: Bobthelost
1) The PSU calc is not accurate

2) The parts are going to be reasonably well divided with your setup

EVGA and all the other manufacturers assume you've got a POS generic one that can't supply more than 80% of it's rated power without exploding. The 7900GS won't draw more than 8W, it doesn't care if it's on an 8W rail or a 100W rail.
Click to expand...

I would appreciate it if you could elaborate on your first 2 points. That is exactly the kind of info I am lookin for.

1): You mean the calc on Extreme PSU Calculator is nor accurate?

2): I would like to know more about which component feeds off which rail. This rail thing remains myterious, as all I see, physically, is one big bunch of power cables coming out of the PSU... So where are the two +12v rails? :disgust:

As for the 7900 GS' power requirements, I guess you meant 8 amps, not watts.
Click to expand...

meaning it's not necessarily a reliable source to measure power requirement. every psu has different amperage so you may have a lot of power left over in some parts and possibly underpowered elsewhere.

i think you'll definitely be fine with the 7600Gt
 
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BernardP

Golden Member
Jan 10, 2006
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Thanks for the comments. Yes, I also think the 7600GT will do, but it seems my precise questions are destined to remained unanswered. I still feel I am guesstimating, and not being able to compute a solid hard answer.

My fallback position is that if I get a card not requiring a PCI-E power connector, total power draw is going to be safely below 75W, and would not be too much for my PSU.

I am going to wait for NVidia's answer to the upcoming ATI X1650XT, such as a GeForce 7650GT. This should be known within a month.
 
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