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View Full Version : PSU Issue - 7800GTs in SLi


Peoples-Agent
09-21-05, 07:30 PM
Well, recently I upgraded my system with a view to going to the SLi platform sometime early in the new year and along with I bought this power supply.

Hiper Power Type R HPU-4M480 480 Watt.

http://www.rbmods.com/Articles/Hiper/Hpu-4m480/1.php

Very nice piece of kit, but my question is.... do you think it will be able to cope with another 7800GT in SLi mode?

I don't run too much in this system, two DVD drives and a single hard drive.

Any ideas?

SuperBORG
09-21-05, 09:38 PM
I think it is enough. I mean, look at my PSU.

Clay
09-21-05, 09:40 PM
You're fine, that PSU even has two separate 12V rails.

acrh2
09-21-05, 11:47 PM
Look at a guy with a 650W PSU giving advice.
The truth is, this psu may or may not be enough, depending on what else you are going to run in your system.
If you are going to OC your CPU to top of the line speeds, 3.8+ for p4 and 2.6+ for A64, it will probably NOT be enough.
Someone has said recently that Nvidia recommends at least 34A on the 12V rail for 7800 sli. You would have 16+18A from the two rails, with combined current of 34 or less, most likely around 30A.
I would definitely go with a beefier PSU.

Clay
09-22-05, 12:01 AM
What does my current PSU have to do with anything? :confused:

Of course it may/may not be enough but he said he already said he doesn't run much on his system. If he is not going to OC his CPU to top of the line speeds then it probably WILL be enough. ;)

Peoples-Agent
09-22-05, 12:10 AM
Nah, no CPU overclocking....definately not!
Defeats the whole point of having SLi in my book.

MikeC
09-22-05, 12:10 AM
Spare us your sarcasm Arch2.


This is what I found in my reviewers guide for the NVIDIA reference 7800GTX preview I did.

"It is critical that systems with high-end graphics cards, such as the GeForce 6800 Ultra and GeForce 7800 GTX, are matched with an adequate power supply to ensure stable operation. Therefore, I have provided the following power supply information, which originated from NVIDIA's GeForce 7800 GTX GPU Reviewer's Guide:

The GeForce 7800 GTX requires a stable, 12-volt power source for best performance, reliability, and enjoyment. Many PC power supplies dedicate most of their 12-volt power to the power rail that goes to the CPU, rather than the peripheral connectors. Many power supplies also do not provide ample overload protection to protect system components such as motherboards and graphics cards.

For a SINGLE GeForce 7800 GTX graphics card, NVIDIA recommends a power supply with at least 350W, 22amps on 12V. For a DUAL SLI GeForce 7800 GTX graphics configuration, NVIDIA recommends a power supply with at least 500W, 30 amps on 12V. A list of SLI-certified power supplies can be found at SLIzone. Due to power distribution limitations of the PCI Express bus, GeForce 7800 GTX boards require a separate PCI Express power plug on the back end of the card."

Clay
09-22-05, 12:18 AM
Thanks for posting that from the NVIDIA Reviewer's Guide Mike. I don't have my copy handy right now. I thought the "Someone has said recently that Nvidia recommends at least 34A on the 12V rail for 7800 sli" sounded incorrect, not to mention vague.

Rytr
09-22-05, 12:23 AM
I doubt you will need more than 28-30A on the +12V rail. I have run 6800GTs in SLI using a FSP 500W v2.0, actually 460W, extensively in test settings as well as some lengthy gaming sessions. I believe the supplied voltage of combined 12V rails are rated at 30A (15/15).

Edit: I was looking up some actual use requirements and basically found that a number of psu's provide ample power for SLI in various combinations of cards ranging from 6600's thru 7800's with ratings ranging from 24A up to 35A. The main thing is that the psu needs to be a quality unit.
I would suggest a unit meeting the v2.0 specifications although I have used an older 20-pin Enermax 550W with no problems.

acrh2
09-22-05, 01:00 AM
Ok. It was my bad when I mentioned 34A incorrectly. The 34A number is for 2 x BFG 7800gtx. And it's a recommendation. It's neither a requirement, nor a guarantee.

Nevertheless, what I said before is true: if you are going to use an SLI system with a power supply rated so closely (or slightly below, see later) the recommended specs, you are getting yourself into a gamble situation. It may or may not work in a stable manner.

To me personally, this is especially true since I am not familiar with this particular manufacturer. If one reads recommendations on PSU brands from a number of hardware forums (hardforum.com, ocforums.com, guru3d.com etc.), names like OCZ, PCP&C, FSP, Seasonic, Enermax are most often mentioned for reliable and proven manufacturers.
As this comparison from PC Power and Cooling website shows, not all PSUs are created equal:
http://www.pcpowercooling.com/pdf/Turbo-Cool_510_vs.pdf
In my reading, dual 12V rails usually means that the total supplied power would be slightly (2-4A) lower than the simple sum of the two numbers. In my opinion, this PSU might only be good for about 30A on the 12V rail. I don't know how good the airflow is in your case, but if it's not very good, you would most likely be looking at even less power than that.

This review of the Hiper PSU in question shows a 12V line drop of 0.2V with a decent amount of load:
http://hi-techreviews.com/reviews/Hiperpower/P5.htm
I'm SURE that a 7800gt sli system would only make things worse.

This is why I want stick to my recommendation: to have a piece of mind and save yourself from possible headaches, get yourself something like this instead:
http://www.newegg.com/Product/Product.asp?Item=N82E16817151025
It seems to be only 30-40 dollars more.

Clay
09-22-05, 01:07 AM
Absolutely, all PSUs are not created equal. Of course there is Fortran which you didn't mention and most know nothing about yet they make many/most components for many of the "name brands" you listed.

$30 to $40 more is likely worth the peace of mind for some, but this guy has already bought this particual Hiper PSU and he'll be just fine with it considering his stated system use/stress level.

Peoples-Agent
09-22-05, 01:07 AM
I already have this PSU in my system currently, decided to take the risk when the time comes , if it doesn't work out ill just use it on another system. Be buying a dual core CPU around the same time so can look towards a second pc anyhow.

Thanks for good cautious advice.

acrh2
09-22-05, 01:10 AM
Of course there is Fortran which you didn't mention and most know nothing about yet they make many/most components for many of the "name brands" you listed.


I did mention Fortran, FSP = Fortran. However, according a number of accounts from owners, Fortran has been flaky lately. OCZ and PCP&C is where it's at.

Clay
09-22-05, 01:12 AM
I did mention Fortran, FSP = Fortran.Gotcha, I missed that.

Peoples-Agent
09-22-05, 01:15 AM
Yeah, I hate overclocking my CPU to be totally honest, always turns out to be a headache to me, recently got it to 2.4ghz comfortably though using the old type 1:1 ratio, any higher and it would fall over no matter what I lowered or tweaked. Even tried lowering the ram to 166mhz and cranking up the FSB giving the CPU vcore more juice and it started freezing and what not. I give up with all that lol

acrh2
09-22-05, 01:16 AM
I already have this PSU in my system currently, decided to take the risk when the time comes , if it doesn't work out ill just use it on another system. Be buying a dual core CPU around the same time so can look towards a second pc anyhow.

Thanks for good cautious advice.

it's quite simple really. If you ever experience shutdowns, or restarts, or hardlocks, it's most likely the PSU. And then again, current forceware drivers for SLI aren't stellar, and I've seen lockups in Dungeon Siege 2 from SLI/incorrect SLI profiles (my own doing). I'm sure, future Forceware will eliminate these minor issues.

smthmlk.
09-22-05, 02:10 AM
I assume you two are talking about "Fortron Source" when you say 'Fortran' :) Or maybe I missed the relevance of that programming language of a similar name....

on topic: I agree that it can't hurt to try it and see how it goes with your current PSU. However, should problems arise and you come into the market for a new PSU, I would only buy those which are SLI certified or the like. And top brands only at that (PC Power&Cooling, Seasonic, Fortron, OCZ, etc). Best of luck to you!

Clay
09-22-05, 09:38 AM
Ahh the typo police. :angel: Yes, I meant Fortron of course.