SLi power supplies

[shivadee]

So some of you may know that I am currently in the process of upgrading 7800GTX to a 512 7900GTX. Ive been noticing these cards ask for an SLi power supply.

Now BEFORE i do this...my current Power SUpply is a 500W non-SLi.

1. Do you think that i am restricting my current gfx card with this?

2. Some on please give me a rundown on these SLi PSU's

[W1nTry]

Basically SLI (read: ready) PSUs are more marketing and really and truly another way for Nvidia to dig the eyes outta their partners. However what you have to be adamant about is the specs really. Like it MUST be dual rail 12V and it MUST have a 24pin main molex. Beyond that, labelling it SLI compliant or ready or whatever other ridiculous tag they have for it, it just marketing. Most/all of the high end PSUs are SLI/crossfire compliant ass they utilize the aforementioned specs. If you're going 7900GTX SLI however I would advise to look in the 600W+ region to be safe. The reason it's always better to go above your requirement are for expansion and for less heating since the PSU tends to be a MAJOR contributing factor to heating your case (unless you running prescHOTt, in which case ISPAT not ready). Here are a few I would suggest:
http://www.newegg.com/Product/Product.asp?Item=N82E16817703002 - costly but considered 1 of the best if not THE best
http://www.newegg.com/Product/Product.asp?Item=N82E16817163108 also very good
http://www.newegg.com/Product/Product.asp?Item=N82E16817104155 more budget friendly

[Spazosaurus]

How about this one...http://www.newegg.com/Product/Product.asp?Item=N82E16817151025

I have a couple 430's in this series and i must say they are running superbly.

[Czar]

... it MUST be dual rail 12V and it MUST have a 24pin main molex.

Agreed...once it conducin dem specs, yuh safe...so save yourself some $$ and jus check out the specs carefully...otherwise, save the hassle and pickup the "SLI-ready" version.

[Beomagi]

SLi ready means
1 - certified by nvidia but seriously, the certification isnt as important as
2 - multiple INDEPENDANT 12V rails
3 - at least 2 independant pci-e power plugs.

http://sg.vr-zone.com/?i=3335&s=8

The 7900gtx isn't as heavy on power as other cards - namely the x1900 series. For the x1900 crossfire, you want 17 to 18 amps per rail minimum. for the 7900gtx, 14 to 16 is fine - keep in mind that this is also taking into account factors such as psu's power dropping as the temperature of your case goes up (can lower efficiency), having other items on your lines (drives, fans etc), power supplies that may overate themselves, and in general you dont want to run a psu at full constantly. Run it easy - say 70%-80% so you always have room.

If the 7900gtx takes 100 watts, you're only talking about having a little over 8 (8.33*12=100watts) amps - but you'll never want to run it on a psu that delivers say 10 amps or only a wee bit more - too much of a voltage drop, to stressed, and it has to power other things.

e.g.
here's the output of the OCZ 620W
+3.3V@28A, +5V@45A, +12V@26A, -5V@0.8A, -12V@1A, +5VSB@2.5A

so you're sharing one 12v line for both cards. That's a bit of a strain - even more so with oc'd versions. I won't ever trust this one for running 2 x1900's, even though it's 620W, the power to the main power eating lines(12V) isn't enough.

I'd recommend an antec trupower 2.0
http://www.newegg.com/Product/Product.asp?Item=N82E16817103931

550 watts, only $75, and very strong 12 volt rails.
3.3V@32A, +5V@40A, +12V1@19A, +12V2@19A, -12V@1A, +5VSB@2A

It's powering my friend's system, and my new one. I bought it for the rail strength.

In terms of brands, i'd look at antec, enermax, pcpower and cooling (the bently of psus), silverstone, zippy, fortran source and maybe a couple others.

Quick way to know if a psu is crappy - pick it up The psu my little brother used and fried the first motherboard and chip i sent him back with was a "awesome power 500W" psu - that on adding rails, only came to 450 or so - but if you add rails, it's suppossed to be MORE than the rating. reason being, as rails go in use they increase the temp and interfere with the maximum the other rails can push - so adding the rails on that antec 550 - sends you into the mid 700W range! You'll be able to deliver 456 watts on just those 2 12 volt rails - though this drops as the other rails come in use. Picking up the el-cheapo-mobo killer my brother bought - shocked how light it was. Picking up the antec, was equally surprising. It's a brick.

[jose]

http://www.jscustompcs.com/power_supply/

A nice simple page to calc psu reqs.

[W1nTry]

It has been seen that 7900GTX SLI has some strange behaviour with <600W so if you going SLI even though the antec is a good solution, don't be too freaked out if you have issues. It has to do with the combination of mobo, cards, ram, etc. Computers are such that sometimes you're unlucky and suffer slight incompatibilty issues with a mobo with specific ram and configuration. I will stick to the 600+ W opinion, but Beo say everything+some you need to know.

I rem searching for a single rail PSU (which I have al now a thermaltake that has >30A on the 5 and >20A on the 12. Meh then original mobo (A7V8X Deluxe) REFUSED to work with anything less... ah well.

Another thing to consider is whether the PSU uses active or passive PFC (power factor correction). To brief you on the significance of this:
PFC is effectively a ratio of real to imaginary power (yeah those nasty sqrt(-1) guys represented in math by j or i) the closer your PF to 1 the more effecient the process (if you're a power plant) is for generation. You add (using capacitance) or remove (using inductance) to the PF to get it close to 1. Also the lower ur PF the less effeciently your appliance will run and the more heat it will generate. Since we're in T&T and the PF sucks (in most places) it's wise to get an PSU that provides Active PFC as it will vary the capacitance or inductance required to maximize the effeciency of the PSU. PAssive will work but is less flexible and will have only a set amt it can compensate.

Just an extra thought.