Cf/SLI Psu question

chromic · Sep 1, 2014

This may seem like a stupid question but im just wonderin (noob at Cf/Sli)

raw example :

750w psu that has 2x (6+2 pin) vga power cables

2x Gtx/Amd hdxxx and 1 card needs (6+6) pin cables.

So my question is.

Psu supplies max of 6+2x2 = 16 pins
cards need 6+6x2 = 22 pins.

how does it work..?

Vexillarius · Sep 1, 2014

It doesn't. You'd need a PSU with more connectors.

junkeymonkey · Sep 1, 2014

psu has 2- 6+2 pin and 1 card needs 2- 6 pins then no cause the psu would need 4 - 6+2 pin to run 2 cards

jimthenagual · Sep 1, 2014

Most high end GPUs need two PCI-e connectors (some 8pin, some 6pin)

For two GPUs you'd need four PCI-e connectors in total. Four connectors like the one in the picture below.

chromic · Sep 1, 2014

The question is cause i have a chieftec ctg-750c
(semi-modular 750w ~82-87% efficient PSU)
and it has 2 vga 6+2 cables, but has around ~56amps/675w @ 12v1+12v2 combined which would be enough for CF + Cpu, the only problem is the cables

junkeymonkey · Sep 1, 2014

if ''each'' card needs 2 pci-e 's to work and the psu only gives you 2 then you will be short 2 for the 2ed card how hard is that??

jimthenagual · Sep 1, 2014

I like to go by the rule "if it doesn't have enough PCI-e connectors, then it wasn't meant to power it". Not always true, but usually. There are 4pin molex -> PCI-e converters though.

What GPU are you trying to CF/SLI?

CGurrell · Sep 1, 2014

PSU Companies know what their PSUs can handle, and they will only attach the cables to match what the PSU can handle. So, if the PSU only has 2 6+2-pin cables, the PSU will likely only be able to power one GPU, regardless of what the output sticker says

junkeymonkey · Sep 1, 2014

PSU Companies know what their PSUs can handle, and they will only attach the cables to match what the PSU can handle. So, if the PSU only has 2 6+2-pin cables, the PSU will likely only be able to power one GPU, regardless of what the output sticker says

I may run 2 cards that only require one pci-e per each like a 7850 that requires just one 6-pin per card

CGurrell · Sep 1, 2014

junkeymonkey :

Fair play lol. OP was looking at cards that need 2 6-pin cables though

chromic · Sep 1, 2014

Probably 5870 CFx or 7870XT CFx

both consume ~200w peak.
I actually watched Vrm out/in wattage in hwinfo while running furmark (7870xt single) and it rise 150w--> and up but dont remember the max (was below 200w if i recall)

It reads on the site & on the box that the PSU is cf/sli ready

junkeymonkey · Sep 1, 2014

bottom line is if you go cheap on your psu you end up like this I got to treat my psu as I do the rest of my build plus head room for upgrades

too many folks buy budget psu's just to find there not too good in the end and there build quality can make it or break it

chromic · Sep 1, 2014

I dont think an ~85% efficient and silent 750w is a bad one.

I sent this question directly to chieftec support, waiting for an answer

junkeymonkey · Sep 1, 2014

ya ? so whats the build quality on it ?? 1st tier 100% jap or 2ed or 3ed tier china/Taiwan ??

I can find a disassembly review on it to back things up so I would be hard pressed to buy it with out knowing [opinion]

Mousemonkey · Sep 1, 2014

CGurrell :

My cards require 2x 6pin PCIe4 each but my PSU only has two, how then have I been able to run my two 660Ti's and the 560Ti's before that?

junkeymonkey · Sep 1, 2014

junkmonky did not say that --- it was a quote from the post above it that did not get the '' ''' put to it ----- sorry

Mousemonkey · Sep 1, 2014

junkeymonkey :

I know, it was CGurrell. I was responding to both of you.

junkeymonkey · Sep 1, 2014

''My cards require 2x 6pin PCIe4 each but my PSU only has two, how then have I been able to run my two 660Ti's and the 560Ti's before that? ''

new and better psu

chromic · Sep 1, 2014

Mousemonkey, how did you do it?

molex to pcie?

junkeymonkey · Sep 1, 2014

its like this will the molex supply the 75w the card needs for the 6 -pin?? or is it a get by thing that may or may not work cause it cant supply the required power to the card at all times ?? and can you be sure the molex is regulated to 75w? as the pci-e is?? to me it seems a lot up in the air on that but i'll look that up

6- pin = 75w 8- pin = 150 pci-e slot 75w

Mousemonkey · Sep 1, 2014

chromic :

Of course, the ability to do so depends not only on the wattage and quality of the PSU but also how much loading those power connectors are actually going to have to take and it seems that people seem to assume that they are going to be at max load at all times which is not the case.

junkeymonkey · Sep 1, 2014

claims here the molex is good to 60w [????]

https://bitcointalk.org/index.php?topic=51740.0

Mousemonkey · Sep 1, 2014

junkeymonkey :

I've not had one melt on me yet.

junkeymonkey · Sep 1, 2014

http://www.techpowerup.com/forums/threads/why-do-6-pin-pcie-adapters-have-2-molex-leads.111509/

junkeymonkey · Sep 1, 2014

well I don't risk my stuff with silly things like that a high quality psu that does the job as required is less cost then doing that jack leg set up and risk damaging the cards and a psu in some way so whats cheaper in the end ?? [plus peace of mind ]

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