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SLI - Which will result in more power?

L

LarryJoe

Platinum Member
According to my Dell documentation on my PSU (750w), I only have two molex connectors chained together coming off the PSU. I have two 6-pin connectors coming separately from the PSU. I am adding a second geforce GTX which requires two 6-pin connections.

Which will result in more juice, splitting the two molex into four to use a two molex to on 6-pin adapters x2 or splitting the 6-pin's using a two 6-pin Y adapters?

What do you think?

This is really a PS question, punting to the Power Supply forum

-ViRGE
 
Reading "Splitting the 12v Rail", this may have answered my question:

So there are no disadvantages to using a PSU with multiple +12V rails?

No! I wouldn't say that at all. To illustrate potential problems, I'll use these two examples:

Example 1:

An FSP Epsilon 700W has ample power for any SLI rig out there, right? But the unit only comes with two PCIe connectors. The two PCIe connectors on the unit are each on their own +12V rail. Each of these rails provides up to 18A which is almost three times more than what a 6-pin PCIe power connector is designed to deliver! What if I want to run a pair of GTX cards? It would have been ideal if they could put two PCIe connectors on each of those rails instead of just one, but instead those with GTX SLI are forced to use Molex to PCIe adapters. Here comes the problem: When you use the Molex to PCIe adapters, you have now added the load from graphics cards onto the rail that's also supplying power to all of your hard drives, optical drives, fans, CCFL's, water pump.. you name it. Suddenly, during a game, the PC shuts down completely.

Solution: To my knowledge, there aren't one-to-two PCIe adapters. Ideally, you'd want to open that PSU up and solder down another pair of PCIe connectors to the rails the existing PCIe connectors are on, but alas... that is not practical. So even if your PSU has MORE than ample power for your next graphics cards upgrade, if it doesn't come with all of the appropriate connectors, it's time to buy another power supply.

There are now 6-pin PCI-E splitters. Check it out at Newegg

It appears that this is exactly the guidance I was looking for. That is, splitting two molex connectors into four is less optimal and that each PCI-e has enough power to be split to power a GTX.

Thoughts?
 
You can use splitters but you must make sure the PSU has enough power to support the upgrade. Sure a 750W PSU may be enough to support a pair of GTXs, but only if it has enough of that power focused on the +12v rails.

Overall its best not to use any splitters. If the unit doesn't have enough PCI-e connectors there is probably a good reason for it. You said you have only 2 free molex connectors? Well them you would only be able to use one molex to PCI-e adapter. It would not be a good idea to ad a splitter to each molex so that you can connect a PCI-e adapter to each splitter. The wires coming from the PSU to those 2 molex connectors are only designed to handle about 6A, maybe as much as 8A. Well a single PCI-e power connector is designed to offer 6A. So with 2 adapters on the same line you'll be pulling anywhere from 8A to 12A depending on the load on the one video card. That kind of overload could start a fire in your system. So like the above mention article says. If the PSU doesn't have the connectors then buy a new PSU.

 
Thanks for the education. In terms of your advice, so anyone who wants to SLI but needs 4 PCI-e's and only has two needs to buy a new power supply?

My system is a Dell XPS 720 and it was offered with either a 750w or a 1000w PSU and optional SLI 8800GTX config. I am guessing if I ordered SLI I would have gotten the 1000w PSU. I think the PSU only has 2 molex plugs because they are becoming fairly obsolote. I have plenty of SATA power connectors for my 3 hard drives and 2 optical drives.

I reached out to Dell and specifically asked them if I could splt each PCI-e into two for SLI and they said yes and added that each PCI-e is capable of pushing 300w. They could not tell me the amps. Now all this said, I take Dell's advice/support with a grain of salt.

Here is a diagram of my PSU (P15 and P16 are only available on the 1000w version), P6/7 are the two molex, I think you can figure out the rest of the connectors:

Dell 750w PSU
 
Sorry about the bad link. I would upoad the image here and post it IF they only had that functionality at Anand like other forums.

Well, all is good. I installed the second GTX and used the PCI-e spitters from Newegg and I am running good. Of course it was only tested for a few hours, but given the power issues I have read about during my research, I am pretty sure it would either trip, not boot, or crash on launch or during plaing a demanding game.

So it seems Dell was right telling me I had enough juice going through each PCI-e to allow me to split them.

Just thought I would follow-up for future reference. I would say that 90% of the advice was that it would not work and that I needed to do the 4 molex to two PCI-e. I'm no expert, but this would appear to dillute the power even more. I guess I may have gotten lucky.
 
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