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Author Topic: PCIe extenders and molex  (Read 1192 times)
cyberlync
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July 12, 2011, 05:02:35 AM
 #1

Hi Forum.

I was wondering, and couldn't find answers, so I ask.

When are the PCIe extenders with molex needed? Without being sure, I think it's when using it on a 5970/6990 or when I have more than 4 gfx cards in the rig. Could someone please correct me if I am wrong, or clarify when exactly they are and are not needed?

Thanks in advance.

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Rob P.
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July 12, 2011, 01:30:06 PM
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You need the molex -> PCI-E Power Adapters (these aren't extenders), when you run out of PCI-E power adapters on your power supply.  Every power supply is different, some have none, some two, and some four.  Usually you'll have to use Molex -> PCI-E power adapters after two cards (not sure of a motherboard with more than 4).

PCI-E Extenders are used when you cannot fit more graphics cards on the motherboard due to space limitations, but you have available PCI-E slots you could still use.

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cyberlync
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July 12, 2011, 04:14:47 PM
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Sorry if I wasn't clear enough. I know what the molex -> PCIe power adaptors are. What I wanted to know is, when are the PCIe extenders with attached molex (the ones cablesaurus sells, for example) needed, and when can the ones without molex be used.

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July 12, 2011, 05:05:01 PM
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Sorry if I wasn't clear enough. I know what the molex -> PCIe power adaptors are. What I wanted to know is, when are the PCIe extenders with attached molex (the ones cablesaurus sells, for example) needed, and when can the ones without molex be used.

The story I read was that some cards consume a lot of power such as the 6990, so much so that the PCIx slot cannot supply enough and so the molex is added to extend the amount of power supplied to the pcix port.  Basically it's for the dual gpu single slot cards.
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July 12, 2011, 05:09:19 PM
 #5

Be careful if you use a molex->PCIe adapter for the purpose of powering your video card.  Doing this will cause your card to share power with the 12v rail it is extended from. The higher end cards can pull 6-10 amps, and may cause the total power draw for the rail to exceed 240W, which will cause the PSU to shut down - e.g. your computer will simply shut off without warning.

Check what other devices share that rail before adding a video card to it.
cyberlync
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July 12, 2011, 08:50:35 PM
 #6

Thanks for the replies.

Yeah I know about the load on rails and such, went with a decent PSU with overhead to be on the safer side, and I know that the efficiency is not optimal when approaching 100% PSU load (from what I could gather, it's usually best around the 50-80% load, varies depending on PSU ofc.), so again some overhead is good.

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cicada
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July 12, 2011, 09:30:14 PM
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PCIe cards can draw ~75W from the PCIe socket.  This is how GPUs got by in the olden days without requiring additional 6/8pin power plugs.

Most cards will still utilize this 75W in addition to the 6/8pin plugs, so 4 cards on a single mobo might be drawing ~300W via the PCIe lanes directly.  That's a lot of current to be pulling through your motherboard.

The PCIe extenders with molex power plugs patched in alleviate this load and allow your PSU to supply this 75W directly, which reduces the strain on your ATX 24pin plug and the motherboard itself.  I've read instances of the 12V leads on the ATX 24pin plug literally melting due to the heat produced from the excessive draw.

If you're going to have more than 3-4 cards in a single motherboard, or several really high-power cards (eg 5970, 6990), it's wise to use these adapters.

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July 12, 2011, 10:08:41 PM
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Be careful if you use a molex->PCIe adapter for the purpose of powering your video card.  Doing this will cause your card to share power with the 12v rail it is extended from. The higher end cards can pull 6-10 amps, and may cause the total power draw for the rail to exceed 240W, which will cause the PSU to shut down - e.g. your computer will simply shut off without warning.

Check what other devices share that rail before adding a video card to it.

yes, this was a problem i ran into.  had to use 2 power supplies in the end.

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