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Author Topic: On providing 12v to the pcie extender ? is 5 GPU too much ?  (Read 1495 times)
Transisto
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August 22, 2011, 07:56:40 PM
 #1

Going to 5 gpu on a board, I'm starting to get concerned.

Ain't this within motherboard's spec ?  Is this to balance the rail going to the motherboard ? Will the trace burn ?

Thanks

I can't find any guide on how to make one,

On cablesaurus website picture it use B1,B2,B3,A2,A3
https://cablesaurus.com/images/DSCN0500a.jpg

The wikipedia page identify B3 as "Reserved, Not presently used, do not connect" so why use it ?
http://en.wikipedia.org/wiki/PCI_Express#Pinout

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Power:

PCI Express cards are allowed a maximum power consumption of 25W (×1: 10W for power-up). Low profile cards are limited to 10W (×16 to 25W). PCI Express Graphics (PEG) cards may increase power (from slot) to 75W after configuration (3.3V/3A + 12V/5.5A).[7] Optional connectors add 75W (6-pin) or 150W (8-pin) power for up to 300W total.
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haploid23
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August 22, 2011, 08:29:59 PM
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that's too much. even 4 for me is too much for a high end card. i measured that each of my 5870 pulls a little over 25w just through the pci-e slots, so when i had 4 of them on there, they pulled over 100w through the motherboard just for the video cards. i can't imagine this not affecting the GPU stability. putting that extra molex power connector like the one sold on cablesaurus will let the GPU pull the power directly from the PSU instead of going through the motherboard first

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August 22, 2011, 10:30:09 PM
 #3

instructions on how it's done ?

ok, A2 A3 B1 B2 but why don't they reroute the negative to the molex too ?
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August 22, 2011, 11:55:59 PM
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24pin ATX has 2 +12V pins and 8 GND pins, rated for 6A each.
So thats a max current of 12A on the 12V side, 48A on the return.
That's the short version of why splitting out GND isn't needed.

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August 23, 2011, 01:19:24 AM
 #5

Thanks

so I conclude it's mostly needed for a 5+ card setup.
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August 23, 2011, 02:09:42 AM
 #6

so I conclude it's mostly needed for a 5+ card setup.

yeah, the wire will barely be able to handle the current draw. but would you really want 144w (more than 4 cards) of power being shoved into the motherbaord before it can be passed through to the pci-e lanes? i would think it's better to use the extra molex power wire to draw directly from the PSU. of course, these things are not very cheap either.

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August 23, 2011, 02:15:47 AM
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that's rather easy to do once you've figured which side is A and B.

But again, what about B3, is it safe to use it for 12v ?
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August 23, 2011, 11:06:33 AM
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Maybe you could measure the voltage on B3 currently? If it's not 12V then I wouldn't provide 12V there, but if it is already providing 12V on that pin one might assume it's safe to do.

Also you might be able to see how loaded the mainboard 12V supply is by measuring voltage on the card extender and comparing to the 12V PSU connectors. If it's dropped more than a bit then that's an indication the traces can't handle the load.

btw, at the bottom of the wikipedia page there is a link to torrent version of the specs. I didn't try that but you may be able to find more info in there on what B3 is reserved for and why, or if perhaps the 2.0 spec changed that (which may explain why the cablesaurus cables appear to use it).

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August 23, 2011, 02:24:04 PM
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This may help.

http://blog.zorinaq.com/?e=44

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