riser with molex or not ?

edited January 2014 in Hardware support

Hi,
My HD7950 have 2 power connector on top for direct connection with the PSU.
If I want to use PCI-e riser, do I need powered riser ?
I want to have 4 cards (I've got 2 pci-e x16 and 2 x1 on my motherboard)

Comments

  • edited January 2014

    the powered riser is to help with the power the gpu draws from the mobo pcie slot and atx socket

    so yes if you going to use 4 x gpu it may well be good to have 2 x powered riser 

    you need to connect the powered riser to the same psu rail as the mobo to avoid connecting two psu rails together 
  • edited January 2014

    My understanding is that :

    - a MB should be able to handle 3 unpowered riser but probably not 4

    - 1x slots are likely to be less able to bring a lot of power to card

    So you'd need only one powered 1x riser and the 3 others unpowered.

    But to be on the safe side, I'd take two powered PCI risers for the 1x slots.

  • I will make powered riser for may 4x280x. I have seen a lot of guides, but none didn't included capacitor? Is with capacitor better or not?
    Atherwise i will do like this guy:
    https://bitcointalk.org/index.php?topic=76121.0
    Is that OK?

  • edited February 2014

    It all depends on your cards and motherboard.


    A 6-pin gpu power connector can supply about 75W, while an 8-pin one can supply 150W

    A pci-e slot can generally handle no more than 75W, and a typical motherboard can only provide about 150W for ALL of the slots.

    So, for example, my 7950 cards take 200W apiece, and have two 6-pin connectors on them.  These can supply a total of 150 of the 200W needed.  The last 50W has to come from the motherboard.  This would say that I could use 3 unpowered risers, but not 4.

    I prefer to play it safe and made all of my risers powered.  No point in risking an expensive motherboard to save on a few $10 cables.  Use the ones that have both yellow and black wires.  The cheap ones only have the yellow +12v wire, forcing the return current to have to flow back through the motherboard via the pci-e connector.
  • My experience with using more than one PCI-E x1 -> powered PCI-E x16 riser is that there are at least 4 different Gigibyte or Asus mobos that will not load Win7 if using more than one such riser.  My experience has been limited to the USB3 cabled risers as opposed to ribbon connections.  If you get it working it'd be nice to know the details of your setup.

    @hastatess: ; The riser that I've seen with a capacitor on the socket are only doing so to simplify the power to the molex connector to two instead of 3 (common 3.3V & 12V ground) or 4 (independent ground runs).  The cap is used to 'clean' the 3.3V leached from the bus/ribbon.  A typical molex run from a PSU has +12VDC, +5VDC and either 1 (jumpered) or two ground lines.  So, if building your own, consider using an SATA power run from the PSU since it has 12, 5 & 3.3VDC which lets you do the job at voltages compatible with the bus spec and no extra components.

  • I was using 4 powered risers, 3 x16 and 1 x1, my motherboard wouldn't recognize the x1 risers in the x16 slots for some reason.  I am using the risers that 3gghead has described with the yellow and black wires with the capacitor and they are running fine, no complaints.  Also these risers seem to of higher quality than those with just the one yellow wire in the molex plug.


    One question that I have not been able to find an answer to is, if I am using powered risers, should I still use the 4 pin molex on the motherboard that supplies additional power to the pci-e slots?
  • Yes!  Those 4-pin motherboard connectors came along well before the high-powered pci-e gpus.  They date back to the Pentium-4 era, and are needed to supply the motherboard's needs too.

  • @3gghead

    Tnx for this info. I will see if i do 3.3V mod too. For start only 12V.

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