Questions about powered risers and splitting power load with 2 PSU

[MeltManBob]

Hey guys I was wondering if it's ok to disconnect the power wires on a riser at the mobo connector side and just run the power wires out of the GPU connector side to a PCIe power connector such as is described in some of the DIY powered riser articles?  I'm asking because that effectively removes any power coming from the mobo.

Hypothetically if I did this with all cards I'm assuming that would put the least stress on the mobo and leave the mobo to only handle communication. Please correct me if I'm wrong here, just trying to verify as I've never built a computer for mining.

Another question I have is regarding whether or not all power connections to a GPU are used equally or if they come on once the connector before it is saturated. For example, typically you have 75w coming through the 16x PCIe slot and then say 2 6 pin connectors on the GPU itself for a 7950.  If the card is only consuming 50 watts, does that come from the PCIe slot or does it come equally from the PCIe slot and the two 6 pin connectors?

There are a few reasons for asking that question but the most relevant reason is to determine how to use 2 PSU's to run 5-7 cards.  You could split the load of each GPU between the 2 PSU's or you could split the total GPU's between the 2 PSU's.  Without ALL of the cards being on powered risers with the power isolated from the mobo, you could potentially have power coming through the mobo from the PSU hooked up to the mobo and power coming from the 2nd PSU through one or more of the 6 pin connectors and I'm not sure if that's a good idea or not.

[MeltManBob]

Am I not getting any responses because no one has any answers or what? I figured this would be the right forum to ask these things in, hoping I'm not left to my best guess it would be nice for someone who has experience with this to chime in.

[Malawi]

I use english as a second language, and that may be part of this - But I cannot understand what you are really asking about.

AFAIK - the power from the PCIE port is for the GPU-memory, while the GPU itself gets the power from 6 or 8 pin molex connectors.

[MeltManBob]

Q1 - One of the DIY articles for powered risers - http://blog.zorinaq.com/?e=44 - has the power wires from the riser cable disconnected from the mobo side and directly connected to a power connector, what I'm wondering is if there is any problems with doing this for all cards so the mobo only handles communication.

Q2 - The PCIe slot has to supply power to more than just the memory because there are cards that don't require supplemental power connections.  I'm trying to figure out if all power connections are drawn from at the same time or if they might kick on as needed ie from 0-75 watts of consumption, power only comes through the PCIe slot, 75-150 comes from supplemental 6 pin connector #1 and 150+ comes from supplemental 6 pin connector #2.  I would assume that power draw comes from all 3 and not in stages but some clarification from someone who knows would be better than my assumptions.  If all else fails, when I get a GPU I can just test it.

[boozer]

Q1 - One of the DIY articles for powered risers - http://blog.zorinaq.com/?e=44 - has the power wires from the riser cable disconnected from the mobo side and directly connected to a power connector, what I'm wondering is if there is any problems with doing this for all cards so the mobo only handles communication.

Q2 - The PCIe slot has to supply power to more than just the memory because there are cards that don't require supplemental power connections.  I'm trying to figure out if all power connections are drawn from at the same time or if they might kick on as needed ie from 0-75 watts of consumption, power only comes through the PCIe slot, 75-150 comes from supplemental 6 pin connector #1 and 150+ comes from supplemental 6 pin connector #2.  I would assume that power draw comes from all 3 and not in stages but some clarification from someone who knows would be better than my assumptions.  If all else fails, when I get a GPU I can just test it.

Q1 - Using powered risers for all cards is fine, but not necessary.  I have 6 running on a mobo and have 3 powered risers and 3 pulling from the board.

Q2 - Good question.  I am not sure how it works.  I can tell you in my setup that my second power supply powers the 3 powered risers, but only 2 cards via the 6 or 8 pin connectors.  I have not had any issues with it, but if someone knows a definite answer to this I would like to know as well.

[Ryu.Hayabusa]

I would assume that power is drawn from all ports simultaneously as needed.  In other words, if you have a card with 2 8 pin connectors, its maximum theoretical draw is 375w (150w + 150w + 75w (from pci-e slot)).  Now just because you have 375w of potential power doesn't mean the card will draw that much, it will draw as much as required to run.

If both the PCI-E slot power and PCI-E connector power are commonly connected (I am not sure if they are or not), then disconnecting the slot power would add an additional 75w load to the PCI-E connectors if the card was drawing the full 375w.  I think it would work, however I am not sure what the potential problems or downsides would be to doing so.

[MeltManBob]

Ryu - I wouldn't be eliminating the PCIe slot power from the GPU, just from the mobo.  The DIY powered riser tutorial has you take the power wires from the riser and add a 6 pin PCIe power connector to it which means your other supplemental power connectors don't have to pick up the extra load, you would now have 3 cables from the PSU to the GPU, assuming you already had 2!

[Ryu.Hayabusa]

Ryu - I wouldn't be eliminating the PCIe slot power from the GPU, just from the mobo.  The DIY powered riser tutorial has you take the power wires from the riser and add a 6 pin PCIe power connector to it which means your other supplemental power connectors don't have to pick up the extra load, you would now have 3 cables from the PSU to the GPU, assuming you already had 2!

I was apparently up way too long when I read the original post, lol.  Yes adding the power to the riser takes the load away from the motherboard slots if you only connect the card's riser wires to the PSU.  The power draw I still believe is equal from all ports, but I am not 100% sure. 

It is best to have power connections from the same PSU for 1 GPU (in other words, don't have 2 power cables from 2 different PSUs connected to the same card) to avoid weird issues.

[MeltManBob]

I kinda figured that's the safe bet but if you read Boozer's post he mentions his second PSU powering the 3 powered risers but only 2 cards with regular 6 pin or 8 pin connectors meaning that the card attached to the 3rd powered riser is getting power from 2 PSU's. Maybe I read that wrong...

I'll keep digging as I really would like a definitive answer about this. I know that using different models etc. would probably be an issue but if/when I build a mining rig it would be using the same model GPU's and PSU's which means their variance is whatever the manufacturing process allows and hopefully everything would be similar enough that it wouldn't be an issue, even a small one that takes a year to actually cause a real problem.

[Ryu.Hayabusa]

You _can_ use 2 different ones on the same card, its just not recommended.

[MeltManBob]

Understood but I want to find out why that is not recommended, what the actual rational reasons are as opposed to conjecture and speculation.

[mistfpga]

Understood but I want to find out why that is not recommended, what the actual rational reasons are as opposed to conjecture and speculation.

The problem as I see it, and why i am still searching for risers that have a ground connected, is what happens when you use two psu's? (one psu with risers should have no problems at all, however watch what you are pulling on the molex lines.)

I use two psu's one seasonc 250 watt (22 amps on the 12v - and can happily pull 300-340, auto shuts down around 360watts) and a 750 seasonic as the power for the gpu's (4x6950 shader unlocked, 1.1v)

Using a motherboard with 4 16x slots, I didnt need powered risers for the 5830's but the 6950's need them after 3 cards (even with full slots) - anyway, my worry is that I am adding power from the 750 seasonic, and this gets grounded through the slot, into the 250 watt psu... This is obviously very bad. I havent risked it to see what happens, i need all my psu's so risers with ground are the only option.

 I think I am going to manually go through the ground pins and see which ones are grounding the 12v lines - then add this to the molex.

This should stop any issues...