When to use powered risers? And if really needed, how to make them?

[BitWalker1]

Hi, is there a rule when it's really necessary to use powered risers, I mean instead of normal not powered ones?
I would like to use up to 5 (maybe even 6) 7970 / R9 280 cards on a Gigabyte 990FXA-UD3 and on a MSI Z87-GD65
And for sure I will use 2 PSUs. I could read that the supply for each powered riser and the associated GPU has to come from the same PSU?

And does someone knows where I can find instructions to make a powered riser out from a not powered? (just by DIY
Here in Europe I had no luck to find a supplier for powered risers, only for normal ones (not powered) 1x and 16x.
But I guess it should not be too difficult to make a powered one out from a normal one...?

Cheers,
BitWalker

[Gazza1]

I just use powered risers on every card exclusively.  I make them myself so it's really no big deal or extra cost.  But if you run more than 3 power hungry cards I would at least use powered risers on the rest as it creates too much load on the mobo.

I run all the powered risers off the same PSU that the mobo is plugged into.  The same way it would be if you weren't using powered risers, the power would all come through the mobo.  I've tried separating them before and it can at times create issues.

Depending on the cards you use figure about 50w +/- per powered riser.  so account that 250w for 5 cards for the main psu.  That is with 7970s/280x's.  R9 290x's and 7990s the power draw could likely be more around 75w+ per riser.

At the same time you can subtract that 50w from each card for the psu power requirement

[BitWalker1]

I make them myself so it's really no big deal or extra cost.  

Okay, sounds reasonable. Can you point me to a source where I can find circuit diagrams?
Are the leads from the Molex connected in parallel with the ones comming from the board, or is it connected exclusively to the Molex?
And do you connect both, ground AND +12V or only the +12V (like I could see already) ?
On some solutions I could also see a capacitor - do you use one too?

Thanks for help in advance!
BitWalker

[Takeshi_Kovacs]

You can buy powered risers from Aliexpress. They will ship to Europe.

If you are mining scrypt, you do need to use powered risers. The edge connector on the graphics card supplies power to the VRAM on the card. Scrypt drives the VRAM much harder than SHA or gaming so there is more power going that way.

Capacitors are not really so important.

Using a powered riser with only the 12V connection can be a bad idea if you have more than one PSU. You could end up with one PSU supplying current out of the plus 12V line but seeing no matching incoming current on the 0V rail while the other PSU would see the opposite effect. I doubt that the designers of the PSUs have even considered what happens under those circumstances. A friend tried it last month and blew up two rigs.

[Pita]

I just use powered risers on every card exclusively.  I make them myself so it's really no big deal or extra cost.  But if you run more than 3 power hungry cards I would at least use powered risers on the rest as it creates too much load on the mobo.

I run all the powered risers off the same PSU that the mobo is plugged into.  The same way it would be if you weren't using powered risers, the power would all come through the mobo.  I've tried separating them before and it can at times create issues.

Depending on the cards you use figure about 50w +/- per powered riser.  so account that 250w for 5 cards for the main psu.  That is with 7970s/280x's.  R9 290x's and 7990s the power draw could likely be more around 75w+ per riser.

At the same time you can subtract that 50w from each card for the psu power requirement

Where do you buy your parts to make the risers yourself? In particular where do you get the x16 and x1 rails (assuming with the PCB board behind them) and what is the exact molex part number for the 4 pin molex connector and pins?

Thanks!

[Tenchi]

according to this guide you don't need powered risers if you use 2 PSUs:

https://litecointalk.org/index.php?topic=11826.0

[hyphenated]

Running more than two GPUs on a mobo is risky. Sure - everything may work fine, but if one connector pin is a bit dicey it will get hot, deteriorate and possibly burn.  This process can take months, and throws more load on the remainder.  You risk a burnt 24-pin connector, stuffed board tracks and even a fire.  Spending 5 minutes with a knife and soldering iron is cheap insurance.

Several guides are out there - all you need is a sharp blade, a steady hand, a soldering iron and a few Molex shells and connectors (you can steal several from unused Y adaptors included with GPUs).  You split a pair of lines on one side, three on the other.

Google is your friend - search 'make your own powered riser guide' 

[Revelations86]

What about 3x 290x cards on a motherboard.  Does it need powered risers for each or unpowered? 

[hyphenated]

If it's a cheap & cheerful mobo you can get away with one unpowered (short) and two powered.

If it's a good quality mobo try two and one.

If it's a super-duper quad crossfire mobo (ie designed with additional 12V feeds) then you would probably be fine with three unpowered, but the mobo price would be such that you would be daft to risk it.

Bear in mind it might be fine for months, then suddenly go sour... and you want to keep the magic smoke in the boxes, right?

All IMHO, you understand.   

[DigitalPackrat]

There are too many variables to give you a flat yes or no. Different cards have different demands from the pcie slot. For instance the FXF 280x that I use pulls about 2.1a ea from the pcie slot where as the reference 290's I use pull about 4.2a ea. With the FXF cards, I am comfortable running 5 per system with unpowered risers on the 290's I only run 2 unpowered and power the remainder off their own PSU. If I were to have a MB with a additional power connector for the PCIe slots, I would figure ~5a per pin max.

This is not a vary detailed picture but basically you just cut 5 wires and connect them to 12v on the PSU. The bottom shelf has the powered risers.