8 pin PCI-e adapter for PSU with only one?

[st4rdust]

I want to drop two 5970s in a box I've been running two 5830s on for about a year now. It's a very basic i3, 2x2gb DDR3, 32gb SSD rig running off of a PC Power and Cooling 650w PSU. I've run this setup thru the PSU calculators and I'm well within the power budget when two 5970s are added in place of the 5830s. It comes out to 494w minimum, 544w recommended. What I'm not quite sure about, though, is the fact that I'd have to add an 8-pin PCI-e adapter since the 650w PSU only has one 8 pin. While I know that theoretically, I'm well within my power budget, I'm not positive on what the 8-pin adapter will actually do when plugged into one of the 5970s. I know they pull more wattage than the 6 pin variant, but wouldn't that be taken into consideration in the PSU calculator estimate? Anyway, I defer to those with a clearer understanding of the PCI-e connectors as to whether or not I'd be safe to run these cards in this machine after adding an 8-pin adapter. (As a side note, I know that 8-pin adapters come in at least two variants, one adapting an existing 6-pin PCI-e connector to 8-pin with a female 6-pin to male 8-pin, and another adapting two 4-pin molex connectors to an 8-pin male connector. Any difference here, or is it the same amount of power being drawn from different connectors?)

Many thanks.

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[Gomeler]

So, the 8 pin PCIe connector contains two additional ground pins. One of my rigs is a quad HD 5970 running on a Corsair AX1200 with a molex -> 6 pin PCIe adapter for the 8th PCIe plug. Pulls 1150w from the wall and no problems for ~1 month now. So long as your power is balanced across the available rails on the PSU AND you aren't exceeding the current draw limits of the PSU then mix and match adapters as you see fit.

I have no experience with 6 to 8 pin PCIe adapters but given how it is duplicating a set of ground pins I imagine whatever you find on Newegg/etc would suffice.

[st4rdust]

I have no experience with 6 to 8 pin PCIe adapters but given how it is duplicating a set of ground pins I imagine whatever you find on Newegg/etc would suffice.

I thought I had read somewhere that 8-pin PCI-e plugs drew more power than 6-pin, though I guess if that were the case and an 8-pin connection drew more power than a 6-pin could supply, there would be no such thing as a 6-pin to 8-pin adapter, so your explanation of the two extra grounds makes a lot more sense than what I had been thinking.

Also, I had forgot to mention that my 12v rail is carrying 46 amps.

[Inaba]

a) It should be safe using a 6pin - 8pin converter.
b) That PSU is going to be on the edge of it's design capabilities with 2 5970's at full bore and may or may not be stable.  That's a quality PSU, and if it's not too old, it should be just about fine, but given that it's so close, I might rethink it if I were you.

Is it a single or multi rail design?

[st4rdust]

Is it a single or multi rail design?

This particular PSU is a single 12v rail. The estimated draw from the machine that I'd be putting these 5970s in only comes out to around 540w including the dual 5970s. The rig itself is very lean (as most dedicated bitcoin rigs tend to be) starting with a 35w 1155 CPU, two 2gb DDR3 sticks, a single 32gb SSD, and aside from those three components the only other power draw comes from the five 120mm fans throughout the box.

[P_Shep]

I thought I had read somewhere that 8-pin PCI-e plugs drew more power than 6-pin, though I guess if that were the case and an 8-pin connection drew more power than a 6-pin could supply, there would be no such thing as a 6-pin to 8-pin adapter, so your explanation of the two extra grounds makes a lot more sense than what I had been thinking.

Also, I had forgot to mention that my 12v rail is carrying 46 amps.

Correct, 6-pin can source 75W, 8-pin 150W. The 2 extra ground pins just tell the gfx card that they can draw up to 150W, or none if the gfx needs 150W and shuts itself down.

A 6 to 8-pin converter may or may not blow up your PSU... unlikely any harm will be done in reality.

[st4rdust]

Well, assuming worst case that the extra 75w was not taken into consideration for my power budget, that still leaves me within the limits if I add the 8-pin. The bottom line is, I know I run the risk of frying the PSU if the strain of the extra power draw proves to be too much. Like many before me, and likely many to follow, I will take my chances and see if I can squeeze by with what resources I have at my disposal at this moment. Then if I do fry the PSU, I likely won't press my luck with PCI-e adapters at the very limit of my power budgets in the future. I will also buy a slightly more powerful PC P&C unit with the adequate number of 6 and 8 pin PCI-e plugs. Thanks to all for sharing your knowledge. I'll post with the results when they become available.

[MrTeal]

Internal to the card, will the PMIC only draw 75W from the 6pin connector and 150W from the 8pin, or are the two connectors internally paralleled.

IE - On a multi-rail supply if I used an 8pin from 12V1 and a 6pin from 12V2, would the card actually draw twice as much power from rail 1?

[dropt]

Internal to the card, will the PMIC only draw 75W from the 6pin connector and 150W from the 8pin, or are the two connectors internally paralleled.

IE - On a multi-rail supply if I used an 8pin from 12V1 and a 6pin from 12V2, would the card actually draw twice as much power from rail 1?

If the card demands it, it will draw ~double V2.

[jake262144]

... a PC Power and Cooling 650w PSU.
...

This one? You're in luck for that is a very decent PSU, you just need to get a PEG 8-pin splitter and perhaps a PEG 6-pin splitter.
Using PEG splitters is by far preferable to drawing power from the molex 4-pin peripheral connectors (three 12V wires instead of just one).

Make sure to measure the actual power usage once you have replaced one 5830 with a 5970 and don't go crazy with overclocking.

[st4rdust]

... a PC Power and Cooling 650w PSU.
...

This one? You're in luck for that is a very decent PSU, you just need to get a PEG 8-pin splitter and perhaps a PEG 6-pin splitter.
Using PEG splitters is by far preferable to drawing power from the molex 4-pin peripheral connectors (three 12V wires instead of just one).

Yes, the PPCMK2S650 is the exact PSU that I'm using. I've always used PC Power & Cooling units for the fact that they have always afforded me rock-solid stability and reliability. I bought this one thinking it was a no-brainer for the rig it was originally intended for (the same setup I'm attempting now, only instead of the 5970s it had dual 5830s) being a PC P&C unit with a solid single 12v rail and 80+ Silver rating. This brand of PSUs has always done right by me, and I anticipate that this one will hold its own when I fire up the new GPU tandem.

Make sure to measure the actual power usage once you have replaced one 5830 with a 5970 and don't go crazy with overclocking.

These were my thoughts exactly. I'll be glad to merely pull off the tandem of dual 5970s in this machine for stable mining. The extra 5970 working at stock speed will be a solid addition to my overall mining fleet; anything beyond stock speed is just gravy. I will also be watching my Kill-A-Watt very closely just to be sure I'm not pushing my PSU too hard.

[st4rdust]

you just need to get a PEG 8-pin splitter and perhaps a PEG 6-pin splitter.
Using PEG splitters is by far preferable to drawing power from the molex 4-pin peripheral connectors (three 12V wires instead of just one).

These are exactly what I'm after, but I'm wondering which combination to buy. Since my PSU has one 6-pin and one 6+2 pin, would there be any difference to buying two 6-pin to 6+2 pin, with the 6+2 pin plug from my power supply going into a 6-pin to 6+2 pin adapter with its own extra 2 pins dangling uselessly, or would it only be appropriate to get a 6-pin to 6+2 pin for my 6-pin, and for my 6+2 pin a full 8-pin to 8-pin adapter, thereby utilizing the 6+2 pin's extra 2 pins, or does this make no difference? If so, buying the two 6-pin to 6+2 pin adapters makes for a slightly better deal.

Also, the 8-pin to 8-pin adapter you listed was designed for the 8-pin power plugs on the motherboard, not 8-pin PCIe plugs. I believe there are differences in these cables, but I could be wrong. Does anybody know if these two 8-pin plugs (the PCIe connection and the motherboard plug) are identical and if their respective cables could be use interchangeably?

[jake262144]

My mistake with the 8-pin splitter, link removed. Ummm... no 8 pin PCIe splitters on NewEgg currently, damn it.

You can safely use the 6 pin to dual 6+2 pin adapter; the two additional ground wires don't carry any load worth mentioning.
The card checks whether they are connected to make sure a PCIe 6-pin connector hasn't been plugged into the 8-pin slot.

Here's why:

No sooner were the PCIe 6-pin connectors introduced into ATX spec (ATX12V v2.1, 2005) when video card manufacturers shocked the ATX committee by announcing they needed even more power and requesting the power limit be raised from 75W to 150W.

A potential issue was brought up where a PSU would deliver the 75W but couldn't handle the increased load of 150W due to rail load limits.
This necessitated introducing yet another connector - the PCIe 8 pin variety. As I already said, the two additional ground wires are merely being used as a safety device.

No semi-decent PSU today should have any problems with supplying 150 watts of power over the 12V rails so the threat 8-pin connectors had been developed to guard us from is irrelevant.

The electrical specs for both PCIe connectors are the same, there are three 12V wires and three ground wires. The connectors use Molex MiniFit Jr contacts, HCS variety, each good for 13A of current. From an engineering standpoint, those connectors are perfectly able to deliver a load of 375 watts indefinitely.

You could only get in trouble if the Chinese manufacturers of those adapters use subpar materials like thinner wire gauge or low quality contacts.
Regrettably, such unfair practices are not unheard of.

[st4rdust]

My mistake with the 8-pin splitter, link removed. Ummm... no 8 pin PCIe splitters on NewEgg currently, damn it.

You can safely use the 6 pin to dual 6+2 pin adapter; the two additional ground wires don't carry any load worth mentioning.
The card checks whether they are connected to make sure a PCIe 6-pin connector hasn't been plugged into the 8-pin slot.

Here's why:

No sooner were the PCIe 6-pin connectors introduced into ATX spec (ATX12V v2.1, 2005) when video card manufacturers shocked the ATX committee by announcing they needed even more power and requesting the power limit be raised from 75W to 150W.

A potential issue was brought up where a PSU would deliver the 75W but couldn't handle the increased load of 150W due to rail load limits.
This necessitated introducing yet another connector - the PCIe 8 pin variety. As I already said, the two additional ground wires are merely being used as a safety device.

No semi-decent PSU today should have any problems with supplying 150 watts of power over the 12V rails so the threat 8-pin connectors had been developed to guard us from is irrelevant.

The electrical specs for both PCIe connectors are the same, there are three 12V wires and three ground wires. The connectors use Molex MiniFit Jr contacts, HCS variety, each good for 13A of current. From an engineering standpoint, those connectors are perfectly able to supply 375 watts of load indefinitely.

You could only get in trouble if the Chinese manufacturers of those adapters use subpar materials like thinner wire gauge or low quality contacts.
Regrettably, such unfair practices are not unheard of.

Okay good, I'll be able to finish out my test with two of the 6-pin to 6+2 cables. Sorry to get so in depth over what probably seem like trivial details, but I've always had a somewhat inherent need to understand things like this down to their fundamental nature. You've done a great job providing all the missing details I've needed. Thanks very much, I will update with the results when the adapter cables arrive.

[st4rdust]

Okay, so I finally took delivery of my 6-pin to 2x6+2 pin adapters the other day. I got to tinkering around with the setup and trying to cover as many different approaches as I could just for the sake of experimenting (you never know what knowledge ends up working to your benefit down the road) including different combinations of the PSU's PCIe connectors, molex to PCIe, PCIe splitters, etc. Anyway, while I didn't have a whole lot of luck in these different variations, my primary goal - operating two 5970s on a 650w PSU with only two PCIe connectors by means of splitting the two PCIe plugs with the aforementioned adapter/splitters - has succeeded in spades thus far. It's been about two days that I've had it running nonstop. I haven't tried any overclocking, however, because with each GPU set at 725/1000 my Kill-A-Watt was hovering right at 650w, which probably would have not even been an issue for my PC Power and Cooling PSU, but still too close for my own personal comfort. With the GPUs at 725/300, the wattage is just below 600w and I can sleep soundly knowing that I'm not abusing the system TOO much.

In conclusion, my thanks go out to all those who offered advice and insight for my problem, and good luck in your own mining endeavors.