Does power draw still go through PCI-E slots?

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WhiteFireDragon

Limp Gawd
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It's been known that older cards will draw some power through the PCI-e slots, even though you've already connected the 6-pin power connector.

However, I've seen some mentions that newer video cards, like the HD7000 series and r9 series get their juice exclusively from the 6/8-pin dedicated power connectors. I did some preliminary quick tests, and this seems to be true.

I haven't read any reputable reviews that even mentions changes in power allocation for current generation cards. Can anyone confirm, or provide some insight to this?
 
I somehow doubt this. I do not have the testing stuff to isolate power from the slot however there are cards in the Radeon 7k-Rx Nvidia 600-700 series that draw power purely from the pci-e I do not see the lower cards drawing power from the slot but the top cards only from the pci-e SUPPLEMENTARY power.

75w for 6pin 150w for 8pin that's 225w, a 7950 as an example can draw ~225w in an overclocked state, 7970 ~260w, most 7970 use 1 6 and 1 8 pin so in the 7950 case you are drawing the "safe" limit max of what the connectors can give, in a 7970 in most cases you would be exceeding it, I am 100% sure AMD would not be this blind in a reference card just as a simple example, they always want to make sure it has over head.

slot 75w 6 pin 75 8 pin 150w=300w heeeyyyy now that adds up better :p
 
There's interesting reading in the PCI-SIG FAQs. In the FAQ for PCI-E 1.0, there's talk about the "PCI Express x16 Graphics 150W-ATX 1.0". This means 75w from the slot, 75w from a 6-pin power connector.

In the FAQ for PCI-E 2.0, we get this: "Slot Power Limit Changes to allow for higher powered slots, which support the newer, high-performance graphics cards; this new feature works in tandem with the 300W Card Electro-mechanical specification". As dragonstongue mentioned above, 75w from the slot, 75w from a 6 pin, 150w from an 8 pin = 300W.

In the PCI-E 3.0 FAQ, it states: "The PCIe Card Electromechanical (CEM) 3.0 specification consolidates all previous form factor power delivery specifications, including the 150W and the 300W specifications." Which sounds to me like "we don't think you actually need any more power at this time."

My guess is that video cards prefer to pull power from the power connectors before they pull any from the slot, so you probably don't normally see much or any power pulled from the slot. But that's just guessing.
 
As far as I know it'll almost always get a power draw from the PCIe slot unless told otherwise. I know for a fact the 150W 660 does given 75W from the PCIe slot and 75W from the 1 x 6 pin adapter break even. Perhaps higher end cards are different.
 
Can a 6 pin really draw 75w max? I always though it can draw more because an 8 pin just has 2 additional ground pins.
 
it can draw more but they limit it for safety reasons as not all psu makers follow spec and for that matter not all gpu makers follow guidelines either, many Nvidia cards over the years went well over these limits by drawing to much from the slot or to much per connector, part of the reason folks were burning out adapted molex connectors, Radeon did as well don't get me wrong but it was not nearly as bad over as many generations.

Anyways. The way I know it, the card draws the 75w from the slot first, then the add in connectors, easier to regulate/filter power delivery this way as it is done through the pci-e lines on the motherboard(cpu especially modern intel ones can more dynamically filter it to reduce draw etc)
 
dragonstongue said:
Anyways. The way I know it, the card draws the 75w from the slot first, then the add in connectors, easier to regulate/filter power delivery this way as it is done through the pci-e lines on the motherboard(cpu especially modern intel ones can more dynamically filter it to reduce draw etc)
Click to expand...

Other way around, it draws the majority of the power through the PCI-e connector and the rest from the slot. If it pulls the majority through the slot it is taking the efficiency hit twice.

On boards that are made for 3-4 +225w GPUs, there is typically 2 extra 4pin molex connectors on the motherboard to supply the additional power needed.

There was a motherboard a long time ago that was actually designed to power 150w through the PCI-e connector. I wanna say it was an early PCI-e 2.0 board and was around or slightly after the RD600, it was an AMD platform.
 
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I have another question for you guys....

What would happen to a system if the PCI-E power was suddenly disconnected while the video card was on and working?

it's a hypothetical situation, and no, I would never unplug the pci-e power connectors while it was on, but I was wondering, if you used 2 psus to power a system, one provides power for the motherboard & a few video cards, and the second is jumpered to power a couple of other video cards.

Now, say, the second psu dies for some defect while the system is on. Would the two gpus that it was powering attempt to pull all the power through the PCI-E slot? And thus, could it damage the motherboard by attempting to pull too much current through the PCI-E slot?

Are video cards designed to only pull 75w from the slot, no matter what?
 
J Macker said:
I have another question for you guys....

What would happen to a system if the PCI-E power was suddenly disconnected while the video card was on and working?

it's a hypothetical situation, and no, I would never unplug the pci-e power connectors while it was on, but I was wondering, if you used 2 psus to power a system, one provides power for the motherboard & a few video cards, and the second is jumpered to power a couple of other video cards.

Now, say, the second psu dies for some defect while the system is on. Would the two gpus that it was powering attempt to pull all the power through the PCI-E slot? And thus, could it damage the motherboard by attempting to pull too much current through the PCI-E slot?

Are video cards designed to only pull 75w from the slot, no matter what?
Click to expand...

What happens when you flick the off switch on the back of your PSU?
 
LordEC911 said:
What happens when you flick the off switch on the back of your PSU?
Click to expand...

Can you read? What exactly are you trying to ask here? Obviously if ONE psu is powering a computer, and the switch is turned off, the whole system turns off.
But if TWO psus are powering a computer it's entirely different. The motherboard & cpu are still getting their full power from the first psu, and thus the system stays on, but the 3rd & 4th video cards don't have any voltage on the 12v rail anymore. During that millisecond, do they attempt to pull more power from the slot?

Try to understand my question before you post a dumb question trying to answer.
 
J Macker said:
Can you read? What exactly are you trying to ask here? Obviously if ONE psu is powering a computer, and the switch is turned off, the whole system turns off.
But if TWO psus are powering a computer it's entirely different. The motherboard & cpu are still getting their full power from the first psu, and thus the system stays on, but the 3rd & 4th video cards don't have any voltage on the 12v rail anymore. During that millisecond, do they attempt to pull more power from the slot?

Try to understand my question before you post a dumb question trying to answer.
Click to expand...
I understood just fine, it is the same situation though.

My buddy has 4 300w GTX680s in his rig but only has two of them with PCI-e connectors plugged in...(coincidentally with two PSUs) They aren't going to try to pull OVER SPEC to try and power themselves.

There is hardware in current GPUs that monitor almost every aspect of it.

Does that guarantee something won't happen in your theoretical example? Nope because there are far too many variables involved but it would be highly unlikely with quality components.
 
Now, say, the second psu dies for some defect while the system is on.
Click to expand...

"Some defect" is where we have to invoke "too many variables". There are countless ways a PSU might die. Some of those ways won't hurt anything, some of them might fry something, some of them will fry something, and some of them will burn your house down. We can't possibly predict which way it would go.
 
I can tell you... with 780 SLI.. If don't connect the 6-pin PCI-e power connector on my Rampage Extreme for extra power to the PCi-e slots, there are stability issues. For whatever that's worth. I used to think those 4-pin molex connectors on motherboards were BS. Well, not on the Rampage Extreme.
 
J Macker said:
I have another question for you guys....

What would happen to a system if the PCI-E power was suddenly disconnected while the video card was on and working?
Click to expand...

Bad Things will happen. Ask someone who did it once in a full-on derp mode. It sparked, killed the GPU, shut down the system, fried the PCI-E slot and ruined the PSU. It was acutally the same situation as you are saying, I had two PSU's running one machine with way too many graphics cards in it for folding.
 
J Macker said:
I have another question for you guys....

What would happen to a system if the PCI-E power was suddenly disconnected while the video card was on and working?

it's a hypothetical situation, and no, I would never unplug the pci-e power connectors while it was on, but I was wondering, if you used 2 psus to power a system, one provides power for the motherboard & a few video cards, and the second is jumpered to power a couple of other video cards.

Now, say, the second psu dies for some defect while the system is on. Would the two gpus that it was powering attempt to pull all the power through the PCI-E slot? And thus, could it damage the motherboard by attempting to pull too much current through the PCI-E slot?

Are video cards designed to only pull 75w from the slot, no matter what?
Click to expand...

I had a friend who had a 7970 and the 2nd PCI-e power cable was loss, what happened was that he would get stability issues at load.

If you flashed a bios that ran the 7970 at low clock and low voltage you might be able to run it with no PCI-e power connectors.
 
Spazturtle said:
I had a friend who had a 7970 and the 2nd PCI-e power cable was loss, what happened was that he would get stability issues at load.

If you flashed a bios that ran the 7970 at low clock and low voltage you might be able to run it with no PCI-e power connectors.
Click to expand...


I haven't seen a card that will even POST without connectors. Hell, some like my 780 had a little screen telling you to connect it but thats as far as it would go.
 
hajalie24 said:
Can a 6 pin really draw 75w max? I always though it can draw more because an 8 pin just has 2 additional ground pins.
Click to expand...

From what I read on it that 6 pin PCIe connector only uses two of the 12v "yellow" 12v sources to power the card. Then two of the grounds are used for them. The additional 2 GND for the 8 pin PCIe connect are sensors letting it know "hey, this is an 8 pin PCIE" and thus allowing the third (I believe the middle one is the one that not being used in the 6 pin) to know it can provide power.

Additionally, adding more grounds reduces resistance which I think is more likely.

Never really looked into it anymore than that. :eek:
 
Skillz said:
From what I read on it that 6 pin PCIe connector only uses two of the 12v "yellow" 12v sources to power the card. Then two of the grounds are used for them. The additional 2 GND for the 8 pin PCIe connect are sensors letting it know "hey, this is an 8 pin PCIE" and thus allowing the third (I believe the middle one is the one that not being used in the 6 pin) to know it can provide power.

Additionally, adding more grounds reduces resistance which I think is more likely.

Never really looked into it anymore than that. :eek:
Click to expand...
There was a pretty big discussion about it on the jonnyguru forums when the 8pin pci-e was first being introduced.
http://www.jonnyguru.com/forums/showthread.php?t=1220
 
I've never seen a PCIe card that doesn't draw power from the slot..
Also, 6 or 6+2 PCIe connectors are called "Auxiliary" sources..
 
J Macker said:
I have another question for you guys....

What would happen to a system if the PCI-E power was suddenly disconnected while the video card was on and working?

it's a hypothetical situation, and no, I would never unplug the pci-e power connectors while it was on, but I was wondering, if you used 2 psus to power a system, one provides power for the motherboard & a few video cards, and the second is jumpered to power a couple of other video cards.

Now, say, the second psu dies for some defect while the system is on. Would the two gpus that it was powering attempt to pull all the power through the PCI-E slot? And thus, could it damage the motherboard by attempting to pull too much current through the PCI-E slot?

Are video cards designed to only pull 75w from the slot, no matter what?
Click to expand...

It happened to me before when the paperclip fell out of my second PSU, it just turned off and was fine on reboot.
 
sorry to bump from the dead. just adding some personal experiences. I'm running a gtx 970 on a dell mb that has a pcie 2.0 slot with a 35w power limit. works perfectly fine, using a corsair cx750 psu.
 
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