How does a GPU choose between power sources

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sshantt

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I'm building a PC, specified here, and I'm planning to rewire the PSU. Also please don't just say get a better PSU, cause this is the form factor I'm looking for, and No im not "cheaping out" as the PSU is $190, and i dont mind spending alot more if it means i dont have to do any modding.

I'm going to connect it as following,

12V1:
ATX24
Two yellow 12v 8-pin CPU header wires (rest on 12V2)

12V3
Three yellow 12v GPU1 (first plug) header wires for 8 pin
One yellow 12v GPU1 (second plug) header wire for 8 pin (rest on 12V2)

12V4
Three yellow 12v GPU2 (first plug) header wires for 8 pin
One yellow 12v GPU2 (second plug) header wire for 8 pin (rest on 12V2)

12V2
Two yellow 12v 8-pin CPU header wires
Two yellow 12v GPU1 (second plug) header wires for 8 pin
Two yellow 12v GPU2 (second plug) header wires for 8 pin

The max current demands for the components are as follows:

Motherboard + fans + ram + M.2 drive: 120W (without pcie)
CPU: 91W
GPU1: 215w
GPU2: 215w

I've planned the above due to the rail limits of 192W each.

However I can't find information of the following things:

1) Can I distribute two CPU pins to one rail and another two to a different rail? (the manufacturer of my PSU seems to have done this, as seen in this picture, which is color coded by rail number)

2) How does a component, such as a CPU, distribute current pull along its four 12V pins? (for example, is it pin1 = 10w, pin2 = 10w, pin3 = 10w, pin4 = 10w, or is it more like pin1 = 15w, pin2 = 5w, pin3 = 18w, pin4 = 2w)

3) Since my planned GPUs have three power sources each (two 8-pin cables (150w each) and PCIE motherboard slot (75w)), how does it prioritize where to pull the current from? For example does it max out the first 8-pin (150w) and if it needs more, starts pulling from the second 8-pin connector (65w)

4) What components on the mother board are accounted for by the 5v and 3.3v pins? because if lets say the m.2 drive relies on current from 5v pin, I can subtract 10w from my forecasted current demand of 12v current.

Sorry about the long as #$$ post but I only started learning about power supplies two days ago and I've got all these road blocks that are really hard to find answers for.

Thanks
 
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madmatt30
Cases are all going to be dependant on how far you're willing to compromise on size.

Yes those 2 cases are good but far from the best , obviously if that's the exact size & form factor you want then options become truly limited.

I've run a gd06 for the last 3 years now

http://www.silverstonetek.com/product.php?pid=293

For me it was about getting something in the living room that would fit under the TV rack , didn't particularly look like a PC , & matched my smoked glass cabinet & my home theatre amp

As soon as that case dropped in store I bought it (& it was damn expensive on release)

It's a case I will probably still own in 20 years time , just can't thing of a singular reason to ever replace it because simply there's still...
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synphul

synphul

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I would consider getting a proper psu to handle your system. A proper psu meaning one that has the connectors in the quantity you need. Rewiring a psu sounds like an incredibly bad idea. I'm not trying to discourage you from building a system but there are certain safety concerns involved here. You have no real idea what internal components are used and on what rails, what their manufacturing tolerance is for carrying current and other things. They built the psu to specific plans with specific components designed to handle current safely at the stated wattages/amps on the various rails. Go monkeying with that and you have no idea if the capacitors can handle it, the mosfets or anything else.

Given a budget that includes such a high end gpu, cpu and everything else I'd work out around $80 or so and pick up a proper psu before you end up potentially blowing components like a $650 gpu, $330 cpu or potentially causing an electrical fire or shock hazard for want of a relatively inexpensive component by comparison. Meaning $80 isn't much savings if you fry a $650 gpu or start an electrical fire that damages you or your home. Safety being a major concern here.
 
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sshantt

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Nov 2, 2016
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I appreciate your concern, but I'm paying about $190 for the current PSU (Hence not "cheaping out"), and I don't mind paying up to $300-400 for a PSU with the form factor I'm looking for (U1 form factor) that has all the connectors needed. The problem is that this is the only 700W U1 PSU out there. I have a very specific build in mind so changing the PSU size is not an option.
 
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Rogue Leader

Rogue Leader

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His point is what you are asking to do is quite complicated, and being hat you just learned about PSUs 2 days ago, undertaking this is something that could end at a minimum of you destroying a $190 PSU, and at worst burning your home down or killing you. I've been working with electronics for the better part of 30 years and I would never consider something like this, WAY too many variables that end in destruction.

1U (its not U1) PSU's are very thin and made for Servers, what are you planning on building? I ask because your build apparently has 2 GPUs in it, so it has to have the room for a real ATX PSU, or at least an SFX one.
 
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sshantt

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The case and build are on this forum, where the developer has successfully modded the PSU. I could just apply the same mods that he has done, however I would like to consider and understand all possible options. Again, I understand the concern, but I will NOT be doing this myself. It will be done in a controlled environment by an electrician with proper testing and soldering tools, keeping in mind the risks of damage etc. I just have a couple of things I do not know (Questions above) about how these components function in terms of power and current. So I would highly appreciate just sticking to the questions.
 
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madmatt30

madmatt30

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I wouldn't be using those particular cards in that case in sli for starters.
They are going to die a death in there temp wise.
Also you should read up on the current problems associated with those particular cards regarding vrm & ram overheating.

http://www.tomshardware.com/news/evga-addresses-geforce-1080-temperature-problems,32967.html

Kill 2 birds with one stone here is my advice.

Go for 2 single 8 pin reference style cards.
Yes they're slightly weaker & generally noisier but you won't need to mod the psu & they exhaust heat straight out of the case which in all honesty is what you want in that setup.
 
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Rogue Leader

Rogue Leader

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According to the thread he isn't even selling the case yet, it not nearly ready for production, I would bet money its gonna be a decent amount of time before you have one in your hands.

Also I saw what he told you about modding it. Of course it is possible, however for someone with no electronics experience this is still a terribly bad idea. And who knows maybe he can convince FSP to make a proper power supply for it.

Finally I don't think anyone here can truly answer those questions for you as power supplies are all engineered by their manufacturers. You would be much better served trying to get in touch with FSP directly if you're still crazy enough to do this.
 
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sshantt

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I dont know if you caught the "PLANNING" part of my post, but to make it clear, I need to plan everything way ahead cause none of the components are available where I live, and it could take 6 months between getting one components and the other. SO IF YOU DON'T MIND, I would like to verify that everything works together waaaaay ahead of time. Not all of us are lucky to have one day amazon delivery!! :)

I only signed up to this forum recently but it seems that getting a straight answer without some BS on the side is quite impossible.

madmatt30 is the only person that suggested something helpful and relevant so thank you for your input madmat30. I'm considering it as an option. Also, since it will take me time to get the GPU's, I think EVGA would have implemented the VRM solution to their products in the market by then.
 
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Rogue Leader

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And Madmatt's idea is the best idea IMO vs trying to rewire a PSU. My suggestion to contact FSP to speak to an engineer is perfectly valid so before you dismiss it as "BS" maybe think for a second. Being that most people (even people here or anywhere else who have a ton of PSU experience) have never done this, getting the answers to how it works from the horses mouth is likely the best idea.

Good Luck.
 
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sshantt

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Nov 2, 2016
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I didn't say everything was BS, i said getting a straight answer without a bit of BS on the side seems impossible. I am planning on talking to FSP, but id rather know as much as possible before hand. :)

Going with madmatt's idea still involves modding the PSU, but that would be clear cut, where each component would get a rail on its own rated higher than one component would ever draw. It is plan B at this point. Ill probs get two and mod one PSU for four 8-pin and artificially load/stress it, and if it backfires, mod the other one for 2 8-pins.

Thanks :)
 
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madmatt30

madmatt30

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^ you'd have to mod it still yes mate but in all honesty getting another single 8 pin PCI connector off there is relatively simple (believe the psu already has 1 x 6-2 pin?)

Theres no need to even open the psu, just take it off the spare 4+4 CPU connector , those wires can be simply cut & spliced with proper electrical crimps to a PCI express plug - it would actually benefit as you could make the cable the exact length you require that way.

See Luke D did make the exact same comment as me regarding using the FTW edition cards - great minds ;-)

Case does look nice though, very well thought out & the flat GPU design using risers is just genius.
 
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sshantt

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Nov 2, 2016
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Lol, yeah I've been getting the FTW comment by a lot lately.

I see what you mean with the light modding, and i thought that was the case in the beginning, but you can see in the PSU pic that the two 8 pin CPU connectors have two yellow cables on each going to the motherboard rail. Which means i wouldn't be utilising the second GPU rail fully or the CPU rail. Also meaning the motherboard rail would be easily overloaded during high stress. I would have to remove one CPU connector and replace it with PCIe on the last rail (just add one yellow cable and remove the other two) and move the two yellow wires (12v) going to the motherboard rail to the CPU rail, allowing the CPU its own rail.

I've been looking for cases the past month, and I think by far the best cases out there are, Ncase M1 and DanCase A4 for single GPU with water and air cooling respectively, and the Project Orthrus case for SLI air cooled.
 
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madmatt30

madmatt30

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Cases are all going to be dependant on how far you're willing to compromise on size.

Yes those 2 cases are good but far from the best , obviously if that's the exact size & form factor you want then options become truly limited.

I've run a gd06 for the last 3 years now

http://www.silverstonetek.com/product.php?pid=293

For me it was about getting something in the living room that would fit under the TV rack , didn't particularly look like a PC , & matched my smoked glass cabinet & my home theatre amp

As soon as that case dropped in store I bought it (& it was damn expensive on release)

It's a case I will probably still own in 20 years time , just can't thing of a singular reason to ever replace it because simply there's still nothing as good about now with that kind of front to back depth & bottom to top height than still takes full size components.

Running
3x3.5inch 3tb tb hard drives
1 x 240gb ssd
Full size optical bluray
Exoc black gtx 970
Overclocked fx CPU with 140mm Pallas cooler
2x120mm intakes
2x 60mm + 1x120mm exhausts
Full size atx 550w xfx PSU

CPU & GPU temps NEVER exceed 60c

Before that I ran a case of similar dimensions that only had 80mm fan mounts & a SFX PSU.
The noise to me was just way too much

I would never buy a case personally now that didn't take a full size psu or120mm fans
I'd take silent case descriptions with a touch of salt , there's no such thing as a silent SFX or tfx PSU or silent 92mm or 80mm fans IMO.

Just about quiet - at a push maybe depending on your tolerance for noise , silent - definitely not.



 
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