Powering CPU with a second PSU

[Machinematrix]

Hi, I bought an XFX R9 280X and I am using a cooler master extreme power plus 550w, I know it's a bad psu and I'll buy a new one in a few weeks. The PC does not reboot or anything but the voltage on the 12v rail dropped to 11.7V when gaming, so I used a second PSU that has 20A on the 12V rail for the ATX12V connector (the 4 pin connector that goes to the motherboard), is it dangerous to do this?

System specs:
AMD FX-6100
XFX R9 280x
WD1002FAEX 1 TB
WD10EARS 1TB
ASUS M5A78L-M LX V2
6GB DDR3 memory

 

[cadaveca]

Yes, it is not a good idea. It can be relatively safe to power GPU from different PSU, but usually you need to have matched PSUs to do so. Many "hardcore" overclockers do use multiple PSUs to power their systems.

 

[dirtyferret]

your 12v rail is within ATX spec +/-5% and while your PSU is sub par (it's an old 500w unit sold as a 550w) at best you are pulling some 275w while gaming, no need to run two PSU if you plan to upgrade in a few weeks.

 

[Frick]

Hi, I bought an XFX R9 280X and I am using a cooler master extreme power plus 550w, I know it's a bad psu and I'll buy a new one in a few weeks. The PC does not reboot or anything but the voltage on the 12v rail dropped to 11.7V when gaming, so I used a second PSU that has 20A on the 12V rail for the ATX12V connector (the 4 pin connector that goes to the motherboard), is it dangerous to do this?

System specs:
AMD FX-6100
XFX R9 280x
WD1002FAEX 1 TB
WD10EARS 1TB
ASUS M5A78L-M LX V2
6GB DDR3 memory

How did you measure that? You cannot trust software.

 

[Machinematrix]

How did you measure that? You cannot trust software.

I used AIDA64, are software readings lower than multimeter readings right? I mean, if aida64 says 11.874 a multimeter would show 11.95? or does that vary from pc to pc?

 

[cadaveca]

It varies. And yes, a 550W should be able to power your system with a single VGA. My rig is a Z170 board with 6700K and dual GTX980's, and pulls just 535W from the wall. Although your CoolerMaster unit isn't the best, as suggested above, it should be fine until you can get a proper replacement. Just do not overclock or anything until you get a better unit.

 

[Machinematrix]

It varies. And yes, a 550W should be able to power your system with a single VGA. My rig is a Z170 board with 6700K and dual GTX980's, and pulls just 535W from the wall. Although your CoolerMaster unit isn't the best, as suggested above, it should be fine until you can get a proper replacement. Just do not overclock or anything until you get a better unit.

OMG does it pull that at full load? I think outervision PSU calculator exaggerates too much with it's results, according to that site, two r9 280x would use 750W with my setup.

 

[cadaveca]

Actually, it pulls less than that at full load; that was the maximum I could get the system to pull under un-realistic loads. That's the beauty of Intel's current platform, and modern NVidia GPUs... they don't need much power-wise, and thereby also do not needs tons of cooling. I reviewed a MSI barebones PC a while ago, with same 6700K and a GTX970, and it has a tiny 300W or 350W(my memory fails me) PSU, and runs like a champ, and is basically silent except when the GTX970 starts to spin up. Fortunately the MSI GAMING GTX970 is nearly silent itself, so the entire unit has a ton of power, is tiny, and makes virtually no noise. I use it daily as a HTPC.

I know you don't have the same hardware, but you do need to keep in mind that power requirements aren't always as high as reported, for sure. Most numbers are there to cover the worst-case scenario, not the ideals.

 

[dirtyferret]

I used AIDA64, are software readings lower than multimeter readings right? I mean, if aida64 says 11.874 a multimeter would show 11.95? or does that vary from pc to pc?

SW readings are never as accurate as a multimeter, then can be under or over the actual figure. I never go by SW readings, I have a gigabyte mobo that tells me my 12v rail is 7v regardless of which PSU I team it with.

 

[Bill_Bright]

There are even some cases that support more than one PSU. The biggest problem is ensuring they have a common ground.

I think outervision PSU calculator exaggerates too much

All PSU calculators pad the results some. But the eXtreme calculator from Outervision is the best and most conservative, IMO - especially now that it has been redesigned. If you are basing your opinion from years back, I recommend you check it out again.

Yes, if you really know what you are doing, and you take the time to properly research all your components (including things like case fans and various types of drives), you can do a better job at calculating power requirements than even Outervisions calc.

That said, there is nothing wrong with padding the results a bit. A little buffer allows the PSU to run at less than near capacity most of the time and that results in less heat and therefore, less fan noise - always good, IMO. Plus some buffer allows for hardware upgrades in 2 or 3 (or 5) years you had not planned on - like more RAM, more drives, or a bigger or perhaps additional graphics card.

according to that site, two r9 280x would use 750W with my setup.

Actually, that is a conservative estimate. If you look here XFX is suggesting a minimum of 750W with just one R9 280X card installed and recommends 850W. While I believe that is high for one card, you said you have two. So 750W sounds very reasonable.

 

[EarthDog]

but usually you need to have matched PSUs to do so.

Why? I haven't seen this recommendation before, nor I have seen matched PSUs for those who do that either. I have only done this twice, but they were completely different PSU OEMs.

but the voltage on the 12v rail dropped to 11.7V when gaming,

5% is ATX 12V spec.... which is 12.4V. But you already know the software results can be iffy at best.

 

[cadaveca]

Why? I haven't seen this reccomendation before, nor I have seen matched PSUs for those who do that either...

Meh. It's what I see all the time in rig pics, and I do recall being recommended that by a PSU manufacturer (likely Corsair or EVGA).

 

[CAPSLOCKSTUCK]

I have used a 2nd PSU to run a GPU and it was fine, they were mismatched PSU s.

You have to power up/down in the right sequence for the gpu to be recognised and then shut down in the right order or you get bsod

 

[cadaveca]

I have used a 2nd PSU to run a GPU and it was fine, they were mismatched PSU s.

You have to power up/down in the right sequence for the gpu to be recognised and then shut down in the right order or you get bsod

That could have been why I was told what I was... it's when you link the two PSUs together so they power on automatically that could have been the issue.

But powering a GPU off of a 2nd PSU is hardly a new idea... you used to be able to buy PSUs for expressly that purpose.

But the OP is suggesting the CPU only... I'm not sure that's a good idea. IIRC, both board and CPU should be connected to the same PSU, auxiliary PSU on GPU plugs only.

 

[CAPSLOCKSTUCK]

How To / Turn ON PC Power Supply WithOut MotherBoard

 

[EarthDog]

But the OP is suggesting the CPU only... I'm not sure that's a good idea. IIRC, both board and CPU should be connected to the same PSU, auxiliary PSU on GPU plugs only.

Why though is my question. What is the logic and science behind it? 12V is 12V is 12V. Just the order of things would matter and make it easier to use as far as my knowledge goes.

 

[Grings]

You can get cables to run 2 at the same time safely
http://www.thermaltake.com/Power_Su...02406/Dual_PSU_24Pin_Adapter_Cable/design.htm

i picked an unbranded one up for a few quid a few years back

 

[cadaveca]

Why though is my question. What is the logic and science behind it? 12V is 12V is 12V. Just the order of things would matter and make it easier to use as far as my knowledge goes.

PSU can pull power from the other on some boards. PSU supplies more than 12V.

That said, that was some time ago, and board designs have changed, but unless we are talking a pure PCIe system (like, perhaps, AMD, where there are still PSU compatibility problems), there can be issues. There is also a difference between hacking shit together, and doing so elegantly.

 

[EarthDog]

Yes, well aware of that. Let me say it different... split your split hairs... 12V is 12V is 12V. 5V is 5V is 5V... 3.3V is 3.3V is 3.3V.

I have heard of AMD platforms not liking Kingston memory, but, never heard of AMD platform not liking PSUs either....

 

[Bill_Bright]

PSU can pull power from the other on some boards.

Huh? Nah! To "pull" power as you describe, the current flow would have to reverse. That's not going to happen in a DC circuit. As EarthDog said, 12V is 12V is 12V. But more correctly, +12VDC is +12VDC is +12VDC. There are diodes all over the place specifically to ensure the current only flows in one direction.

never heard of AMD platform not liking PSUs either....

And you won't either. That's the whole point of the ATX Form Factor standard. And that's why you can mix PSUs too.

 

[cadaveca]

Yes, well aware of that. Let me say it different a split again, your split hairs... 12V is 12V is 12V. 5V is 5V is 5V... 3.3V is 3.3V is 3.3V.

I have heard of AMD platforms not liking Kingston memory, but, never heard of AMD platform not liking PSUs either....

I shipped an Antec PSU to ASRock who confirmed compatibility problem with their AMD motherboard. It's why I have Thermaltake PSUs now.

Huh? Nah! To "pull" power as you describe, the current flow would have to reverse. That's not going to happen in a DC circuit. As EarthDog said, 12V is 12V is 12V. But more correctly, +12VDC is +12VDC is +12VDC. There are diodes all over the place specifically to ensure the current only flows in one direction

Exactly, but not all boards are designed correctly, unfortunately. I have quite a few ES boards here that are questionable at best, even reviewed some (ASRock boards, mostly, such as one that claimed PCIe 3.0, but can only do PCIe 2.0 because wrong bridge chips were used).

Some AMD FM/FM2+ boards use -5VSB, which some PSU do not have, as well.

 

[EarthDog]

So a one off... ok. Good to know. What was the incompatibility? Older PSU new board?

I wouldn't call that common by any stretch of the imagination. Not that you did, but you surely didn't clarify you had one problem with one board and one PSU.

This is getting a bit out of my technical wheelhouse... I can BS with the best of them but.......lol

 

[cadaveca]

So a one off... ok. Good to know. What was the incompatibility? Older PSU new board?

I wouldn't call that common by any stretch of the imagination. Not that you did, but you surely didn't clarify you had one problem with one board and one PSU.

This is getting a bit out of my technical wheelhouse... I can BS with the best of them but.......lol

You know I post blanket statements that cover any and all scenarios. Better safe than sorry.

As to the FM2+ boards needing -5vsb, I have 5, ASRock x2, Gigabyte x2, and ECS. Hardly a one-off. And it's the newer PSUs that are the problem, as they exclude that rail.

 

[OneMoar]

nothing wrong with the CM Extreme Power Plus
I would not do a multi--psu setup unless you really know what you are doing
and 11.7 is within spec so you are wasting your energy

 

[Bill_Bright]

You know I post blanket statements that cover any and all scenarios. Better safe than sorry.

Blanket statements are fine - but not when used to justify or rationalize a "one-off" situation or "exceptions" to the rule.

If a board is not designed correctly, then it is not ATX compliant and there would be many examples of failures and large amounts of RMA - if not total product recalls.

As for your comment about -5VDC, that is no longer an ATX requirement for PSUs and has not been since 2004. If a unique board (perhaps a proprietary sever board) needs -5VDC for some reason, it is no problem reversing that polarity without requiring any changes to the PSU. But then it is no longer ATX compliant either.