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PSU compatability question
https://gladerebooted.net/viewtopic.php?f=5&t=530		 Page 1 of 1
Author:  Telumehtar [ Wed Oct 14, 2009 5:15 pm ]
Post subject:  PSU compatability question
Hey guys, I'm trying to build a system using some parts left over from an old system, namely the case, drives and PSU. I bought a new MB, CPU and memory. My PSU is an OCZ-520-12u.

The MB has a 20+4 pin ATX power socket and an auxillary 8 pin CPU (2x4 ATX) power socket. The PSU has a 20+4 pin ATX power connector, but only a 4 pin CPU connector. I can plug the 4 pin PSU connector into half of the 8 pin socket on the MB and the system starts and runs OK (at least as far as the BIOS screen, I can't get Windows to work but I assume thats a different problem).

My question is, will running this way damage the MB? I watched the power consumption stats in BIOS and everything appears to be OK.
Author:  Darkroland [ Wed Oct 14, 2009 6:36 pm ]
Post subject:  Re: PSU compatability question
Telumehtar wrote:
Hey guys, I'm trying to build a system using some parts left over from an old system, namely the case, drives and PSU. I bought a new MB, CPU and memory. My PSU is an OCZ-520-12u.

The MB has a 20+4 pin ATX power socket and an auxillary 8 pin CPU (2x4 ATX) power socket. The PSU has a 20+4 pin ATX power connector, but only a 4 pin CPU connector. I can plug the 4 pin PSU connector into half of the 8 pin socket on the MB and the system starts and runs OK (at least as far as the BIOS screen, I can't get Windows to work but I assume thats a different problem).

My question is, will running this way damage the MB? I watched the power consumption stats in BIOS and everything appears to be OK.


It should not damage the MB. While it may not be the most stable thing without all it's pins around, I've never seen it hurt anything (typically, if the MB doesn't like the power configuration it won't POST at all), and now you'll have a scapegoat when it crashes! :) Those pins are there for when the high powered CPU's really need their own pipe for electricity. It looks like you've got a pretty hot CPU there, so if you start firing up CPU intense games and the system crashes (or hard reboots), that may be the culprit.
Author:  Stathol [ Thu Oct 15, 2009 2:10 pm ]
Post subject: 
Yes, it's fine to plug your 4-pin ATX connector into half of the EPS12V 8-pin socket as long as you can deliver enough power to the CPU. In most cases, it's just there for EPS12V compatibility rather than out of electrical necessity.

That said, I can't specifically answer your question without knowing more about your graphics card. You can only draw about 50W through the ATX 4-pin aux. At full load (95W TDP for your CPU), it will need to be able to draw at least 45W from the motherboard. In and of itself, that's not a problem. The motherboard has roughly 80W available to it from the +12V rail(s). However, this 80W pool is shared with other devices, most notably the PCI bus. In other words, your GPU can also draw power from the motherboard. If it should try to draw more than about 35W from the motherboard while your CPU is under full (electrical) load, the system may crater. That shouldn't hurt any of the hardware, but it's obviously undesirable.

If you can tell me which GPU you're using and how many external PCI-E power connectors it has, we can do some back-of-the-envelope calculations to be sure.
Author:  Telumehtar [ Fri Oct 16, 2009 2:49 pm ]
Post subject:  Re: PSU compatability question
Hey guys, thanks for the useful replies!

I'm using the onboard video which is AMD's 785G chipset, they call it a "Radeon HD 4200"; I think its similar to a Radeon 3450. I don't intend to put any expansion cards in right now as I intend to use the system strictly for number crunching and don't really need up to date graphics capability.

Thanks again!
Author:  Darkroland [ Fri Oct 16, 2009 3:49 pm ]
Post subject:  Re: PSU compatability question
Telumehtar wrote:
Hey guys, thanks for the useful replies!

I'm using the onboard video which is AMD's 785G chipset, they call it a "Radeon HD 4200"; I think its similar to a Radeon 3450. I don't intend to put any expansion cards in right now as I intend to use the system strictly for number crunching and don't really need up to date graphics capability.

Thanks again!


Then yup, as long as that proc isn't pulling too much power you should be good. A lot of times, it's the extremely high power graphics cards fighting the processor for power, and then all hell is breaking loose.
Author:  Stathol [ Sat Oct 17, 2009 11:20 am ]
Post subject: 
Well, I can't find any specific docs on that chipset. I always have that problem with AMD. "What? Thermal design guide? Why would anyone publish *that*?!" Grrr.

Anyway, the next best thing I could do was dig through some end-user testing. Several reviewers have done power consumption tests with that chipset (with the IG), and the results have pretty consistently shown a total system power consumption of about 105W in combined GPU/CPU stress tests -- using an AMD II X2 250 (TDP 65W).

It's impossible to know exactly how much of that came from the 785G/HD 4200 chipset and how much was coming from the CPU, but pessimistically, we can estimate that the real-world peak power consumption of your system will be roughly 135W (95W TDP - 65W TDP = +30W). Earlier, I said that for the +12V rail, you could get about 80W to the motherboard and 50W to the CPU through the 4-pin connector. At first glance, it sounds like we're right on the edge (130W) with our estimate of 135W. However, the cited 105W was for total system power. Although most of that probably is on the +12V rail, not all of it will be.

So all thing considered, I think you've got just enough power delivery to pull this off, even if you were going to use this for gaming, etc. Ironically, this is one of those weird situations where adding a more powerful graphics card would probably improve your power situation, provided that it had an external 6/8-pin power connector. Your PSU can deliver about 3x more power on the +12V rail than you actually need for this setup -- you just have a distribution problem.

Final thought:

If you're still concerned about it, I'm willing to bet that someone makes an adapter to convert your (currently) unused 6-pin PCI-E connector to a 4-pin connector for the other half of the CPU aux power socket. The 6-pin connector has three +12V conductors and 3 ground conductors. The 4-pin connector has two of each. Both should be using AWG#18 at a minimum. So there's no reason this can't be done.

Edit:

After some googling, it looks like most of the 6-pin to 4-pin adapters you see are for going to the old 4-pin connector used by AGP cards. These had both a +12V and a +5V conductor, though, so this particular solution wouldn't work for the CPU. I still think the right adapter cable has to exist somewhere, though...
Author:  Telumehtar [ Tue Oct 27, 2009 1:41 am ]
Post subject:  Re: PSU compatability question
I've been out of town for over a week, so I havent had a chance to check here. Thanks for your input Stathol, I thought the PSU would have enough power, but the extent of my knowledge about this subject is extremely limited. I managed to get the system running before I left and ran some stress tests which went fine. I might just keep a look out for a good PSU deal.
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