Debunking Power Supply Myths
by Christoph Katzer on September 22, 2008 3:00 AM EST- Posted in
- Cases/Cooling/PSUs
Connectors and the 12V Rail Issue
Okay, we've chosen a few power supplies for our three test systems, but so far we've only looked at performance. There are still two additional issues that can affect your choice of power supply. We have previously shown that it is possible to run even high-end systems with a much lower rated power supply than you might expect. However, look at the offerings from the various manufacturers and you will frequently find that there are insufficient connectors for some configurations. The second issue involves the 12V rail(s), which is primarily responsible for powering the processor and graphics card(s).
Let's start with the first issue: having enough connectors. It is certainly possible to run a GeForce 8800 Ultra using only a 400W power supply; however, no one makes a 400W PSU with the necessary two PEG connectors. In fact, some power supplies in this range might not even have a single PEG connection. You could always use a Molex to PEG adapter(s), but you're probably better off selecting a different power supply.
Our entry-level system used the ATI Radeon HD 3650, which is a nice choice because it doesn't require any PEG connector. In the future users might want to upgrade graphics cards, however. The good news is that all of the entry-level PSUs be selected include a single 6-pin PEG connector, so they should be sufficient for powering up to a midrange (8800 GT/HD 3850) graphics card. If all you need is an average computer system, these power supplies will work fine.
The second issue is the amount of power the unit needs to provide in order to port graphics cards that use a single 6-pin jack. We could have included more cards, but for this example we've selected ATI's HD 3850/70 and NVIDIA's GeForce 8800 GTS and 9600 GT. We've created a table showing how much power these cards consume and where this power comes from under full load.
| GPU Power Requirements by Connector | |||||
| Vendor and Chip | Through 6-pin Jack |
Through PCI-E Slot |
Total Power | ||
| ATI Radeon HD 3850 | 4.4A | 52.8W | 2.4A | 28.8W | 82W |
| ATI Radeon HD 3870 | 5.1A | 61.2W | 2.6A | 31.2W | 92W |
|
NVIDIA GeForce 8800 GTS
|
7.2A | 86.4W | 4.8A | 57.6W | 144W |
| NVIDIA GeForce 9600 GT | 4.9A | 58.8W | 4.0A | 48.0W | 107W |
We need to check if the power supplies come with either one or two 12V rails. PSUs with two 12V rails usually have one rail connected to the 6-pin PEG connector and the other running the 24-pin ATX connector. The 24-pin ATX connector has two yellow cables that supply 12V, and both of these cables supply the PCI-E slots with power. The increasing power requirements of modern GPUs was the driving force behind the switch from 20-pin ATX connectors to 24-pin connectors. Some power supply manufacturers include different color markings on the 12V cables to differentiate rails, so if you have such a power supply you should make sure you connect components to a rail that has sufficient remaining juice.
It's important to have one 12V rail supply the CPU with power and the second rail for the PCI-E slots and 6-pin connector. Unfortunately, many companies make a tremendous mistake when it comes to power distribution. We have seen several power supplies that use one 12V rail for the 6-pin PEG connector and then a second 12V rail for the CPU and 24-pin ATX connector. That means if you have a graphics card that doesn't include a 6-pin jack, both the CPU and GPU will use the same 12V rail for power. In this case, the second 12V rail goes completely unused, and users risk drawing too much current on the remaining 12V rail. In addition, how much power a GPU draws from the 6-pin connector and how much it takes from the PEG slot varies.
Checking the labels of the entry-level units, we see that our selected power supplies should all have no difficulty running any of the above GPUs. Power supplies like the Corsair VX450W that has a single 12V rail have the advantage of being able to fully utilize the rated 33A.
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Christoph Katzer - Monday, September 22, 2008 - link
Yes I guess that was the problem. Thanks for pointing that out!ViRGE - Monday, September 22, 2008 - link
Christoph, thanks for the excellent article. There is one thing I noticed however that has me a bit confused. In a few of your video card tests, you have the cards pulling upwards of 100 watts from the PCIe slot. It has been my understanding, and is mentioned in AT's X38 chipset article*, that the limit for a PCIe slot is 75W. Your results put this value in contention, so I'm not sure what to believe at this point.Is it that I am mistaken that the limit is 75W, or are your video cards pulling too much power from the slot, or is there a problem with the data?
Christoph Katzer - Monday, September 22, 2008 - link
Thanks. The data is correct since the cards draw power through the PCIE slot and the additional PEG connectors (6-pin or 8-pin). Towards the end of the article you will find a list which card drew how much power from the PCIE slot and the PEG connector. 75 watts is however correct for the older PCIE slots, PCIE2.0 can draw up to 150 watts.mobutu - Monday, September 22, 2008 - link
Where is AMD 4850?Also almost all the graph cards used are kind of old, previous generation.
Dobs - Monday, September 22, 2008 - link
Here is a link to a comprehensive list of all graphics cards.Dobs - Monday, September 22, 2008 - link
damn... link didn't workCopy and paste :)
http://archive.atomicmpc.com.au/forums.asp?s=2&...">http://archive.atomicmpc.com.au/forums.asp?s=2&...
Clauzii - Monday, September 22, 2008 - link
Nice lists, thanks :)Christoph Katzer - Monday, September 22, 2008 - link
As stated I didn't have all of the new stuff here and this article anyway focuses primarily on power supplies and how to choose them and not the power consumption of the components... but it might come soon, who knows.LTG - Monday, September 22, 2008 - link
>>our labs don't have the latest GPUs available for testingIsn't this kind of a bad thing for a site with so many enthusiast readers?
First two caveats:
1) There are a lot of article bashers out there who nit-pick, give rude feedback, or are just plain wrong, I don't support these guys or want to be one.
2) The article written was very good and helpful, thank you.
However it doesn't matter that the article "primarily focused on how to choose power supplies".
- Leaving out a GTX 280 was a big omission. Even better to have an stock OC'd product.
- Leaving out very basic overclock numbers was a big omission. For example it's very common for readers to have an Intel QC clocked at 3.6Ghz since it's so doable.
Having the last two data points would be very helpful for those building top systems, and would even be interesting for those who are not.
7Enigma - Monday, September 22, 2008 - link
As mentioned, the article is about the myth that we all need 1k PSU's for the "best experience". Bottom line this article shows that the vast majority of people do not need more than a 500-600w psu. How is leaving out any stock product an omission? Just look at the TDP of the product and go from there. All stock products will fall within a narrow range.Once you get to OC'ing I also think this is a difficult thing to do. Individual chips, even from the same batch can have very different properties and require different voltages to reach the same speeds. I'm sure if the author had put that his Intel quad drew 120w at 3.6GHz, someone would have complained that their's took 135w, and someone else only 110w...Once you go above stock, it is not cut and dry and as the article states you should have a 20-30% extra buffer if you wish to OC.
I feel the scope of the article just doesn't require these extra requests. Sure it would be nice to see just how crazy of a system you can make (if I remember correctly they did that a while ago to actually use the >1000w supplies), but this article was to show what a normal base, mid-grade, and high-end setup would require.
I've complained in the past about articles, but this one I don't see any serious faults.