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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JarredWalton - Monday, September 22, 2008 - link
If you look at all the included components, it should be pretty clear that the latest parts won't be drastically different from the tested components. Sure, 4870 might use 30W more (or 30W less) than the 3870, and the P45 might use +/-10W relative to the P35. Does that really change anything with the information the article conveys? I don't think so. Midrange PCs are still going to use 150-250W for the most part, whether with last year's components or with the latest stuff. If you want to look at the top-end, then GTX 280 will use more power than the listed GPUs, but even then you're not going to break 600W without overclocking.Martimus - Tuesday, September 23, 2008 - link
Thanks. I have since calculated out what is needed for the processor and chipset (for a E8400 and P45 MB) and it came to about 8A. I went on the AMD website to find what the current draw was from the 4870. It doesn't say, but does recommend a 500W PSU. I am a little concerned though, because the EA500 isn't on their approved PSU list from Antec, and I have had issues with their power supplies before, plus they only have 17A available on each rail. that should be fine, but I would like to make sure I am not loading the 12V rail too heavily as well. If those components don't work (those are my plan for the moment), then I can always adjust, but I would like to have that peace of mind before I make the purchases. (the computer is a present, and I bought the Sonata III to save money because I heard good things about the Earthwatts brand, but now I am starting to get worried about this particular model.)Matt Campbell - Monday, September 22, 2008 - link
I would also point out that in addition to all of the data Christoph has pulled together, we have some power consumption numbers on overclocked systems with GTX 280s as well, which seems to be one area people are asking for.4.0 GHz QX9770 with SLI GTX 280s: 579W Max
4.0 GHz QX9650 with Triple 8800 Ultras: 671W Max
JarredWalton - Monday, September 22, 2008 - link
Those figures are power at the outlet, though, right? And they're not tested in the same fashion as what Christoph did. Still, if you max out at 671W at the outlet, even with 88% efficiency you're only using 590W - nowhere near 1200W, which is what NVIDIA certifies for 3-way SLI. Quality over quantity, naturally, but there really aren't many terrible 1000W PSUs out there (which is why they all cost over $200).Torched - Monday, September 22, 2008 - link
For all the hullabaloo about the 12v Rail why nor just recommend a PS that has a single rail. More and more manufacturers are going in that direction anyways since it eliminates the whole power trapping issue.gramboh - Monday, September 22, 2008 - link
Good article. The underlying point is that you do not need 1000-1500W PSUs to run your system like I see many people claiming you do for SLI setups. I see lots of people with 700-800W PSUs with one graphics card. Insane.I've been running a Corsair HX520 (520W) for a year and a half now. My current config is:
Asus P5B-Deluxe (P965) mobo
Q6700 G0 @ 3.3GHz (1.42v actual voltage reported by Speedfan)
4x1GB PC2-6400 DDR2 memory
EVGA GTX280 video card overclocked (675-1350-2422)
3 hard drives (7200.11 1TB + 2x 7200.10 500GB)
1 optical (18x Samsung DVDRW SATA)
3 case fans ~1500rpm + CPU fan 1200rpm
SB X-Fi XtremeGamer audio
Zero issues and I'm sure I have headroom to spare. Some people on forums told me I should upgrade my PSU to at least a 650W Corsair or 750-800 of other brands to run the 6700/GTX280, just goes to show people have fallen for the marketing hype of PSUs.
cesthree - Monday, September 22, 2008 - link
I have had a few different systems in the last 6 months. Using the Zalman MFC-2 I can see the wattage being used in real-time. How accurate it is, I do not know, but it gets me in the ballpark.All systems have had the same 4 WD1600YS HDD, Lite-On DVD burner, 700W OCZ GameXStream PSU, and OCZ DDR3 1333Mhz 2x1 GB RAM. I also run 4 x 120mm fans including the one mounted to my TRUE-120.
Set-ups:
1. EVGA 790i Ultra (JUNK) w/ Q6600 @ 3.2Ghz 1.35VCORE, 1400Mhz FSB, 8800GTS 320MB SLI
Idle Load: 225W, Prime Load: 275W, 3DMARK06: 325W.
Never saw higher than 400-425W during highest loads.
2. DFI X48-T3RS (PWNING 790i ULTRA) w/ Q6600 @ 3.0Ghz 1.32VCORE, 1333Mhz FSB, EVGA 9800GTX
Idle Load: 200W, Prime Load: 250W, 3DMARK06: 290W.
Never saw higher than 350-375W during highest loads.
3. DFI X48-T3RS w/ E8400 @ 3.0Ghz 1.245VCORE, 1333MHZ FSB, EVGA 9800GTX
Idle Load: 175W, Prime Load: 200W, 3DMARK06: 250W.
Never saw higher than 300-325W during highest loads.
BTW, all Watts are averages. Prime Load is averages between blend testing and small FFT's.
I really like the meter on the MFC-2. If it is at all accurate, then it supports the logic that it isn't always necessary to have a 2000KW PSU.
I could see needing a higher wattage, QUALITY PSU, for CF or SLI + the latest GPU'S, non-extreme CPU, with everything OC'd 25-50%, and maybe a single loop WC as well.
Just my 2 cents.
Spacecomber - Monday, September 22, 2008 - link
I was wondering if it might have been helpful to add some comments on the importance (or not) or having 8-pin 12-volt motherboard connectors, instead of the more standard 4-pin motherboard connector. I don't think that this motherboard connector, which I think largely powers the CPU, was given much attention relative to the discussion of the additional 4-pin connector for the main motherboard connector (20 + 4) and the PCI-E power connectors.Overall, this was a very good article. I like how it places technical details about power supplies into a context of everyday use.
CSMR - Monday, September 22, 2008 - link
The power consumption data on page 1 is completely overblown.The tables claim to represent actual power consumption of processors and chipsets but the figures are completely exaggerated.
There are even people who run whole systems on one or two of the chipsets listed on less power than the power the article claims for just the chipset.
Here are actual CPU measurements from behardware and xbitlabs:
http://www.xbitlabs.com/articles/cpu/display/intel...">http://www.xbitlabs.com/articles/cpu/display/intel...
http://www.xbitlabs.com/articles/cpu/display/intel...">http://www.xbitlabs.com/articles/cpu/display/intel...
I haven't seen measurements of chipset power but here is a list of TDPs:
http://www.silentpcreview.com/forums/viewtopic.php...">http://www.silentpcreview.com/forums/viewtopic.php...
Must fix this soon as Anand needs to keep its reputation for good information.
Zoatebix - Monday, September 22, 2008 - link
Seriously. This guy didn't go too far our of his way to make a 30-40 watt system: http://www.silentpcreview.com/forums/viewtopic.php...">http://www.silentpcreview.com/forums/vi...tdays=0&...