Choosing the Right Power Supply

Question

First things first. If you've got a poor-quality and/or faulty power supply, nothing else you do will work to solve your problems. Stick to the basics before you go further...The short answer is to buy a hi-powered, brand name supply, like the new ENERMAX line (430 W or higher) or ANTEC True550. Almost nothing else will do with today's computers. In over 30 years of electronic/computer service, I have found that 85% or more of problems were power-related.
If you want to know more, read on...
Choosing The Right Power Supply
If you’re reading this, there’s a good chance that one of my colleagues or I believe that you could be experiencing problems with your power supply, based upon the symptoms you mentioned in your post, and provided you with this link. Relax, you’re not alone. In 30 years of electronic and computer troubleshooting, I’d say that the majority of the electronic, mainframe, mini, and microcomputer problems I’ve diagnosed and repaired were with the basic power the problematic device was receiving. The symptoms often included random reboots, crashing, the BSOD, lockups, etc.
(As the national support technician for few major computer service companies, working US Defense contracts, I was often the person that had to fly in and correct the problem, or “walk through” the on-site technician as he closely followed my instructions. I achieved success in my career by carefully reading the manuals, knowing where to go for more information that was otherwise unavailable to me, and/or systematically troubleshooting until the problems were discovered and repaired. I never had the option of giving up.)
The most overlooked component when building or upgrading a PC is the power supply unit (PSU). Some people use their old case and PSU when they upgrade. Some use the PSU that came with their new case. Some people even buy a new PSU. And most inexperienced builders all make the same mistake: The PSU that they’re trying to use is simply inadequate for the job.
Suppose you’re upgrading to a new motherboard, CPU, ram, and video card, but still using the old case and PSU. It’s most likely that you’re upgrading in order to build a machine that is more powerful, faster, has a more colorful display, can number-crunch more quickly, play the latest games, etc. These gains in performance all have one thing in common: They require more raw power. However, have you thought about where that power comes from?
Suppose you’re building a new system with a new case and PSU. Has it occurred to you that the company that you bought the case/PSU from might make more money if they skimp on the supply, even if the supply has a large wattage rating? Most bulk power supply manufacturers don’t make good PSU’s. They use older, cheaper technology, and slap on labels that represent the PSU’s peak outputs, and not their continuous output rating. These companies are intentionally misleading you in order to sell you an inferior product. Brands I avoid when building/repairing my friends’ and family’s computers: Allied, Q-Tec, Chieftech, and many others.
For those of you who bought a power supply separately, did you know that you’re only supposed to run a power supply continuously at 30-70% (with 50% being optimal) of its continuous rating for maximum efficiency (which means less heat to you)? Most inexperienced builders either buy PSU’s that are matched to their equipment’s continuous power usage, or ones that are even less powerful than they need. Why? Because they’re trying to save money.
I mean, what’s the fun in a power supply? You don’t get any games with it, there’s no more storage, hardly ever any more bells and whistles, etc. A power supply is boring, and it’s supposed to be, because it’s supposed to provide a stable, reliable platform upon which the rest of the equipment can easily access the amount of power it needs, and when it’s needed. In almost EVERY review of powers supplies, the same point is stressed: Better safe than sorry.
But what does safe vs sorry mean? It can mean that you don’t have to waste money on the wrong PSU in the first place, but it can also mean that you don’t have to replace your expensive ram, CPU, video card, etc. NEEDLESSLY, or because your cheap PSU destroyed them. What? A cheap power supply can wreck your computer? YES IT CAN. A cheap power supply can cause thermal damage, not only from the heat it produces, but also the heat it can create in your components as well. RAM is especially sensitive to heat, and there’s RAM in your CPU, your video cards, and, well, your RAM too. A cheap switching power supply, run at its maximum, or peak, continuously can also destroy components by creating RF (Radio Frequency) signals on your power rails, signals which the components on your peripheral devices were not equipped to handle in the first place.
So this begs the question, how does one choose the right power supply? I’ll illustrate this using my own PC as the example. This is my setup that I use for video processing:
K7N2G-ILSR
Athlon 2500+ Barton @ 2125Mhz
AMD Retail Heatsink/Fan
2 - 512MB DDR333 w/Thermaltake Spreaders (slot 1&3)
MSI TV@nywhere Video Capture
ATI Radeon 9600
120GB Maxtor DiamondMax Plus 9 SATA
30GB Quantum IDE
TEAC DV-W50E DVD/CD-R/W
BTC DVD-ROM Drive
Artec CD-R/W
Using this Power Supply Calculator link:
http://www.jscustompcs.com/power_supply/
I plug in all my equipment values, but some of this can be a little tricky. For example, since I often run the CPU like an XP 3000, I choose the 3000 as my processor; it’s the same chip run at the faster rate. I also choose the ATI Radeon video card, and I select the RAM wattage for 2 sticks of DDR. I also choose every card I have, like my video capture card, but I also select the boxes for the separate cards that correspond to the functions that my ILSR provides as well (and that I use), like sound, USB, Firewire, NIC, etc.  Although I use the onboard SATA controller, I don’t select the SCSI PCI card, because, in truth, I’ve probably made up for it by selecting all the other corresponding devices, including cards that the motherboard replaces. I check the boxes for the fans and drives I use, and I’m done, right?
Not yet.
I just remembered that I plan to upgrade soon, so I go back and change the values to reflect my impending changes. I mean, I want to make sure that I have enough power to begin with so that I don’t have to replace the power supply again, right?
Ok. Done. I look at the bottom and see that it tells me that I need a 468 watt PSU. So a 480 watt supply will do, right? Wrong.
Remember that, for efficiency, long-life, and less heat, you want your actual power consumption to fall between 30-70% of the PSU’s rating, so add 30% (minimum) to the 468, and you get 468 + (468*.30)= 608 Watts! Holy Cow!
However, I’d only need a 608-Watt supply if I was using all the devices at once, and I don’t. But, in truth, with video and audio processing, I often get close when I process, burn, and monitor at the same time. (Hardcore gamers also get close a lot, as they blast the sound and push that video to its limits.) So, let’s take off 10% (maximum) of 608, for a total of 541 Watts.
I need a 550 Watt supply, but not just ANY 550 watt PSU. I need a supply that can give me enough power on the critical 3.3, 5, and 12V rails combined. I also want a supply from a trusted, name-brand manufacturer, so I start hitting the many online reviews. Here are just two from Tom’s Hardware:
http://www6.tomshardware.com/howto/20030609/index.html
http://www6.tomshardware.com/howto/20021021/index.html
Read these in their entirety. I didn’t post them because they’re pretty links.
In the end, I chose Antec, because they’ve got the reputation, the recommendation, and because the Antec True550 has better specs than the rest of the 550 Watt competition. I also bought it from a reputable company I found on Pricegrabber.com, for the lowest price I could find, $95.00 shipped to my door. (In truth, I wanted two mini-redundant supplies, like the hospitals and military use, but they were too expensive.)
The result? Not only are the random reboots, crashing, the BSOD, lockups, etc., gone like magic, but I also now have “peace of mind” in that whatever might happen to my equipment in the future, I know almost for certain that the PSU is NOT the problem. I also bought an UPS, because the East Coast Blackout proved to me that even the Antec True550 isn’t going to provide me any power for emergency shutdown if it doesn’t get its power from somewhere.
Even if your problem doesn’t lie in the PSU completely, it gives you a GREAT platform for troubleshooting further. If you’re not reasonably certain that the supply is the cause, borrow one, or buy one that you can return once you’ve solved the problem. But, above all else, BUY THE RIGHT SUPPLY before you do anything else! Otherwise, you could be plugging and unplugging components, buying and blowing up expensive memory, and causing even further damage, until you give up or die.
I mean, I assume you built your own system to enjoy “more bang for your buck,” right? What’s the fun of a random reboot in the middle of Unreal Tournament 2003?
William Hopkins
Former Staff Sergeant, USAF
B.A., B.S., with Honors
The University of California, San Diego
[email protected]
P.S. It should be noted that while Enermax, ThermalTake, Zalman, Fortron, and others make great PSU’s, and I compared and considered them, the Antec still won out overall in my critical evaluation, like it did in so may others’ reviews. You’d probably be ok if you went with another reputable manufacturer as listed above, but pick a supply that gives you at least 230 watts on the 3.3 and 5V lines combined, and still meets the 30% criteria as stated above. Remember, if the manufacturers don’t give you maximum combined specs up front, they’re untrustworthy right off the bat. With power supplies, you definitely end up getting what you pay for. Don’t say nobody warned you.
P.P.S. Update! After recent developments, it looks like Enermax is the leader, but only the latest line of PSU's.

Answer 1

Ok, as an electrical engineer...I have to step in here! LOL
First, these amp rating are for 2 +12 rails. That is why you see a protection of around 15-18A on the +12 rail. That means each Rail is allowed up to 18A lets say for the new Enermax 1.2 version like the one I have.
Now, Lets say 18A for 12V....well as you know the Abit NF7-S uses the 12V for powering the CPU.
Lets say you have a Barton like me and you want it stable at around 2.4-2.5Ghz. You will have to put lets say around 2V to the cpu to get it stable at that kinda speed, specially if you have high FSB like I do. So 12V * 18Amps = 216W ....well the converter on the NFS-7 is really bad, its loss on the step down convertion is probably around 25% along with the PSU lost cuz its not running at 25oC (another 15%)....you will actually only get around 100-120W for the CPU.
Now, if you go into Sandra and see how much a Barton eats up at 2.4Ghz you will see its around 110Watts.
So, if you wanna push more, dont even think about it! Prime Power test fails and your +12 rail will drop as low as 11.60 Volts.
Now, lets say you got yourself a AMD 64 bit chip and you wanna overclock it....I bet it will need more than 110Watts.
So, what im saying is, dont buy nothing less than a 500 Watt PSU!
You really need around 20-22 A on the main +12 along with really really good cooling on the case and PSU so it is running at a 100%.
http://forums.amdmb.com/showindex.php?s=&threadid=287828
i found this quite interesting especially the bit re the power loss turning the 12v into 1.6v or what ever cpu needs