CHOOSING AND UNDERSTANDING A POWER SUPPLY UNIT
The power supply unit in today’s modern computer assumes a role probably more critical than any other single component in your system even when compared to the CPU and motherboard. Therefore, there are multiple factors that must be evaluated prior to selecting your next power supply or PSU (short for Power Supply Unit). A standing committee of TSF Hardware Team members including Blackduck30 (emeritus), Doby, linderman, Old Rich, Tyree, UncleMacro (emeritus), and Tumbleweed36 have evaluated and compiled data to assist you in understanding how to pick a reliable power supply for your computer.
Today’s PSU market is extremely competitive and replete with information that can mislead home computer builders that have not been fully educated on PSU criteria. We tend to look for wattage ratings as a sign of a good PSU, and that is where we begin to make a mistake in the process of purchasing or choosing wisely.
A power supply converts AC electricity from the wall socket to low voltage DC that computer components need to run. When that PSU is not suitable for a specific computer or does not do an adequate job of providing DC voltage, it does have the capability to bring a system crashing to its knees. An underpowered PSU may cause heat buildup, automatic shutdowns, freezing, BSOD’s, video distortion, system overheating, and a lack of power that may cause expensive top shelf Cpu’s, motherboards, hard drives and ram to burn up. You can lose not only expensive components, but related time loss in a work environment may cost you hundreds of dollars in lost time.
Therefore, make sure you have an adequate PSU to power your unit since power supply systems & voltage requirements have undergone major changes over the years, shifting the major power draw from the 3.3volt & 5 volt rails to the 12volt rail in todays machines.
We will present summaries of information and links with graphics to assist you with selecting an appropriate PSU for your computer. While we think it is important to read and reference all this material, use this guide to select what is appropriate to your needs and understanding from the following areas:
GUIDE AND TOPIC SUMMARIES
Performance Variables of PSU Testing, Efficiency, Compatibility, and Reliability
PSU rating & testing
There are numerous unscrupulous PSU makers in the market that know they can make a larger profit with cheap prices and poor quality merchandise than they can from building and selling high quality units. Therefore, the PSU manufacturer’s who want to sell a higher quantity of power supplies without the quality that is needed to meet the demands of today’s computers, tend to use a testing procedure that is misleading. This is done to make their supplies appear to be of a high quality than they actually are when manufactured.
System heat and the effects on the PSU
Most PSU’s are tested at 25 degrees Celsius, which is unrealistic. There is no PSU running in any computer environment that will be “living” in a 25 degrees Celsius environment. The realistic inside temperature of most computers is higher than room temperature and when fully loaded can sometimes get as high as 50C". That temperature increases under high stress computing like gaming.
The sad truth is very few PSU’s really produce the wattage that the companies advertise. Even if these low quality supplies could produce the wattage they advertise, they could not sustain those outputs for long. The rating and tests are usually only performed at 70% load for 100 hours. That means that PSU’s can’t sustain meager loads greater than 70% for any substantial period of time. In addition, the testing environement is unrealistic, since they test in abnormally cold testing climates of 25C.
An example of heat generated by a PSU is likened to a 50-watt light bulb being used and you feel heat emitting from the light bulb. Heat in a PSU is produced by inefficiency and the way the power conversion process turns AC household current into 12volt DC current in order to be used by computer components. The hotter the PSU gets’ the less clean & stable power the unit is able to produce. When a PSU is subjected to perform its duties within a “box” that has poor circulation the negative effects on its output is dramatically increased.
We have seen documented information that a PSU will actually lose 2-5 watts per one degree of heat above the 25C-testing platform. Please bear in mind, this is a phenomon that happens in all power supplies and those poor quality units tested in a 20C environment will not sustain the power they advertise, because of the de-rating of their stated output.
Therefore, if you are purchasing a power supply, one must add 30% more than the maximum power needed and displayed during your power calculator exercise, or your PSU will not be able to sustain performance during high demand times. They were simply not built nor tested to perform in that manner.
Fans in the power supply
When your PSU is operating inside your computer the unit must be cooled. The majority of PSU’s have a variety of fan configurations that includes either a two-fan or single fan configuration. There are also a limited number of PSU manufacturer’s that make a passive cooling unit that does not have fans. . When you view the total cooling scenario for PSU’s, one must realize that different manufacturers can do either good or bad cooling with one fan, two fans, or no fan at all in the unit.
If you open up a PSU and look at the heatsinks (A good source of pictures is xbitlabs) then you'll see some very boring "T" type heatsinks. PSU internals aren't tight little closed spaces, but they are largely open space with some air flowing past some heatsinks. Therefore, different fan configurations are used by PSU manufacturer’s to cool the components on the PSU and to pull warm air out of the case.
The two fan configurations mostly use one of two types of cooling scenarios. The first has two fans drawing hot air into the PSU from within the computer box. The PSU then moves that airflow across the internal components of the PSU in an effort to cool them while ejecting the even hotter air out the rear of the computer. Another variation of this two-fan model is that a PSU may have two fans that operate independently. One fan operates full time to expell heat from the inside of the case and the second fan only is activated when the heat buildup within the case activates the second fan to initiate more air flow and to further cool the components for optimum operation. This independent fan operation process takes less power and is quieter since the second fan operates only when actually needed. The majority of two-fan configurations use the 80mm size fans.
. A lot of the newer PSU designs have a single 120mm or 140mm size fan, which blows air in the bottom of the PSU. They move more air at a lower given noise level than one or two 80mm fans. Therefore, those who may be looking for quiet operation most likely would need to select one of these single fan models with the 120mm or the 140mm fan.
Efficiency rating of the PSU
You should notice as you begin to shop that PSU’s start with efficiency ratings of 70% and work their way upwards until you get into the more expensive 80-85% efficiency units. A higher stated efficiency rating is normally but not always an indication of a higher quality or better built PSU.
. The new ATX specs are pushing manufacturers to provide higher efficiency for both the lower ranked manufacturers as well as those that presently produce top ranked power supplies. While the efficiency of a PSU is important to the total picture, we feel the brand name and model of the PSU is more important to selecting good power supplies than just looking at the efficienty rating.
What does a Power Supply Certification mean?
The 80-Plus Rating System of a psu is a way to express energy efficiency at specific rated loads. Previous to 2008, the 80-Plus rating was considered the high efficiency standard. After 2008, new standards were added as more efficient power supplies were developed. These higher efficency standards indicate a significant reduction in electricity usage levels compared to less efficient PSU units. Here are the present standards for 80-Plus Certification:
Efficiency Level Certifications for each 80-Plus designation
.........................80 PLUS TestType .........115V Internal Non-Redundant .....230V Internal Redundant
.........................Fraction of Rated Load .......20% ....50% ....100% ..................20% ....50% ....100%
.........................80 PLUS ............................80% .....80% .....80% .........................Not defined
.........................80 PLUS Bronze ................82% .....85% .....82% ...................81% .....85% .....81%
.........................80 PLUS Silver ...................85% .....88% .....85% ...................85% .....89% .....85%
.........................80 PLUS Gold ....................87% .....90% .....87% ...................88% .....92% .....88%
.........................80 PLUS Platinum ..............90% .....92% .....89% ...................90% .....94% .....91%
What this means is if a psu rated for 60% efficiency is using 1000w from the wall outlet, then 400w would be wasted and dissipated as heat. In that same example, if that particular power supply had an 80-Plus rating, only 20% or 200w would be wasted and dissipated as heat. The more efficiency expressed by the 80-Plus Bronze, the 80-Plus Silver, the 80-Plus Gold, and ultimately the 80-Plus Platimum are even more efficient and do not waste electricity.
Therefore, the cost to operate these power supplies goes significantly down as the 80-Plus standard is raised. While one may save a few dollars in purchasing a lower rated power supply, over the lifetime (three or five year warranty) of a power supply, there can be significant savings by purchasing a PSU that has a higher efficiency rating with lowered electricity costs.
The role of Power Factor Correction ( PFC)
PC power supplies are actually switching power supplies A switching power supply converts power from your AC line into the DC voltages needed to run your computer. A standard switching power supply doesn't draw its power from the AC line smoothly. It actually draws sudden gulps of current. That "messes up" your AC power lines. In some offices with lots of equipment with switching power supplies, you can actually overheat wiring and trip current breakers because of the way they gulp power. Power factor correction smooths out the gulps to keep your AC line all nice and clean. If you're in Europe then the power factor correction feature is probably required in your PSU. In most other parts of the world it's your choice. If you just have a run-of-the-mill computer or two in your home then power factor correction isn't nearly as important, but PFC protection will definitely result in "cleaner" AC lines. Your power company will be happier if you have power factor correction because nasty reactive loads like power supplies with no PFC mess up their AC power grid and make their lives more difficult.
There are two kinds of power factor correction: active PFC and passive PFC. Active PFC is more expensive and does a better job of keeping your AC line clean. Passive PFC is cheaper but is still an improvement over not having PFC protection. Power factor correction makes the PSU more expensive so very low priced PSUs rarely come with either type of PFC protection. If you're really looking to clean up your PSU's power consumption habits then active PFC does a much better job than passive PFC to achieve that goal.
Manufacturers that offer numerous Protection options on their PSU’s normally indicate a higher quality product, while a lower quality unit will not offer as many of these features. While this area is often overlooked, experts recommend that you consider these features as you evaluate a power supply.
When you look for Protection features, please understand that manufacturers will often use “acronyms” to identify the protections offered by their company. We have listed the self-explanatory terms and Acronyms to assist you in understanding what features are offered for specific power supplies:
OVP = Over Voltage Protection
OCP = Over Current Protection
OPP = Over Power Protection
SCP = Short Circuit Protection
UVP = Under Voltage Protection
OTP = Over Temperature Protection
Case and power supply combination units
Be very careful with the computer case and PSU combo deals. They are for the most part, a case that can be made very cheaply and a very weak power supply put together simply to make a profit and not provide quality components. Any case combo selling for less than $150.00 is likely to be suspect as a poor quality combination. Therefore, we recommend the policy of buyer beware on case and power supply combination units.
Proprietary PSU units
If you buy a "white box" computer then it always comes with a standard power supply. But some OEM units like Dell, HP, and Compaq (and possibly more) have made some computers, which do not use standard power supplies. Sadly, some large computer vendors like to lock customers into buying upgrades from them rather from a third party. Almost all current OEM models use standard power supplies but many older models do not. If you're replacing a power supply in an OEM computer then it would be wise and prudent to check with the manufacturer to make sure the power supply is a standard power supply. Some of the better-known proprietary power supply situations are listed below.
Dell used to ship many computers which used proprietary power supplies. That's nasty enough but Dell built their non-standard computers and motherboards with what looked similar to standard ATX components. If you plugged a standard ATX power supply into the motherboard then things went kaboom (or frzzzzzap or...). The Dell PCs, which were carrying this ticking time bomb, were made between 1996 and 2000.
More specifically, Dell XPS models use nonstandard supplies. All Gen 2, 3, 4 , XPS 600, XPS 700 series, and up to the XPS 730 have a standard 24 pin connector. However, the second power connector that is used (Often called the CPU power connector) has 20 pins as opposed to the standard ATX 4 pin or EPS 8 pin. There is not an after market replacement for these units.
Compaq is also known to have produced many computers that use proprietary power supplies. Compaq also made some computers that appear to have a standard 24 pin main ATX power connector but it's actually a proprietary connector. Most of Compaq's current computers are standard power supplies.
HP ships mostly standard ATX power supplies. However, some of their power supplies also have an extra 4-pin cable that looks like a floppy power cable. It's usually called a video power cable although it appears to really be used by an EnSoniq soundcard in some Pavilion models.
Compatibility issues for ATX power supplies and motherboards
When ATX was originally introduced there was just one kind of ATX power supply. If you had an ATX power supply and an ATX motherboard then you plugged them together and they worked. At least they did if the power supply delivered enough wattage. Back then ATX life was simple. Nowadays things are a bit more complicated. Now you can get an ATX power supply and ATX motherboard and end up with connectors which don't match. Even if the connectors do match you can still run into trouble even though the power supply appears to have sufficient wattage. This section introduces you to the issues you need to consider to maximize the chance that an ATX power supply and motherboard will work together properly. The information below is just a summary. The green links provide more detailed information on the subject.
The motherboard main power cables
The ATX standard has two different versions of the main power cable: the original 20 pin cable, and the the newer 24 pin cable. The 24 pin cable is just the 20 pin cable with 4 extra wires added to the end to provide extra current. If your power supply main power cable and motherboard main power connector both have the same number of pins then they'll (of course) fit together just fine. But what happens if they don't match? If you plug a 24 pin power cable into a 20 pin motherboard then it will work fine but you often can't get them to physically fit together because something gets in the way. If they don't fit together then you can get an adapter cable which converts a 24 pin power cable into a 20 pin cable. You can always plug a 20 pin power cable into a 24 pin motherboard but whether it works over the long haul depends on how much current your motherboard draws. Those extra 4 pins provide more current carrying capacity. Plugging a 20 pin cable into a 24 pin motherboard can strain the 20 pins that you're using. If the motherboard draws too much current then it will overheat the connector which can burn or melt it. There are adapters which convert 20 pin cables into 24 pin cables but they don't solve the problem and can cause problems of their own. Many newer power supplies come with a 20+4 power cable which has a 24 pin connector which can be split into two pieces: a 20 pin piece, and a 4 pin piece. This kind of power cable is fully compatible with both 20 and 24 pin motherboards. If you're buying a new power supply then try to get one with a 20+4 power cable.
The 12V CPU power cables
CPUs used to be powered by the 20 pin main power cable. Almost all current motherboards power their CPU with a 12 volt CPU power cable. There are two kinds: the 4 pin 12V cable and the 8 pin 12V cable. The 4 pin cable is often called a P4 cable (although it's a very bad name) and the 8 pin cable is called an EPS12V cable. You must plug a cable into this motherboard connector or the CPU will receive no power. The compatibility rules between 4 and 8 pin connectors have a lot in common with the rules for 20 and 24 pin connectors. The best choice is to plug a 4 pin cable into a 4 pin connector, an 8 pin cable into an 8 pin connector, or a 4+4 cable into either kind of connector. You can get adapters which will convert various kinds of power supply cables into both 4 pin and 8 pin 12V cables. You can plug a 4 pin 12V cable into an 8 pin motherboard and it will sometimes work properly but other times it won't work at all or will burn/melt the connector. As with plugging a 24 pin cable into a 20 pin motherboard connector, you can plug an 8 pin cable into a 4 pin connector and leave 4 pins hanging over the end but you can also have the same problems with things preventing it from fitting. It will work fine if it fits. If you're buying a new power supply then the safest choice for the long haul is to get one with a 4+4 power cable because it's compatible with both 4 and 8 pin connectors.
The aux power cable
A cable you're not likely to run into is the 6 pin aux connector. Some older AMD dual CPU motherboards have them. If the motherboard requires this connector then you have to get a power supply which provides one. Most current power supplies don't have aux cables and aux cable adapters apparently don't exist (unless you're good with a soldering iron and can build your own).
Where's most of the load? 3.3/5 volts or 12 volts?
How a computer uses power has gone through a lot of changes over the years. Older machines used to consume most of their power from the 3.3/5 volt rail. Since AMD's socket A and Intel's pentium 4, computers have consumed most of their power from the 12 volt rail. Older power supplies provided the bulk of their wattage on the 3.3/5 volt rail and new supplies deliver it on the 12 volt rail. As a result, you need to be careful when plugging an old power supply into a new computer or a new supply into an old computer. If you know that the power supply won't be used in an older computer then you can use an ATX12V 2.0 or newer power supply which delivers the bulk of its power on the 12 volt rail and not much power (less than 150 watts) on the 3.3/5 volt rail. If you need a power supply for an older computer then you can use an ATX12V 1.3 or earlier power supply which provides most of its power on 3.3/5. There are also many newer ATX12V 2.0 or newer power supplies which provide enough wattage on 12 volts for newer computers as well as enough wattage on 3.3/5 for older computers. Those are the best supplies because they provide enough wattage for all kinds of machines.
If you're easily worried then you might want to skip this paragraph. Most of the things mentioned here are either more detail than you need to know or problems which rarely crop up. But these issues are listed for the thorough types out there (you know who you are). If you've been researching power supplies then you've probably noticed that newer ones usually come with dual 12 volt rails. Sometimes they have three or even four 12 volt rails. This subject of multiple 12 volt rails is more complicated then you may think. And if you have a multiple 12 volt rail power supply in a very high-powered computer then you may have to deal with rail balancing problems. You can have cross loading problems with some power supplies if you draw unbalanced amounts of wattage from the 3.3/5 volt and 12 volt rails. You can even get into trouble if you draw too little wattage. Higher quality power supply units now employ a 12volt single rail design which removes the need for load balancing or cross load worries.
One last cable/case issue to consider
Some large cases have the power supply located in a different area of the case that is extended from the normal peripheral connections. Therefore, one must consider the length of cables coming from the power supply that must be hooked up from those peripherals to the power supply to keep from using extenders that do not normally come with a new power supply.
Analysis, Special considerations, and Application of PSU Data
The Pleasures and Perils of High End Video Cards
Recent High End Video cards that have been developed like the Nvidia GTX 580, ATI HD6950 and HD6970 present a unique challenge for end users. The need to provide a power supply that can meet the demands of these powerful cards can be a real challenge for even the most powerful line of PSU units. Anyone who is under the illusion that a 650 watt power supply will pull these babies without difficulty might want to reconsider before it hits them in the pocketbook from having to replace damaged computer components.
These video cards are a pure pleasure to own and use. The graphics are awesome with uber high definition, fantastic frame rates, and clarity that has been unheard of in recent times. However, with our inclusion of our UBER Ultimate Professional Gaming Class computer, these have become popular quickly as individuals try to stay on top of the heap with the newest and best video card. .
The recent development of the dual engine ATI HD6970 card brings us to the brink of power demands in power supplies as we know them. In addition, even the tri and quad configurations of video cards mentioned above require a very powerful PSU to ensure optimal operation of the computer. The power supply team would like to relay to members who are opting for the HD6970 or some of the tri, quad confurations that one would not be wise to even think about running these setups with less than a 1000 watt high-quality power supply. Our research has indicated that an outstanding 750 watt Corsair will not hold up with the ATI HD6970 over the long haul.
When a buyer chooses to enjoy the visual pleasure of these graphics cards, they must think of a 1000+ PSU if you want that “King of The Hill” card(s) to work correctly and keep your new rig safe from damage.
Identifying your power supply
Members of the forum need to know more about the power supply in their present computer. This can be determined by identifying the brand name on the side or back of the PSU. In addition, one should look on the back or side of the power supply where amps for each rail are listed. This information when provided to somone assisting you will help a qualified tech to understand how the power supply can be expected to perform under normal circumstances. Therefore, problematic areas may be identified more efficiently.
Determining wattage and amperage requirements for my selected machine
The Standard Computer Class would be used for Internet surfing, general office work, and light gaming, with limited hardware upgrades on the computer. The Standard Desktop machine with an integrated/onboard video can perform flawlessly with a quality 400 to 550 watt PSU. These machines which are proprietary machines and/or those with older Intel processors up to and including a Pentium 4 3.0 GHZ CPU that have a FSB of less than 800 MHZ. For AMD enthusiasts, the range would be from the older machines up to and including the Athlon II's and E5xxx/E6xxx Pentiun dual cores as well as the i5 dual cores.
The Power Desktop Computer Class is similar to the Standard desktop computer class plus more demanding gaming usage. One can explore the option of overclocking with this unit. The computer would include numerous Hardware upgrades with multiple drives and a more powerful CPU paired with a fuller featured motherboard. The power desktop computer class is best suited with a 550-750 watt PSU of high quality construction.
Those who choose this class computer should have a dual core or quad core CPU with ONE dedicated video card up to and including any single engine video card an the market demanding up to 125 watts. This would include the Nvidia Nvidia GTs450 or the ATI 5830 video card and above. The Power Desktop Computer Class will run some medium power dual engine video cards, but are not intended to run dual cards like the ATI-X2 or the ATI 5870-X2 video cards.
The Power Gaming Class computer will have the capabilitiy for higher-end video cards, video editing, expected overclocking, and possible useage for SLI or Crossfire configuration.
The optimum CPU for this class would be any dual or quad core CPU that is powered by a high quality 750 to 850 watt power supply. The High-End Power Gaming Class computer will easily power tandem video cards in sli mode like the Nvidia 470/480GTX and/or ATI 5850 and above. If you select the crossfire configuration, these PSU's will also power dual cards like the ATI 5850 or ATI 5870 video cards and newer dual released cards.
The Professional Gaming Class computer is for demanding professionals who desire units that will perform to the maximum level of technological advances while remaining stable at all times. These units are an overclocker's and/or gamer's dream machine.
We would expect those who venture in this area to use quad core CPU's and two or more of the lastest and greatest video cards. The sky is the limit with these machines and the power needs will demand a bare minimum of 900 watts from a power source in order to achieve the optimum performance that is expected by these elite computer enthusiasts. A 900 to 1200+ watt power supply would be the most appropriate for those who want to have the ultimate machine and professional gaming experience.
These powerful machines and the PSU are for those who want to use the very latest video cards in SLI or Crossfire configuration. These units will easily power the newest HD6950 or multiple Crossfire ATI 6970 video cards. Those who favor the SLI configuration will find these units will easily power multiple Nvidia GTX 580 video cards.
Capacitor quality is important in a power supply:
Capacitors are those little parts in a power supply that store an electrical charge and release this energy when needed. There are metal plates with dielectric material between them filled with electrolyte, and this is sealed in a small cannister. When the electrolyte begins to break down with use, it changes to a gas. That transformation may cause the cannister to bulge over time and ultimately fail. Cheap capacitors used in an inferior quality power supplies will permit this breakdown to occur very early in the lifetime of the power supply, while a quality capacitor will not break down as quickly.
Estimated heavy use over time ( ie: 1+ years of usage or 24/7 operation) may cause the electrical charge to deteriate and/or fluxuate as the capacitor ages.. When purchasing a power supply, it is suggested that one should allow an extra 30% in size to compensate for that reduced electrical output as the capacitor naturally deteriates.
Recommended Power Supply Calculator
The power supply calculator at the bottom of this narrative is an important tool to assist you in determining the watts needed by a power supply to power a desktop computer. A calculator should not be totally relied upon in the selection process; other things also need to be considered before purchasing.
The total Watts is important but the Total Amperage Available on the +12V Rail(s) is the most important, followed by the +5V amperage and then the +3.3V amperage. Because of the increased power consumption of the new high-power video cards the recommended minimum for +12V is 26A, for SLI 35A.
We highly recommended that you use the capacitor ageing feature set to 30% or add 30% to the total watts the calculator determines if capacitor aging is set to 0 to cover things such as Capacitor aging, peak efficiency, and future upgrades. Please post any questions or concerns about your power needs on the forum so one of our Hardware Techs may offer a personal evaluation on any power supply selections or answer questions that are appropriate to your needs.
eXtreme Power Supply Calculator
Summary and Reasoning for our TSF Power Supply Recommendations
While there are many other excellent power supply choices out there, we have compiled what we feel is the best power supplies which are the most cost effective in each category as listed below. Should you choose another brand name that is fine with us, but all of the PSU brand names and models listed below are excellent quality PSU’s for the category, which is represented as approriate and cost efficient.
You will find that no other PC component than the PSU has the ability to impact your system in such a critical manner and enhance the margin of quality and performance of your computer. If you make a poor quality choice or try to save a few dollars on your power supply, the chances of failure of not only the initital build but also the components over a long period of time are very real.
A recent fact reported by Anandtech in their review research is that the majority of damaged RAM returned to memory manufacturers is destroyed or damaged by fluctuations in voltage from the PSU unit.
We suggest when purchasing a power supply that you buy the best and forget the rest. Brand names are a very important indicator of quality in this realm.
Please note that all PSU’s listed and recommended here are in Random order with the order of their listing not determining the quality of the supply for that specific area.
Disclaimer: I am not a Research Analyst and not registered with any regulating authority. All posts are for educational purpose only.
Consider us a dumb -dumble guy in this analysis, any type of real time example will be quick-learn approach.