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, Doby, linderman, simpswr, UncleMacro, wrench97, 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 cheap power supply costs about 10 times more than a quality power supply!

Do you know why?

A PSU that is not suitable for a specific computer 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.


Total Power Wattage is important, but there are even more important considerations,


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 fan, which blows air in the bottom of the PSU. They move more air at a 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 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.

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.

Why did you copy this directly from Tech Support Forum (word for word) and not give credit for the people who wrote it. It was written by Tumbleweed36 in conjunction with a team of power supply experts on that forum? Isn't this a form of stealing someone else's work and taking credit for their work?

http://www.techsupportforum.com/foru...on-192217.html

oh Mevan i thought you never ever log on this site