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EMMA HS3 Advanced Hardware Outline Week #3

Review Homework

Hardware - Computer Power Supply Units (PSU)

Amps, Watts, Volts, and Ohms

Plumbing Example

V=IR, P=VI

Computer Power Supply

Wattage

Appearance

External - Connectors – PC Main, 4-Pin Peripheral, Auxiliary, Serial ATA

Internal - Diodes, Capacitors, Transformers, Heat Sinks

AT vs. ATX – Power-on switch

Laptops – Power Brick

Energy Efficiency – 70-80%, Rest is dissipated as heat

False Advertising

Modular

Troubleshooting

AT12V vs. ATX Power Supplies

AT, ATX, ATX12V

24-Pin Main Power

Multiple 12V Rails

Serial ATA Connectors

Power Efficiency

.PC Power Supplies: More Important Than You Think

(This document available on QUIA class page)

Active vs. Passive Cooling

Power Supply Connectors

Modular Cables and Connectors

High End Supply Example – Gigabyte Odin GT Series

Software Control

Modular Cable Management

Online

What are amps, watts, volts and ohms?

ATX12V vs. ATX Power Supplies

Computer Power Supply (Wikipedia)

Homework

Power Supply Online Quiz

Specify Power Supply on Wish List

PC Power Supplies: More Important Than You Think (Online Document)

What are amps, watts, volts and ohms?

The three most basic units in electricity are voltage (V), current (I, uppercase "i") and resistance (r). Voltage is measured in volts, current is measured in amps and resistance is measured in ohms.

A neat analogy to help understand these terms is a system of plumbing pipes. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size.

There is a basic equation in electrical engineering that states how the three terms relate. It says that the current is equal to the voltage divided by the resistance.

I = V/r

Let's see how this relation applies to the plumbing system. Let's say you have a tank of pressurized water connected to a hose that you are using to water the garden.

What happens if you increase the pressure in the tank? You probably can guess that this makes more water come out of the hose. The same is true of an electrical system: Increasing the voltage will make more current flow.

Let's say you increase the diameter of the hose and all of the fittings to the tank. You probably guessed that this also makes more water come out of the hose. This is like decreasing the resistance in an electrical system, which increases the current flow.

Electrical power is measured in watts. In an electrical system power (P) is equal to the voltage multiplied by the current.

P = VI

The water analogy still applies. Take a hose and point it at a waterwheel like the ones that were used to turn grinding stones in watermills. You can increase the power generated by the waterwheel in two ways. If you increase the pressure of the water coming out of the hose, it hits the waterwheel with a lot more force and the wheel turns faster, generating more power. If you increase the flow rate, the waterwheel turns faster because of the weight of the extra water hitting it.

Electrical Efficiency

In an electrical system, increasing either the current or the voltage will result in higher power. Let's say you have a system with a 6-volt light bulb hooked up to a 6-volt battery. The power output of the light bulb is 100 watts. Using the equation above, we can calculate how much current in amps would be required to get 100 watts out of this 6-volt bulb.

You know that P = 100 W, and V = 6 V. So you can rearrange the equation to solve

for I and substitute in the numbers.

I = P/V = 100 W / 6 V = 16.66 amps

What would happen if you use a 12-volt battery and a 12-volt light bulb to get 100 watts of power?

100 W / 12 V = 8.33 amps

So this system produces the same power, but with half the current. There is an advantage that comes from using less current to make the same amount of power. The resistance in electrical wires consumes power, and the power consumed increases as the current going through the wires increases. You can see how this happens by doing a little rearranging of the two equations. What you need is an equation for power in terms of resistance and current. Let's rearrange the first equation:

I = V / R can be restated as V = I R

Now you can substitute the equation for V into the other equation:

P = V I substituting for V we get P = IR I, or P = I2R

What this equation tells you is that the power consumed by the wires increases if the resistance of the wires increases (for instance, if the wires get smaller or are made of a less conductive material). But it increases dramatically if the current going through the wires increases. So using a higher voltage to reduce the current can make electrical systems more efficient. The efficiency of electric motors also improves at higher voltages.

ATX12V vs. ATX Power Supplies

A Look at the Differences in Power Specifications By Mark Kyrnin,

Introduction Over the years, the base components of computer systems have dramatically changed. In order to standardize the design of the system, specifications standards were developed for desktop computers that define the various dimensions, layouts and electrical requirements so that parts could be easily changed between vendors and systems. Since all computer system require electrical power that is converted from high voltage wall outlets to the low voltage currents used by the components, power supplies have very clear specifications.

AT, ATX, ATX12V? Desktop design specifications have been given a variety of names of the years. The original Advanced Technology or AT design was developed in the early PC years with the IBM compatible systems. As the power requirements and layouts changed, the industry developed a new definition called Advanced Technology Extended or ATX. This specification has been used for many years. In fact it has undergone a large number of revisions through the years to deal with various power changes. Now a new format has been developed over the years called ATX12V. This standard is officially known as ATX v2.0 and above.

The primary differences with the latest ATX v2.2 and ATX v1.3 are:

• Use of a 24-pin Main Power Connector over 20-pin Connector for PCI Express Support

• 6-Pin Aux Power Connector Not Required

• Use of Dual 12V Rails if Greater than 18A

• Serial ATA Power Connectors Required

• Minimum Power Conversion Efficiency

24-Pin Main Power This is the most notable change for the ATX12V standard. PCI Express requires a 75 watt power requirement that was not capable with the older 20-pin connector. To handle this, 4 additional pins were added to the connector to supply the addition power through 12V rails. Now the pin layout is keyed such that the 24-pin power connector can actually be used on older ATX motherboards with the 20-pin connector. The caveat is that the 4 extra pins will reside off to the side of the power connector on the motherboard so be sure there is enough clearance for the extra pins to use on such a board.

Dual 12V Rails As the power demands of the processors, drives and fans keeps growing on the system, the amount of power supplied over the 12V rails from the power supply has also grown. At higher amperage levels though, the ability of the power supply to generate a stable voltage was more difficult. In order to address this, the standard now requires any power supply that produces more than 18A of power for the 12V rail to be split into two separate 12V rails to increase stability. Some high wattage power supplies even have three independent 12V rails for increased stability.

Serial ATA Connectors Even through Serial ATA connectors could be found on many ATX v1.3 power supplies, they were not a requirement. With the rapid adoption of SATA drives, the need for the connectors on all new power supplies forced the standard to require a minimum number of connectors on the power supplies. Older ATX v1.3 units typically only provided two while newer ATX v2.0+ units supply four.

Power Efficiency When the current is converted from the wall outlet voltage to the lower voltage levels needed for the computer components, there is bound to be some waste that is transferred into heat. So, even though the power supply may provide 500W of power, it is actually pulling more current from the wall than this. The power efficiency rating determines how much power is pulled from the wall compared to the output to the computer. The newer standards require a minimum efficiency rating of 70% full power efficiency and a recommended rating of 80%. Very few of the latest power supplies are able to reach the greater than 80% efficiency rating at all wattage levels.

Conclusions When buying a power supply, it is important to buy one that meets all of the power specifications for the computer system. In general, the ATX standards are developed to be backwards compatible with older system. As a result, when shopping for a power supply, it is best to purchase one that is at least ATX v2.01 compliant or higher. These power supplies will still function with older ATX systems using the 20-pin main power connector.

Computer power supply

From Wikipedia, the free encyclopedia

A computer power supply unit (Computer PSU), or Modular Power Supply Unit (MPS) is the component that supplies power to a computer. More specifically, a power supply is typically designed to convert 100-120 V (North America and Japan) or 220-240 V (Europe, Asia and Australia) AC power from the mains to usable low-voltage DC power for the internal components of the computer. Some power supplies have a switch to change between 230V and 115V. Other models have automatic sensors that switch input voltage automatically, or are able to accept any voltage between those limits.

The most common computer power supplies are built to conform to the ATX form factor. The most recent specification of the ATX standard is version 2.2, released in 2004. This enables different power supplies to be interchangeable with different components inside the computer. ATX power supplies also are designed to turn on and off using a signal from the motherboard (PS-ON wire), and provide support for modern functions such as the standby mode available in many computers.

[pic]Wattage

Computer power supplies are rated for certain wattages based on their maximum output power. Typical rated wattages range from 200 W to 500 W, although units used by gamers and enthusiasts usually range from 500 W to 800 W, with the highest end units going up to 2 kW for extreme performance computers with multiple processors and graphics cards (ATI CrossFire or NVIDIA SLI).

Appearance

External

Most computer power supplies have the appearance of a square metal box, and have a large bundle of wires emerging from one end. A label on one side of the box lists technical information about the power supply, including maximum wattage.

Connectors

Typically, power supplies have the following connectors:

• PC Main power connector (usually called P1): Is the connector that goes to the motherboard to provide it with power. The connector has 20 or 24 pins. One of the pins belongs to the PS-ON wire mentioned above (it is usually green). This connector is the largest of all the connectors. In older AT power supplies, this connector was split in two: P8 and P9. If you have a power supply with 24-pin connector, you can plug it into a motherboard with a 20-pin connector. In cases where the motherboard has a 24-pin connector, some power supplies come with two connectors (one with 20-pin and other with 4-pin) which can be used together to form the 24-pin connector.

• 4-Pin Peripheral power connectors (usually called Molex for its manufacturer): These are the other, smaller connectors that go to the various disk drives of the computer. Most of them have four wires: two black, one red, and one yellow. Unlike the standard mains electrical wire color-coding, each black wire is a ground, the orange wire is +3.3 V, the red wire is +5 V, and the yellow wire is +12 V.

• 4-Pin Floppy drive power connectors (usually called Mini-connector): This is one of the smallest connectors that supplies the floppy drive with power. In some cases, it can be used as an auxiliary connector for AGP video cards. Its cable configuration is similar to the Peripheral connector.

• Auxiliary power connectors: There are several types of auxiliary connectors designed to provide additional power if it is needed.

• Serial ATA power connectors: a 15-pin connector for components which use SATA power plugs. This connector supplies power at three different voltages: +3.3, +5, and +12 volts.

• Most modern computer power supplies include 6-pin connectors which are generally used for PCI Express graphics cards, but a newly introduced 8-pin connector should be seen on the latest model power supplies. Each PCI Express 6-pin connector can output a maximum of 75 W.

Internal

Inside the computer power supply is a complex arrangement of electrical components, including diodes, capacitors and transformers. Also, most computer power supplies have metal heat sinks and fans to dissipate the heat produced. The speed of the fan is often dependent on the temperature, or less often the power load. It may be dangerous to open a power supply even if it is not connected to an electrical outlet, as high voltages may still be present in charged capacitors. However, for most PSU's this can be fixed by unplugging the PSU and then pressing the power-on button, which will drain the capacitors. Still, care should be taken as some PSU's require a load on the output in order to discharge the capacitors fully. Even when the PC is turned off, a PSU will draw some power from the wall, most of it going to power the 5Vsb (standby) rail.

AT vs. ATX

There are two basic differences between old AT and newer ATX power supplies:

• The PC main connectors (see above description of connectors).

• The soft switch. On older AT power supplies, the Power-on switch wire from the front of the computer is connected directly to the power supply. On newer ATX power supplies, the switch goes to the motherboard, allowing it to control the turning off of the system via a message from the operating system.

Laptops

Most portable computers have power supplies that provide 15 to 100 watts. In portable computers (such as laptops) there is usually an external power brick which converts AC power to one DC voltage (most commonly 19v), and further DC-DC conversion occurs within the laptop to supply the various DC voltages required by the other components of the portable computer.

Energy efficiency

Computer power supplies are generally about 70–75% efficient; to produce 75W of DC output they require 100W of AC input and dissipate the remaining 25W in heat. Higher-quality power supplies can be over 80% efficient; higher energy efficiency uses less power directly, and requires less power to cool as well. As of 2007, 93%-efficient power supplies are available. Resonant-transition or quasi-resonant switching regulators could achieve over 90% energy efficiency, and also reduce radio frequency interference.

It's important to match the capacity of a power supply to the power needs of the computer. The energy efficiency of power supplies drops significantly at low loads. Efficiency generally peaks at about 50-75% load. One rule of thumb is that a power supply which is over twice the required size will be much less efficient, and waste electricity.

Small facts to consider

• Common certification marks for safety are the UL mark, GS mark, TÜV, NEMKO, SEMKO, DEMKO, FIMKO, CCC, CSA, GOST R and BSMI. Common certificate marks for EMI/RFI are the CE mark, FCC and C-tick. The CE mark is required for power supplies sold in Europe.

• Life span may be measured in MTBF and should be at least 100,000 hours. Higher MTBF ratings are preferable for longer device life and reliability. Quality construction consisting of industrial grade electrical components and/or a higher speed fan can help to contribute to a higher MTBF rating by keeping critical components cool thus preventing the unit from overheating. Overheating is a major cause of PSU failure.

• There usually is a sticker on the PSU with a list of certifications, the specification, and a warning not to open the enclosure.

• In computer power supplies that have more than one 12V power rail, it is preferable for stability reasons to spread the power load over the 12V rails evenly to help avoid overloading one of the rails on the power supply.

o Multiple 12V power supply rails are separately current limited as a safety feature; they are not generated separately. Despite widespread belief to the contrary, this separation has no effect on mutual interference between supply rails.

o The ATX12V 2.x and EPS12V power supply standards defer to the IEC 60950 standard, which requires that no more than 240 VA be present between any two accessible points. Thus, each wire must be current-limited to no more than 20 A; typical supplies guarantee 18 A without triggering the current limit. Power supplies capable of delivering more than 18A at 12V connect wires in groups to two or more current sensors which will shut down the supply if excess current flows. Unlike a fuse or circuit breaker, these limits reset as soon as the overload is removed.

o Because of the above standards, almost all high-power supplies claim to implement separate rails, however this claim is often false; many omit the necessary current-limit circuitry, both for cost reasons and because it is an irritation to customers. (The lack is sometimes advertised as a feature under names like "rail fusion" or "current sharing".)

• When the computer is powered down but the power supply is still on, it can be started remotely via Wake-on-LAN and Wake-on-Ring or locally via Keyboard Power ON (KBPO) if the motherboard supports it.

• Most computer power supplies have short circuit protection, overpower (overload) protection, overvoltage protection, undervoltage protection, overcurrent protection, and over temperature protection.

• Some power supplies come with sleeved cables, which is aesthetically nicer, makes wiring easier and cleaner and have less effect on airflow.

• There is a popular misconception that a greater power capacity (watt output capacity) is always better. If all else is equal, this is true, but since supplies are self-certified, a manufacturer's claims may be double or more what is actually provided. Although a too-large power supply will have an extra margin of safety, if it is over twice the needed size, it will be less efficient, and waste electricity. Also, computer power supplies generally do not function properly if they are too lightly loaded. Under no-load conditions they may shut down or malfunction.

• Power supplies do not always live up to what they are marketed. Noise can be measured from different distances and at different room temperatures.

False Advertising

The DIY boom has led to power supply manufacturers marketing their products directly to end users, often with grossly inflated specs. Some of the main tricks employed are...

• advertising the peak wattage, rather than the continuous wattage

• determining the wattage at unrealistically low temperatures (below 40C)

• advertising total wattage as a measure of capacity, when modern systems are almost totally reliant on the number of amps on the 12 volt line(s)

So if...

• PSU A - has a peak rating of 500 watts at 25C, with 25 amps on the 12 volt line

• PSU B - has a continuous rating of 500 watts at 40C, with 33 amps on the 12 volt line

..and those ratings are accurate, then PSU B would have to be considered a vastly superior unit, while PSU A may only be capable of delivering a fraction of its rated wattage under real world conditions.

Modular power supply

A modular power supply is a relatively new approach to cabling, allowing users to omit unused cables. Whereas a conventional design has a continuous cable with two connection points (pcb-to-device), a modular design has two cables, adding another connection point (pcb-to-modular plug-to-device). The main benefits of a modular design, reduced clutter and improved airflow, come at the expense of added resistance, which can reduce power delivery and increase failure rates.

Troubleshooting

A power supply is tasked with providing electricity to every component in a computer, so a faulty or under performing unit will often cause a wide range of symptoms, including, but not limited to:

• Failure to power on.

• General instability.

• Rebooting, either randomly or when the computer is stressed.

• Hardware failure, due to sudden power spikes/dips or long term electronic noise.

• Bad or leaky capacitors. See capacitor plague.

• Noisy fan

Most desktop computer power supplies are equipped with an exhaust fan, which helps to keep internal components cool and operate more efficiently. Abnormal fan noise is generally caused by a lack of internal lubrication or failing motor. While it's relatively easy and inexpensive to lubricate/replace a fan, opening a power supply can be very dangerous and usually voids the warranty, so it's best left to a professional.

PC Power Supplies: More Important than You Think

Marcel Binder August 9, 2007 12:04

Sensible Or Stupid? High-End PSU Buyers

Computers keep increasing their capabilities and their performance. These characteristics not only contribute to increases in their purchase costs, but also their costs of operation, particularly when it comes to power. Although AMD and Intel have curbed their high-flying ways - where CPUs with total power levels of up to 130 Watts were tamed using SpeedStep or Cool'n'Quiet - ATI/AMD and Nvidia's graphics cards continue to consume stratospheric amounts of wattage. As we reported in our German-language coverage Power-hungry Graphics Cards, power consumption levels at or over 200 Watts are not unusual. In dual-card configurations built around SLI or Crossfire technologies, graphics processing can add 500 watts or more to a system's total power consumption.

Such massive needs for power must be satisfied, and power supply manufacturers have reacted to meet them. At this year's Computex Taipei, numerous vendors introduced power supplies - also known as power supply units (PSUs) - rated as high as 2000 W. Gigabyte is one vendor that serves many global markets, and is perhaps best known for its motherboards and graphics cards. At that show, it introduced a new family of power supplies named Odin, after the one-eyed chief of the Norse pantheon, with capacities rated at 550, 680 and 800 Watts.

Power users and case modders alike have quested after the perfect PSU for some time now, driven as much by needs for high-end components as aesthetics and "bling". Thanks to the continuing debate on global climate change, this quest has begun to register for both OEM PC vendors and normal PC users as well. The following questions remain to be answered, however: "Are such monster power supplies really important?" and "Who really needs them, anyway?"

From Necessary Evil To High-Tech Product

For years, power supplies have barely been discussed in the ongoing dialog on computing technologies. People tend to think first about motherboards, processors, hard disks and RAM. The PSU has traditionally been simply necessary but unappreciated component in a system build, usually included as an afterthought if not thrown in as part of a case purchase. These days, however, the PSU is an extremely important PC component, subject to the same kinds of strict specifications and requirements as a motherboard.

You can read about PSU specifications in a document entitled "ATX12V Power Supply Design Guide" at the Web site. Over time, this design guide has continued to track the latest developments in power conservation and delivery technology. This guide was most recently updated in March, 2005, as version 2.2. These latest specifications are comprehensive, in that they encompass not only the form factor and the dimensions for a typical 12-Volt power supply, but also state operating voltages and tolerances, and where cooling fans must be positioned to provide proper ventilation for such devices. These same specifications also require that the voltage sources that a PSU delivers be managed independently of one another. Such individual voltage sources are called rails in PSU-speak.

In certain technical dimensions, the ATX12V specifications are so thorough that the only decisions a power supply vendor has to make deal with which requirements a power supply actually meets. That said, the specifications shouldn't be interpreted as being an instrument for quality control. As always, the quality of a PSU depends on the engineering that the vendor puts into its design, and into the components that go into the device they build to implement that design.

Be Cautious About Fanless PSUs

The power consumption of a computer is as idiosyncratic as the user it serves. To meet all the various needs for energy consumption that a multitude of users presents, vendors typically offer PSUs rated at multiple different wattages. This is an area where vendors have a pretty free hand to anticipate and meet user demands. Most PSU models start at ratings of 300 Watts. For a long time, higher wattages came at increments of 50 watts to deliver higher levels of power. When ratings climb above 500 watts, though, the increments between models also tend to increase as well.

Other technical areas of freedom in PSU design permit the creation of actively and passively cooled models, whose selection and use depend on how a PC will be used. An actively cooled PSU includes at least one fan, and contributes to the ventilation of the entire computer case, as well as handling its own internal cooling needs. This situation plays a role in the evolution of the ATX standards, which requires the PSU to contribute to the overall ventilation of the PC's case. Older power supplies typically include fans that are no more than 80 mm in diameter; they're designed to suck warm air out of the case and blow it out of the back. Today, more and more PSUs include heftier 120 mm fans. Because of the increased diameter, more air gets moved at the same RPMs. Larger PSU fans can also run at lower rotational speeds to produce less noise. Most 120 mm fans are mounted at the bottom center of a PSU, whereas 80 mm models are mounted at the rear, which puts them right at the back of the PC case as well - a longer path from fan to outlet also means less audible noise from that outlet.

Ironically, those who decide to use passively-cooled PSUs in an ATX case must usually install a system fan somewhere else in their PCs. Otherwise, they risk overheating key components to the point where they might be damaged or destroyed.

In some situations, noise output from a PC is a critical factor. For example, Media Center PCs or Home Theater PCs usually reside in a living or family room, where their primary function is entertainment. These kinds of PCs are often equipped with passively-cooled power supplies to keep noise levels to an absolute minimum. If these PCs also omit active cooling entirely, the remaining system components must be carefully chosen to produce less heat than typical desktop system components, to avoid potential overheating problems.

Selection Of PC Components

At some level, the components chosen for specific PCs follow from standard interfaces, which gradually evolve over time to accommodate ever-changing technologies. Thus, in February 2003, the primary power connector for PC motherboards was extended by 4 pins, from 20 to 24. This was necessary to provide sufficient power for PCIe graphics cards, which can draw as much as 75 W through the motherboard. In addition, more powerful PCIe graphics cards can draw still more power directly from the power supply through a secondary 6-pin cable. An everyday example occurs in high-end graphics cards such as the Nvidia 8800 or ATI/AMD 2900 series.

Thanks to the proliferation of Serial ATA (SATA) hard disks, the number of Molex connectors in modern PSUs has been reduced. These connectors are normally designated as part number 0015244048, but are also identified in the ATX12V Power Design Guide as 8981-04P. These wide, four-pin Molex connectors do continue to still be used to deliver power to UltraATA hard disks and other drives as well (primarily, DVD and CD burners or players).

Modular Cables And Connectors

Another PSU trend is to improve cable management, in an attempt to undo the rat's nest of cables that so often unwinds inside PCs. Cheaper PSUs hard-wire the necessary cable bundles directly to their internal components. This leads to the unfortunate consequence that even unused cables must be accommodated inside the PC enclosure - in a worst case situation, this can interfere with air circulation.

For a few dollars more, you can purchase a power supply with a reduced cable bundle that services only the most important components. Additional cables may be plugged into modular outlets as needed. This not only improves air circulation, it also makes the PC's innards much tidier and easier on the eyes.

Where Power Supplies Fail...

Sometimes the specifications are at odds with vendor technical preferences. This explains the situation typical in all cheap PSUs - whose production is more about quantity than quality - that consume more energy than is really necessary, or that fail when used in certain system configurations. On the other hand, high-quality PSUs tend to function flawlessly, in our experience. This also reflects the last endurance testing (by our German staff from February 2007) in which five out of nine models rated at up to 550 W failed before the 24-hour testing period expired.

In part, some vendors tend to exaggerate PSU power outputs. This leads to official ratings that are simply unreachable in actual use. Alas, this can lead to underpowered system components, and contribute to intermittent instability problems. In addition, the components inside a PSU consume energy themselves, which can lead to greater or lesser power consumption and heat production, depending on the quality of those components. As with motherboards, defective condensers in a power supply can bring an entire system crashing down (for more details, see our article How to Fix Your Motherboard for $15).

Power Supply Rating Myths

In theory, the power that a power supply delivers can only be as great as the power the device itself consumes. In reality, this represents 100% efficiency, a performance level that power supplies can never attain. The transformation of 110 V or 220 V A/C power into various D/C voltage levels inside a PSU involves some waste, with the majority of such waste energy making itself felt in the form of heat produced inside the PC's case. This means that the rated wattage that a power supply can deliver must be strictly less than the energy it consumes before it starts the voltage transformation process.

By calculating the ratio between energy consumption and energy production, we produce a number somewhere between zero and one. Thus for example, net energy production of 450 W divided by gross consumption of 550 W at maximum load produces a value of 0.818. This number represents the efficiency of the power supply. Commonly this efficiency index is represented as a percentage value, which may be calculated by multiplying the previous ratio by 100, to produce in our example a value of 76.4%.

Vendor wattage ratings on PSUs always represent the maximum output that the device can deliver. A 350 W PSU with an efficiency rating of 70% must therefore consume a maximum of 500 W, though this occurs only when the components that the power supply drives actually consume the entire 350 watts. The real efficiency of a PSU is not a constant value either; rather, it changes with the amount of power that the device delivers at any given moment. The ATX12V Power Supply Design Guide requires that PSUs deliver minimum efficiency of 65% under light load, 72% under normal load, and 70% at peak load. There is also a recommended efficiency regimen that ups these levels to 75% for light loads, 80% for normal loads, and 77% at peak loads. Here, the term "load" must be understood as the power consumption of the system, as measured in amperes.

Why Are Efficiency Measures So Important?

Two years ago, we conducted a live stress test between an AMD and an Intel system, which measured the power consumption of both systems (among other things). At full or peak load, we measured average gross power consumption at 342 Watts on the Intel system, which included a dual core Intel Pentium Extreme Edition 840, a Gigabyte GA-8N-SLI Royal motherboard, OCZ DDR2 DIMMs, two GeForce 6800GT graphics cards in an SLI configuration, and two 160 GB 7,200 RPM Western Digital hard disks. Following the recommendations of the Power Supply Design Guide that the PSU be 77% efficient at peak load, the average power output from the device was around 263 Watts. One of the consequences of this test was that nearly 80 watts of energy was transformed into waste heat, adding significantly to operating costs for energy.

Let's restate this in plain terms: when it comes to paying for power, we have to cover the costs of gross consumption. In addition, we must also get rid of the waste heat that results from lower efficiency levels, which increases the need for cooling (itself an overhead energy consumer) and increases noise levels as fan speeds go up. If the waste heat isn't expelled from the PC case, this has a negative impact on PSU lifetime, because the lifetimes of its individual components sink as temperatures rise.

Now, let's look at our power supply selection from a different perspective. Power users are less interested in lower efficiency ratings at lower loads than they are in the PSU's ability to deliver sufficient power on demand. A 1,000 W PSU that is used to deliver only 200 W of actual power consumed works best to meet demand, though users must then pay for lower efficiency levels and rising costs of electricity.

With its Odin family of PSUs, Gigabyte seeks to deliver at least 80% efficiency over the entire load range. Over time, this means that when such a device is compared to the total cost of ownership for a cheaper power supply (purchase cost plus energy costs, in other words) it produces net savings. This helps to offset the higher costs of initial purchase, but only if the vendor ratings for the device are accurate. That's why we put the Odin PSUs to the test in our labs.

Proper Sizing For Power Supplies

The total wattage that a power supply must deliver depends on the actual components that go into any given computer. If you start with the requirement that each PCIe x16 slot be allotted up to 75 watts, and then add two or four PCI Express graphics cards into the equation, it's easy to understand how such a system simply wouldn't work at full load with a 300 watt power supply. It's also not hard to conceive that high-end CPUs will always require more power than mainstream models need.

Because determining the effective power consumption of a PC build involves more heavy lifting (and math) than most people are willing to undertake, the process of selecting the proper power supply for a PC should always derive from analysis of the worst case scenario. Most components in a PC operate at 12 Volts, so we simplify matters by assuming that the whole computer is serviced only with a 12 V rail, then use this to analyze the amperage ratings for power supplies.

Starting with the 263 watts that our test rig consumed and taking our assumption that this is delivered over a single 12 V rail into account, this leads to power levels of about 22 Amps (263 W / 12 V = 21,916 mA). Because delivering the total power budget for the system on a single 12 V rail is just a theoretical construct, and doesn't really happen in practice, our assumption leads us to the conclusion that a PSU that can deliver 22 amps on the 12 V rail will indeed suffice to meet the needs of our test system.

Combined Power

Those who take the time to read the ratings on a power supply carefully will immediately notice that they include numerous different values. Among them, 12 volt values appear multiple times, where wattage ratings apply only to the 3.3 V and 5 V rails. Voltages are divided among multiple rails, so we can infer the power levels that these rails carry as a fraction of the total power levels for the device. Moreover, the values for the 3.3 V and 5 V rails are typically presented as a single value, known as combined power. Those who have followed the fortunes of combined power values on power supplies for any length of time can't help but notice the trend toward increasing use of 12 V power in computers. Whereas older 300 W PSUs typically claimed combined power values of 180 to 190 W, current 300 W models are more likely to claim 120 W.

Functional Differences Among PSUs

By continuing to incorporate better-quality components and optimizing their designs, it's becoming hard to distinguish among power supply manufacturers on the basis of quality. These days, the overriding goal among vendors is to maximize efficiency, be it for 300 or 800 W units, even though it's understood that 100% efficiency remains unattainable, especially given today's technologies.

The number of players in the power supply game continues to shrink, so those vendors who remain in the game must continue to innovate and add new features to their products so as to stand out in the field. Gigabyte has added such a feature to its Odin GT series: these power supplies include a USB connection, in addition to their more conventional power cables and connectors. Once a user installs Gigabyte's P-Tuner software on a PC (which is included with the PSU), he or she can employ the USB connection to monitor the inner workings of the power supply, including peak power consumption levels and current wattage input and output values. This display also includes values for the 3.3 V, 5 V, and 12 V rails, and flashes an alarm if any of these slip outside required tolerance ranges.

Gibabyte Odin GT Series With Software Control

The P-Tuner software also enables its users to manage fan behavior in the power supply. Users can choose from among three profiles: performance mode, normal mode, and whisper mode. They can also manage fan voltage in conjunction with temperature values from one of four optional temperature sensors included with the unit, according to their own customized settings.

In addition, the P-Tuner software also permits users to set alarms based on device performance, voltage, current, fan speed and temperature.

Summary And Conclusions

The well-known market principle often stated as "faster, better, cheaper" also applies to power supplies, though added capacity often cancels out cost savings to deliver more functionality today for the same costs paid for less functionality yesterday, instead of driving absolute costs down. Small but potent PSUs can crank out up to two kilowatts of power nowadays. As a consequence, efficiency ratings for PSUs also continue to improve, so that we can use more of the juice we must pay for to actually get something done.

That's also why it's a good idea to draw up a general power budget for a PC build before purchasing a power supply, rather than relying on the sometimes misleading ratings that vendors assign to their PSUs. You can do this by adding up the total energy draw from each of your system's components. CPUs typically fall in a range of 35 to 130 watts, the motherboard from 25-50 watts sans RAM, drives usually fall between 15-20 Watts apiece, and graphics cards may require anywhere from 30 to 200 watts depending on the specific make and model in use. Add 30 percent to this total when you're finished just to be on the safe side. If you want to make room for future components or upgrades, bump this fudge factor even higher, but don't forget that power supplies tend to be somewhat less efficient as loads increase.

Heavy-duty power supplies get expensive pretty quickly, and in view of quad core CPUs that will impose widely different power draws depending on their energy saving regimes (a la SpeedStep and Cool'n'Quiet) switching individual cores on and off, we can only recommend extreme models when they are really necessary.

The big impetus for power supply makers going forward should not be to build ever-bigger and -beefier units, but rather to keep improving efficiency ratings. To be sure, power supplies rated at 600 watts and up have legitimate uses, but the total population of users who need that much power is miniscule comparison to the legions of average users. With a little knowledge and some considered calculations, savvy buyers can save money on both power supply purchase and operating costs.

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