So, you got a GeForce 8800 SLI configuration for this Christmas. Impressive! BUT, does your power supply support such energy-consuming monsters? This article is going to present the basics of computer power supply units and how they work.


The power-tamer

A computer power supply unit (PSU) is typically designed to convert 110 V or 230 V AC (alternative current) power from the mains to usable low-voltage DC (direct current) power for the internal hardware of the computer. The most common computer power supplies are built to conform to the ATX form factor (just like the motherboard one) and 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.

PSUs have a wattage rate based on their maximum output power. The usual rates range from 200 W to 500 W. However, specially designed PSUs for gamers and enthusiasts can reach maximum outputs of 800 W, with the highest end units going up to 1.1 kW. These really come in handy for nVidia quad SLI or ATI Crossfire graphics cards configurations.

The PSU usually comes with a bundle of wires in order to connect to the entire PC hardware. The most important connector is the one that supplies the motherboard with power, being the largest of all the connectors. This connector features a green PS-ON wire. The other connectors are smaller and usually pack only 4 wires: two black, one red and a yellow one. Unlike the standard electrical wire color-coding, each black wire is a ground, the red wire is +5 Volt , and the yellow wire is +12 V.

When opening up a PSU, one could find an intricate arrangement of diodes, capacitors, wires and transformers. In addition, all the actual power supplies have metal heat sinks and fans to dissipate the heat produced. The fans are temperature-sensitive, variating their speeds accordingly. Keep in mind that even when the PC is turned off, a PSU still draws power from the mains.


“Fueling” and comforting the beast

Power supplies are often referred to as "switching power supplies." The foundation of the PSU is represented by the switcher technology, which, as stated before, allows for conversions of the AC power to lower DC voltages. The typical voltages supplied are: 3.3 volts, 5 volts and 12 volts.

The switcher technology dates back to the 1980s, when power supplies were considerably larger and less efficient. Current switcher technology-based PSUs convert the 60 Hz electricity to a much higher frequency, and this translates into more cycles per second. This conversion enables a small, lightweight transformer in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer hardware component. Another important aspect of the PSU is the filtering process that takes place inside the transformer. The step-down electrical current can be rectified and filtered, reducing thus the variances in voltage for the sensitive electronic components in the computer.

Switcher technology is also responsible for the AC to DC conversion, as found in many of the automobile power inverters and in uninterruptible power supplies (UPS).

The last years saw the introduction of a new feature that helps the overall stability of the PC. This is called Advanced Power Management (APM) and offers a set of five different states in which the system can be found. Microsoft and Intel are the initiators of this addition targeted at PC users who wish to conserve power. All the latest system components, including the operating system, basic input/output system (BIOS), motherboard and attached devices have APM compatibilities. The APM support can be activated or deactivated from the BIOS. APM also has an improved version called ACPI (Advanced Configuration and Power Interface), which puts the system OS in charge of all power-related operations, rather than the BIOS.

One should take into consideration the fact that the PSU is probably the most failure-prone piece of hardware that can be found in a PC. It heats and cools each time it is used and receives the first in-rush of AC current when the PC is switched on. A typical failure of a PSU is often noticed as a burning smell just before the computer shuts down. Another problem could be the failure of the vital cooling fan (or fans, as new PSU models have two fans), laving all hardware components in the PSU to overheat. Failure symptoms include random rebooting or failure in Windows for no apparent reason.

In order to improve PSU management, recent motherboards and chipsets permit the user to monitor the revolutions per minute (RPM) of the power supply fan via BIOS and a Windows application supplied by the motherboard manufacturer. The actual monitoring is done by miniaturized detectors that are not always precise. New designs also offer fan control so that the fan only runs the speed needed, depending on cooling needs. These functions are offered via the ACPI interface.

The ACPI interface also controls how your computer is turned on and off. Thus, you turn the power with a little push button, but you usually turn off the machine with a menu option inside the OS. These capabilities were added to standard power supplies several years ago. The operating system can send a signal to the power supply to tell it to turn off. The push button sends a 5-volt signal to the power supply to tell it when to turn on. The power supply also has a circuit that supplies 5 volts, called VSB for "standby voltage" even when it is officially "off", so that the button will work.

Other recent designs include spare PSUs that can be exchanged while the other power supply is in use. Some new computers, usually those designed for use as servers, provide these redundant power supplies, meaning that there are two or more power supplies in the system, with one providing power and the other acting as a backup. The backup supply immediately takes over in the event of a failure by the primary supply. Then, the primary supply can be exchanged while the other power supply is in use.

Tired of that all-wirey PSU design that hinders the overall ventilation inside your PC’s case? There's a solution for you! Meet the modular power supply. Unlike regular PSUs, the modular one has cables that can be plugged into the unit as the user wishes. This helps eliminate unwanted clutter inside the computer. Most companies' modular PSU cables are not compatible with one another. The modular design is being embraced by more and more companies as it provides improved configurability.


PSU's in facts

Here is a series of facts that you should know about PSUs:

– Modern PSUs usually have an On/Off switch of their own on the back of the unit.

– Some power supplies have a passive PFC (power factor correction), some have active PFC. It is better to look for active PFC when buying a new PSU, because this feature offers improved stability and efficiency to the overall power transmission.

– High-end PSUs usually have about 80% energy efficiency and the higher energy efficiency, the better. A power supply with a higher energy efficiency will decrease the amount of energy lost to heat. There currently are power supplies with 85% efficiency.

– Life span - measured in MTBF (mean time before failure) should be at least 100,000 hours for today's PSUs. Higher MTBF ratings are preferable for longer device life.

– Some power supplies have a switch to change between 230v and 115v. Some high-end models have automatic sensors that switch input voltage automatically.

– It is advisable to connect the power supply through a UPS in order to further protect the PSU and the entire PC from power failures and energy surges.

– In PSUs that have more than one 12v power rail, it is preferable to evenly spread the power load over the 12v rails to help avoid overloading only one of the rails on the power supply.

– The most utilized connectors that come with a PSU are:
* a 20 or 24 pin main power connector which plugs into the motherboard.
* 4-pin Molex connectors which plug into peripherals such as hard drives and CD-ROM drives.
* 4-pin auxiliary connectors which can plug into the motherboard to provide additional power. This connector is commonly called the "P4" connector because it was originally used to provide more power to the Intel Pentium 4 based motherboards (and was later adopted by AMD).
* SATA power cables for hard disks which use SATA power plugs.
* at least one 6-pin connector which is generally used for PCI Express graphics cards. Each PCI Express connector can output a maximum of 75 W.

– When the computer is powered down and the power supply is not switched off from its own on/off button, the PC can be started remotely via Wake-on-LAN and Wake-on-Ring, or locally via Keyboard Power ON (KBPO). Again, these features are available through APM or ACPI interfaces.

In addition, here are some newly-introduced features with explanations:

– Short circuit protection (SCP) - is obligatory according to the ATX12V Power Supply Design Guide. This means that it is implemented in all power supplies, even those that don’t explicitly mention such protection.
– Overpower (overload) protection (OPP) – it is obligatory for today's standards and protects the power supply from overload on all of its outputs combined.
– Overcurrent protection (OCP) - protects the separate PSU outputs from overload (but not yet from short circuit). It is available on many, but not all, PSUs, and not for all of the outputs. This protection is not obligatory.
– Overtemperature protection (OTP) - This protects the PSU from overheat. It is not required and is not implemented often.
– Overvoltage protection (OVP) – it is an obligatory protection, but it’s only meant for critical failures. It works only when some output voltage shoots 20-25% above the nominal value. In other words, if your power supply yields 13V instead of 12V, you must replace it as soon as possible, but its protection is not required to react yet because it is designed for even more critical situations.
– Undervoltage protection (UVP) – there are too-low voltages too, but these cannot do much harm to your computer. However, they may cause failures in operation of the hard disks, for example. This protection works when a voltage bottoms out by 20-25%.
– Nylon sleeves - soft braided nylon tubes on the PSU’s output cables help lay them out neatly inside the system case. These are usually equipped on modular cables.

So, now you know the PSU basics and if you are hoping to lay your hands on an ATI Crossfire or nVIdia SLI configuration, you may also want to buy a new PSU. Yep, like that 1.1 kW one I mentioned.

Anyway, it's Christmas time and I wish you Happy Holidays with many, many presents and joyful moments!