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A power supply unit (PSU) converts mains AC to low-voltage regulated DC power for the internal components of a computer. Modern personal computers universally use switched-mode power supplies. Some power supplies have a manual switch for selecting input voltage, while others automatically adapt to the mains voltage.

Most modern desktop personal computer power supplies conform to the ATX specification, which includes form factor and voltage tolerances. While an ATX power supply is connected to the mains supply, it always provides a 5-volt standby (5VSB) power so that the standby functions on the computer and certain peripherals are powered. ATX power supplies are turned on and off by a signal from the motherboard. They also provide a signal to the motherboard to indicate when the DC voltages are in spec, so that the computer is able to safely power up and boot. The most recent ATX PSU standard is version 3.0 as of mid-2022.

The desktop computer power supply converts the alternating current (AC) from a wall socket of mains electricity to a low-voltage direct current (DC) to operate the motherboard, processor and peripheral devices. Several direct-current voltages are required, and they must be regulated with some accuracy to provide stable operation of the computer. A power supply rail or voltage rail refers to a single voltage provided by a PSU.

Some PSUs can also supply a standby voltage, so that most of the computer system can be powered off after preparing for hibernation or shutdown, and powered back on by an event. Standby power allows a computer to be started remotely via wake-on-LAN and Wake-on-ring or locally via Keyboard Power ON (KBPO) if the motherboard supports it. This standby voltage may be generated by a small linear power supply inside the unit or a switching power supply, sharing some components with the main unit to save cost and energy.

First-generation microcomputer and home computer power supply units used a heavy step-down transformer and a linear power supply, as used, in for example, the Commodore PET introduced in 1977. The Apple II, also introduced in 1977, was noted for its switched-mode power supply, which was lighter and smaller than an equivalent linear power supply would have been, and which had no cooling fan. The switched-mode supply uses a ferrite-cored high frequency transformer and power transistors that switch thousands of times per second. By adjusting the switching time of the transistor, the output voltage can be closely controlled without dissipating energy as heat in a linear regulator. The development of high-power and high-voltage transistors at economical prices made it practical to introduce switch mode supplies, that had been used in aerospace, mainframes, minicomputers and color television, into desktop personal computers. The Apple II design by Atari engineer Rod Holt was awarded a patent,

Computer power supplies may have short circuit protection, overpower (overload) protection, over-voltage protection, under-voltage protection, over-current protection, and over-temperature protection.

Power supplies designed for worldwide use were once equipped with an input voltage selector switch that allowed the user to configure the unit for use on local power grid. In the lower voltage range, around 115 V, this switch is turned on changing the power grid voltage rectifier into a voltage doubler in delon circuit design. As a result, the large primary filter capacitor behind that rectifier was split up into two capacitors wired in series, balanced with bleeder resistors and varistors that were necessary in the upper input voltage range, around 230 V. Connecting the unit configured for the lower range to a higher-voltage grid usually resulted in an immediate permanent damage. When the power-factor correction (PFC) was required, those filter capacitors were replaced with higher-capacity ones, together with a coil installed in series to delay the inrush current. This is the simple design of a passive PFC.

Active PFC is more complex and can achieve higher PF, up to 99%. The first active PFC circuits just delayed the inrush. Newer ones are working as an input and output condition-controlled step-up converter, supplying a single 400 V filter capacitor from a wide-range input source, usually between 80 and 240 V. Newer PFC circuits also replace the NTC-based inrush current limiter, which is an expensive part previously located next to the fuse.

The first IBM PC power supply unit (PSU) supplied two main voltages: +5 V and +12 V. It supplied two other voltages, −5 V and −12 V, but with limited amounts of power. Most microchips of the time operated on 5 V power. Of the 63.5 W these PSUs could deliver, most of it was on this +5 V rail.

The +12 V supply was used primarily to operate motors such as in disk drives and cooling fans. As more peripherals were added, more power was delivered on the 12 V rail. However, since most of the power is consumed by chips, the 5 V rail still delivered most of the power. The −12 V rail was used primarily to provide the negative supply voltage to the RS-232 serial ports. A −5 V rail was provided for peripherals on the ISA bus (such as soundcards), but was not used by any motherboard other than the original IBM PC motherboard.

An additional wire referred to as "Power Good" is used to prevent digital circuitry operation during the initial milliseconds of power supply turn-on, where output voltages and currents are rising but not yet sufficient or stable for proper device operation. Once the output power is ready to use, the Power Good signal tells the digital circuitry that it can begin to operate.

Original IBM power supplies for the PC (model 5150), XT and AT included a line-voltage power switch that extended through the side of the computer case. In a common variant found in tower cases, the line-voltage switch was connected to the power supply with a short cable, allowing it to be mounted apart from the power supply.

An early microcomputer power supply was either fully on or off, controlled by the mechanical line-voltage switch, and energy saving low-power idle modes were not a design consideration of early computer power supplies. These power supplies were generally not capable of power saving modes such as standby or "soft off", or scheduled turn-on power controls.

Due to the always-on design, in the event of a short circuit, either a fuse would blow, or a switched-mode supply would repeatedly cut the power, wait a brief period of time, and attempt to restart. For some power supplies the repeated restarting is audible as a quiet rapid chirping or ticking emitted from the device.

When Intel developed the ATX standard power supply connector (published in 1995), microchips operating on 3.3 V were becoming more popular, beginning with the Intel 80486DX4 microprocessor in 1994, and the ATX standard supplies three positive rails: +3.3 V, +5 V, and +12 V. Earlier computers requiring 3.3 V typically derived that from a simple but inefficient linear regulator connected to the +5 V rail.

The ATX connector provides multiple wires and power connections for the 3.3 V supply, because it is most sensitive to voltage drop in the supply connections. Another ATX addition was the +5 V SB (standby) rail for providing a small amount of standby power, even when the computer was nominally "off".

There are two basic differences between AT and ATX power supplies: the connectors that provide power to the motherboard, and the soft switch. In ATX-style systems, the front-panel power switch provides only a control signal to the power supply and does not switch the mains AC voltage. This low-voltage control allows other computer hardware or software to turn the system on and off.

Since ATX power supplies share both, the same width and height (150 × 86 mm (5.9 × 3.4 in)), and the same mounting layout (four screws arranged on the back side of the unit), with the preceding format, there"s no major physical difference preventing an AT case to accept an ATX PSU (or vice versa, if the case can host the power switch needed by an AT PSU), provided that the specific PSU is not too long for the specific case.

As transistors become smaller on chips, it becomes preferable to operate them on lower supply voltages, and the lowest supply voltage is often desired by the densest chip, the central processing unit. In order to supply large amounts of low-voltage power to the Pentium and subsequent microprocessors, a special power supply, the voltage regulator module began to be included on motherboards. Newer processors require up to 100 A at 2 V or less, which is impractical to deliver from off-board power supplies.

Initially, this was supplied by the main +5 V supply, but as power demands increased, the high currents required to supply sufficient power became problematic. To reduce the power losses in the 5 V supply, with the introduction of the Pentium 4 microprocessor, Intel changed the processor power supply to operate on +12 V, and added the separate four-pin P4 connector to the new ATX12V 1.0 standard to supply that power.

Modern high-powered graphics processing units do the same thing, resulting in most of the power requirement of a modern personal computer being on the +12 V rail. When high-powered GPUs were first introduced, typical ATX power supplies were "5 V-heavy", and could only supply 50–60% of their output in the form of 12 V power. Thus, GPU manufacturers, to ensure 200–250 W of 12 V power (peak load, CPU+GPU), recommended power supplies of 500–600 W or higher. More modern ATX power supplies can deliver almost all (typically 80–90%) of their total rated capacity in the form of +12 V power.

Because of this change, it is important to consider the +12 V supply capacity, rather than the overall power capacity, when using an older ATX power supply with a more recent computer.

Low-quality power supply manufacturers sometimes take advantage of this overspecification by assigning unrealistically high power supply ratings, knowing that very few customers fully understand power supply ratings.

+3.3 V and +5 V rail voltage supplies are rarely a limiting factor; generally, any supply with a sufficient +12 V rating will have adequate capacity at lower voltages. However, most hard drives or PCI cards will create a greater load on the +5 V rail.

Older CPUs and logic devices on the motherboard were designed for 5 V operating voltage. Power supplies for those computers regulate the 5 V output precisely, and supply the 12 V rail in a specified voltage window depending on the load ratio of both rails. The +12 V supply was used for computer fan motors, disk drive motors and serial interfaces (which also used the −12 V supply). A further use of the 12 V came with the sound cards, using linear chip audio power amplifiers, sometimes filtered by a 9 V linear regulator on the card to cut the noise of the motors.

Since certain 80386 variants, CPUs use lower operating voltages such as 3.3 or 3.45 V. Motherboards had linear voltage regulators, supplied by the 5 V rail. Jumpers or dip switches set the output voltages to the installed CPU"s specification. When newer CPUs required higher currents, switching mode voltage regulators like buck converters replaced linear regulators for efficiency.

Since the first revision of the ATX standard, PSUs were required to have a 3.3 V output voltage rail. Rarely, a linear regulator generated these 3.3 V, supplied from the 5 V and converting the product of voltage drop and current to heat. In the most common design this voltage is generated by shifting and transforming the pulses of the 5 V rail on an additional choke, causing the voltage to rise delayed and rectified separately into a dedicated 3.3 V railTL431,Zener diode. Later regulators managed all the 3.3, 5 and 12 V rails. Cutting the pulse by the voltage regulator the ratio of the 3.3 and 5 V is controlled. Some of these PSUs use two different chokes, feeding to the 3.3 V rail from the transformer to manage changing loads by pulse with ratio between the 3.3 and the 5 V outputs. In designs using identical chokes, the pulse width manages the ratio.

With the Pentium 4 and newer computer generations, the voltage for the CPU cores went below 2 V. Voltage drop on connectors forced the designers to place such buck converters next to the device. Higher maximum power consumption required the buck converters no longer fed from the 5 V and changed to a 12 V input, to decrease the current required from the power supply.

Entry-Level Power Supply Specification (EPS) is a power supply unit meant for high-power-consumption computers and entry-level servers. Developed by the Server System Infrastructure (SSI) forum, a group of companies including Intel, Dell, Hewlett-Packard and others, that works on server standards, the EPS form factor is a derivative of the ATX form factor. The latest specification is v2.93.

The EPS standard provides a more powerful and stable environment for critical server-based systems and applications. EPS power supplies have a 24-pin motherboard power connector and an eight-pin +12 V connector. The standard also specifies two additional four-pin 12 V connectors for more power-hungry boards (one required on 700–800 W PSUs, both required on 850 W+ PSUs). EPS power supplies are in principle compatible with standard ATX or ATX12V motherboards found in homes and offices but there may be mechanical issues where the 12 V connector and in the case of older boards connector overhang the sockets.

The rule was intended to set a safe limit on the current able to pass through any single output wire. A sufficiently large current can cause serious damage in the event of a short circuit, or can melt the wire or its insulation in the case of a fault, or potentially start a fire or damage other components. The rule limits each output to below 20 amps, with typical supplies guaranteeing 18 A availability. Power supplies capable of delivering more than 18 A at 12 V would provide their output in groups of cables (called "rails"). Each rail delivers up to a limited amount of current through one or more cables, and each rail is independently controlled by its own current sensor which shuts down the supply upon excess current. Unlike a fuse or circuit breaker, these limits reset as soon as the overload is removed. Typically, a power supply will guarantee at least 17 A at 12 V by having a current limit of 18.5 A ± 8%. Thus, it is guaranteed to supply at least 17 A, and guaranteed to cut off before 20 A. The current limits for each group of cables is then documented so the user can avoid placing too many high-current loads in the same group.

Originally at the time of ATX 2.0, a power supply featuring "multiple +12 V rails" implied one able to deliver more than 20 A of +12 V power, and was seen as a good thing. However, people found the need to balance loads across many +12 V rails inconvenient, especially as higher-end PSUs began to deliver far greater currents up to around 2000 W, or more than 150 A at 12 V (compared to the 240 or 500 W of earlier times). When the assignment of connectors to rails is done at manufacturing time it is not always possible to move a given load to a different rail or manage the allocation of current across devices.

Rather than add more current limit circuits, many manufacturers chose to ignore the requirement and increase the current limits above 20 A per rail, or provided "single-rail" power supplies that omit the current limit circuitry. (In some cases, in violation of their own advertising claims to include it.

The requirement was withdrawn as a result, however, the issue left its mark on PSU designs, which can be categorized into single rail and multiple rail designs. Both may (and often do) contain current limiting controllers. As of ATX 2.31, a single rail design"s output current can be drawn through any combination of output cables, and the management and safe allocation of that load is left for the user. A multiple rail design does the same, but limits the current supplied to each individual connector (or group of connectors), and the limits it imposes are the manufacturer"s choice rather than set by the ATX standard.

Since 2011, Fujitsu and other tier-1 manufacturersDC-to-DC conversion, providing 5 V and 3.3 V, is done on the motherboard; the proposal is that 5 V and 12 V supply for other devices, such as HDDs, will be picked up at the motherboard rather than from the PSU itself, although this does not appear to be fully implemented as of January 2012

The reasons given for this approach to power supply are that it eliminates cross-load problems, simplifies and reduces internal wiring that can affect airflow and cooling, reduces costs, increases power supply efficiency, and reduces noise by bringing the power supply fan speed under the control of the motherboard.

At least two of Dell"s business PCs introduced in 2013, the OptiPlex 9020 and Precision T1700, ship with 12 V–only power supplies and implement 5 V and 3.3 V conversion exclusively on the motherboard. Afterwards, Lenovo ThinkCentre M93P adopts 12 V–only PSU and performs 5 V and 3.3 V conversion exclusively on the IS8XM motherboard.

In 2019 Intel released a new standard based on an all-12V design: ATX12VO. The power supply only provides 12 V voltage output;USB, hard disk drive and other devices, are transformed on the motherboard; and the ATX motherboard connector is reduced from 24-pin to 10-pin. Called ATX12VO, it"s not expected to replace current standards but to exist alongside it.CES 2020, FSP Group showed the first prototype based on the new ATX12VO standard.

According to the Single Rail Power Supply ATX12VO design guide officially published by Intel in May 2020, the guide listed the details of 12V-only design and the major benefit which included higher efficiency and lower electrical interruption.

The overall power draw on a PSU is limited by the fact that all of the supply rails come through one transformer and any of its primary side circuitry, like switching components. Total power requirements for a personal computer may range from 250 W to more than 1000 W for a high-performance computer with multiple graphics cards. Personal computers without especially high performing CPUs or graphics cards usually require 300 to 500 W.system power consumption. This protects against system performance degradation, and against power supply overloading. Power supplies label their total power output, and label how this is determined by the electric current limits for each of the voltages supplied. Some power supplies have no-overload protection.

The system power consumption is a sum of the power ratings for all of the components of the computer system that draw on the power supply. Some graphics cards (especially multiple cards) and large groups of hard drives can place very heavy demands on the 12 V lines of the PSU, and for these loads, the PSU"s 12 V rating is crucial. The total 12 V rating on the power supply must be higher than the current required by such devices so that the PSU can fully serve the system when its other 12 V system components are taken into account. The manufacturers of these computer system components, especially graphics cards, tend to over-rate their power requirements, to minimize support issues due to too low of a power supply.

Various initiatives exist to improve the efficiency of computer power supplies. Climate Savers Computing Initiative promotes energy saving and reduction of greenhouse gas emissions by encouraging development and use of more efficient power supplies. 80 Plus certifies a variety of efficiency levels for power supplies and encourages their use via financial incentives. Efficient power supplies also save money by wasting less power; as a result, they use less electricity to power the same computer, and they emit less waste heat which results significant energy savings on central air conditioning in the summer. The gains of using an efficient power supply are more substantial in computers that use a lot of power.

Although a power supply with a larger than needed power rating will have an extra margin of safety against overloading, such a unit is often less efficient and wastes more electricity at lower loads than a more appropriately sized unit. For example, a 900-watt power supply with the 80 Plus Silver efficiency rating (which means that such a power supply is designed to be at least 85% efficient for loads above 180 W) may only be 73% efficient when the load is lower than 100 W, which is a typical idle power for a desktop computer. Thus, for a 100 W load, losses for this supply would be 27 W; if the same power supply was put under a 450 W load, for which the supply"s efficiency peaks at 89%, the loss would be only 56 W despite supplying 4.5 times the useful power.80 Plus Bronze efficiency rating (which means that such a power supply is designed to be at least 82% efficient for loads above 100 W) may provide an 84% efficiency for a 100 W load, wasting only 19 W.[1]

A power supply that is self-certified by its manufacturer may claim output ratings double or more than what is actually provided.at well below the total rating of the power supply. Many power supplies create their 3.3 V output by down-regulating their 5 V rail, or create 5 V output by down-regulating their 12 V rails. The two rails involved are labeled on the power supply with a combined current limit. For example, the 5 V and 3.3 V rails are rated with a combined total current limit. For a description of the potential problem, a 3.3 V rail may have a 10 A rating by itself (33 W), and the 5 V rail may have a 20 A rating (100 W) by itself, but the two together may only be able to output 110 W. In this case, loading the 3.3 V rail to maximum (33 W), would leave the 5 V rail only able to output 77 W.

As of 2012 some high-end consumer PSUs can exceed 90% efficiency at optimal load levels, though will fall to 87–89% efficiency during heavy or light loads. Google"s server power supplies are more than 90% efficient.HP"s server power supplies have reached 94% efficiency.

The energy efficiency of a power supply drops significantly at low loads. Therefore, it is important to match the capacity of a power supply to the power needs of the computer. Efficiency generally peaks at about 50–75% load. The curve varies from model to model (examples of how this curve looks can be seen on test reports of energy-efficient models found on the 80 Plus website Archived 2010-08-28 at the Wayback Machine).

Most desktop personal computer power supplies are a square metal box, and have a large bundle of wires emerging from one end. Opposite the wire bundle is the back face of the power supply, with an air vent and an IEC 60320 C14 connector to supply AC power. There may be a power switch and/or a voltage selector switch. Historically they were mounted on the upper part of the computer case, and had two fans: one, inside the case, pulling air towards the power supply, and another, extracting air from the power supply to the outside. Many power supplies have a single large fan inside the case, and are mounted on the bottom part of the case. The fan may be always on, or turn on and vary its speed depending on the load. Some have no fans, and so are cooled completely passively.

A label on one side of the box lists technical information about the power supply, including safety certifications and maximum output power. Common certification marks for safety are the UL mark, GS mark, TÜV, NEMKO, SEMKO, DEMKO, FIMKO, CCC, CSA, VDE, GOST R mark 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 and India. A RoHS or 80 Plus can also sometimes be seen.

Some power supplies come with sleeved cables, which besides being more aesthetically pleasing, also make wiring easier and have a less detrimental effect on airflow.

ATX motherboard power connector (usually called P1): This 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 (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. A power supply with a 24-pin connector can be used on 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, i.e. 20+4-pin form) which can be used together to form the 24-pin connector.

12V only power connector (labelled P1, though it is not compatible with the ATX 20 or 24 pin connector): This is a 10 or 16-pin Molex connector supplying the motherboard with three or six 12 V lines with common returns, a "supply OK" signal, a "PSU ON" signal and a 12 or 11 V auxiliary supply. One pin is left unused.

ATX12V 4-pin power connector (also called the P4 power connector). A second connector that goes to the motherboard (in addition to the 24-pin ATX motherboard connector) to supply dedicated power for the processor. 4+4-pin For the purpose of backwards compatibility, some connectors designed for high-end motherboards and processors, more power is required, therefore EPS12V has an 8-pin connector.

4-pin peripheral power connector 4-pin Peripheral power connectors: 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 US standard mains electrical wire color-coding, each black wire is a ground, the red wire is +5 V, and the yellow wire is +12 V. In some cases these are also used to provide additional power to PCI cards such as FireWire 800 cards.

4-pin Molex (Japan) Ltd power connectors (usually called Mini-connector, mini-Molex, or floppy drive with power. In some cases, it can be used as an auxiliary connector for Accelerated Graphics Port (AGP) video cards. Its cable configuration is similar to the Peripheral connector.

Auxiliary power connectors: There are several types of auxiliary connectors, usually in 6-pin form, designed to provide additional power if it is needed.

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

6+2-pin For the purpose of backwards compatibility, some connectors designed for use with high end PCI Express graphics cards feature this kind of pin configuration. It allows either a six-pin card or an eight-pin card to be connected by using two separate connection modules wired into the same sheath: one with six pins and another with two pins. Each PCI Express 8-pin connector can output a maximum of 150 W.

12-pin for PCI Express graphics cards, each PCI Express 12-pin connector can output a maximum of 648 W (12V, 9A), 2 150 W 8-pin can be combined via an adapter cable to form one 648 W 12-pin.

16-pin (12VHPWR) for PCI Express graphics cards, each PCI Express 16-pin connector can output a maximum of 662 W (12V, 9.2A), 12 power pins, 4 contact pins. Introduced on ATX 3.0.

A modular power supply provides a detachable cable system, offering the ability to remove unused connections at the expense of a small amount of extra electrical resistance introduced by the additional connector.EPS, though newer supplies marketed as "fully modular" allow even these to be disconnected. The pin assignment of the detachable cables is only standardized on the output end and not on the end that is to be connected to the power supply. Thus, the cables of a modular power supply must only be used with this particular modular power supply model. Usage with another modular power supply, even if the cable prima facie appear compatible, might result in a wrong pin assignment and thus can lead to damage of connected components by supplying 12V to a 5V or 3.3V pin.

The Small Form Factor with a 12 V connector (SFX12V) configuration has been optimized for small form factor (SFF) system layouts such as microATX. The low profile of the power supply fits easily into these systems.

The Thin Form Factor with a 12 V connector (TFX12V) configuration has been optimized for small and low profile Mini ITX and Mini DTX system layouts. The long narrow profile of the power supply fits easily into low profile systems. The cooling fan placement can be used to efficiently exhaust air from the processor and core area of the motherboard, making possible smaller, more efficient systems using common industry components.

Most portable computers have power supplies that provide 25 to 200 W. In portable computers (such as laptops) there is usually an external power supply (sometimes referred to as a "power brick" due to its similarity, in size, shape and weight, to a real brick) which converts AC power to one DC voltage (most commonly 19 V), and further DC-DC conversion occurs within the laptop to supply the various DC voltages required by the other components of the portable computer.

External power supply could send data about itself (power, current and voltage ratings) to the computer. For example, genuine Dell power source uses 1-Wire protocol to send data by third wire to the laptop. The laptop then refuses a non-matching adapter.

Life span is usually specified in mean time between failures (MTBF), where higher MTBF ratings indicate longer device life and better reliability. Using higher quality electrical components at less than their maximum ratings or providing better cooling can contribute to a higher MTBF rating because lower stress and lower operating temperatures decrease component failure rates.

Power supplies for servers, industrial control equipment, or other places where reliability is important may be hot swappable, and may incorporate N+1 redundancy and uninterruptible power supply; if N power supplies are required to meet the load requirement, one extra is installed to provide redundancy and allow for a faulty power supply to be replaced without downtimes.

24-pin ATX motherboard power plug; pins 11, 12, 23 and 24 form a detachable separate four-pin plug, making it backward compatible with 20-pin ATX receptacles

A "power supply tester" is a tool used to test the functionality of a computer"s power supply. Testers can confirm the presence of the correct voltages at each power supply connector. Testing under load is recommended for the most accurate readings.

The voltage of the PSU can be monitored by the system monitor of most modern motherboards.BIOS, or, once an operating system is running, through a system monitor software like lm_sensors on Linux, envstat on NetBSD, sysctl hw.sensors on OpenBSD and DragonFly BSD, or SpeedFan on Windows.

Most of power supply fans are not connected to the speed sensor on the motherboard and so cannot be monitored, but some high-end PSU can provide digital control and monitoring, and this requires connection to the fan-speed sensor or USB port on the motherboard.

Torres, Gabriel (2008-03-15). "How Much Power Can a Generic 500 W Power Supply Really Deliver?". Hardwaresecrets.com. Archived from the original on 2008-05-11. Retrieved 2009-03-28. Our generic 500 W power supply died when we tried pulling 275 W from it, so the maximum amount of power we could extract was 250 W – half the labeled amount!

Nathan Kirsch (2005-03-30). Skyhawk PSU ATX12V & EPS12V Compliance. Legit Reviews. Retrieved 2009-09-24. On the front of the box it says "Triple Rails for +12V" and then goes on to say "Intel ATX 12V Version 2.0 & EPS 12V Version 2.1". It turns out from our investigation that the above power supplies do not meet the ATX12V or EPS12V standards as the packaging claims.

"What is PSU Efficiency and Why is it Important? | Velocity Micro Blog". Custom Gaming & Enthusiast PC Blog | Velocity Micro. 2019-06-12. Retrieved 2020-11-01.

Oklahoma Wolf (September 14, 2007), The Bargain Basement Power Supply Rounup, jonnyGURU.com, archived from the original on July 23, 2009, retrieved 2008-01-31

Rutter, Daniel (2008-09-27). "Lemon-fresh power supplies". dansdata.com. Retrieved 2008-09-28. The lemon-market in PC power supplies has now officially become bad enough that no-name generic "500W" PSUs may actually barely even be able to deliver 250 watts. A realistic constant rating for these units is more like 200 watts. So the capacity inflation factor"s hit 2.5, and it"s still rising.

Murenin, Constantine A. (2007-04-17). Generalised Interfacing with Microprocessor System Hardware Monitors. Proceedings of 2007 IEEE International Conference on Networking, Sensing and Control, 15–17 April 2007. London, United Kingdom: IEEE. pp. 901–906. doi:10.1109/ICNSC.2007.372901. ISBN 978-1-4244-1076-7. IEEE ICNSC 2007, pp. 901—906.

Insert the TF Card to Raspberry Pi, connect the Raspberry Pi and LCD by HDMI cable; connect USB cable to one of the four USB ports of Raspberry Pi, and connect the other end of the USB cable to the USB port of the LCD; then supply power to Raspberry Pi; after that if the display and touch both are OK, it means drive successfully (please use the full 2A for power supply).

After execution, the driver will be installed. The system will automatically restart, and the display screen will rotate 90 degrees to display and touch normally.

（" XXX-show " can be changed to the corresponding driver, and " 90 " can be changed to 0, 90, 180 and 270, respectively representing rotation angles of 0 degrees, 90 degrees, 180 degrees, 270 degrees）

A dual monitor setup isn"t always plug-and-play. Extending your screen to a second or third monitor requires a suitable graphics card with sufficient ports. This is particularly challenging if you"d like to add more than one external screen to your laptop.

Before you start shoving wires into ports in the back of your PC or the sides of your laptop, stop and take a closer look at those ports so that you know what you"re looking at. Modern computers handle video via an HDMI port, but older computers feature a variety of other ports:

DisplayPort: A DisplayPort was originally designed as a superior video port to older VGA and DVI ports, but is far less popular on modern computer systems than the HDMI port.

DVI: The Digital Visual Interface was originally designed by the Digital Display Working Group, and was next-generation to the inferior VGA port. Many older computers have one of several configurations of the DVI port.

Thunderbolt: A Thunderbolt port is a combination of several technologies, including DisplayPort and PCI Express, in combination with a power supply to power the external display if required.

VGA: Older computers typically come with the famous blue VGA port. The 15-pin VGA has been used for computer video for many years, but has been largely replaced by the newer video ports like Thunderbolt and HDMI.

Typically, if your computer has two ports on it, the video card should be capable of sending an output signal to both. If there"s only one port, then it probably can"t. However, your desktop might have available slots for additional video cards. So, even if there"s only one port on your desktop, don"t hesitate to pop the cover and check for any available slots for another video card.

When it comes to laptops, another option is to purchase and use compatible docking stations. You may have a docking station option that could extend your laptop (which may only have a single video port), into a docking station that features two or more ports. So, even if you only have one video port available, definitely research the graphics cards if you want to set up a dual monitor laptop.

If you have two ports, but you can"t seem to get both ports to work at the same time, the next step is to determine if your video card even has the ability to display to multiple monitors.

In the display settings, you may actually see the multiple displays come up, with some disabled and others set up as either your primary display or as an extended display. You can ignore those for now and instead click on Advanced display settings at the bottom of this window.

On the next page, you should see Display information for each connected display if your graphics card "sees" the number of monitors that you"ve plugged in.

If you only see one, then you should quit here because even though you managed to connect multiple displays, the card is only able to utilize one at a time. Much of your success in this comes down to hardware capability, so checking this first is the most important thing.

However, if you"ve confirmed that your video card can "see" all displays that you"ve plugged in, yet you can"t seem to get additional monitors to work, don"t give up. There are a few more things you can try to fix the issue.

If you"re still not certain whether your computer can support dual monitors, another option is to research your graphics card. First, look up the brand of your graphics card.

Head to Google and research the brand of your graphics adapter, followed by the word "multi-display" or "multi-monitor." Hopefully, you"ll be able to find some evidence of whether your graphics card can handle multiple monitors.

If your laptop or PC has a DisplayPort, then you might be able to take advantage of what"s called multi-stream transport (MST) to daisy-chain two monitors off the single DisplayPort. These are the requirements:

Now, in the Display Settings screen, you should see both monitors detected. You can arrange them to either mirror or extend your desktop. There is also the option to split an HDMI signal to multiple monitors.

A docking station is a great way to add additional ports to your laptop, including ports for external monitors. The Anker PowerExpand Elite 13-In-1 Thunderbolt 3 Dock, for example, lets you hook up two 4K 60Hz monitors using its Thunderbolt 3 and HDMI ports. By using a splitter on your Windows laptop, you could even connect a total of three 4K monitors. Other laptop docking stations feature similar options.

Even if your PC or laptop graphics card only supports a single output port, you can still extend your system with a second display using a USB-powered monitor and DisplayLink software. Find an external display that"s USB-powered, plug it in, and install free DisplayLink software to make it all work.

Whichever approach you take to set up a dual monitor setup, you configure it all under Settings (press Windows + I) > System > Display. The example above is a laptop connected to an external monitor using the HDMI port on a USB-C dock.

To determine where your mouse can cross over from one monitor to the next, drag the squares representing your monitors around to rearrange them. Click Apply to save your changes.

To display different things on two or more displays, you need to extend your screen to the other monitors. Under Settings > System > Display, scroll down to Multiple displays and select Extend these displays.

If you want to close your laptop"s lid, but still use your external monitor, for example, to watch a movie, you have to change your laptop"s power options, which are hidden in the Windows 10 Control Panel.

If you want to use your external monitor, select Do nothing next to When I close the lid. Note that you can have different settings, depending on whether your laptop is running on battery or is plugged in.

As you can see, there are a lot of steps in the process where some people would give up and conclude that it just doesn"t work. If that"s still you, consider getting a larger monitor.

But really, it comes down to just making sure your video card is capable of doing this, and then playing around with those display settings and making sure that the monitors are configured correctly. If you have one DisplayPort, consider the daisy-chain option. And if all else fails, buy a USB monitor and install DisplayLink software.

If you travel with a laptop and iPad, you need this app. I needed a second screen, but Duet gives me even more. Full gesture support, customizable shortcuts, Touch Bar, tons of resolution options, and very little battery power. How is this all in one app?

Co-workers can’t believe I can share my desktop on my iPad and my iPhone. Look no further. This is a terrific addition to any office, remote or otherwise.

With the new Air feature that allows remote access to a laptop or desktop computer from anywhere in the world (with internet access), this app has become even more essential.

I just love this app. Especially when I am travelling for work an working from the company branches. Then I use my iPad as second monitor for Outlook, Lync and other chat while I use the laptop big screen for remote desktop to my workstation at the main office. :)

As head of an NGO, I travel a great deal to remote places around the world. It is very difficult to be productive, as power and internet availability are often a challenge. However when I am able to set up, Duet works like charm to improve productivity.

A power supply is an electrical device that converts the electric current that comes in from a power source, such as the power mains, to the voltage and current values necessary for powering a load, such as a motor or electronic device.

The objective of a power supply is to power the load with the proper voltage and current. The current must be supplied in a controlled manner — and with an accurate voltage — to a wide range of loads, sometimes simultaneously, all without letting changes in the input voltage or in other connected devices affect the output.

A power supply can be external, often seen in devices such as laptops and phone chargers, or internal, such as in larger devices such as desktop computers.

A power supply can either be regulated or unregulated. In a regulated power supply, the changes in the input voltage do not affect the output. On the other hand, in an unregulated power supply, the output depends on any changes in the input.

The one thing all power supplies have in common is that they take electric power from the source at the input, transform it in some way, and deliver it to the load at the output.

Direct current (DC) occurs when the current flows in one constant direction. It usually comes from batteries, solar cells, or from AC/DC converters. DC is the preferred type of power for electronic devices.

Alternating current (AC) occurs when the electric current periodically inverts its direction. AC is the method used to deliver electricity through power transmission lines to homes and businesses

Therefore, if AC is the type of power delivered to your house and DC is the type of power you need to charge your phone, you are going to need an AC/DC power supply in order to convert the AC voltage coming in from the power grid to the DC voltage needed to charge your mobile phone’s battery.

It can also be defined as the equivalent DC power needed to produce the same heating effect. Despite its complicated definition, it is widely used in electrical engineering because it allows you to find the effective value of an AC voltage or current. Because of this, it is sometimes expressed as VAC.

Phase: The angular difference between two waves. A complete cycle of a sine wave is divided into 360°, starting at 0°, having peaks at 90° (positive peak) and 270° (negative peak) and crossing the start point twice, at 180° and 360°. If two waves are plotted together, and one wave reaches its positive peak at the same time that the other reaches its negative peak, then, the first wave will be at 90°, while the second wave will be at 270°; this means the phase difference is 180°. These waves are considered to be in antiphase, as their values will always have opposite signs. If the phase difference is 0°, then we say the two waves are in phase.

Alternating current (AC) is the way electric power is transmitted from generating facilities to end users. It is used for power transportation because electricity needs to be transformed several times during the transportation process.

Electric generators produce voltages of about 40,000V, or 40kV. This voltage is then stepped up to anywhere between 150kV and 800kV, to reduce power losses when transporting electric current over long distances. Once it reaches its destination area, the voltage is stepped down to between 4kV and 35kV. Finally, before the current reaches individual users, it is reduced to 120V or 240V, depending on the location.

All these changes in voltage would be either complicated or very inefficient to do with direct current (DC), because linear transformers depend on voltage fluctuation to transfer and transform electrical energy, so they can only work with alternating current (AC).

By using a transformer, the alternating current (AC) input voltage is reduced to a value more suitable for the intended application. Then, the reduced AC voltage is rectified and turned into a direct current (DC) voltage, which is filtered in order to further improve the waveform quality (Figure 2).

Traditional linear AC/DC power supply design has evolved over the years, improving in terms of efficiency, power range, and size — but this design has some significant flaws that limit its integration.

A huge limitation in a linear AC/DC power supply is the size of the transformer. Because the input voltage is transformed at the input, the necessary transformer would have to be very large and therefore very heavy.

At low frequencies (e.g. 50Hz), large inductance values are necessary to transfer high amounts of power from the primary to secondary coil. This demands large transformer cores, which makes miniaturization of these power supplies practically impossible.

A linear AC/DC power supply uses linear regulators to maintain a constant voltage at the output. These linear regulators dissipate any extra energy in the form of heat. For low power, does not pose much of a problem. However, for high power, the heat that a regulator would have to dissipate to maintain a constant output voltage is very high, and would require adding extremely large heatsinks.

New design methodology has been developed to solve many of the problems associated with linear or traditional AC/DC power supply design, including transformer size and voltage regulation.

Switching power supplies are now possible thanks to the evolution of semiconductor technology, especially thanks to the creation of high-power MOSFET transistors, which can switch on and off very quickly and efficiently, even if large voltages and currents are present.

AC/DC power supplies that are designed using switching power converters are called switched-mode power supplies. AC/DC switched-mode power supplies have a slightly more complex method for converting AC power to DC.

In switching AC power supplies, the input voltage is no longer reduced; rather, it is rectified and filtered at the input. Then the DC voltage goes through a chopper, which converts the voltage into a high-frequency pulse train. Finally, the wave goes through another rectifier and filter, which converts it back to direct current (DC) and eliminates any remaining alternating current (AC) component that may be present before reaching the output (see Figure 3).

When operating at high frequencies, the transformer’s inductor is able to transfer more power without reaching saturation, which means the core can become smaller and smaller. Therefore, the transformer used in switching AC/DC power supplies to reduce the voltage amplitude to the intended value can be a fraction of the size of the transformer needed for a linear AC/DC power supply.

Switching AC/DC power converters can generate a significant amount of noise in the system, which must be treated to ensure it is not present at the output. This creates a need for more complex control circuitry, which in turn adds complexity to the design. Nevertheless, these filters are made up of components that can be easily integrated, so it does not affect the size of the power supply significantly.

Smaller transformers and increased voltage regulator efficiency in switching AC/DC power supplies are the reason why we can now convert a 220V¬RMS AC voltage to a 5V DC voltage with a power converter that can fit in the palm of your hand.

If unregulated, transformer losses are the only significant causes for efficiency loss. If regulated, high power applications will have a critical effect on efficiency.

Unregulated power supplies may have significant noise caused by the voltage ripple, but regulated linear AC DC power supplies can have extremely low noise. That is why they are used in medical sensing applications.

When transistors switch very quickly, they generate noise in the circuit. However, this can be either filtered out, or the switching frequency can be made extremely high, above the limit of human hearing, for audio applications

A three-phase power supply is composed of three conductors, called lines, which each carry an alternating current (AC) of the same frequency and voltage amplitude, but with a relative phase difference of 120°, or one-third of a cycle (see Figure 4). These systems are the most efficient at delivering large amounts of power, and are therefore used for delivering electricity from generating facilities to homes and businesses all around the world.

A single-phase power supply is the preferred method to supply current to individual homes or offices, so as to distribute the load evenly between lines. In this case, the current flows from the power line through the load, then back through the neutral wire. This is the type of supply found in most installations, except large industrial or commercial buildings. Single-phase systems cannot transfer as much power to loads and are more prone to power failures, but single-phase power also allows use of much simpler networks and devices.

There are two configurations for the transmission of power through a three-phase power supply: delta $(\Delta)$ and wye (Y) configurations, also referred to as triangle and star configurations, respectively.

Delta connections offer greater reliability, but Y connections can supply two different voltages: phase voltage, which is the single-phase voltage supplied to homes, and line voltage, for powering larger loads. The relationship between phase voltage (or phase current) and line voltage (or line current) in a Y configuration is that the line voltage (or current) amplitude is √3 times larger than the phase magnitude.

Because a standard power distribution system must supply power to both three-phase and single-phase systems, most power distribution networks have three lines and a neutral. This way, both homes and industrial machinery can be supplied with the same transmission line. Therefore, the Y configuration is the most commonly used for power distribution, whereas the delta configuration is typically used to power three-phase loads, such as large electric motors.

The voltage at which the power grid delivers single-phase electric power to its users has various values, depending on the geographical location. That is why it is very important to check a power supply’s input voltage range before buying or using it, to ensure that it is designed to work in your country’s power grid. Otherwise, you could damage the power supply or the device connected to it.

*Japan has two frequencies in its national grid because of the origins of its electrification in the late 19th century. In the western city of Osaka, electricity suppliers bought 60Hz generators from the United States, while in Tokyo, which is in the east of Japan, they bought 50Hz German generators. Both sides refused to change their frequency, and to this day Japan still has two frequencies: 50Hz in the east, 60Hz in the west.

As mentioned before, three-phase power is not only used for transportation, but is also used to power large loads, such as electric motors or charging large batteries. This is because the parallel application of power in three-phase systems can transfer much more energy to a load, and can do so more evenly, due to the overlapping of the three phases (see Figure 6).

Single-phase chargers are plugged into the alternating current (AC) mains and converted to direct current (DC) by the car’s internal AC/DC power converter (also called an on-board charger). These chargers, are limited in power by the grid and the AC socket.

The limitation varies from country to country, but is typically less than 7kW for a 32A socket (in EU, 220 x 32A = 7kW). On the other hand, three-phase power supplies convert the power from AC to DC externally, and can transfer over 120kW to the battery, enabling super-fast charging.

AC/DC power supplies are everywhere. The main job of an AC/DC power supply is to transform the alternating current (AC) into a stable direct current (DC) voltage, which can then be used to power different electrical devices.

Alternating current is used to transport electric power all across the electric grid, from generators to end users. An alternating current (AC) circuit can be configured as a single-phase or a three-phase system. Single-phase systems are simpler, and can deliver enough power to supply an entire house, but three-phase systems can deliver much more power in a more stable way, which is why they are frequently used to supply power for industrial applications.

Designing an efficient AC/DC power supply is no easy task, as current markets demand high-power, extremely efficient, minuscule power supplies that are capable of maintaining efficiency over a wide range of loads.

Methods for designing an AC/DC power supply have changed over time. Linear AC/DC power supplies are limited in size and efficiency, because they work at low frequencies and regulate the output temperature by dissipating the excess energy in the form of heat. By contrast, switching power supplies have become extremely popular, because they use switching regulators to convert AC to DC power. Switching power supplies work at higher frequencies and convert electrical power far more efficiently than previous designs, which has enabled the creation of palm-sized, high-power AC/DC power supplies.