 A power supply unit (PSU) is the component that supplies power to the other components in a computer. More specifically, a power supply unit is typically designed to convert general-purpose alternating current (AC) electric power from the mains
(100-127V in North America, parts of South America,
Japan, and Taiwan; 220-240V in most of the rest of the world) to usable low-voltage DC power for the internal components of the computer. Some power supplies have a switch to change between 230 V and 115 V.
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 .
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 , and provide support for modern functions such as the standby mode available in many computers. The most recent specification of the ATX standard
PSU as of mid-2008 is version 2.31.

 Standard power supplies turn the incoming 110V
or 220V AC (Alternating Current) into various DC
(Direct Current) voltages suitable for powering the computer's components.
Power supplies are quoted as having a certain power output specified in Watts, a standard power supply would typically be able to deliver around 350 Watts
 By using a PSU that delivers more power than required means it won't be running at full capacity, which can prolong life by reducing heat damage to the PSU's internal components during long periods of use.

 These are brand new and original boxed power supplies. It should be a a fairly simple job to change the supplied output connector to one which will suit your particular laptop or other power supply requirement, anyone with basic soldering skills should be able to do this. Input is via a standard IEC connector, and will accept any mains from around the world. The output is 16V at 1.5A continuous,
2.5A peak. No mains cable is supplied, you should be able to obtain this very cheaply locally.

 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
(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 4pin) which can be used together to form the 24pin connector.

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

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Unlike the 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 Berg power connectors (usually called
Mini-connector or "mini-Molex"): 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.

 6-pin 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.
 6+2 pin For the purpose of backwards compatibility, some connectors designed for use with PCI Express graphics cards feature this kind of pin configuration.

It allows either a 6-pin card or an 8-pin card to be connected by using two separate connection modules wired into the same sheath: one with 6 pins and another with 2 pins.
 A C14 IEC connector with an appropriate C13 cord is used to attach the power supply to the local power grid.

Motherboard Power Connectors
 One of the most important connections in the
PC is that between the power supply and the motherboard. It is through this connection (or set of connections) that the various voltages and other signals are sent between these two important devices. (You may want to familiarize yourself with these signals in the section on power supply functions if necessary.) Different form factors use different numbers, types, shapes and sizes of connectors between the power supply and motherboard.

 Before we look at the connectors, let's talk a bit about the wires that run between the power supply and the connectors themselves. Pretty much all wires within the PC are made from copper, due to its excellent conductivity, relative low expense, and flexibility. The most important characteristic of a wire is its size, and more specifically, its cross-sectional area. The reason is that the resistance of the wire is inversely proportional to the cross-sectional area of the wire

Thicker wires can carry more current, while the higher resistance of small wires causes heating when they are subjected to a high current, which can be hazardous. Since some wires need to carry more power than others, they are given different thicknesses. In addition, most motherboard connectors have multiple wires for the main voltage levels.
This allows for more current, spread out between the different wires.

In the electronics world one standard used for wire thicknesses is American
smaller the AWG number, the larger the wire. These numbers go from 0
(below 0 actually) to 50 and above, but for electronics the most common gauges are between 8 and 24.

For motherboard connectors the wires are usually AWG 16, 18, 20 or 22. The table below shows these four sizes and some relevant statistics. You'll notice that the numbers are not linear with the actual size of the wire; AWG
16 wire is almost four times the crosssectional area of AWG 22 wire.

 An alternating current (AC, also ac) the movement of electric charge periodically reverses direction. In
(DC), the flow of electric charge is only in one direction .

 AC is the form in which electric power is delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave . In certain applications, different waveforms are used, such as triangular or square waves . Audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated ) onto the
AC signal.

 Direct current (DC) is the unidirectional flow of electric charge . Direct current is produced by such sources as batteries , thermocouples , solar cells , and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through semiconductors , insulators , or even through a vacuum as in electron or ion beams .
The electric charge flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was
Galvanic current.

 Direct current may be obtained from an alternating current supply by use of a currentswitching arrangement called a rectifier , which contains electronic elements (usually) or electromechanical elements (historically) that allow current to flow only in one direction. Direct current may be made into alternating current with an inverter or a motor-generator set.

 The first commercial electric power transmission
(developed by Thomas Edison in the late nineteenth century) used direct current. Because of significant historical advantages of alternating current over direct current in transforming and transmission, electric power distribution was nearly all alternating current until a few years ago. In the mid 1950s, HVDC transmission was developed, which is now replacing the older high voltage alternating current systems. For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current.

 Direct current is used to charge batteries, and in nearly all electronic systems as the power supply. Very large quantities of direct-current power are used in production of aluminum and other electrochemical processes. Direct current is used for some railway propulsion, especially in urban areas. High voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.

 First decide what you need to power with DC voltage.
 Most electronics circuits or devices you purchase have voltage protection built into the circuit. If the circuit requires a 6VDC input, the acceptable range may actually be 5 to 8 volts DC. Check with the manufacturer's specifications for the input voltage.
If you are designing your own circuit and you want to save money and time by not including voltage protection on your board you will have to purchase a more expensive power supply to compensate.

 Determine the maximum load that your circuit will require to operate.
The power rating of the AC to DC power supply must exceed the maximum DC power consumption of the circuit. Calculate the total load (current) of the components on your circuit by determining the maximum load rating for each item such as motors, servos, resistors, lights, etc. If you are purchasing a premanufactured circuit or electronic device, the DC load in Amps will be identified. To calculate the current use Ohm's law I=V/R (where current equals voltage divided by resistance).

 Determine the type of power supply you want to use, first based on the physical type.
There are four main physical types of AC to DC power supplies. The physical types include individual circuit boards, brick-type switching power supplies, wall plugs, and power cords with an AC to DC adapter box (type used for laptop computers). If you want to keep the electronics in one box then use either circuit boards or bricktype switching power supplies. If you have a smaller electronic device or you need to keep the heat produced from the power supply away from the electronic components use either wall plug or a power cord with an AC to DC adapter box.
Select the physical type of power supply desired for your project, based on these criteria, and on price and availability.

 Select the power supply output type from the decision making from steps 1-4 and consider the overall power accuracy you need for your electronics device.
There are several different AC to DC power supply output types including unfiltered
(linear), filtered (linear), and regulated
(switching).

 Unfiltered power supplies are the least expensive but can also result in variable power output. Some circuits have regulation built into the design but others do not.
Filtered power supplies are better since they are designed to remove some of the high frequency noise from the power output. Both of these types of power supplies are linear and will have a voltage rating. Be careful because the voltage rating is fully loaded. If the circuit is not using the full current, then the voltage can increase to a much higher value.

 Switching power supplies are better since an
IC uses pulse width modulation to regulate the output voltage. Brick type switching power supplies are usually have even better output than wall plug and cord with transformer switching power supplies. The brick type switching power supplies will regulate the output voltage under varying loads very well.

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If your power supply does not provide enough current for the circuit, the electronics in your device can be damaged.
Working with electricity can be dangerous.
Take all necessary safety precautions when working with both AC and DC voltage.