A power supply (sometimes known as a power supply unit or PSU) is a device or system that supplies electrical or other types of energy to an output load or group of loads. The term is most commonly applied to electrical energy supplies.
The complete range of power supplies is very broad, and could be considered to include all forms of energy conversion from one form into another. Conventionally though, the term is usually confined to electrical or mechanical energy supplies. Constraints that commonly affect power supplies are the amount of power they can supply, how long they can supply it for without needing some kind of refueling or recharging, how stable their output voltage or current is under varying load conditions, and whether they provide continuous power or pulses.
The voltage regulation of power supplies is done by incorporating circuitry to tightly control the output voltage and/or current of the power supply to a specific value. The specific value is closely maintained despite variations in the load presented to the power supply's output, or any reasonable voltage variation at the power supply's input.
This term covers the mains power distribution system together with any other primary or secondary sources of energy such as:
Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company (see electricity generation) to a well-regulated lower voltage DC for electronic devices. For examples, see switched-mode power supply, linear regulator, rectifier and inverter (electrical).
Chemical fuel cells and other forms of energy storage systems
Generators or alternators (particularly useful in vehicles of all shapes and sizes, where the engine has rotational power to spare, or in semi-portable units containing an internal combustion engine and a generator) (For large-scale power supplies, see electricity generation.) Low voltage, low power DC power supply units are commonly integrated with the devices they supply, such as computers and household electronics.
The top cover has been removed to show the internals of a computer PSU. 
A computer power supply typically is designed to convert 110 V or 230 V AC power from the mains to usable low-voltage DC power for the internal components of the computer. The most common computer power supplies are built to conform with 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 (PS-ON wire), and provide support for modern functions such as the Standby mode of many computers.
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 1 kW.
Most computer power supplies have a large bundle of wires emerging from one end. One connector attached to the opposite end of some wires goes to the motherboard to provide power. The PS-ON wire is located in this connector, and usually green. The connector for the motherboard is the largest of all the connectors. There are also other, smaller connectors, most of which have four wires: two black, one red, and one yellow. Unlike the standard electrical wire color-coding, each black wire is a Ground, the red wire is +5 V, and the yellow wire is +12 V.
Inside the computer power supply is a complex arrangement of electrical components, including diodes, capacitors and transformers. Also, computer power supplies have metal heatsinks and fans to dissipate the heat produced. It is dangerous to open a power supply while it is connected to an electrical outlet as high voltages may be present. These may remain even while the unit is switched off, as there can be charge stored in the capacitors. However, this can be fixed by unplugging the PSU and then pressing the on button, which will drain the capacitors. Even when the PC is turned off, a PSU will draw power from the wall.
In desktop computers, the power supply is a box inside the computer; it is an important part of the computer because it provides electrical power in a form that is suitable for every other component inside or attached to the computer in order for it to work. In portable computers 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.
The original ATX main connector is a 20-pin connector. Most newer boards have a 24-pin connector; however many will function perfectly with only the original 20 pins connected. Similarly, most PSUs split the connector into 20+4, so that it will be compatible with both past and future motherboards.
A power supply (or in some cases just a transformer) that is built into the top of a plug is known as a wall wart, power brick, or just power adapter.
A simple AC powered linear power supply uses a transformer to convert the voltage from the wall outlet to a lower voltage. A diode circuit (generally either a single diode or an array of diodes called a diode bridge but other configurations are possible) then rectifies the AC voltage to pulsating DC. A capacitor smooths out most of the pulsating of the rectified waveform to give a DC voltage with some ripple. Finally depending on the requirements of the load a linear regulator may be used to reduce the voltage to the desired output voltage and remove the majority of the remaining ripple. It may also provide other features such as current limiting.
In a switched-mode power supply the incoming power is passed through a transistor and transformer network that switches on and off at typical rates of 10kHz to 1 MHz. This means that a smaller, less expensive, lighter transformer can be used, because the voltage is being made to alternate faster, and thus a smaller magnetic core can be used. The switch mode PSU is also significantly more efficient than the linear design. Typically 80 - 90% compared to 50 - 60%. The downside to this design is that the switching on and off at high speeds introduces electrical noise into the system which can cause interference with the power supply load or other connected systems.
Switching power supplies can be used as DC to DC converters. In this application, the power supply is designed to accept a limited range DC input and then output a different DC voltage. This is particularly useful in portable devices, as well as power distribution in large electronic equipment. A transformerless switching power supply that outputs a voltage higher than its input voltage is typically called a boost converter. A transformerless switching power supply that outputs a voltage lower than its input voltage is typically called a buck converter. These transformerless switching power supplies use an inductor as the primary circuit element in converting the voltage. Circuitry is used to pass current through the inductor to store a certain amount of electrical energy as a magnetic field. The current flow is then stopped, and the magnetic field collapses causing the stored energy to be released as current again. This is done rapidly (up to millions of times per second). By carefully metering the amount of energy stored in the inductor, the current released by the inductor can be regulated thus allowing the output voltage to be tightly regulated. A switching power supply incorporating a transformer can provide many output voltages simultaneously, and is typically called a flyback converter. Switching power supplies are typically very efficient if well designed, and therefore waste very little power as heat. Because of these efficiencies, they are typically much smaller and lighter than an equivalently rated linear supply. |