Power Supply Unit (PSU)

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Power Supply Unit(PSU)
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Power Supply Unit

• 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.

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• 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.

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Parts of a PSU
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Desktop PSU
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• 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.

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Laptop PSU
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• 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.

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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
  (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)
  which can be used together to form the 24-pin
  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.

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• 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.

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• 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.
• 62 pin For the purpose of backwards
  compatibility, some connectors designed for use
  with PCI Express graphics cards feature this kind
  of pin configuration.

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• 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.

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• 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.

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• 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

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• 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.

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• In the electronics world one standard used for
  wire thicknesses is American Wire Gauge, or AWG
  for short. The 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.

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• 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 cross-sectional
  area of AWG 22 wire.

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AC Power (Alternating Current)

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

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• 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.

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DC Power (Direct Current)

• 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.

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Types of Direct Current

• Direct current may be obtained from an
  alternating current supply by use of a
  current-switching 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.

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• 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.

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• 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.

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Convert AC to DC Power
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Step 1
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• 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.

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Step 2
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• 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
  pre-manufactured circuit or electronic device,
  the DC load in Amps will be identified. To
  calculate the current use Ohm's law IV/R (where
  current equals voltage divided by resistance).

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Step 3
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• 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
  brick-type 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.

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Step 4
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• 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).

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• 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.

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• 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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Tips and Warnings

• 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.