Download Group 1

Power Supply
Unit
(PSU)
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.
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.
Parts of a PSU
Desktop PSU
 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.
Laptop PSU
 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.
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 4pin) which can be used together to form the 24pin connector.
 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.
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
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.
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.
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.
 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.
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.
Types of Direct 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.
Convert AC to DC Power
Step 1
 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.
Step 2
 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).
Step 3
 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.
Step 4
 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.
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.