Transcript Power Supply

Power Supply
.
Introduction
• Power is supplied to your computer in two stages.
• First, power comes from ZESA to the wall, and through
the black power cord to the PC.
• Then, the internal power supply transforms this
standard household electricity into the form that the
PC needs to function.
• Electrical power is taken for granted by most as well as
the internal power supply, which is usually just
considered part of the case and given little attention.
• Please note:: The power supply is not part of the case
Introduction ctd.
The importance of the quality of power
supplied to the PC. This pie chart shows
the leading causes of data loss by
category.
Introduction ctd.
• If PC use is light, it is fine not to pay too much attention to power.
• However, as the old computer saying goes, "garbage in, garbage
out".
• If the motherboard and components are being supplied poorquality power, problems will arise that they wouldn't have if they
received proper, high-quality power.
• If PC use is heavy, or if data is important, or if upgrading is a future
option, attention must be paid to the quality of power!
• Power issues are responsible for more PC problems than probably
any other single source, even though most people don't realize that
the power is responsible.
• This slide takes a look at power, both internal and external to the PC
External Power
• The matter of power begins with a look at the power delivered to
the case.
• The power that comes into your home is normally quite reliable,
but it can be surprising how many quality problems it often has.
Spikes, surges, blackouts, brownouts, line noise—all are common
power problems that you don't generally notice.
• Usually, the electrical devices manage to deal with most of them
• The computer's power supply has some tolerance to these
problems as well;
• the more rigid the system, the more the power supply can tolerate
without failure.
• If the power supply is inexpensive, however then poor power can
lead directly to system troubles that manifest themselves in
unsuspecting ways
PSU Cost Vs. Other PC Components
Many users would pay a lot to
get a CPU 10% faster than
another one, (even though it
will make almost no
noticeable difference in
performance and will be
obsolete in months) but won't
pay half that for a quality
power supply that will last for
years and years
PC Problem Causes
These two pie charts show
vividly why there is need to pay
attention to power. Despite their
low cost compared to other PC
components, power and cooling
are responsible for a large
percentage of overall system
problems
Internal Power
• Despite its critical role, the power supply is one of
the most ignored and under-studied components
in the PC. In fact, some people don't even bother
to check out what power supply is included when
they purchase a case! Upon these faulty
foundations, important PC systems are built.
Don't let that happen to you.
• This section will help to understand the power
supply, what makes up the power supply, how it
works in detail and what to look for when
purchasing one
The Power Supply
• The internal power supply is responsible for
converting your standard power into a form that
the computer can use.
• The power supply is responsible for powering
every device in the computer;
• If it has a problem or is of low quality difficulties
may arise that you a user may not realize are
actually the fault of the electrical system.
• The power supply plays an important role in the
following areas of your system:
The PSU: Stability
• A high quality power supply with sufficient capacity to
meet the demands of the PC will provide years of
stable power
• A poor quality or overloaded power supply will cause
all sorts of problems that are particularly insidious,
because they occur in other, seemingly unrelated, parts
of the system.
• For example, power supplies can cause system crashes,
can make hard disks develop bad sectors, or cause
software bugs to appear, problems which can be very
difficult to trace back to the power supply.
The PSU: Cooling
• The power supply contains the main fan that
controls the flow of air through the PC case.
• This fan is obviously a major component in
your PC's cooling system.
The PSU: Energy Efficiency
• Newer PC power supplies work with your
computer's components and software to
reduce the amount of power they consume
when idle.
• This can lead to significant savings over older
systems.
The PSU: Expandability
• The capacity of your power supply capacity is
factor that will determine your ability to add new
drives or upgrade to a more powerful
motherboard or processor.
• Important to note that a high-speed Athlon CPU
and motherboard consume way more power
than a similar Pentium-based system, and the
power supply needs to be able to provide this
power.
• A power supply that barely meets the system’s
needs may have to be replaced on upgrading
Power Supply Functions and Signals
• The power supply's main function is to take the power
input to it from the power system of your home or
office, and turn it into a form that the PC can use.
• The PSU must provide several different voltages, at
different strengths, and must also manage some
additional signals that the motherboard uses.
• The following sections will discuss what the power
supply does, the voltages and secondary signals it
generates, power supply output and ratings and power
signal quality issues
AC-DC Voltage Conversion
• The electricity from Zesa is in the form of alternating current (AC), while
the electricity your PC requires is direct current (DC).
• The primary function of power supply is to convert AC power into a DC
form that the PC can use.
• In fact, while almost everything in your home runs off standard AC power,
many devices actually use DC internally. Examples of this are answering
machines, many types of audio equipment, battery chargers, certain types
of printers, external modems and other peripherals.
• Two indicators that a device actually uses DC inside are: the ability of the
device to run on batteries, and the presence of a device outside the unit
that powers it.
• These small adapters with one plug for the wall and another for the
device, often called AC adapters, are really DC power supplies that
converts the AC of the wall into DC for the device
• The difference between this sort of DC power supply and the PC PSU
though is the design.
AC-DC Voltage Conversion ctd.
AC adapters are linear power
supplies. These supplies are cheap
and simple to make. The main
problem with them is that they are
tremendously wasteful; typically,
50% or more of the energy
supplied to one of these adapters is
wasted as heat. A hot AC adapter
means electricity is being wasted.
This is an acceptable compromise
for small appliances, but
unacceptable for a PC power
supply
An AC adaptor (linear power
supply) for a battery charger
AC-DC Voltage Conversion ctd.
• Instead of the linear design, PCs use switching
power supplies.
• The switching power supply uses a transistor
switch and a closed feedback loop to produce
DC output that is properly regulated
regardless of the load on it, with only the
amount of AC power required to draw the DC
load being taken from the utility.
AC-DC Voltage Conversion ctd.
Simplified block diagram of one design for a switching power supply. Note the
voltage sense line and circuit, which act as a closed feedback loop to control the
output of the supply, increasing it as demand increases, decreasing it as load is
reduced
AC-DC Voltage Conversion ctd.
• The main advantage of a switching power supply is that it is far
more efficient than a linear design, especially when dealing with
hundreds of watts of power
• Second advantage is that all the energy wasted in the power supply
as heat has to be removed by the PC's cooling system.
• Therefore, more efficient power supplies produce less heat that the
system has to exhaust.
• The main disadvantage of a switching supply is that it generates
high-frequency signals within it as part of its conversion process,
which can radiate out of the unit and cause interference to other
electronic devices (inside or outside the PC).
• For this reason, you will always see PC power supplies encased in
metal boxes for shielding
Standard Output Voltages
• In fact, the supply provides several different voltage levels to meet the
demands of different components in the machine.
• PCs use several different voltages to power their various components. The
core voltages have mostly remained unchanged over time, though a
couple of the less-used voltages have essentially been dropped, and an
important new one has been added.
• The power supply provides each of these voltages, in varying amounts
depending on the model, directly from its circuitry.
• Most of the power provided by the power supply is in the form of positive
voltages, but some is in the form of negative voltages.
• Negative voltage is a slightly strange concept when used in reference to a
DC current. In a nutshell, it just means that the voltage potential is
measured from ground to the signal, instead of the signal to ground. It's
essentially like turning a battery upside-down: same voltage, the current
just goes backwards.
Standard Output Voltages ctd.
Various voltages provided by a typical, modern power supply. The color of
each line corresponds to the color normally given wires carrying that
voltage in the supply's motherboard connectors. The black zero voltage line
represents the system's ground, which is the reference point
Standard Output Voltages ctd.
• The amount of current provided at each voltage level is important
because of its impact on determining the supply's ability to provide
sufficient power for your system. Below are the details on the
various voltages provided:
• -12 V: This voltage is used on some types of serial port circuits,
whose amplifier circuits require both -12V and +12V. It is not
needed on some newer systems, and even on older ones not very
much is used, because the serial ports require little power. Most
power supplies provide it for compatibility with older hardware, but
usually with a current limit of less than 1A.
• -5 V: An outdated voltage, -5 V was used on some of the earliest
PCs for floppy controllers and other circuits used by ISA bus cards. It
is usually provided, in small quantity (generally less than 1A), for
compatibility with older hardware. Some form factor power
supplies such as the SFX no longer supplies it (systems using the SFX
power supply are intended not to have ISA bus slots).
Standard Output Voltages ctd.
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0 V: Zero volts is the ground/ earth/ common of the PC's electrical system. The
ground signals provided by the power supply are used to complete circuits with
the other voltages. They provide a plane of reference against which other voltages
are measured.
+3.3 V: The newest voltage level provided by modern power supplies, it was
introduced with the ATX form factor and is now found on the ATX, NLX, SFX and
WTX form factors. It is not found in Baby AT or older form factors. Originally, the
lowest regular voltage provided by the power supply was +5 V, which was used to
provide power to the CPU, memory, and the motherboard components. Starting
with the second generation Pentium chips, Intel went to a reduced 3.3V voltage in
order to reduce power consumption as the chips got faster. This required
motherboard manufacturers to put voltage regulators on their boards to change
the +5 V to +3.3 V. The regulators produced a great deal of waste heat and having
to do this reduction on the motherboard was very inefficient, so now the power
supply provides +3.3 V directly. It is used to run most newer CPUs, as well as some
types of system memory, AGP video cards and other circuits.
Standard Output Voltages ctd.
• +5 V: On older form factor systems (Baby AT and earlier) ,
this is the voltage used to run the motherboard, the CPU
(directly or indirectly) and the other components in the
system. On newer systems, many of the components,
especially the CPU, have migrated to the lower +3.3 V, but
the motherboard and many of its components still use +5 V.
• +12 V: This voltage is used primarily to power disk drive
motors. It is also used by fans and other types of cooling
devices. It is in most cases not used by the motherboard in
a modern PC but is passed on to the system bus slots for
any cards that might need it. Of course, drives are
connected directly to the power supply through their own
connectors.
Power Good Signal
• When the power supply first starts up, it takes some time for the
components to get "up to speed" and start generating the proper DC
voltages that the computer needs to operate.
• Before this time, if the computer were to boot up, strange results could
occur since the power might not be at the right voltage. It can take a
second or longer for the power to stabilize, but this is an eternity for a
processor that can run half a billion instructions per second!
• To prevent the computer from starting up prematurely, the power supply
puts out a signal to the motherboard called "Power Good" after it
completes its internal tests and determines that the power is ready for
use.
• Until this signal is sent, the motherboard will refuse to start up the
computer.
• In addition, the power supply will turn off the Power Good signal if a
power surge or glitch causes it to malfunction. It will then turn the signal
back on when the power is OK again, which will reset the computer.
Power Good Signal ctd.
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If you've ever had a brownout where the lights flicker off for a split-second and the
computer seems to keep running but resets itself, that's probably what happened.
Sometimes a power supply may shut down and seem "blown" after a power
problem but will reset itself if the power is turned off for 15 seconds and then
turned back on.
The nominal voltage of the Power Good signal is +5 V, but in practice the allowable
range is usually up to a full volt above or below that value. All power supplies will
generate the Power Good signal, and most will specify the typical time until it is
asserted.
Some extremely cheap power supplies may "fake" the Power Good signal by just
tying it to another +5 V line. Such a system essentially has no Power Good
functionality and will cause the motherboard to try to start the system before the
power has fully stabilized.
Needless to say, this type of power supply is to be avoided. Unfortunately, unless
one tests their equipment, one cannot tell about these issues. Fortunately, if you
buy anything but the lowest-quality supplies you don't really need to worry about
this.
Soft Power (Power On and 5V Standby
Signals)
• Early PCs using the PC/XT, AT, Baby AT and LPX form factors use a
mechanical switch to turn the computer on and off.
• Newer form factors, starting with the ATX/NLX, and including the
SFX and WTX, have changed the way the power supply is turned on
and off.
• Instead of using a physical switch, these systems are turned on by a
signal from the motherboard telling the power supply what to do.
In turn, the motherboard can be told to change this signal under
software control. This is what allows Windows to shut the power
down to a PC, or what allows such features as turning a PC on from
a button on the keyboard.
• This feature is called "Soft Power" and the signal that controls the
power supply is called "Power On"
Soft Power (Power On & 5V Standby
Signals)
• This feature would seem to create a small "chicken and egg" situation
however. How can the motherboard tell the power supply to turn on,
electronically, when the motherboard is also off due to not having any
power from the supply?
• The answer is the other "Soft Power" signal, which is called "+5 V Standby"
(or "+5VSB", or "5VSB", etc.)
• This signal is the same output level as the regular +5 V lines from the
power supply, but is independent of the other provided voltages and is
always on, even when the rest of the power supply is turned off.
• A small amount of current on this wire is what allows the motherboard to
control the power supply when it is off. It also permits other activities that
must occur while the PC is off, such as enabling wakeup from sleep mode,
or allowing the PC to be turned on when activity is detected on a modem
("Wake on Ring") or network card ("Wake on LAN").
• The WTX form factor also includes a similar standby signal for +3.3 V. See
the next section for more.
Additional Power Signals
• Some power supply form factors define additional power signals
beyond the standard voltage outputs and the signals mentioned
earlier.
• Most of these are signals that can be implemented optionally by
the power supply manufacturers, "optional" meaning that they are
not required by the form factor for the power supply to meet the
specification. In practice, this means that they are left off of most
power supplies, especially less expensive models, to save cost.
However, they are present on some supplies, so it's useful to
understand what they are..
• Note: For these functions to do you any good, you need a
corresponding capability in other components of your system, most
importantly your motherboard. Normally this means a matching
socket for any optional motherboard connectors used by these
power supplies to supply the additional signals.
Additional Power Signals ctd.
• The following additional signals are specified for ATX/NLX systems:
• +3.3 V Sense: This signal is used to detect the voltage level of the
+3.3 V signal being provided to the motherboard. This allows the
power supply to "fine tune" the +3.3 V output in the event of
excessive voltage drop between the supply and the components
that use +3.3 V. This is more needed for +3.3 V than the other
signals probably because CPUs use +3.3 V
• FanC: This is a fan control signal, which allows the motherboard
(and hence the system) to control the speed of the power supply
fan. If implemented, when the voltage on this signal is less than 1
volt, the fan is turned off. As the voltage is increased the fan spins
faster, and when it is over 10.5 V, the fan is run at full speed. This
can be used to shut the fan off if the system is put into a sleep
mode, or to allow the fan's speed to be increased or decreased
based on the temperature of the system, which helps in saving
power and reducing unnecessary noise
Additional Power Signals ctd.
• FanM: A companion to FanC, this is the fan monitor signal,
which allows the motherboard to keep track of the current
speed of the power supply fan. Sort of a "power supply fan
tachometer" for fans designed to implement it. This could
be used to provide a warning to the user if the main cooling
fan in the power supply failed.
• 1394V and 1394R: This pair of signals provides a separate,
unregulated voltage circuit for powering IEEE-1394
("FireWire") peripherals. It is not used by the motherboard.
• The SFX form factor defines just one optional signal, called
"Fan ON/OFF", which is essentially the same as the "FanC"
ATX signal described above.
Additional Power Signals ctd.
• WTX, reflecting its status as a high-end form factor, includes
several additional signals. These include the +3.3 V Sense,
FanC and FanM signals described as above for ATX/NLX, as
well as these:
• Sleep: Puts the power supply into sleep mode. This is used
for power savings, to power down parts of the power
supply. It is used in conjunction with the Power On signal.
• +3.3 VAUX: This is a standby +3.3 V signal just like the +5 V
Standby signal defined for standard ATXSoft Power.
• +5 V Sense: Just like the +3.3 V Sense signal, but for +5 V.
The WTX form factor also provides special, dedicated
grounds (called returns) for its sense lines.
Parts of the Power Supply
• This section takes a look at the various parts
that comprise the power supply both inside
and outside the box.
• The exact contents of any supply vary
depending on both the supply's form factor
and its individual design, but most of them
have the same general components.
Parts of the Power Supply ctd.
An ATX/NLX power supply showing most of the major external components
Case and Cover
• Every PC power supply comes surrounded by a metal case with a metal
cover.
• The cover is normally secured with screws and comes up off the top of the
case
• In many ways, the case of the power supply is to the power supply what
the system case is to the PC as a whole. It has several functions.
• The case isolates the components inside the power supply from the rest of
the PC, which serves to keep harmful electromagnetic interference inside
the box. This is important because the switching design used for PC power
supplies can otherwise cause emissions that will wreak havoc on other
components inside and outside the PC.
• The case also keeps prying fingers outside the box where they will remain
safely non-electrocuted.
• Power supplies are usually intended to be considered as "black boxes" and
not serviced by individual PC owners
Case and Cover ctd.
A baby AT case, viewed from the
rear. The cover is on top; secured
with only two screws, one visible in
the foreground and one in the
background. The ventilation holes for
the power supply fan are prominent,
and the power cord receptacle and
power pass-through can be seen as
well
Case and Cover ctd.
• The design of the case and cover are also important
because they play a role in cooling the power supply
components, and to some extent, the whole PC.
• Ventilation slots or holes are placed into the case in key
locations to allow the power supply fan to provide air
flow over critical components.
• Warning: In addition to other warnings about not
opening the power supply for safety reasons, another
reason could be warranty related if you open the
power supply. Look for small "warranty void if
removed" stickers around the perimeter of the cover
Power Cord and Power Pass-Through
• Virtually all PCs come with a standard black
power cord that runs from a receptacle on the
power supply to a power outlet in the wall, for
a conscious user preferably, a power
protection device or UPS.
• This cord is unchanged since the creation of
the personal computer. It has a special keyed
shape on the end that plugs into the power
supply. All PC power cords are three-pronged
Power Cord and Power Pass-Through
ctd.
A standard PC power cord,
oriented to show its two ends
Power Cord and Power Pass-Through
ctd.
• Some power supplies, especially older ones, have a "passthrough" connector on the back into which the monitor's
power cord can also be plugged if it has the right shape.
• Some adapters can also convert a standard outlet plug into
the shape needed to go into the back of the computer's
supply.
• When this is done, the monitor is turned on and off using
the computer's power switch.
• This feature cannot be employed in newer form factor
power supplies, since they use soft power mechanism and
not a mechanical switch to turn the PC on and off but was
quite common on PC/X and AT systems, allowing the PC to
use only one power outlet instead of two
Power Cord and Power Pass-Through
ctd.
View of the power cord receptacle (left)
and pass-through (right) on a tower-style
Baby AT power supply. The red button
between them is the voltage selector
switch . Note the input current ratings
printed on the right-hand side.
Note: The monitor is not driven by the
computer's internal power supply
when you use the power pass-through;
the only thing you gain is the ability to
have the switch turn the monitor on
and off with the PC and save one
outlet.
Power Switch
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Older form factor desktop cases had the power switch at the back of the machine, usually on the
right side of the case. This switch was actually inside the power supply itself, with a hole cut out in
the case so that it could be reached from the outside. Users hated having to reach to the back of
the machine to turn it on or off!
With the AT form factor tower cases changed to a remote, physical toggle power switch that was
connected to the power supply using a cable. AT desktop cases retained the old style case in the
back of the PC, but clone manufacturers soon began to use a remote switch on these units as well.
The switch is normally mounted to the front of the case. Some "slimline" systems actually use a
mechanical plastic stick that is pushed on by the button on the front of the case, and presses
against the real power switch on the power supply itself, in the back of the machine.
The remote switch cable has four leads that run to it (with a fifth ground lead, to ground the power
supply to the case, optional) . One pair of these (the brown and blue) run to the power cord on the
back of the power supply. The other pair (black and white) run from the switch to the power supply
circuitry. When the switch is on, brown connects to black, and blue connects to white, and the
power supply is energized. These wires attach to spade connectors on the body of the switch.
Power Switch ctd.
Warning: The brown and blue leads to the remote
power switch on an AT-style system carry live 110V
(or 220V) AC power whenever the power supply is
plugged in, even when the power is off! You should
not work inside the computer with the power
plugged in
A remote PC power supply switch,
showing its four spade connectors and
attached wires
Warning: Switching the pairs of wires from one set
of spade connectors to another will cause no
problems as long as you exchange black with
brown, and white with blue. However, if you
accidentally make any other type of change, say,
swapping black with blue and white with brown,
the results will range from a blown fuse or circuit
breaker, to smoke
Power Switch ctd.
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From the ATX/NLX form factor, the way the power switch works has been changed altogether.
Instead of using a physical toggle switch connected to the power supply, on modern systems the
power switch is electronic.
It connects to the motherboard, much the way that the reset switch does, using a feature called
soft power. On an ATX system, when one presses the power switch, they aren't turning on the
power supply; it is more like sending a "request" to the motherboard to turn the system on.
The switch has wires carrying only low-current DC power, removing the potential risks inherent in
the AT-style switch
One consequence of the "soft power" method of operating the power supply comes into play if
there is a power failure. Imagine that you have a PC running unattended. There's a power outage,
and the system shuts down. Several hours later, the power comes back on. With an old-fashioned
mechanical power switch, the system would immediately restart, because as soon as the power
was restored the power supply would turn on.
With an ATX/NLX, SFX or WTX form factor supply however, the power supply would sit there
waiting for a "turn on" signal from the motherboard! This is not much of an issue for most personal
PCs, but is a big problem for business servers and other machines running without users physically
present.
To solve this problem, some high-end power supplies include an auto-restart feature that powers
up the system immediately when the system detects that the AC power has returned after a power
failure.
External Voltage Selector Switch
• PC power supplies support 110V input, 220V input or
both.
• Dual-voltage supplies normally have a selector in the
back that controls which voltage to be used; important
that it is set correctly.
• There are also some supplies that will automatically
support either 110V or 220V without a selector switch,
but are often found on more expensive units
• Power supplies that support dual voltage input are
preferable since they are flexible, although few people
transport their PCs overseas (other than laptops).
External Voltage Selector Switch ctd.
Warning: If your power supply does have a 110/220
switch, make sure it is set correctly, or else! This is usually
done by the manufacturer of the case (which normally
contains the power supply). Running a power supply set to
220 on 110 V power will probably cause it just to not work,
but if you set the switch to 110 and run it on 220 V,
damage might result
A manual voltage selector switch. This
design wisely makes it clear right on the
switch the current voltage setting
Power Conversion Circuitry
• The "guts" of the power supply is usually a circuit board with
various electrical components on it, mounted inside the metal box
of the supply.
• All the cables going into and out of the power supply go to this
circuit, including those of the remote power switch, if any
• This circuitry is what is responsible for the work of converting AC to
DC within the power supply and also manages the other power
supply functions of course.
• In newer supplies, many of the features of the power supply are
combined into special integrated circuits to reduce space
requirements and eliminate manufacturing costs.
• The circuitry inside the case of the power supply relies on the
power supply fan for ventilation and cooling.
Power Conversion Circuitry ctd.
Some of the internal circuitry in a typical power supply.
Note the tan-colored circuit board in the background, into
which most of the components are connected. The silvercolored "fence" a third of the way down from the top of
the image is part of a heat sink used for cooling of
some of the components. The cable bundle in the
foreground goes to the remote power switch. On the lefthand side, in the middle background, is an integrated
circuit
Motherboard Power Connectors
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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.
Different form factors use different numbers, types, shapes and sizes of connectors between the
power supply and motherboard.
A bit about the wires that run between the power supply and the connectors themselves.
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.
Motherboard Power Connectors ctd.
• 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.
• Note: 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
Motherboard Power Connectors ctd.
AWG
Diameter
(mm)
Cross-sectional
area (mm²)
Approximate
Maximum Current
(A)
16
1.29
1.31
19
18
1.02
0.82
15
20
0.81
0.52
10
22
0.644
0.33
8
Relative
Size
Note: The relative sizes of the four wires shown are
to scale, meaning that their relative sizes are
accurate. Also, the current capacities are
approximate maximums and probably not what
would be considered safe or reasonable to use in a
responsible design
Motherboard Power Connectors ctd.
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The other issue of interest is the color of the wire insulation.
There are standards established for the colors of various wires, to avoid confusion
by those who work with different components and PCs. While not all
manufacturers follow these conventions, most do. If they do not, problems can
easily occur when a technician sees a black wire, assumes it is a ground (which it
usually would be)
Below are diagrams that show the configuration of pins for the various connectors
used by different form factors between the power supply and motherboard.
In each diagram the pins on the power supply connector are shown in their correct
orientation. The color of each pin is the color of the wire established as a standard
for that pin. Outside the rectangular outline of each connector,
Note: the diagrams are not to scale.
Note: diagrams are shown from the perspective of the connector coming from the
power supply. For those connectors with two columns of pins, the mating
motherboard connector will have its pins in a mirror-image configuration.
Motherboard Power Connectors ctd.
• The biggest problem with the design IBM
used for these power connectors is that
there are two of them and they are the
same size and shape.
• The connectors are physically keyed so
they cannot be inserted backwards, but it
is very possible to accidentally swap
them.
• If so, ground wires will be put where the
motherboard expects live power and
vice-versa, and the results are
catastrophic.
• Thus, technicians working with older
systems developed the well-known
mantra: "black wires together in the
middle"!
The two "AT style" power connectors, P8
(left) and P9. On the PC/XT, pin #2 of P8
is left unconnected
Motherboard Power Connectors ctd.
• So this would be the proper installation of the two
AT-style power connectors to a motherboard.
• Notice the four black wires together in the middle.
• Incidentally, in this picture the connectors are shown
upside-down from the diagrams above; pin #1 of P8
is at the bottom, pin #6 of P9 at the top.
• In the background is an ATX-style motherboard
connector; this board can work with either form
factor of power supply
Motherboard Power Connectors ctd.
• With the ATX/NLX power supply, Intel removed
the potential P8/P9 risk by making a single
piece connection and using different shapes on
any other Power Supply and Motherboard
connections
• The regular ATX power supply connection uses
a 20-pin connector with a square hole for pin
#1 and round AWG 18 holes for the other 19
pins, except for pin #11.
• There, the specification calls for two AWG 22
wires in the same pin, a +3.3 V signal (orange)
and the default +3.3 V sense signal (brown).
• Note that the connector has 20 pins, but 21
wires
The main ATX/NLX power connector.
Motherboard Power Connectors ctd.
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The ATX specification also defines
an auxiliary 6 AWG 16 wire (1x6
configuration) and an optional 6 AWG 22
wire (2x3 configuration) connectors.
The auxiliary is intended for boards that
require lots of power to run their
components (250+W); it consists of more,
thicker wires for the +3.3 V and +5 V signals.
If the optional ATX connector is used, the
+3.3 V sense signal on the 20 pin connector
can be omitted and a regular +3.3 V line put
into pin #11
The optional connector also carries
additional signals
The ATX auxiliary and optional connectors. FanM is
plain white, FanC is white with a blue stripe, +3.3V
Sense is white with a brown stripe, 1394R is white
with a black stripe and 1394V is white with a red
stripe
Motherboard Power Connectors ctd.
• The SFX power supply, similar to the
ATX main connector uses a main 20 pin
(AWG 18 except for the pin #11
combination, which are AWG 22)
connector.
• The only difference is that pin #18 is
omitted, since the SFX does not call for
a -5 V signal.
• The SFX optional connector only
provides the Fan ON/OFF signal, on pin
#2, with AWG 22 wire.
• No auxiliary connector because SFX is
not intended for use in systems that
require a lot of power.
SFX main and optional connectors.
Motherboard Power Connectors ctd.
• WTX has a large number of
connections to carry the
tremendous amount of current
that the power supplies are
capable of providing.
• WTX power supplies therefore
have a different motherboard
interface.
• The two primary connectors are
the 24-pin "main" connector (P1)
and 22-pin "additional" connector
(P2).
• Despite P2's name, it is
mandatory since all the control
signals are on it
WTX main (left) and additional connectors, P1 and P2. The main
connector's wires are AWG 18 except for the two low-power
standby signals, which are AWG 20. Similarly, the additional
connector uses AWG 18 for the 12V and ground power lines, and
AWG 22 for the signals and sense lines
Motherboard Power Connectors ctd.
• In addition to the above, three more
connectors are defined.
• P3 is an 8-pin optional connector (6
pins used) that provides +12 V power
to optional power modules or DC-toDC converters used for additional
processors and/or memory within the
system.
• P4 and P5 are 6-pin optional
connectors used in a similar fashion,
to provide additional current for
multiple-CPU motherboards or other
applications
WTX optional connectors; P3 (left) and P4/P5.
Note that unlike other connectors, the wire color
standard for these +12 V signals is white, not yellow.
All wires are AWG 18
Note:: In addition to the all connectors described
above, the newer form factors that use soft power also
have a connection from the power switch on the case,
back to the motherboard
Drive Power Connectors
• The power supply provides power to the hard disk, floppy disk,
CD/DVD and other drives directly, through four-wire connectors
that are designed to attach to the rear of each drive.
• The four wires provide +5 V and +12 V power, along with two
grounds, to the various drives that use them.
• The connectors themselves come in two basic styles.
• The larger size, often called a Molex connector (after the name of
one of the big connector companies) is keyed by virtue of the
connector itself being "D-shaped", and is used on most internal
drives, including hard disk, CD/DVD, Zip and other removable media
drives, and the older 5.25" floppy disk drives.
• The smaller size, typically called a "mini-plug", is used for the newer
style of 3.5" floppies. It is also keyed, but in a different way than the
larger connector, and actually secures to its mating connector with
a retention clip of sorts
Drive Power Connectors ctd.
• Large style hard disk drive cable connector
• the small style is wired the same, the shape and
size are just different, as shown below.
• The wires used for these connectors vary widely
from system to system.
• For the larger style connector, AWG 16 to 18 are
common.
• For the smaller size, AWG 18 down to AWG 22 are
sometimes specified (since floppies draw little
current).
Drive Power Connectors ctd.
Large and small disk drive connectors. The tab in the middle bottom of
the small connector fits into a retention clip on the floppy disk drive and
holds it in place. The large connectors secure simply by friction
Drive Power Connectors ctd.
• The number of connectors that come with each power supply varies
considerably.
• In general, the bigger the supply, the more devices expected to run, so the
more connectors included.
• Totals can range from 3 or 4 connectors to as many as a dozen.
• Another factor is just general quality; some makers skimp on the
connectors to save money, and make you buy adapters to let you run
additional drives.
• These adapters, usually called "Y-splitters" or "Y cables" after their general
shape contain one male and two female large-style hard disk connectors
are available in most electronics or computer stores. They are increasingly
needed in modern systems, because not only do newer systems have
more drives, they also have more fans and cooling devices, which also
often attach using a disk drive power connector.
• Note:: Adding a Y-splitter doesn't magically increase your power supply's
output capacity, It just gives you more connectors!
Drive Power Connectors ctd.
A Y-splitter cable for running two drives
off a single large hard disk cable
connector
Advisable to avoid Y-splitters for the following reasons ::
• Incorrectly wired Y-splitters, which have the potential to
damage equipment. It's easy to see if the adapter has been
wired correctly by inspecting it carefully. Using an
ohmmeter to test for correct connectivity is even better
• They are an additional potential source of failure in the
system, as they are often cheaply made and hard to align
and plug in properly.
• They can further clutter the inside of the case.
• Whenever two devices share a single connector, all the
power drawn by the two devices has to travel down the
same set of wires from the power supply. Chaining three or
four drives off the same connector using multiple splitters
may exceed the current rating for the wires and/or
connector damaging it
Drive Power Connectors ctd.
• Most systems only come with a single mini-plug
connector--because most systems come with only a single
floppy drive that uses it--but some may come with two.
• If one needs an additional mini-plug connector in a
system with only one, or if the first one breaks, you can
employ a simple adapter to change a large style into a
small style.
An adapter to change a large-style
drive power connector into a small
style connector
Power Supply Fan
• Since the earliest PCs, the power supply fan has been the primary
cooling source for the entire PC.
• Today's PCs incorporate additional cooling methods, including auxiliary
fans and CPU cooling devices, but the power supply fan remains an
important factor in the overall cooling equation.
• The fan is traditionally located at the rear of the power supply, and
special vents are provided for it in the case of the supply itself to allow
for it to exhaust.
• Most fans use +12 V power to operate, despite the fact that the wires
that run to them are normally red for the +12 V line, and black for the
ground (not yellow and black as you might expect from the wire colour
standards elsewhere in the PC)
Power Supply Fan ctd.
• In addition to the regular fan found in the power supply, most
newer systems include auxiliary fans for improved air flow and
system cooling and they are typically mounted at various locations
around the system case.
• The standard size of a PC cooling fan is about 3.25" or 80 mm
square, but they come in other sizes as well.
• An important quality consideration when it comes to PC cooling
fans is the quality of construction of their motors, particularly the
motor bearings.
• Cheaper fans use sleeve bearings that are much less durable than
their ball bearing counterparts.
• While "sleeve vs. ball" isn't the only dimension upon which to
measure cooling fan quality, it is an important one as sleeve bearing
fans can lock up after as little as a year of use, while ball bearing
fans typically last many years.
Power Supply Fan ctd.
Power supply fan viewed from the
inside of a tower AT form factor
power supply
A ball bearing fan. The "silencer“ label identifies it as a
fan specifically engineered to reduce the noise level of
the system. This can be used as an auxiliary cooling fan
anywhere that it can be mounted in the case, or as a
replacement for a failed internal power supply fan.
Power Supply Fan ctd.
• Another quality consideration of a fan is how much air it can move,
measured in cubit feet per minute (CFM). The higher the rating, the more
work the fan is accomplishing. Fan speed can be controlled on some
systems through the use of the FanC, FanM or the Fan On/Off signals.
Many power supplies also have automatic thermal control of the power
supply fan: they reduce or increase its speed based on internal
temperature without any intervention required by the rest of the system
• The fan is the component most likely to go first in a power supply. The
usual cause of this is dirt that gets into the motor of the fan. The average
time until failure is greatly increased if the PC is used in a very dirty or
dusty environment, or if the PC is never cleaned. When the fan stops
working, overheating of the components within the power supply as well
as components in the rest of the PC are likely. A PC that makes use of the
optional fan monitoring signal FanM can detect a fan failure and sound an
alarm to the user, or shut down the PC. Another way of detecting an
overheating condition is through hardware that monitors the internal
temperature of the system.
Power Supply Fan ctd.
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The power supply fan is probably the only component that can be replaced by an end-user
(although no recommended) An alternative solution to a bad fan that does not involve opening the
power supply is an add-on external fan. These fans plug into the wall directly and are typically
heavier-duty and higher-capacity than a standard power supply fan. They are actually designed for
improving the cooling of the existing fan even when it is running
Another consideration regarding the power supply fan is the direction in which it circulates air.
Older power supplies are designed to exhaust air out the back of the PC, which for machines using
older, slower CPUs, this worked fine. However, starting with the 486, separate cooling started to
become required for the processor. In response, Intel designed the ATX form factor to reverse the
flow of air and move the power supply fan to the inside edge of the supply case, with the goal of
using the power supply fan to also cool the CPU. Later, when it became obvious that the newest
CPUs still needed their own cooling and having the power supply fan blow already-heated air on
them wouldn't get the job done, Intel made the fan direction (and location) optional
One advantage of a power supply fan that blows into the case is that it provides much better
control over the air that enters the system. Instead of being drawn in through all the holes and
cracks in the case as with a fan that pushes air out the back of the system, the air entering the case
all comes in from the power supply fan intake. This intake can be filtered to dramatically reduce
the amount of dirt that accumulates within the system.
Power Supply Fuse
• Some power supplies come with their own integrated fuse.
The fuse is designed to protect the circuits in the power
supply from damage should an over-current situation arise.
• If there is a problem with the electrical system (surge,
lightning strike) or internal fault within the power supply,
the fuse will blow. It can then be replaced and if it did its
job properly, the supply should operate normally.
• Unfortunately, many PC power supplies don't have fuses at
all, a short sighted cost-savings measure.
• Even many power supplies that do have fuses hide them
from the user within the power supply case.
• It's a good idea though to search the back of your system to
see if there is a user-replaceable power supply fuse.