Power Supply

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Objectives

• In this chapter, you will:– Understand power supply– The most common computer power supplies

are built to conform with the ATX form factor.

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Power Supply: Switcher Technology

• Instead of the linear design, PCs use switching power supplies.

• The full name for this sort of design is actually "constant-voltage, half-bridge forward-converting switching power supply", but don't worry, that won't be on the final exam. :-)

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Power Supply• Warning: Power supplies

generate high voltages internally and can be dangerous.

• Unless you have been specifically trained to work inside power supplies, you should not open one.

• Even with the unit unplugged dangerous electricity can remain stored within its components for some time

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Power Supply• Power supplies, often referred to as

"switching power supplies", use switcher technology to convert the AC input to lower DC voltages.

• However, the PC power supply's job isn't as simple as that of a standard power converter, – such as the kind you might plug into your

car's lighter socket to let you run a TV off your car battery.

• The power supply in your PC provides all of the different voltages your computer needs to operate properly.

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Watts• The main specification of a power supply is in

watts. • A watt is the product of the voltage in volts

and the current in amperes or amps. – The electrons that flow through the wire is referred to as

Current, measured in AMPS.

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1W=1V×1A

Power Supply• A Power Supply does not supply

power; electrical power comes from your local utility company.

• The Power Supply converts the power from alternating current to direct current at levels that the system requires.

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AC vs. DC• Alternating current works fine for certain

appliances and has the added advantage of being easy to distribute over long distances, but many devices will not work on AC.– Just a few examples of this are telephone answering

machines, many types of audio equipment, some kinds of battery chargers, and in the PC world, 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 "bricks" with one plug for the wall and

another for the device are often called AC adapters

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Power Supply: Switcher Technology• The switching power supplies used today are

much smaller and lighter. They convert the 60-Hertz (Hz, or cycles per second) current to a much higher frequency, meaning more cycles per second. – This conversion enables a small, lightweight transformer

in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer component.

• 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.

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Voltages• Voltage is the pressure that pushes the

electrons through the conductor (wire). Voltage is measured in Volts.

• PC usually uses120 volt power to the power supply.

• The typical voltages supplied are: – 3.3 volts – 5 volts – 12 volts

• The 3.3- and 5-volts are typically used by digital circuits,

• The 12-volt is used to run motors in disk drives and fans.

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Power Supply Standardization• Over time, there have been at least six

different standard power supplies for personal computers.

• Recently, the industry has settled on using ATX-based power supplies.

• ATX is an industry specification that means the power supply has the physical characteristics to fit a standard ATX case and the electrical characteristics to work with an ATX motherboard.

• PC power-supply cables use standardized, keyed connectors that make it difficult to connect the wrong ones.

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Power Supply Wattage• Computer power supplies are

rated based on their maximum output power. – Small Form Factor - 15A (250W)– Mini-Tower - 25A (300-350W)– Mid-Tower - 35A (400-500W)– Full Tower - 40A (600-650W)– Dual Video Card (SLI) - 60A (850W+)– Power supplies used by gamers and

enthusiasts mostly range from 450 W to 1400 W.

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Power Supply Wattage• Power Requirements for typical systems. • Item Power requirement Voltage used • OLDER CPUs 20-40 Watts 3.3 Volts • Athlon XP – Pentium IV 60-90 Watts 12 Volts • motherboard 20-60 Watts 3.3, 5.0, 12 Volts • RAM 20 Watts /256 MB 3.3 Volts • PCI Card 5–10 Watts 3.3 Volts • AGP Video 20–50 Watts and more 5, 12 Volts • CPU Case Fan 2 – 4 Watts 12 Volts • 5400 RPM Hard Drive 5 – 10 Watts 5, 12 Volts • 7200 RPM Hard Drive 5 – 15 Watts 5, 12 Volts • 10000 RPM Hard Drive 5 – 20 Watts 5, 12 Volts • Floppy Drive 5 Watts 5, 12 Volts • CD / DVD 10 – 20 Watts 5, 12 Volts

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Power Supply Wattage• A 400-watt switching power supply

will not necessarily use more power than a 250-watt supply.

• A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case.

• It is not a good idea to have a 250-watt supply if you have 250 watts total in devices, since the supply should not be loaded to 100 percent of its capacity.

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Mother Board with a Pentium 4 processor, 256 MB RAM, a 7200 Rpm Hard Drive, a floppy drive, a CD and an AGP Card.

o Mother Board ----------------------------- 50 Watts

o Pentium 4 ---------------------------------- 90 Watts

o 256 MB RAM ------------------------------ 20 Watts

o Hard Drive --------------------------------- 15 Watts

o Floppy Drive ------------------------------- 5 Watts

o CD -------------------------------------------- 20 Watts

o AGP Video Card -------------------------- 50 Watts

Total System Power: 250 Watts

Power Supply Problems• The PC power supply is probably the most

failure-prone item in a personal computer. • Typically, a stalled cooling fan is a predictor

of a power supply failure due to subsequent overheated components.

• A typical failure of a PC power supply is often noticed as a burning smell just before the computer shuts down.

• Failure symptoms include random rebooting or failure in Windows for no apparent reason.

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Power Supply Problems• 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!

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Case and Cover• Every PC power supply comes

surrounded by a metal case with a metal cover.

• The case isolates the components inside the power supply from the rest of the PC.

• Power supplies are usually intended to be considered as "black boxes" and not serviced by individual PC owners.

• 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.

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PC Power Switch• Older form factor desktop XT

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.

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Baby AT power supply

Turn on PC• Today you turn on the power with a

little push button, and you turn off the machine with a menu option.

• These capabilities were added to standard power supplies several years ago.

• The operating system can send a signal to the power supply to tell it to turn off. The push button sends a 5-volt signal to the power supply to tell it when to turn on.

• The power supply also has a circuit that supplies 5 volts, called VSB for "standby voltage" even when it is officially "off", so that the button will work.

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remote PC power supply switch

PC Power 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!

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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 you are using; obviously, you want to make sure it is set correctly.

• There are also some supplies that will automatically support either 110V or 220V without a selector switch, but these are often found only on more expensive units.– 100-127V in North America, parts of South

America, Japan, and Taiwan;– 220-240V in most of the rest of the world

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Connectors• Typically, power supplies have the following 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.

• 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.

• 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.

• 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.

• 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.

• 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.

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Connectors• Typically, power supplies have the following 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.

• 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.

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Connectors• 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.

• 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.

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Connectors• 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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Connectors• 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.

• 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.

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Power outage• Power outages are categorized into three

different phenomena, relating to the duration and effect of the outage:– A dropout is a momentary (milliseconds to seconds)

loss of power typically caused by a temporary fault on a power line. Power is quickly (and sometimes automatically) restored once the fault is cleared.

– A brownout is a drop in voltage in an electrical power supply, so named because it typically causes lights to dim. Such malfunctions are particularly damaging to electric motors. • The voltage from power companies is generally between 115

and 120 Volts AC, during a brownout the power could drop to less than 110 volts.

– A blackout refers to the total loss of power to an area and is the most severe form of power outage that can occur.

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Protecting computer systems from power outages• Computer systems and other electronic

storage devices are susceptible to data loss or hardware damage that can be caused by the sudden loss of power.

• To protect against this, the use of an Uninterruptible power supply or UPS can provide a constant flow of electricity in the event that a primary power supply becomes unavailable for a short period of time.

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Power Surge• An unexpected increase in the amount of

electrical voltage in an electrical line. A power surge can provide too much power to a computer or electrical device generally causing damage to that device making the device not operate at all or improperly. – In electrical engineering, spikes are fast, short

duration electrical transients in voltage (voltage spikes), current (current spike), or transferred energy (energy spikes) in an electrical circuit.

• Users can help prevent power surges with surge protectors.

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Review• Alternation Current – The type of power delivered by the electric company where

current flows in one direction then stops and reverses direction • AMP - The measure of current in an electrical circuit • Blackout – Total loss of power • Brownout - When the voltage delivered by the power company drops to a less than

expected level • Direct Current – The type of power delivered by batteries, the voltage is constant • EMI – Electromagnetic Interference is electrical interference caused by nearby devices

that have electromagnets, like motors, or transformers • RFI – Radio Frequency Interference is electrical interference caused by other electronic

devices operating nearby • Spike – When the power increases for a very short period of time • Surge - When the voltage increases to higher than normal for a duration of time • Surge Suppressor – Device used to absorb voltage spikes • Volt - The Measure for Voltage • Watt - The measure of Power

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