Chapter 10 Power Supplies

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In this chapter, you will learn

how to

■ Explain the basics of electricity

■ Describe the details about

powering the PC

■ Install and maintain power

supplies

■ Understand power supply

troubleshooting and fire safety

As simple as this appears on the surface, power supply issues are of critical

importance for techs. Problems with power can create system instability,

crashes, and data loss—all things most computer users would rather avoid!

Good techs therefore know an awful lot about powering the PC, from

understanding the basic principles of electricity to knowing the many variations

of PC power supplies. Plus, you need to know how to recognize power problems

and implement the proper solutions. Too many techs fall into the “just plug

it in” camp and never learn how to deal with power, much to their clients’

unhappiness.

CompTIA A+ certification exams

could refer to a power supply

as a PSU, for power supply unit. A

power supply also falls into the

category of field replaceable unit

(FRU), which refers to the typical

parts a tech should carry, such

as RAM and a hard drive.

Understanding Electricity Electricity is a flow of negatively charged particles, called electrons, through

matter. It is like water.

the pressure of the electrons in the wire is called voltage and is measured

in units called volts (V).

The amount of electrons moving past a certain

point on a wire is called the current or amperage, which is measured in

units called amperes (amps or A).

The amount of amps and volts needed so that a particular device will function is expressed as how much wattage

(watts or W) that device needs. The correlation between the three is very

simple math: V × A = W (or VA = W)

Wires of all sorts—whether copper, tin, gold, or platinum—have a slight

resistance to the flow of electrons, just as water pipes have a slight amount

of friction that resists the flow of water. Resistance to the flow of electrons

is measured in ohms (Ω).

■ Pressure = voltage (V)

■ Volume flowing = amperes (A)

■ Work = wattage (W)

■ Resistance = ohms (Ω)

A particular thickness of wire only handles so much electricity at a time.

If you push too much through, the wire will overheat and break, much as an

overloaded water pipe will burst. To make sure you use the right wire for

the right job, all electrical wires have an amperage rating, such as 20 amps.

If you try to push 30 amps through a 20-amp wire, the wire will break and

electrons will seek a way to return into the ground. Not a good thing, especially

if the path back to the ground is through you!

Circuit breakers and ground wires provide the basic protection from

accidental overflow. A circuit breaker is a heat-sensitive electrical switch

rated at a certain amperage. If you push too much amperage through the

circuit breaker, the wiring inside detects the increase in heat and automatically

opens, stopping the flow of electricity before the wiring overheats and

breaks

An electrical outlet must

have a ground wire to be

suitable for PC use.

Many years ago, your electrical supply used fuses instead

of circuit breakers. Fuses are small devices with a tiny filament

designed to break if subjected to too much current. Unfortunately,

fuses had to be replaced every time they blew, making

circuit breakers much more preferable. Even though you no

longer see fuses in a building’s electrical circuits, many electrical

devices—such as a PC’s power supply—often still use fuses

for their own internal protection

Most electronic devices use DC power, but all power companies

supply AC power because AC travels long distances much

more efficiently than DC.

Your PC uses DC voltage, so some conversion process must take place

before the PC can use AC power from the power company. The power supply

in a computer converts high-voltage AC power from the wall socket to

low-voltage DC

The power supply connects to the

power cord (and thus to an electrical outlet) via a standard IEC-320 connector.

In the United States, standard AC comes in somewhere between 110

and 120 V, often written as ~115 VAC (volts of alternating current). The rest

of the world uses 220–240 VAC, so most power supplies are dual-voltage

and compatible with both.

The IEC-320 plug has

three holes, called hot, neutral, and ground.

These names describe the function of the

wires that connect to them behind the wall

plate.

The hot wire carries electrical voltage,

much like a pipe that delivers water.

The

neutral wire carries no voltage, but instead

acts like a water drain, completing the circuit

by returning electricity to the local source,

normally a breaker panel.

The ground wire makes it possible for

excess electricity to return safely to the ground.

When testing AC power,

you want to check for three things: that the hot outputs approximately 115

V (or whatever the proper voltage is for your part of the world), that the

neutral connects to ground (0 V output), and that the ground connects to

ground (again, 0 V).

The V with the solid line above a dashed line, for

example, refers to direct current.

The V~ stands for alternating

current.

Every multimeter offers at least four types of electrical

tests: continuity, resistance, AC voltage (VAC), and

DC voltage (VDC).:::::::::::

Continuity tests whether electrons

can flow from one end of a wire to the other end. If so, you have continuity; if not, you don’t

Testing AC and DC voltages is a

matter of making sure the measured voltage is what it should be.

Two essential devices handle

spikes and sags in the supply of AC: surge suppressors and uninterruptible

power supplies.

---Surges or spikes are far more dangerous than sags

Given the seriousness

of surges, every PC should use a surge suppressor device that absorbs the

extra voltage from a surge to protect the PC. Most people tend to spend a lot of money on their PC and for some

reason suddenly get cheap on the surge suppressor. Don’t do that! Make

sure your surge suppressor has the Underwriters Laboratories UL 1449 for

330-V rating to ensure substantial protection for your system.

Additionally, check the joules rating before

buying a new surge suppressor. A joule is a

unit of electrical energy. How much energy

a surge suppressor can handle before it fails

is described in joules. Most authorities agree

that your surge suppressor should rate at a

minimum of 800 joules—and the more joules,

the better the protection. My surge suppressor

rates out at 1,750 joules.

A joule is a unit of electrical energy. How much energy a surge suppressor can handle before it fails is described in joules.

UPS An uninterruptible power supply (UPS) protects your

computer (and, more importantly, your data) in the event of

a power sag or power outage. A UPS essentially contains a big battery that provides

AC power to your computer regardless of the power coming

from the AC outlet.

CONFUSING::::::All uninterruptible power supplies are measured in both

watts (the true amount of power they supply in the event of a

power outage) and in volt-amps (VA). Volt-amps is the amount

of power the UPS could supply if the devices took power

from the UPS in a perfect way

UPS Technologies

There are two main types of

UPS: online, where devices are

constantly powered through the

UPS’s battery, and standby,

where devices connected to the

UPS receive battery power only

when the AC sags below ~80–90

V. POWER TO THE MOTHERBOARD::::::

Modern motherboards use a 20- or 24-pin P1 power

connector

POWER TO THE PERIPHERALS::::

he typical PC power supply has

up to three types of connectors that plug into peripherals: Molex, mini, and

SATA

Molex Connectors:::::::::::::::::

The most common type of power connection for devices

that need 5 or 12 V of power is the Molex connector. (has notches)

Mini Connectors::::::::::::::::::::::

All power supplies have a second type of

connector, called a mini connector (see Figure 10.15), that supplies

5 and 12 V to peripherals, although only floppy disk

drives in modern systems use this connector. Drive manufacturers

adopted the mini as the standard connector on 3.5-inch

floppy disk drives. Often these mini connectors are referred to

as floppy power connectors.

SATA Power Connectors::::::::::::

Serial ATA (SATA) drives need a special 15-pin

SATA power connector :::::: are L shaped,

ATX When plugged in, ATX systems have 5 V running to the motherboard.

They’re always “on” even when powered down. The power switch you

press to power up the PC isn’t a true power switch like the light switch on

the wall in your bedroom. The power switch on an ATX system simply tells

the computer whether it has been pressed. The BIOS or operating system

takes over from there and handles the chore of turning the PC on or off. This

is called soft power.

Rails Generally, all of the PC’s power comes from a single transformer

that takes the AC current from a wall socket and converts it into DC current that is split into three primary DC voltage rails: 12.0

V, 5.0 V, and 3.3 V

ATX12V 2.0: he ATX12V 2.0 standard incorporated many of the good

ideas of EPS12V into the ATX world, starting with the 24-pin connector.

Active PFC Visualize the AC current coming from the

power company as water in a pipe, smoothly moving back

and forth, 60 times a second

Good PC power supplies come with active power factor

correction (active PFC), extra circuitry that smoothes

out the way the power supply takes power from the power

company and eliminates harmonics (see Figure 10.26).

Never buy a power supply that does not have active PFC—

all power supplies with active PFC will announce it on

the box

As a general recommendation for a new system, use at least a 500-W

power supply. This is a common wattage and gives you plenty of extra

power for booting as well as for whatever other components you might add

to the system in the future.

Handling ATX power supplies requires special consideration. Understand

that an ATX power supply never turns off.

the power supply

will continue to supply 5 V to the motherboard.