The Power Supply






The Power Supply

This section concentrates on the first exam objective listed at the beginning of the chapter: recognizing PC power supply terms, concepts, and functions. This section covers a few questions you should expect on the test.

The power supply is a black or silver box with a fan inside and cables coming out of it. It’s located either at the back of a desktop case or at the top of a tower or mini-tower case. The power supply is distinctive because of its big yellow warning label with scary-looking symbols and warnings. The purpose of this label is to warn you not to try to fix a power supply.

 Instant Answer  A caution sticker (you’ll know it — it says “Caution”) on an electric or electronic device in a PC alerts you to possible equipment damage. A warning label (says “Warning” and has lightning bolts, a skull and crossbones, or the like) alerts you to possible electrocution, which could hurt you.

 Shocking Information  Just to quench your curiosity, inside the power supply, one part should keep you out: a 1000-microfarad capacitor. Capacitors store electricity, even when the power is off. This capacitor performs line conditioning by absorbing any power coming in above the normal level and using it to replace power below normal levels. If you were to touch the capacitor, it would shock you — potentially with bodily harm or worse. Because you can buy a new power supply in the range of $25 to $80, I’m not sure it’s worth risking your life to open up the power supply to try to fix it. A good quality power supply should last for years, providing the computer with stable electrical current, if it has been protected adequately. On the other hand, a low quality, faulty, or overloaded power supply can cause all kinds of problems in a system. A bad power supply can cause hard disk drives to develop bad sectors and affect memory to cause what seem like software bugs — problems that are usually hard to pin on the power supply.

Anatomy of the power supply

Be familiar with the parts of the power supply for the exam.

The features you can access on the outside of the power supply are incredibly standard even between form factors. The primary power supply components are

  • Power cord: I think you know what this is.

  • Passthrough connectors: Located on the back of the power supply. In the past, these connectors were used primarily to plug a monitor into the power supply, which enables you to turn the monitor on and off with the computer’s power switch. This feature has all but disappeared from PCs, because it’s no longer necessary to plug the monitor into the power supply.

  • Power switch: Where the main power switch is located on a PC depends on its form factor and its age:

    • On older PCs, this switch extended through the case wall from the power supply on a back corner of the PC.

    • In the newer ATX power supply, the switch is electronic, not physical. You don’t so much turn on or off the computer as you request the motherboard to do it.

  • 110V/220V Selector switch: Allows you to select between the two voltages. If a power supply has one, be sure it’s set correctly. This switch is handy when you jaunt to Europe with your PC.

 Tip  When a monitor is plugged into the power supply’s passthrough connector, the monitor is not being powered by the PC’s internal power supply. It’s called a passthrough plug because it passes the AC power through. You have only gained the convenience of turning the monitor on and off with the PC.

Cooling it

The power supply also contains the main cooling fan that controls airflow through the PC case. The power supply fan is the most important part of a PC’s cooling system. Air is forced to flow through the computer case and over the motherboard and electronic components, which generate heat as they work. Any interruption to the airflow can cause sensitive components to degrade or fail. The power supply fan should be kept clean and clear.

 Instant Answer  Only with the case closed and intact does the PC cooling system function at its optimum level.

 Tip  If a power supply’s wattage rating is sufficient to supply the computer’s electrical requirements, the fan should be adequate to handle the computer’s cooling needs, although Pentium-class processors require additional cooling or heatsinks of their own.

As I cover in more detail later in this chapter, two popular form factors for power supplies exist: the Baby AT and the ATX. I bring up form factors here only because these two types of power supplies cool the system differently. A form factor defines the size, shape, and fit of the components of a case, motherboard, and power supply combination:

  • The Baby AT, which has been the standard until the past year or two, cools the system by pulling air into the case and blowing it out through the fan. You can feel the air blowing out of the fan on this type of power supply. If you’ve ever opened up an old PC AT computer that has been in use for a while without the case being opened, you know firsthand a primary problem with this type of cooling. Room dust, smoke, chalk in school settings, and all else are sucked into the computer to accumulate on internal components such as grills, wires, circuit boards, memory modules, and so on. This buildup can affect the cooling system’s capability of cooling the motherboard and drives by restricting the airflow. Buildup also can possibly short out the motherboard or other components.

  • The ATX form of power supply sucks air into the case. This method helps keep the case clean by pressurizing the inside of the case. The power supply is situated on the board so that air blows straight over the processor and RAM. This was originally intended to eliminate the need for a CPU fan, but nearly all Pentium-class processors include their own fans and heatsinks.

In either case, all expansion slot filler slides should be in place and the case should be in place and intact to allow the cooling system to do its job.

Cooling it beyond cold

Liquid cooling systems usually use ordinary water; some use liquid nitrogen, which is about the coldest liquid there is (except maybe for Lake Tahoe in the early spring), around –196 degrees Celsius.

  • Water cooling systems, called waterblocks (see Figure), are available for CPUs, video processors, and chipsets. They do what you expect: circulate water in a closed system through a cooling agent and over the processor, where the water absorbs heat from the processor.

    Click To expand
    Figure: A CPU waterblock is the component of a water-cooling system that attaches to the heat sink and CPU.

Image courtesy of Swiftech

  • The most sophisticated systems used liquid nitrogen to cool the heat sinks and the processor. Just to frame a reference for you, water freezes solid at 0 degrees Celsius (32 degrees Fahrenheit) and boils at 100 degrees Celsius (212 degrees Fahrenheit). So –196 degrees Celsius is nearly 200 degrees (Celsius) colder than the temperature needed to freeze water. Obviously, this is not something you want to just cobble together for your PC.

See Chapter 6 for more information you need to know for the exam about the methods and devices used to cool a microprocessor.

Converting power

What the power supply does is simple: It converts AC to DC. Many devices can convert current; for example, the power converter I plug into my van’s dashboard so my kids can watch videotapes as we motor along. Another type allows my portable CD player to plug into the wall when the batteries are dead. Another lets my notebook PC run from an AC supply while it charges the batteries. If only it were as simple for the desktop PC as it is for the notebook PC. The PC power supply must provide several voltages at different strengths and manage some power-related signals for the motherboard.

Standard voltages

 Tip  Know this stuff for the test. The power supply provides the following voltages to the motherboard and drives:

  • +5V: The standard voltage of motherboards with all processors below 100 MHz (such as early Pentium, 486, and 386) and many peripheral boards.

  • +12V: Used primarily for disk drive motors and similar devices. Modern motherboards also pass this voltage to ISA bus expansion slots.

  • –5V and –12V: Included in most power supplies for compatibility with older systems. Most modern motherboards don’t use either of these voltages. Power supplies that produce these values do so at very low (less than 1 amp) amperage. Check the label on your power supply.

  • +3.3V: This is the standard voltage level for motherboards compatible with 100 MHz processors and faster. Upgraded motherboards must convert the 5V signal from the power supply into 3.3V for the processor, requiring a voltage regulator on the motherboard. Newer power supplies provide the 3.3V power for the CPU directly.

 Time Shaver  Concentrate on which voltages are used with which types of devices, especially the voltage of all Pentium-class processors, listed in Figure. For example, disk drives use the +12V lines, the Pentium processor uses 3.3V, and the other lines (+/–5V and –12V lines) are primarily used for backward compatibility.

Processor voltages

Processors use two levels of power:

  • External voltage or I/O voltage: Powers the devices mounted to motherboards compatible with a certain processor.

  • Internal voltage: The voltage level used by the micro-components of the processor. The internal voltage number is important; because the higher the voltage, the more heat the processor generates.

Figure shows a trend of smaller internal voltage levels for the Pentium-class processors.

Figure: Processor Voltage Levels

Processor

Version

External

Internal

Pentium

60–66

5

5

Pentium

75–200

3.3/3.52

3.3/3.52

Pentium

MMX

3.3

2.8

6x86

 

3.3

3.3

AMD

K5

3.52

3.52

Pentium Pro

150

3.1

3.1

Pentium Pro

166+

3.3

3.3

Pentium II

(Klamath)

3.3

2.8

Pentium II

Deschutes/Celeron/Xeon

3.3

2.0

Pentium III

Katmai

3.3

2.0

Pentium III

Coppermine/Xeon

3.3

1.65–1.75

Celeron II

Coppermine 128

3.3

1.5–1.65

Pentium IV

423-pin

3.3

1.75

Pentium IV

478-pin 256KB L2

3.3

1.75

Pentium IV

478-pin 512KB L2

3.3

1.5–1.525

AMD Athlon

Socket A

3.3

1.6–1.8

AMD Athlon

XP

3.3

1.75

AMD Duron

Socket A

3.3

1.6

AMD

K6-2+/K6-III+

3.3

1.8–2.0

AMD

K6-III

3.3

2.2

AMD K6-2

w/3DNow

3.3

2.2

AMD K6

266/300

3.3

2.2

AMD K6

233

3.3

3.2

6x86 MX

 

3.3

2.9

Regulating power to the processor

 Remember  Virtually every motherboard has a voltage regulator to control the flow of voltage to the microprocessor. On some motherboards, a replaceable module, called a Voltage Regulator Module, or VRM, is used and on others the voltage regulator is a built-in component of the motherboard itself.

Fitting it in the box

Form factor refers to the shape and dimensions of a device. PC cases are designed to hold a power supply form factor. The power supply must match both the designed form factor of the case and the motherboard’s power requirements. The form factor of a power supply is seldom an issue (except for upgrades and build-your-owns) because the power supply is usually purchased already installed in the case. The form factor of the case is usually more of an issue.

In general, the form factors of the motherboard are the same for the case and the power supply. The most common form factors used today are

  • Baby AT: The oldest standard that has been used for PCs until recently

    The Baby AT is what most people think of as the standard desktop case and power supply. Figure shows the back of a Baby AT power supply. On the left side of the power supply is the power switch intended to extend through a hole in the system case.

    Click To expand
    Figure: The rear panel of a Baby AT power supply.

  • ATX: The newest form factor

    The ATX form factor has essentially replaced the Baby AT for new systems. The ATX power supply differs from the Baby AT power supply in three primary ways:

    • ATX has additional voltage and power lines that signal and control the power supply.

    • The fan blows into the case instead of out as with the Baby AT, which helps keep the case clean.

    • The ATX power supply turns on and off with electronic signaling and not a physical power switch. It can also be switched on and off by software, such as a Windows shutdown.

    Figure shows the back of an ATX power supply.

    Click To expand
    Figure: The rear panel of an ATX power supply.



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