A case can be confining. It can keep just about everything from escaping, including the heat that electronic circuits produce as a by-product of performing their normal functions. Some of the electricity in any circuit (except one made from superconductors) is turned into heat by the unavoidable electrical resistance of the circuit. Heat is also generated whenever an element of a computer circuit changes state. In fact, nearly all the electricity consumed by a computer eventually turns into heat.

Inside the protective (and confining) case of the computer, that heat builds up, thus driving up the temperature. Heat is the worst enemy of semiconductor circuits; it can shorten their lives considerably or even cause their catastrophic failure. Some means of escape must be provided for the excess heat. In truth, the heat build-up in most computers may not be immediately fatal to semiconductor circuits. For example, most microprocessors shut down (or simply generate errors that shut down your computer) before any permanent damage occurs to them or the rest of the components inside your computer. However, heat can cause circuits to age prematurely and can trim the lives of circuit components.

The design of the case of a computer affects how well the machine deals with its heat build-up. A case that’s effective in keeping its internal electronics cool can prolong the life of the system.

Passive Convection

The obvious way to make a computer run cooler is to punch holes in its case to let the heat out—but to keep the holes small enough so that other things such as mice and milkshakes can’t get in. In due time, passive convection—less dense hot air rising with denser cool air flowing in to take its place—lets the excess thermal energy drift out of the case.

Any impediment to the free flow of air slows the passive cooling effect. In general, the more holes in the case, the merrier the computer will be. Remove the lid, and the heat can waft away along with temperature worries.

Unfortunately, your computer’s case should be closed. Keeping a lid on it does more than just restrict cooling—it is also the only effective way to deal with interference. It also keeps your computer quieter, prevents foreign objects and liquids from plummeting in, and gives your monitor a lift.

Moreover, passive cooling is often not enough. Only low-power designs (such as notebook and Green computers) generate little enough heat that convection can be entirely successful. Other systems generate more heat than naturally goes away on its own.

Active Cooling

The alternative to passive cooling is, hardly unexpectedly, active cooling, which uses a force of some kind to move the heat away from the circuits. The force of choice in most computers is a fan.

Usually tucked inside the power supply, the computer’s fan forces air to circulate both inside the power supply and the computer. It sucks cool air in to circulate and blows the heated air out.

The cooling systems of early computers, however, were particularly ill-conceived for active cooling. The fans were designed mostly to cool off the heat-generating circuitry inside the power supply itself and only incidentally cooled the inside of the computer. Moreover, the chance design of the system resulted in most of the cool air getting sucked in through the floppy disk drive slots. Along with the air came all the dust and grime floating around in the environment, polluting whatever media you had sitting in the drive. At least enough air coursed through the machine to cool off the small amount of circuitry that the meager power supply of the computer could provide.

Modern computers do much better. Many have carefully designed air channels to route cooling air over the places most apt to get work (such as memory chips). Some manufacturers opt to put one or more additional fans (besides the one in the power supply) into their systems to keep the overall chassis cool.

Modern microprocessors generate so much waste heat that a single system fan can’t keep them cool. They require an additional means to keep air circulating. Most microprocessors in desktop computers now use fans integrated into their heatsinks to keep the chips cool. Each heatsink design requires a fan custom tailored to it.

Large cooling fans make noise. Some computer manufacturers try to minimize this noise by making their fans thermostatically controlled. They switch on only when your computer needs cooling. Sometimes this feature is controlled through the BIOS; other times it is an invariant feature. If you suspect fan failure (discussed later in this chapter in more detail), be sure your system does not have a thermostatically controlled fan that simply has not switched on.

Many people believe you cannot overdo cooling—more fans is always better. In fact, the cooler you keep your chips, the longer they will last. Heat is their worst enemy. The only downside to blowing hurricanes through your computer is that air is inevitably fouled with dust and lint. A substantial amount of foreign material may accumulate inside your computer, and it may alter or stop airflow in some places in the system, causing them to overheat. To prevent such problems, fans may be equipped with filters. In any case, whenever a computer is operated in an environment likely to cause dust and lint contamination (say, for instance, you have a cat), you should vacuum the inside of your computer periodically to remove any accumulation.

Fan Failure

The fan inside a computer power supply is a necessity, not a luxury. If it fails to operate, your computer won’t falter—at least not at first. But temperatures build up inside. The machine—the power supply in particular—may even fail catastrophically from overheating.

The symptoms of fan failure are subtle but hard to miss. You hear the difference in the noise your system makes. You may even be able to smell components warming past their safe operating temperature.

Should you detect either symptom, hold you hand near where the air usually emerges from your computer. (On most computers, that’s near the big round opening that the fan peers through.) If you feel no breeze, you can be certain your fan is no longer doing its job.

A fan failure constitutes an emergency. If it happens to your system, immediately save your work and shut the machine off. Although you can safely use it for short periods, the better strategy is to replace the fan or power supply as soon as you possibly can.

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Normal line voltage is often far from the 115-volt alternating current you pay for. It can be a rather inhospitable mixture of aberrations such as spikes and surges mixed with noise, dips, and interruptions. None of these oddities is desirable, and some can be powerful enough to cause errors in your data or damage to your computer. Although you cannot avoid them, you can protect your computer against their ill effects.

Power Line Irregularities

Power line problems can be broadly classed into three basic categories: over-voltage, under-voltage, and noise. Each problem has its own distinct causes and requires a particular kind of protection.

Over-Voltage

The deadliest power-line pollution is over-voltage—lightning-like high potential spikes that sneak into your computer and actually melt down its silicon circuitry. Often the damage is invisible—except for the very visible lack of image on your monitor. Other times, you can actually see charred remains inside your computer as a result of the over-voltage.

As its name implies, an over-voltage gushes more voltage into your computer than the equipment can handle. In general—and in the long run—your utility supplies power that’s very close to the ideal, usually within about ten percent of its rated value. If it always stayed within that range, the internal voltage regulation circuitry of your computer could take its fluctuations in stride.

Short duration over-voltages larger than that may occur too quickly for your utility’s equipment to compensate, however. Moreover, many over-voltages are generated nearby, possibly within your home or office, and your utility has no control over them. Brief peaks as high as 25,000 volts have been measured on normal lines, usually due to nearby lightning strikes. Lightning doesn’t have to hit a power line to induce a voltage spike that can damage your computer. When it does hit a wire, however, everything connected to that circuit is likely to take on the characteristics of a flash bulb.

Over-voltages are usually divided into two classes by duration. Short-lived over-voltages are called spikes or transients and last from a nanosecond (billionth of a second) to a microsecond (one millionth of a second). Longer-duration over-voltages are usually termed surges and can stretch into milliseconds.

Sometimes power companies do make errors and send too much voltage down the line, causing your lights to glow brighter and your computer to teeter closer to disaster. The occurrences are simply termed over-voltages.

Most AC-power computers are designed to withstand moderate over-voltages without damage. Most machines tolerate brief surges in the range of 800 to 2,000 volts. On the other hand, power cords and normal home and office electrical wiring break (by arcing between the wiring conductors) at potentials between about 4,000 and 6,000 volts. In other words, electrical wiring limits the maximum surge potential your computer is likely to face to no more than about 6,000 volts. Higher voltage surges simply can’t reach your computer.

Besides intensity and energy, surges also differ in their mode. Modern electrical wiring involves three conductors: a hot, neutral, and ground. Hot is the wire that carries the power; neutral provides a return path; and ground provides protection. The ground lead is ostensibly connected directly to the earth.

A surge can occur between any pairing of conductors: hot and neutral, hot and ground, or neutral and ground. The first pairing is termed normal mode. It reflects a voltage difference between the power conductors used by your computer. When a surge arises from a voltage difference between hot or neutral and ground, it is called common mode.

Surges caused by utility switching and natural phenomena—for the most part, lightning—occur in the normal mode. They have to. The National Electrical Code requires that the neutral lead and the ground lead be bonded together at the service entrance (where utility power enters a building) as well as at the utility line transformer, typically hanging from a telephone pole near your home or office. At that point, neutral and ground must have the same potential. Any external common mode surge becomes normal mode.

Common mode surges can, however, originate within a building because long runs of wire stretch between most outlets and the service entrance, and the resistance of the wire allows the potential on the neutral wire to drift from that of ground. Although opinions differ, recent European studies suggest that common mode surges are the most dangerous to your equipment. (European wiring practice is more likely to result in common mode surges because the bonding of neutral and ground is made only at the transformer.)

Under-Voltage

An under-voltage occurs when your equipment gets less voltage than it expects. Under-voltages can range from sags, which are dips of but a few volts, to complete outages or blackouts. Durations vary from nearly instantaneous to hours—or even days, if you haven’t paid your light bill recently.

Very short dips, sags, and even blackouts are not a problem. As long as they are less than a few dozen milliseconds—about the blink of an eye—your computer should purr along as if nothing happened. The only exceptions are a few old computers that have power supplies with very sensitive Power Good signals. A short blackout may switch off the Power Good signal, shutting down your computer even though enough electricity is available.

Most computers are designed to withstand prolonged voltage dips of about 20 percent without shutting down. Deeper dips or blackouts lasting for more than those few milliseconds result in a shutdown. Your computer is forced to cold start, booting up afresh. Any work you have not saved before the under-voltage is lost.

Noise

Noise is a nagging problem in the power supplies of most electronic devices. It comprises all the spurious signals that wires pick up as they run through electromagnetic fields. In many cases, these signals can sneak through the filtering circuitry of the power supply and interfere with the signals inside the electrical device.

For example, the power cord of a tape recorder might act like an antenna and pick up a strong radio signal. The broadcast could then sneak through the circuitry of the recorder and mix with the music it is supposed to be playing. As a result, you might hear a CB radio maven croaking over your Mozart.

In computers, these spurious signals could confuse the digital thought coursing through the circuitry of the machine. As a practical matter, they don’t. All better computers are designed to minimize the leakage of their signals from inside their cases into the outside world to minimize your computer’s interfering with your radio and television. The same protection that prevents signals leaking out works well in warding off other signals from getting in. Personal computers are almost automatically well-protected against line noise. You probably won’t need a noise filter to protect your computer.

Then again, noise filtering doesn’t hurt. Most power-protection devices have noise filtering built in to them because it’s cheap, and it can be an extra selling point (particularly to people who believe they need it). Think of it as a bonus. You can take advantage of its added protection—but don’t go out of your way to get it.

Over-Voltage Protection

Surges are dangerous to your computer because the energy they contain can rush through semiconductor circuits faster than the circuits can dissipate them—the silicon junctions of your computer’s integrated circuits fry in microseconds. Spike and surge protectors are designed to prevent most short-duration, high-intensity over-voltages from reaching your computer. They absorb excess voltages before they can travel down the power line and into your computer’s power supply. Surge suppressors are typically connected between the various conductors of the wiring leading to your computer. Most short out surges that rise above a preset level.

The most important characteristics of over-voltage protection devices are how fast they work and how much energy they can dissipate. Generally, a faster response time or clamping speed is better. Response times can be as short as picoseconds—trillionths of a second. You get better protection from devices that have higher energy-handling capacities, which are measured in watt-seconds or joules. Devices claiming the capability to handle millions of watts are not unusual.

Four kinds of devices are most often used to protect against surges: metal-oxide varistors (MOVs), gas tubes, avalanche diodes, and reactive circuits. Of these, the MOVs dominate because the parts are inexpensive and they work effectively against most surges.

The MOV is a disc-shaped electronic component typically made from a layer of zinc-oxide particles held between two electrodes. The granular zinc oxide offers a high resistance to the flow of electricity until the voltage reaches a break-over point. The electrical current then forms a low-resistance path between the zinc-oxide particles that shorts out the electrical flow. The energy-handling capability can be increased simply by enlarging the device (typical MOVs are about an inch in diameter; high-power MOVs may be twice that). Figure 31.1 shows a typical MOV.

The downside to MOVs is that they degrade. Surges tend to form preferred paths between the zinc-oxide particles, reducing the resistance to electrical flow. Eventually, the MOV shorts out, blowing a fuse or (more likely) overheating until it destroys itself. The MOV can end its life in flames or with no external change at all—except that it no longer offers surge protection.

Thanks to the laws of thermodynamics, the excess energy in a surge cannot just disappear; it can only change form. With most surge suppression technologies (all except reactive devices), the over-voltage is converted into heat that’s dissipated by the wiring between the device and the origin of the surge as well as inside the surge suppressor itself. The power in a large surge can destroy a surge suppressor so that it yields up its life to protect your computer.

Because they degrade cumulatively with every surge they absorb, MOVs are particularly prone to failure as they age. Eventually, an MOV will fail, sometimes in its own lightning-like burst. Although it’s unlikely this failure will electrically damage the circuits of your computer, it can cause a fire—which can damage not just your computer, but your home, office, or self.

An MOV-based surge suppressor also can fail more subtly—it just stops sucking up surges. Unbeknownst to you, your computer can be left unprotected. Many commercial surge suppressors have indicators designed to reveal the failure of an internal MOV.

In any case, a good strategy is to replace MOV-based surge suppressors periodically to ensure that they do their job and to lessen the likelihood of their failure. How often to replace them depends on how dirty an electrical diet you feed them, but most MOV-based surge protectors should work effectively for a few years before needing replacement.

Blackout Protection

Protecting against total blackouts requires a local source of electricity, either a generator or storage batteries. A local generator is the choice when you want to continue to work as normal, running from local power for hours or days. Battery backup lasts only as long as the batteries, typically a few minutes to allow the orderly shutdown of your computer so you can start back up when normal utility power returns.

Battery backup systems are often called uninterruptible power systems (UPSs) because they supply power continuously, without interruption. UPSs are often used in conjunction with generators to bridge over the few seconds that power would otherwise not be available while the generator is starting. A battery backup system is built around powerful batteries that store substantial current. An inverter converts the direct current from the batteries into alternating current that can be used by your computer. A battery charger built in to the system keeps the reserve power supply fully charged at all times. Long runtime UPSs, which have extra batteries, sometimes substitute for generators, keeping computer servers (particularly those in remote locations where they are not readily accessible) running for hours.

Although the term UPS has become the industry standard for any kind of battery backup system, there are actually two kinds of UPSs, only one of which provides truly uninterruptible power. An offline or standby power system switches the input of your computer from utility power to backup power when the utility fails. An online power system keeps your computer constantly connected to the backup power source, so it never has to switch.

Offline Backup Systems

As the name implies, the standby power system constantly stands by, waiting for the power to fail so that it can leap into action. Under normal conditions—that is, when utility power is available—the battery is offline and its charger draws only a slight current to keep its source of emergency energy topped off. The AC power line from which the offline supply feeds is directly connected to its output, and thence to the computer. The batteries are out of the loop.

When the power fails, the offline supply switches into action—switch being the key word. The current-carrying wires inside the power supply that lead to your computer are physically switched (usually by a mechanical relay) from the utility line to the current coming from the battery-powered inverter.

Most offline power systems available today switch within one-half of one cycle of the AC current they are supplied—that’s less than ten milliseconds, quick enough to keep nearly all computers running as if no interruption occurred. Although the standby power system design does not protect again spikes and surges, most offline power systems have other protection devices installed in their circuitry to ensure that your computer gets clean power.

Line-interactive power systems are offline designs with an added feature. They react to changes in line voltage and compensate when the voltage gets too high or too low. To change the voltage, they use multitap autotransformers. When the voltage falls low, the line-interactive UPS switches to a tap that boosts the voltage back to the appropriate level. When the voltage gets too high, it switches to a different tap that bucks the voltage down, reducing it. If the line voltage is too far from normal for the transformer taps to compensate, the UPS reacts as if it suffered a power failure, switching to battery power. The line-interactive design offers the big benefit of compensating for most brownouts without draining its battery reserves.

Online Backup Systems

Online backup systems use several designs to guarantee that there is never an interruption in their output power. The traditional design is the double-conversion UPS. These devices earn their name from converting the power twice. First, incoming utility power is transformed down to battery level and rectified to direct current. Then this direct current is inverted and transformed back up to utility voltage to be supplied to your computer. The process would seem to be wasteful and redundant except that a set of batteries connects in the middle. When utility power is available, the constant supply at low voltage keeps the batteries charged. When utility power fails, the batteries maintain the low voltage that feeds the inverter, so the power to your computer is never interrupted.

This traditional double-conversion design is wasteful and expensive. It requires two large transformers capable of carrying the entire load (your computer’s and its peripherals), which are very costly and resistant to miniaturization technology. UPS-makers consequently have shifted to a slightly different design that eliminates the transformers. These newer double-conversion UPSs simply rectify the incoming utility voltage to DC and then send it to an inverter, which changes it back to AC. The batteries connect in the middle, but instead of a direct link, they couple through a DC-to-DC converter that matches their voltage to the power line. DC-to-DC converters are high-tech electronic devices that cost substantially less than conventional transformers. As a result, this kind of double-conversion UPS not only has replaced other online designs, but it can also be cost-competitive with offline designs.

In effect, a double-conversion UPS acts like your computer’s own generating station, one that is only inches away from the machine it serves. It keeps your system safe from the polluting effects of lightning and load transients found on long-distance power lines. Moreover, dips and surges can never reach the computer. Instead, the computer gets a genuinely smooth, constant electrical supply, exactly like the one for which it was designed.

An alternate online design is more like an offline standby system but uses clever engineering to bridge over even the briefest switching lulls. These UPSs connect both the input power and the output of their inverters together through a special transformer, which is then connected to your computer or other equipment to be protected. Although utility power is available, this kind of UPS supplies it through the transformer to your computer. When the utility power fails, the inverter kicks in, typically within half a cycle. The inductance of the transformer, however, acts as a storage system and supplies the missing half-cycle of electricity during the switchover period.

The double-conversion UPS provides an extreme measure of surge and spike protection (as well as eliminating sags) because no direct connection bridges the power line and the protected equipment—spikes and their kin have no pathway to sneak in. Although the transformer in the new style of UPS absorbs many power-line irregularities, overall it does not afford the same degree of protection. Consequently, these newer devices usually have other protection devices (such as MOVs) built in.

Specifications

The most important specification to investigate before purchasing any backup power device is its capacity as measured in volt-amperes (VA) or watts. This number should always be greater than the rating of the equipment to which the backup device is to be connected.

In alternating current (AC) systems, watts do not necessarily equal the product of volts and amperes (as they should by the definition that applies in DC systems) because the voltage and current can be out of phase with one another. That is, when the voltage is at a maximum, the current in the circuit can be at an intermediary value. So the peak values of voltage and amperage may occur at different times.

Power requires both voltage and current simultaneously. Consequently, the product of voltage and current (amperage) in an AC circuit is often higher than the actual power in the circuit. The ratio between these two values is called the power factor of the system.

What all this means to you is that volt-amperes and watts are not the same thing. Most backup power systems are rated in VA because it is a higher figure thanks to the power factor. You must make sure the total VA used by your computer equipment is less than the VA available from the backup power system. Alternatively, you must make sure that the wattage used by your equipment is less than the wattage available from the backup power system. Don’t indiscriminately mix the VA and watts in making comparisons.

To convert a VA rating to a watt rating, multiply the VA by the power factor of the backup power supply. To go the other way—watts to VA—divide the wattage rating of the backup power system by its power factor. (You can do the same thing with the equipment you want to plug into the power supply, but you may have a difficult time discovering the power factor of each piece of equipment. For computers, a safe value to assume is 2/3.)

Both online and offline backup systems also are rated as to how long they can supply battery power. This equates to the total energy (the product of power and time) that they store. Such time ratings vary with the VA the backup device must supply—because of finite battery reserves, it can supply greater currents only for shorter periods. Most manufacturers rate their backup systems for a given minutes of operation with a load of a particular size instead of in more scientific fashion using units of energy. For example, a backup system may be rated to run a 250 volt-ampere load for 20 minutes.

If you want an idea of the maximum possible time a given backup supply can carry your system, check the ratings of the batteries it uses. Most batteries are rated in ampere-hours, which describes how much current they can deliver for how long. To convert that rating to a genuine energy rating, multiply it by the nominal battery voltage. For example, a 12-volt, 6 amp-hour battery could, in theory, produce 72 watt-hours of electricity. That figure is theoretical rather than realistic because the circuitry that converts the battery DC to AC wastes some of the power and because ratings are only nominal for new batteries. However, the numbers you derive give you a limit. If you have only 72 watt-hours of battery, you can’t expect the system to run your 250 VA computer for an hour. At most, you could expect 17 minutes; realistically, you might expect 12 to 15.

You probably will not need much time from a backup power system, however. In most cases, five minutes or less of backup time is sufficient because the point of a backup supply is not to keep a system running forever. Instead, the backup power system is designed to give you a chance to shut down your computer without losing your work. Shutting down shouldn’t take more than a minute or two.

UPS-makers warn that no matter the rating of your UPS, you should never plug a laser printer into it. The fusers in laser printers are about as power hungry as toasters—both are resistive heaters. The peak power demand when the fuser switches on can overload even larger UPSs, and the continuing need for current can quickly drain batteries. Moreover, there’s no need to keep a print job running during a power failure. Even if you lose a page, you can reprint it when the power comes back at far less expense than the cost of additional UPS power capable of handling the laser’s needs. Some printers, such as inkjets, are friendlier to UPSs and can safely be connected, but you’ll still be wasting capacity. The best strategy is to connect only your computer, your monitor, and any external disk drives to the UPS. Plug the rest of your equipment into a surge suppresser.

To handle such situations, many UPSs have both battery-protected outlets and outlets with only surge protection. Be sure to check which outlets you use with your equipment, making sure your computer has battery-backed protection.

Interfaces

An ordinary UPS works effectively if you’re sitting at your computer and the power fails. You can quickly assess whether it looks like the blackout will be short or long (if the world is blowing away outside your window, you can be pretty sure any outage will be prolonged). You can save your work, haul down your operating system, and shut off your computer at your leisure. When a computer is connected to a network or is running unattended, however, problems can arise.

During a prolonged outage, a simple UPS only prolongs a disaster with an unattended computer—it runs another dozen minutes or so while the power is off, then the UPS runs out of juice and the computer plummets with it. Of course, if a server crashes without warning, no one is happy, particularly if a number of files were in the queue to be saved.

To avoid these problems, better UPSs include interfaces that let them link to your computer, usually through a serial port. Install an appropriate driver, supplied by the UPS-maker, and your computer can monitor the condition of your power line. When the power goes off, the software can send messages down the network warning individual users to save their work. Then, the UPS software can initiate an orderly shut down of the network.

Some UPSs will continue to run even after your network or computer has shut itself down. Better units have an additional feature termed inverter shutdown that automatically switches off the UPS after your network shuts down. This preserves some charge in the batteries of the UPS so that it can still offer protection if you put your computer or network back online and another power failure follows shortly thereafter. A fully discharged UPS, on the other hand, might not be ready to take the load for several hours.

Battery Life

The gelled electrolyte batteries most commonly used in uninterruptible power systems have a finite life. The materials from which they are made gradually deteriorate, and the overall system loses its ability to store electricity. After several years, a gelled electrolyte battery will no longer be able to operate a UPS, even for a short period. The UPS then becomes nonfunctional. The only way to revive the UPS is to replace the batteries.

Battery failure in a UPS usually comes as a big surprise. The power goes off and your computer goes with it, notwithstanding your investment in the UPS. The characteristics of the batteries themselves almost guarantee this surprise. Gelled electrolyte batteries gradually lose their storage capacity over a period of years, typically between three and five. Then, suddenly, their capacity plummets. They can lose nearly all their total storage capability in a few weeks. Figure 31.2 illustrates this characteristic of typical gelled electrolyte batteries.

Note that the deterioration of gelled electrolyte batteries occurs regardless of whether they are repeatedly discharged. They deteriorate even when not used, although repeated heavy discharges will further shorten their lives.

To guard against the surprise of total battery failure, better UPSs incorporate an automatic testing mechanism that periodically checks battery capacity. A battery failure indication from such a UPS should not be taken lightly.

Phone and Network Line Protection

Spikes and surges affect more than just power lines. Any wire that ventures outside holds the potential for attracting lightning. Any long wiring run is susceptible to induced voltages, including noise and surges. These over-voltages can be transmitted directly into your computer or its peripherals and cause the same damage as a power line surge.

The good news is that several important wiring systems incorporate their own power protection. For example, Ethernet systems (both coaxial and twisted pair) have sufficient surge protection for their intended applications. Apple LocalTalk adapters are designed to withstand surges of 2000 volts with no damage. Because they are not electrical at all, fiber-optical connections are completely immune to power surges.

The bad new is that two common kinds of computer wiring are not innately protected by surges. Telephone wiring runs long distances through the same environments as the power-distribution system and is consequently susceptible to the same problems. In particular, powerful surges generated by direct lightning hits or induction can travel through telephone wiring, through your modem, and into the circuitry of your computer. In addition, ordinary serial port circuitry includes no innate surge suppression. A long unshielded serial cable can pick up surges from other cables by induction.

The best protection is avoidance. Keep unshielded serial cable runs short whenever possible. If you must use a long serial connection, use shielded cable. Better still, break up the run with a short-haul modem, which will also increase the potential speed of the connection.

Modem connections with the outside world are unavoidable in these days of online connectivity and the Internet. You can, however, protect against phone-line surges using special suppressors designed exactly for that purpose. Better power-protection devices also have modem connections that provide the necessary safeguards. Standalone telephone surge suppressors are also available. They use the same technologies as power-line surge suppressors. Indeed, the voltage that rings your telephone is nearly the same as the 110–120 volt utility power used in the United States. Most phone-line suppressors are based on MOV devices. Better units combine MOVs with capacitors, inductors, and fuses.

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With few advances in battery storage density expected in the near-term future, computer-makers have relied on reducing the power consumption of their notebook computers to extend the time a machine can operate between battery charges.

Engineers can use two basic strategies to reduce the power consumption of computers. They can design circuits and components to use less power, and they can manage the power used by the devices. Managing power needs usually means switching off whatever system components aren’t being actively used. Although the two design methods can be used separately, they are generally used in tandem to shrink computer power needs as much as possible.

Microprocessors, the most power hungry of computer circuits, were among the first devices to gain built-in power management. System Management Mode endowed processors with the ability to slow down and shut off unnecessary circuits when they were idle. Similarly, makers of hard disk drives have added sleep modes to spin down their platters and reduce power needs. Most computers also incorporate timers to darken their screens to further conserve power.

Although these techniques can be successful in trimming power demands, they lack a unified control system. In response, the industry developed the Advanced Power Management (APM) interface to give overall control to the power-savings systems in computers. More recently, APM has been updated and augmented by the Advanced Configuration and Power Interface specification.

Advanced Power Management

The Advanced Power Management interface specification was jointly developed by Intel and Microsoft to integrate the control of hardware power-saving features with software control. First published in January 1992, as the APM BIOS Interface Specification, the current version, 1.2, was published in February 1996.

Although nominally a BIOS interface, the APM specification describes a layered control system that manages computer devices to reduce power consumption using both BIOS and API interfaces. To be fully functional, APM requires a compatible BIOS and hardware devices that recognize APM control. In addition, hardware devices may have their own built-in automatic power-management functions that are not controlled by your computer’s software. For example, a hard disk drive may automatically power down after a given period without a specific command from your computer. The APM specification tolerates but does not affect these built-in functions.

States

APM is an overall system feature. Although it has the ability to individually control the features of each device it manages, the basic-design APM controls all devices together to conserve power. It manages system power consumption by shifting the overall operating mode of the computer, called APM states. APM shifts the operating state of the system based on the needs of the system as determined from a combination of software commands and events. The various APM states provide for power savings that occur in five levels (six, if you count normal power-hungry operation). The APM specification gives each of these levels a specific state name.

The first state, Full On, means that the system is operating at full power without any management at all. The APM software is not in control, and no power savings can be achieved. A system without APM or with its APM features disabled operates in Full On state.

When the APM system is active, all devices run in their normal, full-power consumption modes. The system is up and ready to do business, operating in what the specification calls the APM Enabled state.

In the APM Standby state, the microprocessor may stop, and many of the system devices are turned off or operate at reduced power. The system usually cannot process data, but its memory is kept alive and the status of all devices is preserved. When your activity or some other event requires system attention, the computer can rapidly shift from the APM Standby to the APM Enabled state.

In the APM Suspend state, the system shifts to its maximum power-savings mode—most devices that follow the APM standard are switched off, and the microprocessor switches to its lowest power state with its clock turned off. Your computer becomes a vegetable.

Hibernation is a special implementation of the APM Suspend state that allows the system to be switched entirely off and still be restored to the point at which it entered the APM Suspend state. When entering the APM Suspend state, the system saves all its operating parameters. In entering the Hibernation state, the system copies memory and other status data to nonvolatile storage, such as the hard disk, allowing you to switch off memory power. A system event can shift back to the APM Enabled state from APM Suspend or Hibernation, but changing modes from APM Suspend to APM Enabled takes substantially longer than from APM Standby to APM Enabled.

The Off state is exactly what the name implies. Power to the system is entirely off. The computer is more a mineral than vegetable. The only event that restores the system is turning it back on. If you enter the Off state directly—say, by switching your computer off—no status information or memory gets saved. The system must run through the entire boot-up process and starts with a clean slate.

Structure

APM adds a layered control system to give you, your software, and your hardware a mechanism to shift states manually or automatically.

The bottom layer of the system is the APM BIOS, which provides a common software interface for controlling hardware devices under the specification. APM specifies that the BIOS have at least a real-mode interface that uses interrupt 15(hex) to implement its functions. In addition, the APM BIOS may also use 16- or 32-bit protected mode using entry points that are returned from the protected-mode connection call using the real-mode interrupt.

The APM BIOS is meant to manage the power of the motherboard. Its code is specific to a given motherboard. Under the APM specification, the APM BIOS can operate independently of other APM layers to effect some degree of power savings in the system by itself. Your computer’s operating system can switch off this internal BIOS APM control to manage system power itself, still using the APM BIOS interface functions to control hardware features.

Linking the APM BIOS to your operating system is the APM driver. The driver provides a set of function calls to the operating system, which it translates to BIOS interrupts. The driver is more than a mere translator, however. It is fully interactive with both the BIOS and operating system. For example, the BIOS may generate its own request to power down the system, and the driver then checks with the operating system to determine whether it should permit the power-down operation.

The APM system has a built-in failsafe. The APM driver must interact with the BIOS at least once per second. If it does not, after a second, the BIOS assumes the operating system has malfunctioned and takes self-contained control. The driver can regain control by sending the appropriate commands (interrupts) to the BIOS.

Certain system events termed wake-up calls tell the APM system to shift modes. Interrupts generated by events such as a press of the resume button, the modem detecting an incoming telephone ring, or an alarm set on the real-time clock can command the APM BIOS to shift the system from the APM Suspend to the APM Enabled state.

Advanced Configuration and Power Interface

Customizing the power use of your computer is a natural part of setting up your system. Understanding that, Intel, Microsoft, and Toshiba decided that the best way to integrate power management with your system was to combine it with the computer’s setup facilities and to give both a common interface. The result is called the Advanced Configuration and Power Interface, a formal specification that was first published by the threesome as ACPI version 1.0 in December 1996.

ACPI is an integral part of the Microsoft-inspired OnNow initiative, which was created to minimize the delays inherent in starting up and shutting down a computer burdened with megabytes of operating system overhead, to let the computer run tasks while it appears to be off, and to lower the overall power requirement of the computer. At the time OnNow was proposed, operating systems required time to test the host computer, to check out Plug-and-Play devices, and to set up their structures. These functions required a lengthy period to carry out, so booting your system took so long it seemed to be warming up from absolute zero. OnNow was designed to eliminate that wait. The first inkling of OnNow appeared in Windows Me, and Windows XP continues to develop the concept.

OnNow also sought to integrate the power and configuration interfaces of modern (meaning Windows) operating systems so that programmers can write to a common standard.

To bring these features to life, the OnNow design moves the operating system to the center of power management using ACPI and builds a new table structure for storing and organizing configuration information.

As a power-management system, the ACPI specification can accommodate the needs of any operating system, integrating all the necessary power-management features required in a computer, from the application software down to the hardware level. It enables the operating system to automatically turn on and off and adjust the power consumption of nearly any peripheral, from hard disk drives to displays to printers. It can reach beyond the computer to other devices that may be connected into a single system some time in the future—televisions, stereos, VCRs, telephones, and even other appliances. Using the Smart Battery specification, under ACPI the operating system takes command of battery charging and monitoring. It also monitors the thermal operation of the system, reducing speed or shutting down a computer that might overheat.

The ACPI standard itself defines the interface for controlling device power and a means of identifying hardware features. The interface uses a set of five hardware registers that are controlled through a higher-level application programming interface through the operating system. The descriptive elements identify not only power management but also device features through a nested set of tables. The ACPI standard supplements Plug-and-Play technology, extending its existing structure with an architecture-independent implementation, and replaces the Plug-and-Play BIOS with a new ACPI BIOS.

Soft Off

The fundamental and most noticeable change made by ACPI is the power button on the front of new computers. In systems equipped to handle ACPI, this is a soft switch or set of two switches. Although one of these switches may be labeled “Power” and imply that it is an on/off switch, in the ACPI scheme of things the power switch does not actually switch the power to the system on and off. Rather, it sends a command to the system to shut itself off—and not exactly what you think is off.

Using the front panel Off button actually puts the computer in a new mode called Soft Off. In this mode, the computer acts like you’ve shut it off and requires rebooting to restart it. But it doesn’t remove all power from the system. A slight bit of power continues to be supplied to the motherboard and expansion boards, enabling them to monitor external events. For example, a network board will still listen to network traffic for packets targeted at it. A modem or fax board may lie in wait for a telephone call. Or you may set a time (such as midnight) at which the tape backup system starts. When any of these designated external events occurs, the computer automatically switches itself back on to deal with it.

ACPI envisions that some manufacturers will also put a sleep switch (or standby button) on the front panel or on the keyboard. Pressing it puts the computer in a sleep mode that uses somewhat more power than Soft Off but allows the system to resume operation more quickly.

States

As with APM, the ACPI design works by shifting modes called ACPI states. The states differ substantially from those in APM. Under ACPI, four basic types of states are defined—Global, Special Sleep, Microprocessor, and Device—and these can be further subdivided. Most importantly, ACPI lets the operating system control all aspects of the power consumption of a computer by shifting the single devices or the entire system between these states.

The ACPI Global states most closely correspond to the APM modes, affecting the entire operation of the computer and how you deal with it. Formally, the states are termed G0 through G3, but they are effectively the same as the APM states, with the addition of Soft Off. The chief operational difference between them is how long it takes your computer to move from each state to normal operation, which can be instantaneous in Sleeping state (corresponding to Suspend), a bit longer in Soft Off (similar but not identical to Hibernation), and a full boot-up from Power Off. Table 31.4 lists the Global ACPI states.

Under ACPI, each device, such as your hard disk, modem, or display screen, operates under several similar states (but termed with the letter D) corresponding to various power-saving modes. Similarly, the microprocessor (designed with state names starting with C) can switch to various power-saving states. As a device is needed, your operating system can switch it on or off using the ACPI protocols. Most of these changes happen invisibly—you may only detect a slight (or longer) lag as a device powers up from a lower state.

Configuration

To handle its configuration function, ACPI must manage a tremendous amount of data, not only about the power needs and management capabilities of the system but also describing the features available for all the devices connected to the system. ACPI stores this information in a hierarchy of tables.

The overall master table is called the Root System Description Table. It has no fixed place in memory. Rather, upon booting up, the BIOS locates a pointer to the table during the memory scan that’s part of the boot-up process. The Root System Description Table itself is identified in memory because it starts with the signature “RSDT.” Following the signature is an array of pointers that tells the operating system the location of other description tables that provide it with the information it needs about the standards defined on the current system and individual devices.

One of these tables is called the Fixed ACPI Description Table. In it the operating system finds the base address of the registers used for controlling the power-management system. In addition, the Fixed ACPI Description Table also points to the Differentiated System Description Table, which provides variable information about the design of the base system. Some of the entries in this table are differentiated definition blocks, which can contain data about a device or even a program that sets up other structures and defines new attributes. ACPI defines its own languages for programming these functions.

Energy Star

Rather than a technology, the Energy Star standard is a goal. The Energy Star standard itself was created by the United States Environmental Protection Agency in 1992 with the goal of encouraging manufacturers to create business equipment that minimizes power consumption. Since then, the standard has been adopted by Japan, New Zealand, and Sweden.

Energy Star is a certification program that allows compliant computers and peripherals (as well as other devices unrelated to computers) to wear a certification badge. The program specifically covers computers, monitors, printers, fax machines, copiers, scanners, and multifunction devices—as well as telephones, home appliances (including washing machines), and even entire buildings. Equipment conforming to the Energy Star standard must meet strict guidelines on power consumption published by the Environmental Protection Agency.

Manufacturers have a strong incentive to embrace the Energy Star standard and put the label on their products—some businesses and many federal contracts require that new PC equipment meet the Energy Star standards.

All that said, the actual Energy Star standards for PCs are quite simple. Energy Star version 2.0, which applies to products shipped after October 1, 1995, asks only that a PC or monitor be able to switch to a low-power mode that consumes less than 30 watts after 15 to 30 minutes of inactivity (a default you are allowed to adjust). Combination monitor-and-PC units are allowed the full 60 watts. Printers able to generate seven or fewer pages per minute must reduce their drain to 15 watts after 15 minutes; 14 or fewer ppm, 30 watts after 30 minutes; faster or high-end color printers, 45 watts after an hour.

You can find out more about the Energy Star program at its Web site, www.energystar.gov

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Outside of the physical difference in packaging, notebook computers differ from desktop machines most significantly in that they are designed with self-contained power systems. They hold their own electricity and can run on batteries. The power versatility is what gives the notebook machine the ability to compute anywhere.

But batteries play a larger role in modern computing. Wireless keyboards and mice, digital cameras, and MP3 players all rely on batteries as their primary source of power (as likely does your cell phone and a host of other electronic gadgets you take for granted). It’s enough to make you want to buy stock in a battery company—and shudder to think what all those throw-away batteries are doing to landfills.

Batteries represent the primitive side of electricity, the chemical side. It’s a territory strewn with things that get your hands dirty, including smudgy black carbon and a host of toxic materials you hope you never do get on your fingers. Batteries have stubbornly refused to give in to micro-miniaturization, yielding only small increases in capacity with every investment in research and development.

Electrical Characteristics

We often forget the chemical nature of batteries because the chemistry is all sealed away, usually permanently. To use the battery usually is a small cylinder that produces electricity (or, in the case of batteries for notebook computers, an expensive plastic shell that doesn’t hold nearly enough electricity). In any case, the outward manifestations of the battery are physical and electrical. Its physical size and shape determine where it will fit. Its electrical ratings determine what it can run.

The most popular batteries for small electronic devices come in standard sizes. The battery packs for notebook computers are usually tailored to a specific model of machine, although many such packs comprise a set of standard batteries permanently connected together. All batteries produce the same kind of electricity—direct current—but they vary in the amount of energy they can store and several other electrical characteristics.

Cell Types

Batteries can be divided into two types: primary and secondary or storage. In primary batteries, the creation of electricity is irreversible; one or both of the electrodes is altered and cannot be brought back to its original state except by some complex process (such as re-smelting the metal). Secondary or storage batteries are rechargeable; the chemical reaction is reversible by the application of electricity. The electrons can be coaxed back from whence they came. After the battery is discharged, the chemical changes inside can be reversed by pumping electricity into the battery again. The chemicals revert back to their original, charged state and can be discharged to provide electricity once again.

In theory, any chemical reaction is reversible. Clocks can run backwards, too. And pigs can fly, given a tall enough cliff. The problem is that when a battery discharges, the chemical reaction affects the electrodes more in some places than others; recharging does not necessarily reconstitute the places that were depleted. Rechargeable batteries work because the chemical changes inside them alter their electrodes without removing material. For example, an electrode may become plated with an oxide, which can be removed during recharging.

Primary and secondary (storage) batteries see widely different applications, even in computers. Nearly every modern computer has a primary battery hidden somewhere inside, letting out a tiny electrical trickle that keeps the time-of-day clock running while the PC is not. This same battery also maintains a few bytes or kilobytes of CMOS memory to store system configuration information. Storage batteries are used to power just about every notebook computer in existence. (A few systems use storage batteries for their clocks and configuration memory.)

Voltage

The most important of these is voltage, which describes the electrical potential of a battery, the force with which the battery can move electrons through circuitry. The technical term is electromotive force (EMF), but most people usually talk about its direct measure, volts.

All batteries have a voltage rating that is both unchangeable and varying. That is, the voltage of a battery cell is characteristic of the cell design and the chemical reaction taking place inside, and this reaction does not change. But the voltage produced by the reaction varies with temperature (most batteries produce lower voltage as the temperature declines), the age of the cell (most batteries produce lower voltage as they age), and load (most batteries produce lower voltages when they are called upon to deliver more current).

These factors result in battery voltage varying widely from the nominal or rated voltage. Cells may start life producing 1.8 volts and remain useful until their output falls to half that. Because of the wide variance of cell voltage, most equipment that uses battery power is either insensitive to the exact voltage supplied or regulates the supplied voltages so that the internal circuitry of the equipment sees a constant voltage no matter the exact voltage produced by battery cells. Consequently, typical commercial cells that use carbon-zinc (nominally rated at 1.5 volts), nickel-cadmium (nominally rated at 1.2 volts), and lithium disulfide (nominally rated at 1.6 volts) are essentially interchangeable.

Depending on the chemistry used, a single cell can produce anywhere from a small fraction of a volt to somewhat more than three volts. Batteries rated with voltages higher than about three are composites of several cells linked together. (Technically, the term battery describes a collection of several individual electrochemical cells, although in common usage a commercial battery may only be a single cell.)

Current

Current describes the number of electrons the potential can push, the quantity of electricity. Current is measured in amperes (named after the French mathematician and physicist André Marie Ampére, 1775–1836), usually clipped to the term amps.

Battery cells are limited in the current they can produce by their designs and chemistries. In theory, if the entire chemical reaction in a battery cell occurred instantly, the cell would produce unlimited current—for an instant. Practical factors limit the chemical reaction rate and the current a cell can produce. Chief among these are the basic reaction rate of the chemicals, the design of the cell, and the area over which the reaction takes place. Consequently, some cells are inherently able to produce high currents. Others can only product weak currents. For example, the currents produced by lead-acid batteries and nickel-cadmium cells are so high, such batteries can melt metals and start fires when shorted out. Put an unpackaged ni-cad battery in your pocket, and it may short out against your keys and loose change. The high current and heat from the short circuit could start a fire. Consequently, these high-current cells often wear warning labels.

Cell size is also an important factor in determining the reaction area of the cell chemistry and consequently the current-creating capabilities of the cell. Making a cell larger increases the current it can produce, so heavy-duty applications often require large cells. “D” cells can produce more current than “AA” cells.

The various factors in cell design and chemistry essentially reduce to a single mathematical factor—the equivalent internal resistance of the cell, which determines current capabilities. A low internal resistance allows high currents.

Energy and Capacity

Voltage and current are instantaneous values that describe battery characteristics that are relevant when determining what kind of device the battery can power. Electrical circuits and motors vary in their voltage and current needs, and they must be tailored to match the battery used to power them.

The actual power that a battery can produce is the product of the voltage and current and is described in watts (named in honor of Scottish inventor and engineer James Watt, 1736–1819). A battery’s power is independent of its size—a battery designed to produce a high current can generate tremendous power, although briefly. For example, even a small AA ni-cad battery can create enough current to melt metal, but it wouldn’t last very long when challenged with the task of running a notebook computer.

A more relevant measure is the energy a given battery can produce. Energy is the amount of power a battery can produce over an extended period. A common measure is the watt-hour, the steady production of one watt of power for one hour.

The rated capacity of a cell or battery is the amount of electricity or electric charge it can produce when fully charged under specified conditions. As with voltage, the actual amount of charge the battery can produce varies with its temperature and the discharging current.

In science, the standard unit for measuring battery capacity is the coulomb (named after French physicist C. A. de Coulomb, 1736–1806), which describes the time the battery can produce a given current. One coulomb is one ampere produced for one second. In practice, however, cell or battery capacity is more commonly expressed in ampere-hours (AH) or milliampere-hours (mAH), equal to 3600 times the coulomb rating. The total energy in a battery is its capacity multiplied by its voltage (which results in a measurement of watt-hours).

Storage Density

The ratio of capacity to the weight (or size) of the battery is called the storage density of the battery. For you, as a battery user and the person charged with carrying around a notebook computer weighed down with batteries, this is a most important measure. The storage density of a battery determines how heavy a load of batteries your computer needs for a given runtime. The higher the storage density of a battery, the more energy that can be stored in a given size or weight of a cell and, hence, the more desirable the battery.

The chemistry used by a battery to store or produce electricity is the primary factor in determining the storage density of a battery. Table 31.2 lists the storage density of the major chemical systems used in storage batteries for personal computer and cell phone applications, expressed in watt-hours per kilogram of weight (Wh/kg).

The storage density of each chemistry falls into a range of values because the actual construction of a battery cell—the materials used and the layout—also affects its storage density.

Shelf Life

No battery stores energy forever. The chemicals inside the cells inevitably react and slowly degrade. As a result, the charge stored by the battery degrades as well. The degradation takes two forms.

Some of the chemical reactions permanently affect the ability of the cell to store chemical energy. After a while, the battery loses its usefulness and becomes nothing more than a colorful piece of clutter. The period during which the cell remains useful is termed its shelf life. The chemistry and construction of the cell determine its shelf life, as do the conditions of storage. Some cells, such as modern lithium designs, have shelf lives in excess of ten years, whereas other cells may deteriorate in a matter of weeks (for example, zinc-air batteries once activated). Poor storage conditions—especially high temperatures—usually accelerate the degradation of cells, whereas refrigeration (and, with some chemistries, freezing) often prolongs shelf life.

In secondary cells, the reversible chemical reactions that produce electricity slowly take place even when the cell is not used. These reactions discharge the cell as if it were being used and are consequently called self-discharge. As with normal discharge of the cells, the reactions of self-discharge are reversible by simply recharging the cells. The self-discharge rates of batteries vary with the same factors affecting shelf life, although in modern cells the chemistry and cell design are the major determinants. Some chemistries lose as much as 10 percent of their charge in a day; others less than 1 percent.

Chemical Systems

The chemical reactions in the cell are the most important factor constraining energy density and the usefulness of batteries. In fact, the entire history of battery technology has been mostly a matter of finding and refining battery chemistries to pack more energy in ever-smaller packages. Today’s batteries use a variety of chemical systems, some dating from the late 19th Century, as mentioned previously, and some hardly a decade old. The diversity results from each having distinct benefits for particular applications. The following battery chemistries are the most popular for portable computer, cell phone, power system, and peripheral applications.

Carbon-Zinc

The starting point for battery technology is the carbon-zinc cell, the heir of Georges Leclanché’s 1866 invention of the wet cell for producing electricity. Carbon-zinc cells are probably the most common batteries in the world, known under a variety of names, including dry cell and flashlight battery. When you think of batteries, it’s likely that carbon-zinc cells first come to mind. One company alone, Energizer, sells over six billion carbon-zinc cells each year. They are the lowest priced primary cells. They also have the lowest storage density of any common battery.

One reason carbon-zinc cells are so popular is that the name actually describes two or three different chemistry systems. These include Leclanché cells, zinc chloride cells, and alkaline batteries.

The name describes the basic chemistry of the cells. In the basic carbon-zinc cell, the “carbon” in the name is a cathode current collector—a carbon rod in the center of the cell. The actual material of the cathode is a mixture of manganese dioxide, carbon conductor, and electrolyte. The zinc serves as the anode and often serves as the metal shell of the battery. The electrolyte is a complex mixture of chemicals that typically includes ammonium chloride, manganese dioxide, and zinc chloride.

The electrolyte is the chief difference between Leclanché and zinc-chloride cells. The former use a slightly acidic mix of ammonium chloride and zinc chloride in water. The electrolyte in zinc-chloride cells is mostly zinc chloride. Zinc chloride cells produce a slightly higher open-circuit voltage than Leclanché cells (1.6 versus 1.55 volts).

Although zinc-chloride cells typically have a greater capacity than Leclanché cells, this difference shrinks under lighter loads, so zinc-chloride cells are often termed heavy-duty. In any case, the efficiency of any carbon-zinc cell decreases as the load increases—doubling the current drain more than cuts in half the capacity of the cell. The most efficient strategy is to use as large a cell as is practical for a given application. That’s why power-hungry toys demand “D” batteries and low-drain transistor radios make do with “AA” cells.

Alkaline batteries, no matter the advertised claims, are little more than an enhancement of 19th Century carbon-zinc technology. The biggest change in chemistry is an alteration to the chemical mix in the electrolyte that makes it more alkaline (what did you expect?). This change helps to increase storage density and shelf life of the cells.

The construction (as opposed to chemistry) of alkaline cells differs significantly from ordinary carbon-zinc cells, however. Alkaline cells are effectively turned inside-out. The shell of the alkaline battery is nothing more than that—a protective shell—and it does not play a part in the overall chemical reaction. The anode of the cell is a gelled mixture of powered zinc combined with the electrolyte (itself a mixture of potassium hydroxide—a strong alkali—and water), and the combination is linked to the negative terminal of the cell by a brass spike running up the middle of the cell. The cathode, a mixture of carbon and manganese dioxide, surrounds the anode and electrolyte, separated by a layer of nonwoven fabric such as polyester.

Depending on the application, alkaline cells can last for four to nine times the life of more traditional carbon-zinc cells. The advantage is greatest under heavy loads that are infrequently used—that is, something that draws heavy current for an hour once a day rather than a few minutes of each hour.

Carbon-zinc cells nominally produce 1.5 volts, but this full voltage is available during the initial discharge of the cell. The voltage of the cell diminishes as the load to the cell incr

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The intermediary that translates AC from your electrical outlets into the DC that your computer’s circuits need is called the power supply. As it operates, the power supply of your computer attempts to make the direct current supplied to your computer as pure as possible, as close to the ideal DC power produced by batteries. The chief goal is regulation, maintaining the voltage as close as possible to the ideal desired by the circuits inside your computer.

The power needs of the circuitry of desktop and notebook computers are the same. Both are built with the same circuitry with the same voltage and similar current requirements. Ultimately, both kinds of computers draw power from the same source—electric utilities. But they differ in how they handle the power between your outlet and their circuits. Desktop systems provide a direct route. Utility power gets converted to circuit power once and for all inside a little tin box called a power supply. Notebook computers add an auxiliary power source—their batteries—to the mix. In so doing, the power circuitry of the computer system is rearranged to suit the needs of both the logic circuitry of your computer and its batteries.

In effect, the notebook computer has two power sources: utility power when you’re near an outlet (and plugged in) and battery power when you’re in the field (or in an airplane). But they are not entirely independent. The same circuits regulate the voltage going to the logic of your computer whether your notebook machine is running from AC or battery power. In fact, should you remove its batteries and the charging and control circuits, the power functions of a notebook are the same as a desktop machine. Only the location of the power circuits has been rearranged to suit the system designers. The same technologies that bring a desktop power supply to life also are at work in notebook machines.

Desktop Power Supplies

Most computers package their power supplies as a subassembly that’s complete in itself and simply screws into the chassis and plugs into the system board and other devices that require its electricity. The power supply itself is ensconced in a metal box perforated with holes that let heat leak out but prevent your fingers from poking in.

In fact, the safety provided by the self-contained and fully armored computer power supply is one of the prime advantages of the original design. All the life-threatening voltages—in particular, line voltage—are contained inside the box of the power supply. Only low, nonthreatening voltages are accessible—that is, touchable—on your computer’s system board and expansion boards. You can grab a board inside your computer even when the system is turned on and not worry about electrocution (although you might burn yourself on a particularly intemperate semiconductor or jab an ill-cut circuit lead through a finger).

Grabbing a board out of a slot of an operating computer is not safe for the computer’s circuits, however. Pulling a board out is apt to bridge together some pins on its slot connector, if but for an instant. As a result, the board (and your computer’s motherboard) may find unexpected voltages attacking, possibly destroying, its circuits. In other words, never plug in or remove an expansion board from a computer that has its power switched on. Although you may often be successful, the penalty for even one failure should be enough to deter your impatience.

In many desktop computers, the power supply serves a secondary function. The fan that cools the power supply circuits also provides the airflow that cools the rest of the system. This fan also supplies most of the noise that computers generate while they are running. In general, the power supply fan operates as an exhaust fan—it blows outward. Air is sucked through the other openings in the power supply from the space inside your system. This gives dust in the air taken into your computer a chance to settle anywhere on your system board before getting blown out through the power supply.

The prevailing standard for power supplies of today’s computers is ATX. This standard defines not only the power supply itself but also the connectors and voltages available for the motherboard and devices inside the computer.

Technologies

In electronic gear, two kinds of power supplies are commonly used: linear and switching. The former is old technology, dating from the days when the first radios were freed from their need for storage batteries in the 1920s. The latter rates as high technology, requiring the speed and efficiency of solid-state electronic circuitry to achieve the dominant position it holds today in the computer power market. These two power supply technologies are distinguished by the means used to achieve their voltage regulation.

Linear Power Supplies

The design first used for making regulated DC from utility-supplied AC was the linear power supply. At one time, this was the only kind of power supply used for any electronic equipment. When another technology became available, it was given the linear label because it then used standard linear (analog) semiconductor circuits, although a linear power supply need not have any semiconductors in it at all.

In a linear power supply, the raw electricity from the power line is first sent through a transformer that reduces its voltage to a value slightly higher than required by the computer’s circuits. Next, one or several rectifiers, usually semiconductor diodes, convert the now low-voltage AC to DC by permitting the flow of electricity in only one direction, blocking the reversals. Finally, this DC is sent through the linear voltage regulator, which adjusts the voltage created by the power supply to the level required by your computer’s circuits.

Most linear voltage regulators work simply by absorbing the excess voltage made by the transformer, turning it into heat. A shunt regulator simply shorts out excess power to drive the voltage down. A series regulator puts an impediment—a resistance—in the flow of electricity, blocking excess voltage. In either case, the regulator requires an input voltage higher than the voltage it supplies to your computer’s circuits. This excess power is converted to heat (that is, it’s wasted). The linear power supply achieves its regulation simply by varying the waste.

Switching Power Supplies

The design alternative is the switching power supply. Although more complex, switching power supplies are more efficient and often less expensive than their linear kin. Although designs vary, the typical switching power supply first converts the incoming 60 hertz utility power to a much higher frequency of pulses (in the range of 20,000Hz, above the range of normal human hearing) by switching it on and off using a transistors.

At the same time, the switching regulator increases the frequency of the commercial power; it regulates the commercial power using a digital technique called pulse width modulation (PWM). That is, the duration of each power pulse is varied in response to the needs of the computer circuitry being supplied. The width of the pulses is controlled by the electronic switch; shorter pulses result in a lower output voltage. Finally, the switched pulses are reduced in voltage down to the level required by the computer circuits by a transformer and then turned into pure direct current via rectification and filtering.

Switching power supplies earn their efficiency and lower cost in two ways. Switching regulation is more efficient because less power is turned into heat. Instead of dissipating energy with a shunt or series regulator, the switching regulator switches all current flow off, albeit briefly. In addition, high frequencies require smaller, less expensive transformers and filtering circuits. For these two very practical reasons, nearly all of today’s personal computers use switching power supplies.

Power Needs

Modern computer logic circuits operate by switching voltages with the two different logic states (true or false, one or zero, for example) coded as two voltage levels—high and low. Every family of logic circuits has its own voltage standards.

The primary consumers of power inside a computer are its logic circuits. At one time, nearly all logic circuits used five volts of direct current. This power level was set by the design of the electronic circuit components they used, based around the requirements of transistor-transistor logic (TTL). In a TTL design, high refers to voltages above about 3.2 volts, and low means voltages lower than about 1.8. The middle ground is undefined logically, an electrical guard band that prevents ambiguity between the two meaningful states.

To reduce the power needs of today’s high-speed circuits, computer designs are shifting to 3.3-volt logic and require power supplies that deliver that voltage level (often in addition to 5-volt power). The ATX design and those derived from it (SFX and TFX) provide for both 5-volt and 3.3-volt supplies.

Some computer circuits, such as microprocessors, run at even lower voltages. The levels required by their circuits are not available from most computer power supplies. Instead, motherboard-makers use voltage regulators to reduce a 12-, 5-, or 3.3-volt power source to the level required by the chips. This design allows standard power supplies to work with chips rated for almost any voltage.

In addition to the basic logic voltage, computers often require other voltages as well. The motors of most disk drives (hard and floppy) typically require 12 volts to make them spin. Other specialized circuits in computers sometimes require bipolar electrical supplies. A serial port, for example, signals logic states by varying voltages between positive and negative in relation to ground. Consequently, the mirror image voltages (–5 and –12 volts) are usually available inside every computer.

In notebook computers, most of which have no room for generic expansion boards, all these voltages are often unnecessary. For example, many new hard disks designed for notebook computers use 5-volt motors, eliminating the need for the 12-volt supply. The custom-tailored power systems of notebook computers supply only the voltages required by circuits actually built in to the computer.

Voltages and Ratings

The power supplies you are most likely to tangle with are those inside desktop computers, and these must produce all four common voltages to satisfy the needs of all potential combinations of circuits. In older desktop computers, the power supply typically produces four voltages (+5, –5, +12, and –12) that are delivered in different quantities (amperages) because of the demands associated with each. A separate voltage regulator on the motherboard produces the lower voltage in the 3.3-volt range required by Pentium-level microprocessors and their associated circuitry. In some systems, the output voltage of this regulator may be variable to accommodate energy-saving systems (which reduce the speed and voltage of the microprocessor to conserve power and reduce heat dissipation). Newer power supplies, such as those that follow the ATX design standard, sometimes also provide a direct 3.3-volt supply.

The typical computer has a lot of logic circuitry, so it needs copious quantities of 5-volt power, often as much as 20 to 25 amperes). Many disk drives use 12-volt power; the typical modern drive uses an ampere or so. Only a few components require the negative voltages, so most power supplies only deliver a few watts of each.

Most power supplies are rated and advertised by the sum of all the power they can make available, as measured in watts. The power rating of any power supply can be calculated by individually multiplying the current rating of each of the four voltages it supplies and summing the results. (Power in watts is equal to the product of voltage times current in amperes.) Most modern full-size computers have power supplies of 150–220 watts. Notebook computers may use from 10 to 50 watts when processing at full speed.

Note that this power rating does not correspond to the wattage that the power supply draws from a wall outlet. All electronic circuits—and power supplies in particular—suffer from inefficiencies, linear designs more so than switching. Consequently, a power supply requires a wattage in excess of what it provides to your computer’s circuits—at least when it is producing its full output. Computer power supplies, however, rarely operate at their rated output. As a result, efficient switching power supplies typically draw less power than their nominal rating in normal use. For example, a computer with a 220-watt power supply with a typical dosage of memory (say, 4MB) and one hard disk drive likely draws less than 100 watts while it is operating.

When you’re selecting a power supply for your computer, the rating you require depends on the boards and peripherals with which you want to fill your computer. Modern computers are not nearly so power hungry as their forebears. Nearly all computer components require less power than the equivalents of only half a dozen years ago. The one exception is the microprocessor. Greater performance requires more power. Although Intel’s engineers have done a good job at reducing the power needs of the company’s products by shifting to lower-voltage technologies, the reductions have been matched by increasing demands. Chips need as much power as they ever have, sometimes more.

In a desktop computer, a 200-watt power supply essentially loafs along; most individual computers (as opposed to servers or workstations) could get along with 120 watts without straining. A system board may require 15–25 watts; a floppy disk drive, 3–20 (depending on its vintage

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