Power Supplies

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); a hard disk, 5 “50 (also depending on its vintage); a memory or multifunction expansion board, 5 “10. Table 31.1 summarizes the needs of both vintage and modern computer components.

Table 31.1. Typical Device Power Demands

Device Class	Device Type	Power	Example
Floppy disk drive	Full height, 5.25 inch	12.6 watts	IBM PC diskette drive
	Half height, 5.25 inch	12.6 watts	QumeTrak 142
	One-inch high, 3.5 inch	1.4 watts	Teac FD-235J
Graphics board	Two-board old technology	16.2 watts	IBM 8514/A
	High performance, full length	13.75 watts	Matrox MGA
	Accelerated half-card	6.5 watts	ATI VGA Wonder, Graphics Ultra+
Hard disk	Full height, 5.25-inch	59 watts	IBM 10MB XT hard disk
	Half height, 5.25-inch	25 watts	[Estimated]
	One-inch high, 3.5 inch	12.25 watts (peak)	maxtor DiamondMax Plus 9
	2.5 inch	2.2 watts	Quantum Go-Drive 120AT
	PCMCIA card	3.5 watts	Maxtor MXL-131-III
	Full height, 3.5 inch	12 watts	Quantum ProDrive 210S
Memory	1MB SIMM	4.8 watts	Motorola MCM81000
	4MB SIMM	6.3 watts	Motorola MCM94000
	8MB SIMM	16.8 watts	Motorola MCM36800
	128MB SoDIMM	1.8 watts	Micron MT8LSD3264
Modem	PCMCIA card	3.5 watts	MultiTech MT1432LT
	Internal, half-card	1.2 watts	Boca V.32bis
Network adapter	Ethernet, half-card	7..9 watts	Artisoft AE-2/T
System board	286, AT size	25 watts	[Estimated]
	386, XT size	12 watts	Monolithic Systems MSC386 XT/AT
	486 or Pentium, AT size	25 watts	[Estimated]

Reserve power is always a good thing, and with switching power supplies, it comes without a penalty (except in the cost of making the power supply). In other words, although you could get along with a smaller power supply in your desktop computer, the quasi-standard 200 watts remains a good choice.

Supply Voltage

Most power supplies are designed to operate from a certain line voltage and frequency. In the United States, utility power is supplied at a nominal 115 volts and 60 hertz. In other nations, the supply voltage and frequency may be different. In Europe, for instance, a 230-volt, 50Hz standard prevails.

Most switching power supplies can operate at either frequency, so that shouldn't be a worry when traveling. (Before you travel, however, check the ratings on your power supply to be sure.) Linear power supplies are more sensitive. Because their transformers have less reactance at lower frequencies, 60Hz transformers draw more current than their designers intend when operating on 50Hz power. Consequently, they are liable to overheat and fail, perhaps catastrophically.

Most computer power supplies are either universal or voltage selectable. A universal power supply is designed to have a wide tolerance for supply current. If you have a computer with such a power supply, all you need to do is plug the computer in, and it should work properly. Note that some of these universal power supplies accommodate any supply voltage in a wide range and will accept any standard voltage and a line frequency available in the world ”a voltage range from about 100 to 250 volts and a line frequency of 50 to 60Hz. Other so-called universal supplies are actually limited to two narrow ranges, bracketing the two major voltage standards. Because you are unlikely to encounter a province with a 169.35-volt standard, these dual-range supplies are universal enough for worldwide use.

Voltage-selectable power supplies have a small switch on the rear panel that selects their operating voltage, usually in two ranges ”115 and 230 volts. If your computer has a voltage-selectable power supply, make sure that the switch is in the proper position for the available power before you turn on your computer.

CAUTION

When traveling in a foreign land, always use this power supply switch to adjust for different voltages. Do not use inexpensive voltage converters. Often these devices are nothing more than rectifiers that clip half the incoming waveform. Although that strategy may work for light bulbs , it can be disastrous to electronic circuitry. Using such a device can destroy your computer. It's not a recommended procedure.

Standards

The metal modules that serve as power supplies for desktop computers were one of the first components to be standardized. The technology for building a power supply is common and straightforward, and reverse-engineering a power supply for given computer is almost trivial. There are no strange codes or complex logic signals coming from a power supply, just a few, easily measured voltages. Each popular case used a characteristic power supply that became a de facto industry standard, both in size (the power supply had to match the case mechanically) and connectors (to plug into motherboards and drives). The earliest of these matched models of IBM computers ”PC, XT, and AT ”the last being the reigning standard for a decade .

The standards used by modern computers arose when Intel standardized its motherboard offerings and inspired the industry to follow suit with its ATX design. The new motherboard broke with the past with a different physical layout that required a new case design, which in turn dictated a different size for the power supply. In addition, Intel chose to break with the past standards for power connectors to create a new one of its own for the ATX motherboard. The choice was more than Intel wishing to re-create the industry to suit itself. A new connector was overdue ”the old design could not accommodate the 3.3-volt supplies that new logic designs required.

The result was the ATX power supply. Roughly patterned after previous designs, the ATX power supply is a steel -cased modular design that incorporates the computer's primary (and usually, only) external power connection. It supplies all the voltages required by both past and current logic circuitry: positive direct-current voltages of 5 and 12 volts in addition to the new 3.3-volt supply as well as negative supplies of 5 and 12 volts. But on its new 20-pin motherboard connector, the ATX design has added several signals aimed at the needs of modern computers:

Power Good . This signal is a carryover from older designs. The power supply sends out the Power Good signal to indicate that its output voltages are at the correct level and safe for computer circuits. A computer won't turn on unless the Power Good signal is present. The technical designation of this signal under the ATX standard is PWR_OK.
On . This is a signal from the motherboard telling the power supply to switch its main outputs on. This signal allows an external switch to control the computer power. More importantly, it allows the motherboard to control the power supply. The motherboard can shift the system to standby and cut its power consumption by switching off the On signal, and it can shift back to normal operation by restoring it. This mode of operation is essential for features such as wake-on-alarm, wake-on-modem, and wake-on-network that shift the system to standby awaiting a specific event (an alarm, incoming call, or network request). The engineering name for this signal is PS_ON#.
Standby Supply Voltage . This is an auxiliary source of logic voltage that is not controlled by the On signal. It provides the power the motherboard needs when it is on standby and the main voltages from the power supply have been switched off. The engineering name for this signal is +5 VSB.

The ATX standard also defines an auxiliary power connector with six pins. This connector is aimed at high-current power supplies. It gives another channel to the main 5 and 3.3 volts when the current from the supply exceeds the safe carrying capacity of the 16-gauge wires recommended for the connections. This connector is used when the 3.3-volt output exceeds 18 amperes or the 5-volt output exceeds 24 amperes.

ATX power supplies also include several peripheral connectors for disk drives (and, sometimes, auxiliary fans and other accessories). Two styles are usually present: large four-pin Molex connectors to match hard disk drives and old floppy disks, and miniaturized four-pin connectors for modern floppy disk drives.

With the introduction of ATX version 2.0, Intel added a variation on the ATX power supply called ATX12V . The new design differs from ordinary ATX power supplies only in the presence of one or more new +12V power connectors. These four-pin connectors provide a high-current 12-volt supply designed to feed the voltage regulators of high-current processors or other demanding peripherals.

The ATX specification allows for two types of power supplies ”those meant to cool not only themselves but also the microprocessor with their fans, and those with fans that cool only the power supply. The only difference between the two is the location of ventilation holes. Otherwise, all ATX power supplies look much the same and measure 5.8 inches (150 millimeters) wide, 5.5 inches (140 mm) long, and 3.4 inches (86 mm) thick.

The ATX design served as a pattern for two additional power supplies designed for cases more compact than those used by standard ATX motherboards. These include the SFX and TFX designs. No matter the physical package, these power supplies offer the same connectors and signals as the basic ATX power supply.

The SFX and SFX12V power supplies are small form-factor designs meant for smaller computers (hence the SF in the designation). The SFX power supply standard envisions computers needing from 90 to 180 watts. The standard defines two basic packages. One is called the 40-millimeter profile design that's about 5 inches (125 mm) long, 4 inches (100 mm) wide, and 2 inches (50 mm rather than 40 mm) thick. In addition, the standard also allows for a 60-millimeter profile version that's actually 63.5 millimeters (2.5 inches) thick. The standard also allows manufacturers to add external fans to the 60-millimeter profile power supplies either as external or internal fans. In either case, the fan adds about 17 millimeters to the thickness of the power supply. The SPX12V design provides one or more four-pin +12V power connectors, which the ordinary SPX does not.

The TFX12V is a thin form-factor design (hence the designation) made to match microATX and FlexATX motherboards and fit into low-profile chassis. Under the standard, a TFX12V power supply measures 6.9 inches (175 millimeters) long, 3 inches (75 mm) high, and 2.6 inches (65 mm) thick. The standard does not define the total output of the supply (or the current to be supplied at any of its output voltages) but rather envisions supplies with outputs in the range of 180 to 220 watts. The "12V" in the name indicates that each TFX12V power supply makes available one or more four-pin +12V power connectors.

The official specifications for ATX, SFX, and TFX power supplies are available on the Web at www.formfactors.org.

In modern computer designs, only the WTX power supply design does not follow the connector and signal standards popularized by ATX. WTX power supplies are meant to match WTX motherboards in workstations and servers, applications demanding more power than the ATX family can deliver.

WTX recognizes two sizes of power supply: single-fan power supplies capable of delivering from 400 to 460 watts, and a two-fan design meant to supply from 550 to 850 watts. In addition to harboring an additional fan, two-fan designs are physically larger. A single-fan WTX power supply measures 9.1 inches (230 mm) long, 5.8 inches (150 mm) wide, and 3.4 inches (86 mm) thick. A two-fan supply measures 9.1 inches (230 mm) long, 8.8 inches (224 mm) wide, and 3.4 inches (86 mm) thick.

Either size of WTX power supply offers five power connectors that do not match the ATX standard. These include a primary 24-pin connector that supplies most of the power, a 22-pin connector with control signals (as well as some power), an eight-pin connector meant to supply voltage converters for low-voltage memory and processors, and two six-pin connectors that supply high-current 12VDC to power pods for high-current processors and similar applications.

The complete WTX specification is available on the Web at www.wtx.org. This standard is now being phased out. The Server System Infrastructure group at www.ssi.org has developed a new set of specifications for servers and their power supplies.

Portable Computer Power

In comparison to desktop computers, notebook and subnotebook computers would seem to have it easy when it comes to power supplies. Notebooks work with battery power, which is generated inside the battery cells in exactly the right form for computer circuits ”low voltage DC. But notebook computers actually have much more complex power systems, more complex than desktop computers because of their use of two power systems, utility and battery.

Even though battery power is smooth direct current, battery- powered computers require built-in voltage regulation because the voltage from batteries varies as cells discharge . In addition, most computer batteries are rechargeable , so they need to get electricity from somewhere. Moreover, most of the time laptop and notebook computers are close to electrical supplies when they are used, so it makes sense to use utility power rather than battery power ”or charge batteries at the same time. Consequently, notebook computers also use power supplies, but such supplies have a significantly different design, one that splits the desktop power supply design into pieces.

The power supplies of most notebook computers have an external half that reduces utility voltage to a safe, near-battery level and rectifies it into DC. These external power supplies generally create only one voltage, one that will substitute for battery power. But that's not the end of it. Inside the notebook computer's case are several voltage regulators that keep these low voltages at the constant levels logic circuits require. These voltage regulators wallpaper over the wide variations that occur to the voltage output of a battery as it discharges. The regulators also ensure that the low-voltage power from the external power supply exactly matches battery power before it gets to critical circuits. In addition, notebook computers have another, specialized regulator that desktop computers lack, one that charges the notebook computer's battery reserves and keeps the batteries topped up when external utility power is available.

Unlike the power supplies and voltage regulators for desktop computers, those for notebooks do not follow any industry standards. Each make and model of machine is different. In general, the power circuitry isn't even in one place or one module (as with desktop computers) but rather spread throughout the computer system.

The notebook computer is more concerned with minimizing power waste rather than regulating. Today's power-management functions, which save energy in both desktop and notebook computers, had their origins in the power-saving features of notebook machines.

External Power Supplies

In essence and operation, the external power supply of a notebook computer is little more than a repackaged version of those inside desktop computers. Line voltage AC goes in, and low voltage DC (usually) comes out. The output voltage is close to that of the system's battery output, always a bit higher. (A slightly higher voltage is required so that the batteries are charged to their full capacity.)

In most rudimentary form, the external power supply is nothing more than a transformer that supplies low-voltage AC to a computer. This approach has two benefits. It moves the heaviest element of the power supply out of the computer, reducing its weight and making it more portable. In addition, it keeps dangerous high voltages out of the computer itself, not only making the computer inherently safer but also reducing the number of hurdles the manufacturer must leap to get regulatory approval for the entire computer package.

Current external power supply designs are more elaborate. Instead of AC, they supply DC to the computer. More importantly, they benefit from the latest power technologies and eliminate the large, heavy (and expensive) power transformers required by older designs. As a result, the external power supply weighs less and is more energy efficient.

Despite this design change, many people still call the external power supply a transformer . Another common name is power brick because of the shape of the supply (although the brick-like weight left the design along with the power transformer).

No standard exists for the external battery chargers/power supplies of notebook computers. Every manufacturer ”and often every model of computer from a given manufacturer ”uses its own design. They differ as to output voltage, current, and polarity. You can substitute a generic replacement only if the replacement matches the voltage used by your computer and generates at least as much current. Polarity matching gives you two choices ”right and wrong ”and the wrong choice is apt to destroy many of the semiconductors inside the system. In other words, make extra certain of power polarity when plugging in a generic replacement power supply. (With most computers, the issue of polarity reduces to a practical matter of whether the center or outer conductor of the almost-universal coaxial power plug is the positive terminal.)

Most external power supplies are designed to operate from a single voltage (a few are universal, but don't count on it). That means you are restricted to plugging in and charging your portable computer to one hemisphere (or thereabouts) or the other. Moving from a 117-volt to a 230-volt electrical system requires a second, expensive external charger. Experienced travelers often buy a second external supply/charger at travelling voltage.

Car and Airplane Adapters

Utility power is not the only external source available for powering computers. Both automobiles and airplanes have their own electrical systems that can be used as an external power source for notebook computers. However, you must still match the available power to the requirement of your computer.

Today, the standard power system in automobiles is direct current of approximately 12 volts. The 12-volt rating is only nominal, however. When your car is running and charging its battery, the voltage in the electrical system is often as high as 16 volts. When the car is starting, the available voltage may dip below 8 volts. In fact, even a so-called 12-volt car battery, when fully charged, actually delivers 13.2 volts.

A few computers managed to deal with the wide range of voltages in automotive electrical systems directly. They required only a simple cable with a plug that fit a cigarette lighter to operate from automotive power. But most current computers are more demanding in their power requirements, often needing voltages greater than available from standard 12-volt automotive systems. Although you can power these computers from an automobile cigarette lighter, you need a more costly kind of adapter called an inverter .

An inverter is a device that converts DC to AC and increases the voltage to that of utility-supplied power ”inverting what you expect a power supply to do. You simply plug your notebook computer's external power supply into the inverter exactly as you would plug into utility power; then you plug your computer in normally.

In some commercial airplanes ”primarily in first- and business-class sections but occasionally in economy ”you'll find power jacks meant to supply electricity to portable computers. These jacks supply the equivalent of automobile power, regulated to a constant 15 volts of direct current (usually limited to about 75 watts per seat). This power is not meant to directly operate notebook computers but to supply an adapter, which in turn powers the computer. Each computer model is supposed to use its own adapter.

The airline jacks use a special connector manufactured by the Hypertronics Corporation but commonly referred to as an Empower connector or ARINC 628 power connector. One exception is American Airlines, which uses an automotive-style cigarette lighter connector for in-seat power.

You can avoid the need for a special computer power adapter by using an inverter. Several manufacturers now offer small inverters with both airline-style and automotive connectors. The inverters produce the equivalent of normal house current into which you can plug your computer's standard AC adapter.

Note that these airplane power jacks are meant as a convenience for passengers. The main electrical power system of most airplanes does not operate at 15 volts nor is it compatible with house current.