How Pc Power Supplies Work

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How PC Power Supplies Work If there is any one component that is absolutely vital to the operation of a computer, it is the power supply. Without it, a computer is just an inert box full of plastic and metal. The power supply converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer. In this article, we'll learn how PC power supplies work and what the wattage ratings mean PowerSupply In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan.

This is a power supply removed from its PC case. The small, red switch at right, above the power-cord connector, is for changing line voltages in various countries.

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The interior of a power supply. Power supplies, often referred to as "switching power supplies", use switcher technology to convert the AC input to lower DC voltages. The typical voltages supplied are: 3.3 volts 5 volts 12 volts The 3.3- and 5-volts are typically used by digital circuits, while the 12-volt is used to run motors in disk drives and fans. The main specification of a power supply is in watts. A watt is the product of the voltage in volts and the current in amperes or amps. If you have been around PCs for many years, you probably remember that the original PCs had large red toggle switches that had a good bit of heft to them. When you turned the PC on or off, you knew you were doing it. These switches actually controlled the flow of 120 volt power to the power supply. Today you turn on the power with a little push button, and you turn off the machine with a menu option. These capabilities were added to standard power supplies several years ago. The operating system can send a signal to the power supply to tell it to turn off. The push button sends a 5-volt signal to the power supply to tell it when to turn on. The power supply also has a circuit that supplies 5 volts, called VSB for "standby voltage" even when it is officially "off", so that the button will work. Switcher Technology Prior to 1980 or so, power supplies tended to be heavy and bulky. They used large, heavy transformers and huge capacitors (some as large as soda cans) to convert line voltage at 120 volts and 60 hertz into 5 volts and 12 volts DC.

PC POWER SUPPLY. ACEC KOMNA 2 The switching power supplies used today are much smaller and lighter. They convert the 60-Hertz (Hz, or cycles per second) current to a much higher frequency, meaning more cycles per second. This conversion enables a small, lightweight transformer in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer component. The higher-frequency AC current provided by a switcher supply is also easier to rectify and filter compared to the original 60-Hz AC line voltage, reducing the variances in voltage for the sensitive electronic components in the computer.

In this photo you can see three small transformers (yellow) in the center. To the left are two cylindrical capacitors. The large finned pieces of aluminum are heat sinks. The left heat sink has transistors attached to it. These are the transistors in charge of doing the switching -- they provide high-frequency power to the transformers. Attached to the right heat sink are diodes that rectify AC signals and turn them into DC signals. A switcher power supply draws only the power it needs from the AC line. The typical voltages and current provided by a power supply are shown on the label on a power supply.

Personal computer power supply label. VSB is the standby voltage provided to the power switch. Switcher technology is also used to make AC from DC, as found in many of the automobile power inverters used to run AC appliances in an automobile and in uninterruptible power supplies. Switcher technology in automotive power inverters changes the direct current from the auto battery into alternating current. The transformer uses alternating current to make the transformer in the inverter step the voltage up to that of household appliances (120 VAC).

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Power Supply Standardization Over time, there have been at least six different standard power supplies for personal computers. Recently, the industry has settled on using ATX-based power supplies. ATX is an industry specification that means the power supply has the physical characteristics to fit a standard ATX case and the electrical characteristics to work with an ATX motherboard. PC power-supply cables use standardized, keyed connectors that make it difficult to connect the wrong ones. Also, fan manufacturers often use the same connectors as the power cables for disk drives, allowing a fan to easily obtain the 12 volts it needs. Colorcoded wires and industry standard connectors make it possible for the consumer to have many choices for a replacement power supply.

A PC power supply removed from its PC case. Cables and connectors at right supply DC voltages. Advanced Power Management Advanced Power Management (APM) offers a set of five different states that your system can be in. It was developed by Microsoft and Intel for PC users who wish to conserve power. Each system component, including the operating system, basic input/output system (BIOS), motherboard and attached devices all need to be APM-compliant to be able to use this feature. Should you wish to disable APM because you suspect it is using up system resources or causing a conflict, the best way to do this is in the BIOS. That way, the operating system won't try to reinstall it, which could happen if it were disabled only in the software.

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Power Supply Wattage A 400-watt switching power supply will not necessarily use more power than a 250-watt supply. A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices, since the supply should not be loaded to 100 percent of its capacity. According to PC Power & Cooling, Inc., some power consumption values (in watts) for common items in a personal computer are: PC Item

Watts

Accelerated Graphics Port (AGP) card

20 to 30W

Peripheral Component Interconnect (PCI) card

5W

small computer system interface (SCSI) PCI card

20 to 25W

floppy disk drive

5W

network interface card

4W

50X CD-ROM drive

10 to 25W

RAM

10W 128M

per

5200 RPM Integrated Drive Electronics (IDE) hard 5 to 11W disk drive 7200 RPM IDE hard disk drive

5 to 15W

Motherboard (without CPU or RAM)

20 to 30W

550 MHz Pentium III

30W

733 MHz Pentium III

23.5W

300 MHz Celeron

18W

600 MHz Athlon

45W

Power supplies of the same form factor ("form factor" refers to the actual shape of the motherboard) are typically differentiated by the wattage they supply and the length of the warranty.

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Power Supply Problems The PC power supply is probably the most failure-prone item in a personal computer. It heats and cools each time it is used and receives the first in-rush of AC current when the PC is switched on. Typically, a stalled cooling fan is a predictor of a power supply failure due to subsequent overheated components. All devices in a PC receive their DC power via the power supply. A typical failure of a PC power supply is often noticed as a burning smell just before the computer shuts down. Another problem could be the failure of the vital cooling fan, which allows components in the power supply to overheat. Failure symptoms include random rebooting or failure in Windows for no apparent reason. For any problems you suspect to be the fault of the power supply, use the documentation that came with your computer. If you have ever removed the case from your personal computer to add an adapter card or memory, you can change a power supply. Make sure you remove the power cord first, since voltages are present even though your computer is off.

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Power Supply Improvements Recent motherboard and chipset improvements permit the user to monitor the revolutions per minute (RPM) of the power supply fan via BIOS and a Windows application supplied by the motherboard manufacturer. New designs offer fan control so that the fan only runs the speed needed, depending on cooling needs. Recent designs in Web servers include power supplies that offer a spare supply that can be exchanged while the other power supply is in use. Some new computers, particularly those designed for use as servers, provide redundant power supplies. This means that there are two or more power supplies in the system, with one providing power and the other acting as a backup. The backup supply immediately takes over in the event of a failure by the primary supply. Then, the primary supply can be exchanged while the other power supply is in use.

Power Supply Improvements Recent motherboard and chipset improvements permit the user to monitor the revolutions per minute (RPM) of the power supply fan via BIOS and a Windows application supplied by the motherboard manufacturer. New designs offer fan control so that the fan only runs the speed needed, depending on cooling needs. Recent designs in Web servers include power supplies that offer a spare supply that can be exchanged while the other power supply is in use. Some new computers, particularly those designed for use as servers, provide redundant power supplies. This means that there are two or more power supplies in the system, with one providing power and the other acting as a backup. The backup supply immediately takes over in the event of a failure by the primary supply. Then, the primary supply can be exchanged while the other power supply is in use.

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Power Power is supplied to your computer in two stages. First, power is conveyed to the case from your electrical utility to your wall, and through the black power cord to the PC. Then, the internal power supply transforms this standard household electricity into the forms that your computer needs. Most people take electrical power for granted and don't think too much about it. This is also true of the internal power supply, which is usually just considered part of the case and given little attention. (The power supply is not part of the case!) The power from the utility itself is taken for granted and rarely given a second thought--that is, until disaster strikes. It is my hope that by taking the time to explain in more detail how electricity works and how it powers your PC, this subject will be given more attention in the future.

How important is the quality of power you supply to your PC? This graph, which shows the leading causes of data loss by category, gives you a pretty good idea. (Source: Contingency Planning, via American Power Conversion) If you are going to use your PC lightly, it is fine not to pay too much attention to power. However, as the old computer saying goes, "garbage in, garbage out". If your motherboard and components are being supplied poor-quality power, you will have problems that you wouldn't have if they received proper, high-quality power. If you plan on using your PC heavily, or if your data is important, or if you are looking for upgradeability in the future, you must pay attention to power! Power issues are responsible for more PC problems than probably any other single source, even though most people don't realize that the power is responsible. This chapter takes a look at power, both internal and external to the PC. External Power The matter of power begins with a look at the power delivered to the case. The power that comes into your home is normally quite reliable, but it can be surprising how many quality problems it often has. Spikes, surges, blackouts, brownouts, line noise—all are common power problems that you don't generally notice (well, you notice a blackout :^) ). Usually, the electrical devices you use manage to deal with them (at least, most of them.) Your computer's power supply has some tolerance to these problems as well; the more expensive the system, the more the power supply can tolerate without failure. If your power supply is inexpensive, however (and too many of them these days are) then poor power can lead directly to system troubles that manifest themselves in ways that would never lead you to suspect power as the cause. Taking some steps to improve the quality of the power going into the system can help prevent problems no matter what PC you use.

What Is Electricity?

PC POWER SUPPLY. ACEC KOMNA 8 You use it every day, but what exactly is electricity? To understand it, we must go down to the very building blocks of matter itself: the atom. Without getting too complicated (and going down to the level of quantum mechanics), matter is comprised of atoms. Atoms are made up of three basic particles: protons, which are positively-charged particles; electrons, which are negatively-charged particles; and neutrons, which have no charge. An atom has its neutrons and protons clustered together at its center, called the nucleus, while the electrons orbit around the nucleus. Electrons are negatively-charged, and protons positively-charged. In a stable substance, therefore, the charges balance out and the item comprised of that substance will have no net charge. This is why an item at rest will not spontaneously generate electricity: it is stable. Neutrons and protons normally remain static within the nucleus of most (nonradioactive) substances. Electrons, on the other hand, can easily be removed from atoms and can move between those atoms and adjacent ones. This is most readily observable through the phenomenon of static electricity. In the classic experiment that you can do at home, a balloon is is rubbed on a child's head. When this is done, electrons are stripped from atoms in the hair, and moved to the balloon (or is it the other way around? :^) ). Once the two are separated, one is left with a positive charge and the other with a negative charge. Iif the balloon is then moved near that child's head again, the hair will stick up as the remaining, charged atoms are attracted to those in the balloon. What causes electrons to move? It is two things: first, the attraction of the negativelycharged electrons towards a positive charge; and second, a means of allowing the electrons to flow. Electricity is then at its heart, a flow of electrons. Whether it is the powerful discharge of a lightning bolt, or the flow of electricity in an appliance, or even the small flow of electricity in a battery-powered wristwatch, they are all at their core the same. Electricity flows readily in some materials but not in others. What differentiates materials is primarily the atomic structure of the matter that comprises them. Some conduct electricity readily; they are of course called conductors. Typical good electrical conductors include copper, aluminum, gold and other metals, and water. Materials that do not conduct electricity are called insulators. Common insulators include wood, glass, plastic, and air (though many people don't think of air as an insulator, it is actually one of the best.) Some materials conduct electricity better than others because some atoms hold on more tightly to their electrons than others. However, any substance, even one that normally insulates so well as to stop the flow of electricity, will conduct electricity if the charge is strong enough--even the air. This brings us to the concept of a circuit. A circuit is any combination of objects that allows electricity to flow. A battery by itself is not a circuit; there is no flow of electricity (this is sometimes called an open circuit. Attach a wire from one end of the battery to the other though, and you have a closed circuit and electricity will immediately flow.

Voltage, Current, and Resistance In order to talk meaningfully about electricity, and especially how we can use electricity, we need to be able to measure its fundamental properties. There are three primarily characteristics that describe the nature of electrical flows. The first is voltage, usually abbreviated "V" and measured in volts (also abbreviated "V".) Voltage, also sometimes called potential difference or electromotive force (EMF), refers to the amount of potential energy the electrons have in an object or circuit. In some ways, you can think of this as the amount of "push" the electrons are making to try to get towards a positive charge. The more energy the electrons have, the stronger the voltage. If we draw an analogy to a waterfall, the voltage would represent the height of the waterfall: the higher it is, the more potential energy the water has by virtue of its distance from the bottom of the falls, and the more energy it will possess as it hits the bottom. The second primary characteristic of electricity is current, usually abbreviated "I" ("C" is reserved for the principle of charge, the most fundamental building block of electricity.)

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ACEC KOMNA 9 Current is measured in amperes or amps, abbreviation "A". Current refers to how much electricity is flowing--how many electrons are moving through a circuit in a unit of time. If we think about our waterfall example, the current would represent how much water was going over the edge of the falls each second. The third primary characteristic of electricity is resistance, normally abbreviated "R" and measured in ohms, abbreviated using the Greek letter omega ( ). Resistance refers to how much the material that is conducting electricity opposes the flow of electrons. The higher the resistance, the harder it is for the electrons to push through. In the waterfall analogy, resistance would refer to any obstacles that slowed down the flow of water over the edge of the falls. Perhaps there are many rocks in the river before the edge, to slow the water down. Or maybe a dam is being used to hold back most of the water and let only a small amount of it through. These three characteristics are directly related through a mathematical principle known as Ohm's Law. Its usual formulation is: V=I*R meaning that the voltage of a circuit is equal to the current through the circuit times its resistance. Another way of stating Ohm's Law, that is often easier to understand, is: I=V/R which means that the current through a circuit is equal to the voltage divided by the resistance. This makes sense, if you think about our waterfall example: the higher the waterfall, the more water will want to rush through, but it can only do so to the extent that it is able to as a result of any opposing forces. If you tried to fit Niagara Falls through a garden hose, you'd only get so much water every second, no matter how high the falls, and no matter how much water was waiting to get through! And if you replace that hose with one that is of a larger diameter, you will get more water in the same amount of time. Getting back to electrical circuits, what does resistance mean in practical terms? First, conductors have relatively low resistance; that's what makes them conductive. Insulators have high resistance. Let's consider a D-cell battery. This is a device that chemically creates a charge differential; one end of the battery is positively charged, and the other is negatively charged, so it has a voltage (typically 1.5V for alkaline cells). What prevents the electrons from jumping straight to the positive charge and neutralizing the battery? It is the conductor that separates them: the air itself. Air is one of the best insulators around. Now if you attach a wire from one terminal to the other, you have replaced the high resistance of the air with the low resistance of the wire, and you will get a very high current as a result through the wire. How high? Well, directly short-circuiting a battery in this manner can cause burns! How can electricity cause burns? Some of the electrical energy is converted to heat. How much heat is created depends on the current through the circuit, and the resistance of the conductor. In fact, this is all that an incandescent lightbulb is: a closed circuit is made to the bulb. Inside the bulb is a filament made of a special material with a fairly high resistance. When electricity flows through it, some of it is converted to heat, which causes the filament to glow and, combined with the gases in the bulb, produce light. When you turn off the switch, you open the circuit by replacing part of the wire in the circuit with air, and the electrical flow stops. There's one final thing to note about current: even though it is comprised of negatively-charge particles (electrons) moving towards a positive charge, by convention current is considered to be the opposite: positive charges moving towards a negative charge. I believe that this is the case because in the early days of research into electricity, scientists believed the positive charges are what moved in the circuit, and not the negative ones. It doesn't really matter too much actually, except that you will normally see, for example, battery voltages referred to as positive and not negative. All voltages are measured in reference to a common zero point, normally called ground potential because it is usually connected to the ground of the earth, directly or indirectly.

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Direct and Alternating Current There are two different ways that electricity is produced, and they are used in most cases for very different purposes. They can also be converted from one form to another, as discussed in this section. The first and simpler type of electricity is called direct current, abbreviated "DC". This is the type of electricity that is produced by batteries, static, and lightning. A voltage is created, and possibly stored, until a circuit is completed. When it is, the current flows directly, in one direction. In the circuit, the current flows at a specific, constant voltage (this is oversimplified somewhat but good enough for our needs.) When you use a flashlight, pocket radio, portable CD player or virtually any other type of portable or battery-powered device, you are using direct current. Most DC circuits are relatively low in voltage; for example, your car's battery is approximately 12 V, and that's about as high a DC voltage as most people ever use.

An idealized 12 V DC current. The voltage is considered positive because its potential is measured relative to ground or the zero-potential default state of the earth. (This diagram drawn to the same scale as the AC diagram below.) The other type of electricity is called alternating current, or "AC". This is the electricity that you get from your house's wall and that you use to power most of your electrical appliances. Alternating current is harder to explain than direct current. The electricity is not provided as a single, constant voltage, but rather as a sinusoidal (sine) wave that over time starts at zero, increases to a maximum value, then decreases to a minimum value, and repeats. A representation of an alternating current's voltage over time is shown in the diagram below. While simple direct current circuits are generally described only by their voltage, alternating current circuits require more detail. First of all, if the voltage goes from a positive value to a negative value and back again, what do we say is the voltage? Is it zero, because it averages out to zero? That would seem to imply that there is no energy there at all. But imagine, if you will, a wave of water flowing across the surface of the sea. The peaks and troughs of the wave seem to "cancel each other out", but the wave clearly exists and has energy. The same is true of alternating current. The way the science world measures the energy in an AC signal is to compute what is called the root mean square (RMS) average of the voltage. In simple terms, the RMS value of an electrical current is the number which represents the same energy that a DC current at that voltage would produce; it is in essence an average of the alternating current waveform. Whenever you see an AC voltage specification, they are giving you the RMS number unless they say otherwise specifically. So for example, in North America, most homes have 115 VAC electricity. This is AC electricity equivalent in energy to a 115 V DC circuit. (This is an approximate number, and standard household electricity in North America is also sometimes called 110VAC or 120VAC; it's the same thing.) Other parts of the world use different voltages ranging from 100VAC to 240VAC, and of course, heavy equipment anywhere can use much higher voltages. The other key characteristic of AC is its frequency, measured in cycles per second (cps) or, more commonly, Hertz (Hz). This number describes how many times in a second the voltage alternates from positive to negative and back again, completing one cycle. In North America,

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ACEC KOMNA 11 the standard is 60 Hz, meaning 60 cycles from positive to negative and back again in one second. In other parts of the world the standard is 50 Hz.

Three cycles of an idealized North American 115 VAC, 60 Hz alternating current signal (black curve). Note that each cycle represents 16.67 milliseconds of time, because that is 1/60th of a second. The curve actually goes from -170 V to +170 V in order to provide the average (RMS) value of 115 V. The RMS equivalent is shown as a green horizontal line. To demonstrate what RMS means, look at the blue shaded area, which shows the total energy in the signal for one cycle. The green shading is the area between the RMS line and the zero line for one cycle, and represents the energy in an equivalent 115 V DC signal. The definition of the RMS value is that which makes the green and blue areas equal. (This diagram drawn to the same scale as the DC diagram above.) Why does standard electricity come only in the form of alternating current? There are a number of reasons, but one of the most important is that a characteristic of AC is that it is relatively easy to change voltages from one level to another using a transformer, while transformers do not work for DC. This capability allows the companies that generate and distribute electricity to do it in a more efficient manner, by transmitting it at high voltage for long lengths, which reduces energy loss due to the resistance in the transmission wires. Another reason is that it may be easier to mechanically generate alternating current electricity than direct current. PCs use only direct current, which means that the alternating current provided by your utility must be converted to direct current before use. This is the primary function of your power supply. Work, Power and Apparent Power If we have electricity, what can we do with it? Well, we can do work, which in physics is defined as transferring energy from one object or another through the application of force. Essentially, any time you have a circuit with electricity flowing in it and you are doing

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ACEC KOMNA 12 something with it, you are accomplishing work. The basic unit of work (or energy) is a joule ("J"). Now, finally, we get to where we wanted to go in our discussion of electricity: the definition of power. Power is simply the rate at which work is done. The more power you have in a system, the more work you can get done in the same period of time. In terms of electricity, increasing power means the ability to do more electrical work (for example, running more appliances, or spinning a motor faster, or running a faster CPU, etc.) in the same number of seconds. Power is measured in watts ("W"). Since power is the rate at which work is done, one watt equals one joule of energy expended in one second: Power (W) = Work (J) / Time (seconds) Conversely, the amount of energy used by a device can be computed as the amount of power it uses multiplied by the length of time over which that power is applied: Work (J) = Power (W) * Time (seconds) Computing electrical power can be very simple or very complicated, depending on the type of electricity you are looking at. Let's start with direct current. Here, power (in watts) is just the product of the voltage (in volts) and the current (in amps) of the circuit: P (W) = V (V) * I (A) Fairly simple stuff, and it makes sense: you do more work when you have electrons pushing with more force (higher voltage) and also when you have more of them per period of time (higher current). Since P = V*I, and I = V/R, another way to express power is: P = V² / R For example, if you have a simple 5 V circuit running through a 20 ohm simple resistance, you will have 250 mA of current, and the total power is 5*0.250=1.25 W. Double the voltage to 10 V, and the power doesn't double; it increases by a factor of four, because doubling the voltage while leaving the resistance the same will also double the current. The new power is 5 W. Unsurprisingly, with alternating current the answer is more complicated. To understand it, it is necessary to introduce the concept of phase, which I will try to do without overcomplicating things (not an easy task! :^) ). As illustrated on this page, alternating current is a wave of voltage that swings between a large positive and negative value. The current also makes this sinusoidal trip the same number of times per second. However, sometimes the current and voltage don't peak at the exact same time. The timing relationship between current and voltage of a flow is called its phase, and is expressed in degrees. Why degrees? Well, a cycle of a sine wave is analogous to a circle. 360 degrees is a full cycle, 180 degree half a cycle, and so on. Now, what determines the phasing between the current and voltage? Primarily, it depends on the kinds of loads being powered. Simple loads, such as light bulbs, heater elements and the like, are said to be primarily resistive. These loads will cause the phase between the current and voltage to be close to zero. When the phase angle is zero, the voltage and current applied to the load is equal to the voltage and current used by the load.

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115 VAC current and voltage driving a purely resistive load. The phase angle between voltage and current is about 0 degrees. Note that the voltage and current peak together. Other loads, particularly items such as motors, are said to be reactive. Reactive loads are caused by more complex opposition to the flow of alternating current such as that produced by capacitors and inductors. They can cause the current and voltage to be out of phase, in theory by as much as 90 degrees.

115 VAC current and voltage driving a (theoretical) purely reactive load. The current is lagging behind the voltage by about 90 degrees. (It is possible for the current to be leading by 90 degrees also.)

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ACEC KOMNA 14 Note that whenever one of voltage or current hits a peak, the other one is at zero! If the phase angle between current and voltage is 90 degrees, then whenever voltage is at its peak (either positive or negative), current is zero, and vice-versa. This is a "worst-case" situation that doesn't normally arise in the real world because real loads aren't purely reactive. A more typical situation is where the phase angle is about 45 degrees.

115 VAC current and voltage driving a partly resistive and partly reactive load. The current is lagging behind the voltage by about 45 degrees, making this an inductive load. If the load were capacitive, the current would be leading the voltage. See here for more on inductors and capacitors. "Alright, alright," you are saying. "Why do I care about all of this?" Well, here's one important reason: PC power supplies are partly reactive loads, and often exhibit a phase difference between voltage and current of about 45 degrees. This means that the voltage and current applied to the load do not equal the voltage and current used by the load, and you cannot compute the power used by the supply by simply multiplying the current and voltage. OK, now here's where it gets interesting. :^) The voltage and current applied to the load can be multiplied together to yield what is called apparent power, measured in Volt-Amps (VA): Apparent Power (VA) = V (V) * I (A) Apparent power represents the voltage and current being sent to the device, and is used to measure draw from the utility, for determining heat generation by equipment under use, and for sizing wires and circuit breakers. The actual power used by the load is called "true" power, or just power, and is measured in Watts. (Even though Watts = Volts * Amps, apparent power is measured in VA to differentiate it from true power.) The relationship between power and apparent power is expressed using this formula: P (W) = cosine(phase) * Apparent Power (VA) where "cosine" is the trigonometric function. "cosine(phase)" is also called the power factor of the load. Let's try an example. Let's suppose we are trying to run a power supply and the power supplied is 115V voltage and 2A of current. The apparent power is 115 * 2 = 230 VA. If the nature of the power supply is that its voltage and current are out of phase by 50 degrees, then the power factor is cosine(50º) = 0.642 (sometimes expressed as 64.2%) and the power used by the load is 148 W.

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ACEC KOMNA 15 There's a particular place where all of this comes into play, and that is in the capacity and sizing of uninterruptible power supplies. UPSes are normally specified in terms of apparent power (VA), whereas PC power supplies are specified in terms of true power (W). Many people use the numbers interchangeably, when they most definitely are not the same! Now that you understand the difference between the two, and you know what a power factor is, you are light years ahead of 95% of the population when it comes to figuring out how to purchase a properlysized UPS or similar device.

Voltage Conversion and Inversion As discussed here, there are two types of electricity: direct current and alternating current. Each has its advantages and disadvantages, and most types of devices will only run on one or the other. Therefore, it would be useful to be able to change electricity from one form to the other. Fortunately, devices have been created that enable us to do exactly that. The process of changing AC into DC is called conversion (actually, this is an imprecise term because "conversion" also refers to changing one DC voltage to another, and other things as well, but it will do for our purposes.) Devices that perform this process are called converters, but are also sometimes called adapters, and if being used for charging batteries, they are often just called chargers. Changing DC into AC is the opposite process and is called inversion. A device that does this is, of course, called an inverter. Most people use converters on a daily basis even if they don't realize it, while inverters are used only for special applications. The reason for this is pretty simple: most people have AC power in their houses and therefore have little need for a device that creates AC from a DC source. However, inverters are useful for a wide range of applications, including letting you run small 115 VAC household appliances from your car's battery or electrical system, which is DC. In the PC world, inverters are a major component in uninterruptible power supplies, changing stored battery energy into a form your AC-powered PC power supply can use. Another important thing to keep in mind is that every time you convert or invert electricity, there is some loss of energy due to waste heat in the components. The very best inverters are only about 90% efficient, meaning 10% of the energy is lost during the inversion; cheaper ones are less efficient. Converters can be as efficient as inverters, but are often much worse, often being no better than 50% efficient--half the power input to them is radiated as heat.

Uninterruptible Power Supplies While there are many less-expensive methods you can employ to provide some degree of protection for your PC from power problems, none of them can insulate your system from power troubles as well as a good uninterruptible power supply (UPS). The idea behind a UPS is pretty obvious from the name. In addition to filtering, enhancing or modifying the utility power, special circuitry and batteries are used to prevent the PC from losing power during a disruption (blackout) or voltage sag (brownout). These units are called different names depending on their exact design, but all fit into the general category of backup power. Once considered an expensive luxury, UPSes are now available quite inexpensively. While once I would have recommended a UPS to only those whose systems really needed them, I can say now that anyone who uses their PC for any work purpose should be thinking seriously about getting one. If you consider your time valuable, a UPS can pay for itself the first time the power flickers or goes out. Note: Some people pronounce "UPS" using the three letters sounded out (like the name of a famous parcel delivery company) while others pronounce them as a word ("ups"). I do the former, so you will see "a UPS" and not "an UPS" on this web site. Warning: I have received information that warns about possible equipment failure and even safety hazards as a result of plugging surge suppressors into the output jacks of a UPS. You

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ACEC KOMNA 16 should never do this, as they are not designed to be used this way (there is no need to anyway) and a hazardous situation could result.

Uninterruptible Power Supply Overview Before delving into the details of how an uninterruptible power supply works, let's take a quick look at the basics of this type of equipment. The fundamental purpose of a UPS is to provide an uninterruptible source of power for the equipment it protects. How exactly is this done? An electric device plugged into the wall (or into a surge suppressor plugged into the wall) has only one source of power. If there is a blackout, the electricity is cut and the device obviously goes off immediately. A UPS changes this equation by providing its equipment two sources of power. UPSes are designed so that there is one source of power that is normally used, called the primary power source, and another source that kicks in if the primary is disrupted, called the secondary power source. The power from the wall is always one of these sources, and the battery contained within the UPS is the other. A switch is used to control which of these sources powers the equipment at any given time. The switch changes from the primary source to the secondary when it detects that the primary power has gone out. It switches back from the secondary power source to the primary when it detects that the primary power source has returned.

Very basic block diagram of a UPS, showing the basic design: two power sources, controlled by a switch. Contrary to what you might think, the wall AC power is not always the primary power source and the battery the secondary. Which source is primary and which is secondary depends on the type of UPS. Of course, the power that comes from the wall is AC, and your PC uses AC power as well. All batteries, however, provide DC power. Therefore, circuitry is provided within all UPSes to convert AC power to DC to charge the battery. A device called an inverter is also provided to change the battery's stored DC electricity to AC to run your equipment. These components of the UPS, and others, are discussed in detail in the section covering the various parts of the UPS. UPSes come in many different sizes and shapes. The size of the UPS is primarily dictated by the size of the battery; the larger the battery, the more time your equipment can run on battery power before shutting down. Larger units not only can power equipment for more time, they can also handle a larger total demand for power. Different UPSes have various other additional

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ACEC KOMNA 17 features, including warning signals, PC control software, and conditioning circuitry for the AC power source. Most newer UPSes also include a feature to shut down your PC in the event that both of its power sources fail, to avoid possible operating system problems caused by the power going out suddenly to the PC. UPS features are discussed in this section.

Uninterruptible Power Supply Types Most people know that UPSes exist, but many seem to think that there is just one kind of device that goes by that name. In fact, there are several different major designs in use for UPS models. Those who sell these devices share much of the blame for this situation, because too often, the different kinds of UPS are all called the same, generic name. (Although it is usually possible to determine which type of UPS a given model is, if you do your homework.) This section takes a look at the different UPS types, discusses their basic design principles, and attempts to draw comparisons between them to help you understand them better. Note: In addition to the types I cover here, there are some other, esoteric designs, some of which are hybrids of the units described in this section. I don't cover every variant here, just the most common architectures. Standby UPS / Standby Power Supply The standby UPS is the simplest and least expensive UPS design. In fact, some don't even consider a standby UPS to really be a UPS, calling it instead a standby power supply (SPS). However, many of the most common consumer-grade devices marketed as UPSes, particularly on the lower end of the budget scale, in fact use this general design. They are sometimes also called offline UPSes to distinguish them from online UPSes. In this type of UPS, the primary power source is line power from the utility, and the secondary power source is the battery. It is called a standby UPS because the battery and inverter are normally not supplying power to the equipment. The battery charger is using line power to charge the battery, and the battery and inverter are waiting "on standby" until they are needed. When the AC power goes out, the transfer switch changes to the secondary power source. When line power is restored, the UPS switches back.

Block schematic of a standby UPS. The primary power source is filtered and surge-suppressed to protect against line noise and other problems that would not cause a switch to battery power. Image © American Power Conversion Corp. Image used with permission.

While the least desirable type of UPS, a standby unit is still a UPS and will serve well for most users. After all, if standby UPSes didn't work, they wouldn't sell. For a very critical function, however, such as an important server, they are not generally used. The issue with a standby UPS is that when the line power goes out, the switch to battery power happens very quickly, but not instantly. There is a delay of a fraction of a second while the switch occurs, which is called the switch time or transfer time of the UPS. While rare, it is possible for the UPS to not make the switch fast enough for the PC's power supply to continue operation uninterrupted. Again, in practice this does not normally occur or nobody would bother to buy these units. Still, you

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ACEC KOMNA 18 should compare the unit's transfer time to the hold (or holdup) time of your power supply unit, which tells you how much time the power supply can handle having its input cut off before being interrupted. If the transfer time is much less than the hold time, the UPS will probably work for you. Standby UPSes are usually available in a size range of up to about 1000 VA. Warning: If you use a standby UPS, make sure it incorporates surge suppression and filtering features for when the machine is running off standard power, as shown in the block diagram above. Otherwise, under normal cases (i.e., any time you aren't experiencing a blackout) your system is, in essence, plugged directly into the wall.

Electric power is defined as the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt. Electrical power is distributed via cables and electricity pylons like these in Brisbane, Australia.When electric current flows in a circuit with resistance, it does work. Devices convert this work into many useful forms, such as heat (electric heaters), light (light bulbs), motion (electric motors) and sound (loudspeaker). Electricity can be produced by generation or from storage such as batteries.

Mathematics of electric power [edit] In circuits Electric power, like mechanical power, is represented by the letter P in electrical equations. The term wattage is used colloquially to mean 'electric power in watts'. In direct current resistive circuits, instantaneous electrical power is calculated using Joule's Law, which is named after the British physicist James Joule, who first showed that electrical and mechanical energy were interchangeable. where P is the power (watt or W) I is the current (ampere or A) V is the potential difference (volt or V) For example: . Joule's law can be combined with Ohm's law to produce two more equations: where R is the resistance (Ohm or Ω). For example: and In alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow. The portion of power flow that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as real power (also referred to as active power). That portion of power flow due to stored energy, that returns to the source in each cycle, is known as reactive power. Power triangle The components of AC power The relationship between real power, reactive power and apparent power can be expressed by representing the quantities as vectors. Real power is represented as a horizontal vector and reactive power is represented as a vertical vector. The apparent power vector is the hypotenuse

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ACEC KOMNA 19 of a right triangle formed by connecting the real and reactive power vectors. This representation is often called the power triangle. Using the Pythagorean Theorem, the relationship among real, reactive and apparent power is: (apparent power)2 = (real power)2 + (reactive power)2 The ratio of real power to apparent power is called power factor and is a number always between 0 and 1.

[edit] In space Electrical power flows wherever electric and magnetic fields exist in the same place. The simplest example of this is in electrical circuits, as the preceding section showed. In the general case, however, the simple equation P = IV must be replaced by a more complex calculation, the integral of the vector cross-product of the electrical and magnetic fields over a specified area, thus: The result is a scalar since it is the surface integral of the Poynting vector.