Electricity And Your Computer This article presents a basic explanation of the considerations involved in powering and protecting your personal computer. To minimize complexity, it is deliberately limited in scope to typical home or small office computing situations. It begins with a simplified explanation of electrical units and terminology that we often see misused. Electricity 101 This section provides a basic understanding of common electrical terms and how they relate to the devices we use daily. The terms voltage, current, wattage, capacitance and resistance are frequently used but not completely understood by most who use them. What is electricity anyway? Electricity is made up of electrons, a fundamental part of the atom. They are negatively charged particles that orbit about the atomic nucleus. Certain materials have atoms in which some of the electrons are able to break away and travel from atom to atom, these are the materials that can conduct electricity. When you gather a lot of roaming electrons (6.24 X 10^18) it is called a coulomb, a term you may quickly forget after I tell you that when one coulomb of electrons passes through a point in an electrical circuit in one second, it is called a current of one ampere of electricity. What makes the electrons move through a circuit in the first place? Because electrons carry a negative charge they will be attracted to a positively charged point in a circuit, and repelled by a negatively charged point. The amount of difference between the positive and negative points is known as the potential or electromotive force and is measured in volts. Electrical power refers to how much energy is expended performing work, and is equal to the current (amperes) multiplied by the force (volts) , and is measured in watts, for example: 12 volts X 5 amperes = 60 watts. A common analogy to make this all more understandable in familiar terms is by comparing it to water in a hose. If we connect a hose to a faucet and turn the faucet on, water will fill the hose and eventually come out the other end. Let the water represent the electrical current, the pressure that pushes the water when we turn on the faucet represents the voltage. If we eliminate the pressure by turning off the faucet, the flow stops. The amount of water passing through the hose multiplied by the pressure pushing it determines how much work the flow can do. A + inch garden hose at 50 pounds pressure can sweep pebbles from your driveway, a 4 inch fire hose at the same pressure can knock a person over. We often talk about Ground, but what is Ground? True ground refers to Earth itself, but just driving a metal rod into the earth is not necessarily a true ground point. Dry, sandy soil can be well above true ground electrically. The same rod driven deep enough to be into the water table will be very close to a true ground. In an electronic device ground has an entirely different meaning. In this case ground is just a common point where power supplies and metal supports such as a case or chassis are connected together, a common reference point. The term Ground potential however means there is no difference in potential (voltage) between a circuit point and the earth. Electricity comes in two flavors, Alternating Current (AC), and Direct Current (DC). DC maintains a constant value above (+), or below (-) a zero reference level. DC always travels in just one direction. AC current alternates between a positive and a negative value over a time period. AC is constantly reversing direction. AC is almost universally used to send electricity over distances because transformers can be used to boost it to very high voltages at relatively low amperages. This reduces the losses caused by the heating of the high voltage transmission wires and results in more of the power arriving at its destination. Once the power reaches the end of the trip, other transformers reduce it to whatever voltage is required. Most electronic equipment operates on DC power, at relatively low voltages. The job of conditioning the electricity into the type and values needed is the work of the power supply, which we will take a closer look at next. There is a lot more we could go into such as electrical resistance, measured in ohms, and electrical storage, or capacitance measured in farads, but for the most part they are beyond the needs of our discussion here, and I can sense your eyes starting to glaze over. I'll finish this section by giving one relationship: it requires a force of one volt for a current of one ampere to pass through a resistance of one ohm. Computer Power Supplies All computers need a source of electrical power. Our portable computers rely on battery power when an electrical outlet is not handy. Our desktop machines use a power supply, which really doesn't supply the power, but converts the power the electric company delivers to our homes and businesses into the form our computers use. The electricity delivered to most of our homes in the United States is alternating current (AC) at a cycle rate of 60Hz, and at 240 volts. Except for appliances that have heavy power requirements like air conditioners, electric stoves, etc., this is split into separate circuits of 120 volts each. Most other countries use between 220 to 240 volts at 50Hz. Our computer power supplies change the AC we get at the wall outlet to the 12 volt and 5 volt direct current (DC) that our computers run on. They are a type of supply known as a switching power supply, which is relatively inexpensive to make and very efficient in the amount of power available in a fairly small, light weight unit. Our power supplies are rated in watts of power capacity. The early PC's only had 65 watt supplies, but we have come a long way from those days. Today we have machines with CPU's that contain tens of millions of transistors, video processors that have huge current needs Floppy, hard, and CD drives filling every available space, and enough memory to give an elephant an inferiority complex. To top it off we need a bunch of fans to keep this whole mess from melting down. It all adds up to a lot of watts! Even a modest computer these days should have at least a 250 watt supply. A fairly loaded, high speed machine will choke on less than a 300 to 400 watt supply. A switching power supply will deliver only as much power as the load requires. A 400 watt supply in a computer that only requires 250 watts will run at 250 watts. This means your electric bills will not increase because you installed a more powerful supply, and the supply will run cooler and last longer because it is operating well below its design level. There is a big difference in quality and price in power supplies. A top of the line supply can cost five times as much as a supply of the same wattage in a generic brand. Is it worth the price difference? Yes, that expensive supply will be made of better quality components, run quieter, and cooler, and have more precisely regulated output voltages. Do you need a top of the line supply? Probably not, as long as the one you buy is decent quality, and approved for the CPU you intend to use, but buy the best you can justify and you will not be likely to regret it. Remember, a failing power supply can take a lot of the equipment you've invested in with it. There is more than one type of computer power supply. The original AT supply connects to an AT motherboard with two six pin connector plugs, and has a two position power switch with an off or on position. The newer ATX style power supply connects to the ATX motherboard with a twenty pin plug, and usually has a momentary push button type switch. The ATX supply is designed to always deliver a voltage to the motherboard, even when the computer is turned off. It is therefore important to disconnect an ATX machine from the wall outlet whenever any work is done inside the case to prevent damage to internal components. Calculating how big a power supply you actually need is a bit messy since you have to know just how much power each component in your computer needs. Here are some typical values for common components: Motherboard without CPU or RAM 30 Watts Floppy Drive 5 Watts IDE Drive 5400RPM 10 Watts IDE Drive 7200RPM 15 Watts SCSI drive 7200RPM 25 Watts SCSI drive 10000RPM 40 Watts 128MB SDRAM 10 Watts IDE 50X CD or 10X DVD Drive 20 Watts High Performance AGP Card 30 Watts Network Card 10/100 5 Watts Pentium III 700MHz 25 Watts Athlon 600MHz 45 Watts To determine the size of the power supply you need, add up the total power demand of your computer, multiply that total wattage by 1.5 so that you do not exceed 70% of the supply's capacity. Surge Protection The power the electric utilities provide to our homes has some problems. The actual voltage is legally allowed to vary over a range of +/- 5% at the generating station. Large power loads going on- or offline can create voltage surges or sags called transients. Conditions like a hot summer day with everyone running their air conditioning full blast can tax total generating capacity causing serious brownouts or drops in available current. These can play havoc with sensitive electronic equipment, and cause extensive damage. Add to this the surges caused by electrical storms, and your computer starts to look like a duck during hunting season! At the very least every computer should be protected by a good quality surge suppressor. This does not refer to the $3.00 bargain at the local hardware store. Expect to pay at least about $20.00 for real protection. Look for a suppressor that meets the UL (Underwriters Laboratories) 1449 rating of 330 volts or lower. This means that the suppressor will block voltage spikes exceeding 330 volts from reaching your connected equipment. It should also meet the UL 1283 specification for EMI/RFI noise reduction. This refers to Electro Magnetic noise, and Radio Frequency noise that can be picked up on the power lines acting like a big antenna, and carried on the lines into your home. If you use a modem, it is also very important to protect it from surges, especially from electrical storms, being carried by the telephone lines. This is covered by UL497A specification for Secondary Telecommunications. Most of the surge devices available for home use utilize Metal Oxide Varistors (MOV), these are electronic components that change resistance according to the voltage applied to them. They work very well, but they will eventually be damaged by the voltage spikes they intercept. For this reason, the suppressor should have an indicator light to show if the unit is still offering protection, If that light goes out or begins to flicker, the unit should be replaced. The amount of energy the unit can suppress is rated in joules, the higher the number of joules the better. Finally, be sure the MOV protection covers all three legs of the circuit including ground, Better Protection - The UPS Far greater protection is available with an Uninterruptable Power Supply (UPS). A UPS starts off by having a very good quality surge protection circuit, and in addition contains one or more large rechargeable batteries. If the electric power goes off, or drops below a safe voltage, a high speed sensing circuit boosts or replaces it with battery power in a tiny fraction of a second preventing damage to the connected equipment or data loss, and give you time to save your data, and shut down. If the power is off long enough to begin exhausting the battery, and the computer is unattended, most better UPSs will safely shut down the computer. In order to accomplish this, the UPS must be connected to the computer through a serial port, and software supplied with the UPS must be installed on the computer. A UPS is one of the best investments a computer owner can make. A typical computer will experience several power glitches a day ranging from very minor to major. In addition to preventing a disaster, a UPS will prevent many of the mysterious data losses computers are prone to, and make all your connected components last longer, and run cooler. If you run your computer constantly or unattended, a UPS is a must. All UPS and most better surge protectors carry insurance that will replace any equipment that is damaged while connected to them. Because any battery has a finite amount of energy stored, only the most important components should be connected to the battery backed outlets. This means the computer itself and the monitor, along with any external modems. Never connect a laser printer to the battery outlets, since they have very high current requirements, and will quickly drain the battery. Most UPSs have outlets that are not battery backed but are surge protected for your other equipment. The amount of time that a UPS can keep a system running depends on the capacity of the UPS and the current drawn by the connected equipment. In other words, the bigger your monitor and more powerful your computer, the bigger the UPS you will need to have a reasonable run time when the power fails. A UPS is rated in watts or VA (Volts X Amperes) or both. The best way to determine how big a UPS you need is to go to the UPS manufacturer's website. You will find a calculator to fill in listing what components you own, and the calculator will determine what size UPS will provide a specific number of minutes of battery protection. Decide the length of backup time you wish and buy that size UPS. A simple rule of thumb is to buy a UPS that has VA rating at least twice the computers power supply rating in watts. For example, a computer with a 300 watt supply would need at least a 600VA UPS. Electrical Safety In electronics we must be concerned with the protection of out equipment from damage and ourselves from electrical shock or worse. Let's discuss protecting ourselves first, since if we get killed we won't be able to finish whatever project we are working on. All of the circuits you can come into contact with inside a computer are harmless to human beings because they operate at 12 volts or less. The sole exception to this is inside the power supply where the connections to house current are made, and before it is reduced to the low voltage DC supplied to the computer circuits. Anyone foolish enough to take apart the power supply without disconnecting it from the power cord may as well go for the Darwin Award for eliminating stupidity from the gene pool. Human skin has a surface resistance to electricity of about 22,000 ohms. Because of this you would need to contact about 25 volts before you would even start to feel anything. The component of electricity that can hurt you is the current. Interestingly, small currents, well under an ampere are the most dangerous. A shock of an ampere or more will cause the heart to clamp (stop) and often it will start beating again on its own as soon as the current is eliminated. A shock in the 20 to several hundred milliamp range can cause the heart to fibrillate (quiver) and in that condition it is much more difficult to restart. Many of you have been told that charged capacitors are dangerous. Only large capacity ones at fairly high voltages, as stated above, can overcome skin resistance. No such capacitors exist inside a computer. Further, safety agencies such as UL require that any capacitors that can supply a dangerous shock must have a bleeder resistance across them that will quickly discharge the capacitor to a safe level once supply power is disconnected. The only place you need to be worried about shock is inside a CRT based monitor where the anode and focus supplies are measured in the thousands of volts but at a very low current. The picture tube (CRT) can retain a shock danger for a long time, even after being shut off. Static Electricity Although the computer offers you little danger, you can be electrically dangerous to your computer! Some materials will generate an electrical charge when friction occurs between them. The simple act of a person walking on a nylon carpet can quickly build up a static charge on the surface of the persons body. Because the charge has no path to ground, it can reach many thousands of volts, though at an extremely low amperage. If that person then touches an electrical circuit the static can discharge to ground through the circuit. Certain types of electronic components are extremely sensitive to static discharge and can be destroyed as a result. To prevent such damage it is important to touch a ground point, such as the metal computer case (before it is unplugged) prior to touching any circuit component in or outside of the computer, This procedure will eliminate any static charge harmlessly. This discussion could easily go on to book length, but we have covered the basics sufficiently to remove some of the mystery of electricity, and allow for intelligent decisions in powering our computers while protecting them, and ourselves, from damage. There are a lot of myths constantly repeated by well meaning people due to a lack of understanding technology. Electronics is a fascinating subject that is touching our lives far more than ever before. http://www.PCNineOneOne.com