Just as you usually use a pound or so of apples to bake your average pie or several tons of steel to build a suburban office park, in electronics, some things just naturally come in small measurements and others in large measurements. That means that you typically see certain combinations of prefixes and units over and over. Here are some common combinations of notations for prefixes and units: ߜ Current: pA, nA, mA, µA, A ߜ Inductance: nH, mH, µH, H ߜ Capacitance: pF, nF, mF, F ߜ Voltage: mV, V, kV ߜ Resistance: Ω, kΩ, MΩ ߜ Frequency: Hz, kHz, MHz, GHz 24 Part I: Getting Started in Electronics Exploring some new terms Although we discussed resistance, voltage, and current earlier in this chapter, some other terms in this section may be new to you. Capacitance is the ability to store a charge in an electric field. This stored charge has the effect of making decreases or increases of voltage more gradual. You can use components called capacitors to provide this property in many cir- cuits. This figure shows the signal that occurs when you decrease voltage from +5 volts to 0 volts, both with and without a capacitor. + 5 VOLTS 0 VOLTS WITHOUT CAPACITOR WITH CAPACITOR Frequency is a measurement of how often an AC signal repeats. For example, voltage from a wall outlet undergoes one complete cycle 60 times a second. The following figure shows a sine wave. In this figure, the signal completes one cycle when the current goes from -5 to +5 volts then back down to -5 volts. If a signal repeats this cycle 60 times a second, it has a frequency of 60 hertz. TEAM LinG - Live, Informative, Non-cost and Genuine ! Using the information in Tables 1-1 and 1-2, you can translate these notations. Here are some examples: ߜ mA: milliamp or 1 thousandth of a amp ߜ µV: microvolt or 1 millionth of a volt ߜ nF: nanofarad or 1 billionth of a farad ߜ kV: kilovolts or 1 thousand volts ߜ MΩ: megohms or 1 million ohms ߜ GHz: gigahertz or 1 billion hertz The abbreviations for prefixes representing numbers greater than 1, such as M for mega, use capital letters. Abbreviations for prefixes representing num- bers less than 1, such as m for milli, use lowercase. The exception to this rule (there’s always one) is k for kilo, which is lowercase even though it stands for 1,000. 25 Chapter 1: From Electrons to Electronics Inductance is the ability to store energy in a magnetic field; this stored energy resists changes in current just as the stored charge in a capacitor resists changes in voltage. Components called inductors are used to pro- vide this property in circuits. Power is the measure of the amount of work that electric current does while running through an electrical component. For example, when voltage is applied to a light bulb and current is driven through the filament of the bulb, work is done in heating the filament. In this example, you can calculate power by multiplying the volt- age applied to the light bulb by the amount of current running through the filament. − 5 TO + 5 VOLT AC SINE WAVE 0 VOLT + 5 VOLTS − 5 VOLTS 1 CYCLE TEAM LinG - Live, Informative, Non-cost and Genuine ! The use of capital K is a special case reserved for kilohms; when you see a capital K next to a number such as 3.3k, this translates as 3.3 kilohms. You have to translate any measurement expressed with a prefix to base units to do any calculation, as you can see in the following sections. Understanding Ohm’s Law Say that you’re wiring a circuit. You know the amount of current that the component can withstand without blowing up and how much voltage the power source applies. So you have to come up with an amount of resistance that keeps the current below the blowing-up level. In the early 1800s, George Ohm published an equation called Ohm’s Law that allows you to make this calculation. Ohm’s Law states that the voltage equals current multiplied by resistance, or in standard mathematical notation V = I x R Taking Ohm’s Law farther Remember your high school algebra? Remember how if you know two things (such as x and y) in an equation of three variables, you can calculate that third thing? Ohm’s Law works that way; you can rearrange its elements so that if you know any two of the three values in the equation, you can calcu- late the third. So, here’s how you calculate current: current equals voltage divided by resistance, or I R V = You can also rearrange Ohm’s Law so that you can calculate resistance if you know voltage and current. So, resistance equals voltage divided by current, or R I V = So far, so good. Now, take a specific example using a circuit with a 12-volt bat- tery and a light bulb (basically, a big flashlight). Before installing the battery, you measure the resistance of the circuit with a multimeter and find that it’s 9 ohms. Here’s the formula to calculate the current: I R V 9 ohms 12 volts 1.3 amps== = 26 Part I: Getting Started in Electronics TEAM LinG - Live, Informative, Non-cost and Genuine ! What if you find that your light is too bright? A lower current reduces the brightness of the light, so just add a resistor to lower the current. Originally, we had 9 ohms; adding a 5-ohm resistor to the circuit makes the total resis- tance 14 ohms. In this case, the formula for current is . 14 09I R V ohms 12 volts amps== = Dealing with numbers both big and small Say that you have a circuit with a buzzer that has resistance of 2 kilohms and a 12-volt battery. You don’t use 2 kilohms in the calculation. To calculate the current, you have to state the resistance in the basic units, without using the “kilo” prefix; in this example that means that you have to use 2,000 ohms for the calculation, like this: , . 2 000 0 006I R V ohms 12 volts amps== = You now have the calculated current stated as a fraction of amps. After you finish the calculation, you can use a prefix to restate the current more suc- cinctly as 6 milliamps or 6 mA. Bottom line: You have to translate any measurement expressed with a prefix to base units to do a calculation. The power of Ohm’s Law Ohm (never one to sit around twiddling his thumbs) also expressed that power is related to voltage and current using this equation: P = V x I; or power = voltage x current You can use this equation to calculate the power consumed by the buzzer in the previous section: P = 12 volts x 0.006 amps = 0.072 watts which is 72 milliwatts (or 72 mW) What if you don’t know the voltage? You can use another trick from algebra. (And you thought Mrs. Whatsit wasted your time in Algebra 101 all those years ago!) Because V = I x R, you can substitute I x R into this equation, giving you P = I 2 x R; or power = current squared x resistance 27 Chapter 1: From Electrons to Electronics TEAM LinG - Live, Informative, Non-cost and Genuine ! You can also use algebra to rearrange the equation for power to show how you can calculate resistance, voltage, and current if you know power and any one of these parameters. Do you really hate algebra? Did Mrs. Whatsit fail you those many years ago? You’re probably happy to hear that online calculators can make these calcu- lations much easier. Try searching on www.google.com using the keyword phrase “Ohm’s Law Calculator” to find them. Also, check out Chapter 18. It provides ten of the most commonly used electronics calculations. 28 Part I: Getting Started in Electronics TEAM LinG - Live, Informative, Non-cost and Genuine ! Chapter 2 Keeping Humans and Gadgets Safe In This Chapter ᮣ Using common sense when working with electronic components ᮣ Avoiding electrocution ᮣ Keeping watch over static ᮣ Working with AC current ᮣ Measuring safely with a multimeter ᮣ Soldering without fear ᮣ Wearing the right clothes for safety Y ou probably know that Benjamin Franklin “discovered” electricity in 1752 by flying a kite during a lightning storm. But actually, Franklin already knew about electricity. He was really just testing a form of lightning conductor. Though his experiment proved modestly successful, it was anything but safe. Franklin almost killed himself, and if he had, whose picture would be on the $100 bill? Respect for the power of electricity is necessary when working with electron- ics. In this chapter, we take a look at keeping yourself — and your electronic projects — safe. This is the one chapter that you really should read from start to finish, even if you already have some experience in electronics. The Sixth Sense of Electronics The sixth sense of electronics isn’t about seeing dead people. In this case, the sixth sense is common sense, the smarts that help you stay among the living. Common sense is that voice inside you that tells you not to stick your fingers in an empty lamp socket without first unplugging the lamp. TEAM LinG - Live, Informative, Non-cost and Genuine ! No book can ever teach you common sense. You have to cultivate it like an exotic flower. But a few words to the wise may help get you started in your quest for electronics common sense. For starters: ߜ Never assume. Always double-check. Pretend that your soldering pencil is out to get you. Your family may think you’re crazy, but you’re less likely to burn or electrocute yourself. ߜ If you’re not sure about how to do something, read up on it first. Not everything in electronics is as obvious as it first appears. ߜ Don’t take chances. If you can make a 50/50 bet that something is plugged in, give some thought as to what happens if you lose the bet this time. Never let your guard down. Don’t ruin the fun of a wonderful hobby or voca- tion because you neglected a few basic safety measures. The Dangers of Electrical Shock By far, the single most dangerous aspect of working with electronics is the possibility of electrocution. Electrical shock results when the body reacts to an electrical current — this reaction can include an intense contraction of muscles (namely, the heart) and extremely high heat at the point of contact between your skin and the electrical current. The heat leads to burns that can cause death or disfigurement. Even small currents can disrupt your heartbeat. The degree to which electrical shock can harm you depends on a lot of fac- tors, including your age, your general health, the voltage, and the current. If you’re over 50 or in poor health, you probably won’t stand up to injury as well as if you’re 14 and as healthy as an Olympic athlete. But no matter how young and healthy you may be, voltage and current can pack a wallop, so it’s important that you understand how much they can harm you. Electricity = voltage + current To fully understand the dangers of electrical shock, you need to know the basics of what makes up electricity. In Chapter 1, we state that electricity is made up of two elements: voltage and current. Voltage and current work hand-in-hand and in ways that directly influence the severity of electrical shock. Consider the analogy of water flowing through a pipe. Think of the water as representing the electricity. Increasing the diame- ter of the pipe to let more water through is like increasing current. Imagine being under a drain pipe for the Hoover Dam! Increasing the pressure of the 30 Part I: Getting Started in Electronics TEAM LinG - Live, Informative, Non-cost and Genuine ! water in the pipe is like increasing voltage. You know that even small amounts of water at high pressure can be devastating. The same is true of electricity, where even low voltages at high currents can potentially kill you. Is it AC or DC? You can describe electrical current as being either of the following ߜ Direct (DC): The electrons flow one way through a wire or circuit. ߜ Alternating (AC): The electrons flow one way, then another, in a continuing cycle. If this stuff is new to you, you may want to go back and read Chapter 1 for a more detailed discussion. Household electrical systems in the U.S. and Canada operate at about 117 volts AC. This significantly high voltage can, and does, kill. You must exercise extreme caution whenever you work with it. Until you become experienced working with electronics, you’re best off avoiding circuits that run directly off household current. Stay with circuits that run off standard-size batteries, or those small plug-in wall transformers. Unless you do something silly, like lick the terminal of a 9-volt battery (yes, you get a shock!), you’re fairly safe with these voltages and currents. The main danger of household current is the effect it can have on the heart muscle. High AC current can cause severe muscle contraction, serious burns, or both. And many electrocution accidents occur when no one is around to help the victim. Burns are the most common form of injury caused by high DC current. Remember that voltage doesn’t have to come from a souped-up power plant to be dangerous. For example, don’t be lulled into thinking that because a transistor battery delivers only nine volts, it’s harmless. If you short the ter- minals of the battery with a piece of wire or a metal coin, the battery may overheat — and can even explode! In the explosion, tiny battery pieces can fly out at high velocity, burning skin or injuring eyes. Trying to not get electrocuted Most electrocution accidents happen because of carelessness. Be smart about what you’re doing, and you will significantly reduce the risk of being hurt by electricity. 31 Chapter 2: Keeping Humans and Gadgets Safe TEAM LinG - Live, Informative, Non-cost and Genuine ! Here are a few handy electrocution prevention tips: ߜ Avoid working with AC-operated circuits. Of course, you can’t always do this. If your project requires an AC power supply (the power supply converts the AC to lower-voltage DC), consider using a self-contained one, such as a plug-in wall transformer. They’re much safer than a home- made power supply. ߜ Physically separate the AC and DC portions of your circuits. This helps to prevent a bad shock if a wire comes loose. ߜ Make sure you secure all wiring inside your project. Don’t just tape the AC cord inside the project enclosure. It may pull out sometime, exposing a live wire. Use a strain relief or a cable mount to secure the cord to the enclosure. A strain relief clamps around the wire and prevents you from tugging the wire out of the enclosure. You can buy a strain relief for elec- trical cords at almost any hardware store or electronics shop. ߜ Whenever possible, use a metal enclosure for your AC-operated pro- jects, but only if the enclosure is fully grounded. You need to use a 3- prong electrical plug and wire for this. Be sure to firmly attach the green wire (which is always the ground wire; ground is used as a reference for all voltages in a circuit) to the metal of the enclosure. ߜ If you can’t guarantee a fully-grounded system, use a plastic enclosure. The plastic helps insulate you from any loose wires or accidental electro- cution. For projects that aren’t fully grounded, only use an isolated power supply, such as a wall transformer (a black box with plug prongs which is attached to a wire, such as you may have on your cell phone charger). You plug the transformer into the wall, and only relatively safe low-voltage DC comes out. ߜ Don’t be the class clown. Be serious and focused while you’re working around electricity. ߜ Don’t work where it’s wet. “Yeah, duh!” you say. But you’d be surprised what people sometimes do when they’re not paying attention. And remember, just because you put liquid in a cup, that doesn’t mean you don’t run the risk of knocking it over and getting things wet; consider leaving your soft drink or coffee on an out of the way shelf when working on your electronics project. Practice the buddy system. Whenever possible, have a buddy nearby if you’re working around AC voltages. You want someone around who can dial 9-1-1 when you’re lying on the ground unconscious. Seriously. Getting a first aid chart Of course, you’re the safest person on earth, and you will never be electro- cuted. But just in case, get one of those emergency first aid charts that 32 Part I: Getting Started in Electronics TEAM LinG - Live, Informative, Non-cost and Genuine ! includes information about what to do if anyone else (not you, of course) ever pokes his finger into a wall outlet. You can find these charts on the Internet; try a search for “first aid wall chart.” You can also find them in school and industrial supply catalogs. Helping someone who has been electrocuted may require cardio-pulmonary resuscitation, otherwise known as CPR. Be sure that you’re properly trained before you administer CPR on anyone. Otherwise, you may cause more harm than good. Check out www.redcross.org to get more information about CPR training. Zaps, Shocks, and Static Discharge One type of everyday electricity that is dangerous to both people and elec- tronic gizmos is static electricity. They call it static because it’s a form of current that remains trapped in some insulating body, even after you remove the power source. With conventional AC and DC current, static electricity disappears when you turn off the power source. The ancient Egyptians discovered static electricity when they rubbed cat fur against the smooth surface of amber. After they rubbed the materials together, they tended to cling to one another by some unseen force. Similarly, two pieces of cat fur that they rubbed against the amber tended to separate from each other when the Egyptians drew those pieces together. Although the Egyptians didn’t understand this mysterious force, they were aware of it. And they had the scratched-up arms to prove it! (Note to Pharaohs: Best not to use live cats for your electricity experiments.) 33 Chapter 2: Keeping Humans and Gadgets Safe Carpets don’t shock, people do Carpet shock hasn’t killed anyone (that we know of, anyway). The amount of current is usually too low to harm your body. But, because of their extremely small size, the same isn’t true for electronic components. Static electricity of just a few thousands volts, a mere tingle to you (because the current is so very, very low), can cause great harm to sensitive electronic components. As an electronics experimenter, remember to take specific precautions against electrostatic discharge, or ESD. See the section “Tips for reducing static electricity,” later in this chapter, for specific pointers. You can all but eliminate damage from static discharge by taking just a few simple steps to protect yourself, your tools, and your projects from static buildup. The cost for protecting against static electricity is mini- mal; without knowing it, you may already be on the road to preventing dangerous static buildup in your electronics workshop. TEAM LinG - Live, Informative, Non-cost and Genuine ! . used electronics calculations. 28 Part I: Getting Started in Electronics TEAM LinG - Live, Informative, Non-cost and Genuine ! Chapter 2 Keeping Humans and. that electricity is made up of two elements: voltage and current. Voltage and current work hand-in-hand and in ways that directly influence the severity of