Electronics All-in-One For Dummies. Doug Lowe

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СКАЧАТЬ isn’t controlled by a switch, unplug the light from the wall outlet.

       If an extension cord becomes frayed or damaged in any way, discard it. When the insulation begins to rub off of an extension cord, the shock hazard is very real.

       Never perform electrical wiring work while the circuit is energized. If you insist on changing your own light switches or electrical outlets, always turn off the power to the circuit by turning off the circuit breaker that controls the circuit before you begin. Many people die every year because they think that they can be careful enough to safely work with live power.IS IT TRUE THAT CURRENT, NOT VOLTAGE, KILLS?There is an old adage that “it’s the current that kills, not the voltage.” Although this statement may be technically true, it’s also dangerously misleading. In fact, it stems from a fundamental misunderstanding of what current and voltage are. It can cause you to take dangerous risks if you don’t understand the relationship between current and voltage.The danger from electric shock occurs as current passes through vital parts of your body — specifically, your heart. It takes only a few milliamperes of current to stop your heart. At somewhere around 10 mA, muscles seize up, making it impossible to let go if you’re holding a live wire. At around 15 mA, the muscles in your chest can seize up, making it impossible to breath. And at around 60 mA, your heart can stop. It takes only a few moments of exposure for these effects to occur.So yes, it is current passing through your body that can kill you.But current is inseparable from voltage. Current can’t happen without voltage, and all other things being equal, the greater the voltage, the greater the current. As a result, it’s very difficult to receive a lethal shock from three volts even if you’re dripping wet and standing on bare concrete. But under those conditions, 30 volts may be enough to create a painful and damaging shock.Saying “it’s the current that kills, not the voltage” is kind of like saying “it’s lack of oxygen, not water” that causes drowning. Although it may be technically true, isn’t it the water that causes the lack of oxygen?

       Never work on an AC-powered appliance when it has power applied. Simply turning the appliance off isn’t enough to be safe. If the appliance has a power cord, unplug it before you work on it. If it doesn’t have a power cord, turn off the power to the appliance by throwing the circuit breaker on your home’s electrical panel.

       Take extra precautions when you’re working with your own AC circuits. In Book 4, I tell you more about working with AC circuits; I say more about AC safety then.

      Even relatively small voltages can hurt you

      Most of the projects in this book work with AA batteries, usually two or four of them tied together to produce a total of three or six volts. That’s not enough voltage to do serious harm. Even if you do get a shock with three or six volts, you will probably barely feel it.

      However, it’s possible to injure yourself with voltages even as low as three or six volts. If you accidentally create a short circuit between the two poles of a battery, a lot of current will flow very fast. This will very likely cause the wire connecting the two ends of the battery to get very hot, and the battery itself may also heat up. The heat may be enough to inflict a nasty burn.

       If the racing current goes unchecked, there’s also the possibility that the battery will explode. Trust me; you don’t want to be nearby if that happens. You really don’t want to make a trip to the emergency room to have fragments of an exploded battery removed from your eyes.

      As a result of this danger, you should take the following precautions when working with the battery-powered circuits described in this book:

       Don’t connect power to the circuit until the circuit is completely finished and you’ve reviewed your work to ensure that everything is connected properly.

       Don’t leave your circuits unattended when they’re connected to power. Always remove the batteries before you walk away from your workbench.

       Periodically touch the batteries with your finger to make sure they aren’t hot. If they’re getting warm, remove the batteries and recheck your circuit to make sure you haven’t made a wiring mistake.

       If you smell anything burning, remove the batteries and recheck your circuit.STAYING SAFE BY STAYING DRYWe’ve all seen murders committed on TV crime dramas by throwing a plugged-in electrical appliance such as a hair dryer into a bathtub while the victim was taking a bath. I’ve always wondered how often that really happens, and how likely it’s fatal. For example, how quickly would the circuit breaker kick in and cut power to the hair dryer? Would the special GFCI-protection devices required in all modern bathrooms work as designed and cut power to the hair dryer in time?I’ve never wanted to conduct an experiment to actually find out — nor should you, under any circumstances. Water and electricity are a very bad combination because water is an excellent conductor of electricity, and it flows everywhere.Strictly speaking, pure uncontaminated water is actually an insulator. But pure water is very rare. Most water is filled with contaminates, and those contaminants turn the water into an excellent conductor. Thus, it’s true that you should avoid water when working with electrical current. Here are a few tips for staying safe by staying dry:Make sure the floor is dry. Don’t work on electronic or electrical devices in an area where the floor is wet.Beware of high humidity, especially if it condenses into moisture on your projects.Dry your hands before working with electrical current. Even a small amount of sweat on your hands can lower your body’s natural resistance and accentuate the danger of electrical shocks from lower voltages.

       Always wear protective eyewear to protect yourself against exploding batteries. (Under the right circumstances, other components can explode as well!)

      Sometimes voltage hides in unexpected places

      One of the biggest shock risks in electronics comes from voltages that you didn’t expect to be present. It’s easy enough to keep your eye on the voltages that you know about, such as in your power supply or batteries, but some electronic circuits are designed to amplify voltages. So even though your circuit runs on 6-volt batteries, there may be much larger voltages at specific points within your circuit.

In addition, some electrical devices can actually store an electric charge long after the power from your circuit has been disconnected. The most notorious device with this characteristic is the capacitor, which alternately builds up and then releases electrical charges. Thus, you should be wary of any circuit that contains capacitors — especially if the capacitors are large. Common ceramic-disk capacitors, which are typically smaller than a tiddlywink, don’t store much charge. However, if your circuit has capacitors the size of batteries, you should be very careful when working around them. Such capacitors can hold large charges long after the power has been cut off.

      Here are some safety points concerning capacitors:

        One of the most common places to find large capacitors is in the power-supply circuit. Any electronic device that plugs into a household electrical outlet has a power-supply circuit that may contain a large capacitor. Be very careful around these capacitors. In fact, if the power-supply circuit is inside its own enclosed box, don’t open the box. Instead, replace the entire power supply if you suspect it’s bad.

       Another common place to find high-voltage capacitors is in a flash camera. Even though the battery may be just 1.5 V, the capacitor that drives the flash unit may well be holding a charge of 300 V or more.

       Before working on a circuit that contains a capacitor, always discharge the СКАЧАТЬ