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Electronics for dummies - McComb G.

McComb G., Boes E. Electronics for dummies - Wiley publishing, 2005. - 433 p.
ISBN: 0-7645-7660-7
Download (direct link): electronicsfordummies2005.pdf
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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 diameter 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
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Chapter 2: Keeping Humans and Gadgets Safe
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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
Direct (DC): The electrons flow one way through a wire or circuit.
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, youíre 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!), youíre 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 doesnít have to come from a souped-up power plant to be dangerous. For example, donít be lulled into thinking that because a transistor battery delivers only nine volts, itís harmless. If you short the terminals of the battery with a piece of wire or a metal coin, the battery may overheat ó 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 youíre doing, and you will significantly reduce the risk of being hurt by electricity.
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Part I: Getting Started in Electronics
Here are a few handy electrocution prevention tips:
Avoid working with AC-operated circuits. Of course, you canít 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. Theyíre much safer than a homemade power supply.
^ Physically separate the AC and DC portions of your circuits. This helps to prevent a bad shock if a wire comes loose.
^ Make sure you secure all wiring inside your project. Donít 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 electrical cords at almost any hardware store or electronics shop.
^ Whenever possible, use a metal enclosure for your AC-operated projects, 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.
^ If you canít guarantee a fully-grounded system, use a plastic enclosure.
The plastic helps insulate you from any loose wires or accidental electrocution. For projects that arenít 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.
^ Donít be the class clown. Be serious and focused while youíre working around electricity.
^ Donít work where itís wet. ďYeah, duh!Ē you say. But youíd be surprised what people sometimes do when theyíre not paying attention. And remember, just because you put liquid in a cup, that doesnít mean you donít 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 youíre working around AC voltages. You want someone around who can dial 9-1-1 when youíre lying on the ground unconscious. Seriously.
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