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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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64
Part II: Aisle 5, Component Shack: Stocking Up
Viva (a Resistors
Electric current is simply the motion of electrons from one place to another through a wire. The more electrons that are flowing, the higher the current. Resistors have an apt name: They “resist” the electrical current going through them. You can think of resistors as “brakes” for electrons. By controlling the electrons going through a resistor, you can make a circuit do different things.
Resistors may be the primary building block of circuits, so you see them quite a bit in electronics projects. Here are some of the things you can use them for:
Limiting current to another component: Some parts, such as light emitting diodes (LEDs), eat up current. Like a kid eats candy bars they try to gobble up as much as you give them. But LEDs run into a problem — they burn themselves out if they eat too much current. You can use a resistor to limit the amount of current that reaches an LED.
Reducing voltage to part of the circuit: In many circuits, you need to provide different voltages to different parts of the circuit. You can do this easily with resistors. Two resistors joined, as Figure 4-1 shows you, form what’s called a voltage divider. Assuming that you have two identical resistors, that is, they apply their brakes in the same amount, the voltage in between the two is exactly half that of the rest of the circuit.
Controlling the voltage/current going into another component:
Combine a resistor and a capacitor, for example, and you create a kind of hourglass timer. Or put a resistor at the input of a transistor to control how much the transistor amplifies a signal. Or . . . well, you get the idea.
Figure 4-1:
Use two resistors to create this voltage divider, a common technique to produce different voltages for different parts of a circuit.
V OUT
+
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Chapter 4: Getting to Know You: The Most Common Electronic Components
Protecting the inputs of sensitive components: Too much current destroys electronic components. By putting a resistor at the input of a sensitive transistor or integrated circuit, you limit the current that reaches that transistor or circuit. Although not foolproof, this simple technique can save you a lot of time and money that you would lose fixing accidental blow-ups of your circuits.
Ohming in on resistor Values
If resistors act like brakes, then you have to have some way to change how hard you push the pedal, in order to have control over the flow of electrons. That control involves modifying the resistance of a resistor.
Electronics dabblers know the amount of resistance in a resistor as the ohm, typically represented by the Greek capitalized letter omega: Q. The higher the ohm value, the more resistance the component provides.
To understand how you can adjust resistance, you should know that there are two basic types of resistors, fixed and variable. Here’s how they differ:
A fixed resistor supplies a pre-determined resistance to current. Color coding identifies the value of most fixed resistors. The color coding starts near the edge of the resistor and is comprised of four, five, and sometimes six bands of different colors. Figure 4-2 shows the order of bands marked on the body of the resistor along with what each represents.
A variable resistor, called a potentiometer, allows for the continual adjustment from virtually no ohms to some maximum value. The potentiometer usually has the maximum value printed on it somewhere. See the section “Dialing with potentiometers,” later in this chapter, for detailed info on these puppies.
1st DIGIT MULTIPLIER
Figure 4-2:
Color coded bands are used to denote the value in a fixed resistor.
2nd DIGIT TOLERANCE
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66
Part II: Aisle 5, Component Shack: Stocking Up
Not all resistors use color coding. Sometimes, the exact value may be printed on the resistor. This is typical of so-called precision resistors: The actual resistance of the component is very close to or exactly what you see printed on them. You can read more about the precision of resistors in the following section.
Color me red, green, and blue
As we noted in the previous section, the vast majority of resistors use color coding to tell you what resistance, in ohms, they provide. The color code is a world-wide standard, and we’ve been using it in electronics for many decades. Although the colors are standardized, a resistor can have either four or five bands of color, depending on whether it’s standard-precision or high-precision.
Standard-precision resistors use four color bands. These resistors come within at least 2 percent of their marked value. That is, the markings on the resistor and the actual value of the resistor when you test it fall within at least 2 percent of one another. You use standard-precision resistors for 99 percent of your hobby projects. High-precision resistors have five color bands, and they come within 1 percent or less of their marked value. You can find out more about high-precision resistors in the section “A word (or two) about high-precision resistors,” later in this chapter.
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