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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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Chapter 4: Getting to Know You: The Most Common Electronic Components
69
in high-load applications, such as motor or lamp control, require higher wattages than those resistors that you use in low-current applications. The majority of resistors that you use for hobby electronics are rated at X or even watt.
High-wattage resistors take many forms, some of which you can see in Figure 4-3. Resistors over five watts commonly come encased in epoxy or other waterproof and flameproof coating and have a rectangular, rather than cylindrical, shape. Higher-wattage resistors may even include their own metal heat sink where the fins help draw heat away from the resistor.
Figure 4-3:
High-wattage resistors come packaged in many different forms.
Dialing with potentiometers
Variable resistors, more commonly known as potentiometers (or in electronics slang as pots), let you “dial in” a resistance. The upward value of the potentiometer determines the actual range of resistance. Most potentiometers are marked with this upward value — 10K, 50K, 100K, 1M, and so forth. For example, with a 50K potentiometer, you can dial in any resistance from 0 to
50,000 ohms. Bear in mind that the range on the potentiometer is approximate only. If the potentiometer lacks markings, you need to use a multimeter to figure out the component’s value. (You can read about how to test resistances, using a multimeter, in Chapter 9.)
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70
Part II: Aisle 5, Component Shack: Stocking Up
Virtually all potentiometers are the dial type. You sometimes use the other type, the slide potentiometer, on gadgets like stereo equipment. The dial kind is typically easier to mount into your own projects.
With the dial type of potentiometer, you can rotate the dial nearly 360 degrees, depending on the specific qualities of the potentiometer that you’re using. At one extreme, the potentiometer has zero resistance going through it; at the other extreme, the resistance is the maximum value of the component. Your television volume control or electric blanket control are typical examples of the dial pot.
Capacitors: Reservoirs for Electricity
After resistors, capacitors are the second most common component in the average electronic device. Capacitors are interesting little gadgets. They store electrons by attracting them to a positive voltage. When the voltage is reduced or removed, the electrons move off. When a capacitor removes or adds electrons to the circuit in this fashion, it can work to smooth out voltage fluctuations. In some cases you can use capacitors combined with resistors as timers (read more about this in Chapter 7). Capacitors make possible all kinds of circuits, such as amplifiers and thousands of others.
Capacitors are used for all sorts of neat applications, including
Creating timers: A kind of electronic metronome, a timer most often pairs up with a resistor to control the speed of the tick-tick-tick.
Smoothing out voltage: Power supplies that convert AC current to DC often use capacitors to help smooth out the voltage so that the voltage stays at a nice, constant level.
Blocking DC current: When connected inline (in series) with a signal source, such as a microphone, capacitors block DC current but pass AC current. Most kinds of amplifiers use this function, for example.
Adjusting frequency: You use capacitors to make simple filters that reject AC signals above or below some desired frequency. By adjusting the value of the capacitor, it’s possible for you to change the cut-off frequencies of the filter.
A quick look inside a capacitor
Though they many sound complicated because of all the things that you can use them for, capacitors are really very simple devices. The typical capacitor has two metal plates inside it. The plates don’t touch. Instead, a dielectric material, which is a fancy term for an insulator, separates the plates.
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Chapter 4: Getting to Know You: The Most Common Electronic Components
71
Common dielectrics used in capacitors include plastic, mica, and paper. (We talk more about the dielectric in the section “Dielectric this, dielectric that,” later in this chapter.)
Farads big and small
You probably realize by now that, just as politicians have an excuse for everything, electronics types have units of measure for absolutely everything. The electronics world rates capacitors by capacitance, expressed in farads. The higher the value, the more electrons the capacitor can store at any one time. The farad is a rather large unit of measurement, so the bulk of capacitors available today come rated in microfarads, or a millionth of a farad. You may even come across an even smaller rating — the picofarad, or a millionth of a millionth of a microfarad. Using the Greek “micro” character, electronics documentation most often shortens the microfarad to |iF, as in 10|iF. You shorten the picofarad to the simple pF.
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