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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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The following sections talk about the nitty-gritty reason for using a multimeter: Testing resistors, capacitors, and the other main components of a circuit.
To get more detailed information on what resistors, capacitors, and other parts that we discuss in the following sections do, see Chapter 4.
Gee, it looks all burned out!
Because the goal of testing is to determine if you have a good component to begin with, start by making first judgments based on the overall appearance of the component. In some cases, a part may be so obviously destroyed that any more testing wastes your time. You have a good sign that youíre dealing with a bad electronic component when it looks burned out. If an electronic component overheats, usually as a result of soaking up too much current, it can melt or erupt. It sometimes even catches on fire! When you find a burned-out component, you need to consider why the component burned out so that you can prevent it from happening again.
Hereís what you need to look for to spot damaged components:
^ On a resistor, see if it has an obviously bulging center, with or without a distinct discoloration.
^ On a capacitor, check for a bulge on the top or sides, with or without gooey electrolyte material seeping out. Donít worry what this gunk looks like: anything coming out of a capacitor is bad news.
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Chapter 9: Making Friends with Your Multimeter 201
On a diode, transistor, or integrated circuit, look for any obvious discolorations on the circuit board caused by extreme overheating of the component.
Donít overlook any component that you find in two or more pieces! (Okay . . . duh . . .)
Avoid contact with the syrupy liquid inside an electrolytic capacitor. Itís caustic, which means it can burn you. Wash your hands immediately with warm water and soap if you do touch this liquid. Donít get any in your eyes! If you do, flush your eyes out right away and seek immediate medical attention.
Of course, looks alone can be deceiving. Your component may have internal damage, even if you donít see visual signs of burn-out. Therefore, use a visual examination only to find obvious faults and not as a way to ultimately determine if your component has a problem. Donít assume that because everything looks okay on the outside that the component doesnít have an internal problem.
Testing resistors
Resistors are the components that limit current through a circuit or divide voltages in a circuit. Resistors come in a lot of values; you can find the value marked on the body of the resistor. Sometimes, you need to verify that the markings are accurate or that the resistor hasnít gone bad.
You can readily test resistors with a multimeter by following these steps:
1. Set the multimeter to read ohms.
If you donít have an auto-ranging meter, start at a high range and work down.
2. Position the test probes on either end of the resistor.
Be sure that your fingers donít touch the test probes or the leads of the resistor; if you do, you add the natural resistance of your own body into the reading, giving you an inaccurate result.
3. Take the reading.
A bad resistor can be either completely open inside, in which case you may get a reading of infinite ohms, or it can be shorted out, in which case you get a reading of zero ohms.
When testing a resistor, check its marked value against the reading provided by the meter. The reading should fall within the tolerance range of the resistor. For example:
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202 Part IV: Getting Your Hands Dirty
Figure 9-15:
Connect the meter probes to the first and center, center and third, and first and third terminals of the pot.
If the resistor has a tolerance of 10 percent and is marked as 1K ohms, acceptable test readings fall in the range of 900 to 1,000 ohms. Tolerance is 10 percent of 1,000, or 100 ohms.
^ If the resistor has a tolerance of one percent (you call these low-tolerance resistors precision resistors), acceptable test readings fall in the range of 990 to 1,010 ohms. Tolerance is one percent of 1,000, or 10 ohms.
Testing potentiometers
A potentiometer is a variable resistor. Like a resistor, you can test potentiometers (also called ďpotsĒ) with your multimeter. As you can see in Figure 9-15, you can connect the meter to either end of the conductive material. With the multimeter applied to points 1 and 2, turning the dial shaft in one direction increases resistance. But with the meter applied to points 2 and 3, turning the dial in the other direction decreases resistance.
The material used for the conductive surface of the pot can take many forms, including cermet (a combination of ceramic, glass, and precious metals), carbon, wire, and conductive plastic. This conductive surface can break off, get dirty, or burn out. A pot sometimes goes bad because something damages the surface. As you turn the shaft of the potentiometer, use your multimeter to make note of any sudden changes in resistance, which may indicate an internal fault. If you find such a fault, you should replace the pot with a new one.
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