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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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You call an SPST switch “single-pole” because it switches only a single part of the circuit. You use “single-throw” to describe the switch because it has only a single variation in its operating positions, on or off.
Some switches are double-pole, double-throw, or both. In a double-pole switch, the switch controls two separate circuits (say, one 12 VDC circuit and one 5 VDC circuit). In a double-throw switch, the switch may be the on-on type, or it may have a center off position, as in on-off-on. Figure 9-13 shows some variations in switch designs that you may test. Other switches may have additional poles and even three, four, or five positions, but these switches usually just
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Chapter 9: Making Friends with Your Multimeter 197
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come as variations on the theme (and switches with additional poles aren’t common). So we don’t spend extra time on them in this book.
In Chapter 5 we talk about switches and their variations in more detail.
Figure 9-12:
Connect the meter probes to each of the two terminals on a switch.
Follow the procedures in Table 9-1 when testing various types of switches. The physical location and function of the terminals on each switch may differ. Often, in a double-pole switch, the center terminal serves as a common; you turn the switch one way to route the current to the left terminal and turn the switch the other way to route the current to the right terminal. However, not all switches are designed this way, and only your own experimentation will help you identify the differences.
Table 9-1 Switch Types
Number of Terminals Type Notes
1 SPST Metal body of the switch/second terminal. To test, connect one lead to body of switch and other lead to lone terminal.
2 SPST To test, connect two probes of the meter to two terminals.
3 SPDT To test, connect one lead to center terminal and other lead to one of remaining terminals. Set switch to one position and make note of the results.
4 DPST Like three-terminal switch, but test both independent switching circuits.
6 DPDT Like three-terminal switch, but test both positions.
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198 Part IV: Getting Your Hands Dirty
Figure 9-13:
Connect the meter probes as the figure shows to test SPDT, DPST, and DPDT switches.
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Chapter 9: Making Friends with Your Multimeter 199
Some Words to the wise
When testing the various types of switches, use the following tips to help you:
With the switch in the off position, the meter should show no continuity (infinite ohms). You’ll encounter this reading with both poles in a doublepole switch.
With the switch in the on position, the meter should show continuity (zero ohms). If the meter doesn’t show continuity when you place the switch in the on position, you have a fairly good indication that you’re working with a bad switch.
You can most easily test switches when you work with them out of a circuit. If you have the switch wired in a circuit, the meter may not show infinite ohms when you place the switch in the off position. Instead, you may get a reading of some value other than 0 (zero) ohms. (To understand why, read the sidebar “Even wire resists the flow of electrons,” earlier in this chapter.)
If you have a double-throw variety of switch, you may not have an off position. Instead, the switch has two on positions. You can test this type of switch as if it were two single-throw switches combined by making two tests rather than just one. If the switch has a center-off position, you should get a no-continuity reading for only the center position.
Testing fuses
If a circuit begins to draw too much current, it can get very hot, not only destroying itself in the process, but also possibly causing a fire. Fuses are designed to protect electronic circuitry from damage caused by excessive current flow and, more importantly, to prevent a fire if a circuit overheats.
A fuse is designed to blow (or become an open circuit) when the current going through it exceeds the safe level for that fuse.
Fuses blow for reasons other than a circuit going haywire. Sometimes, they blow because of some intermittent problem, like a momentary rise (called a spike) in voltage from a distant (or not-so-distant!) lightning strike. When fuses blow, you need to replace them with a fuse of the same rating. You can find the fuse rating printed on the component.
To test a fuse, dial the meter to either Ohms or Continuity. Touch each end of the fuse with the meter probes, as Figure 9-14 shows. The meter should read 0 (zero) ohms. If the meter reads infinite ohms (beyond what the meter can read) it means you have a burned-out fuse and you need to replace it.
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200 Part IV: Getting Your Hands Dirty
Figure 9-14:
Connect the meter probes to either end of the fuse.
Testing Resistors, Capacitors, and Other Electronic Components
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