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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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With the typical logic probe, one light glows when the circuit is low and another light glows when the circuit is high (see Figure 10-2). Most logic
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 209
Figure 10-2:
Logic probes use lights to indicate a low or high signal.
probes also include a tone feature. The tone toggles between two states to indicate low or high. You donít have to take your eyes off the circuit; just listen to the probe as it sings to you!
Logic probes also let you know when the circuit has no signal at all, either low or high. If the circuit has no signal, then neither probe light glows and the probe doesnít make a sound. (However, this lack of response from the probe doesnít always mean that you have a bad circuit, as we talk about in the section ďWhat if the indicator doesnít indicate?Ē). With a multimeter, the lack of any signal may appear as zero volts (indicating a possible low). This difference makes a logic probe the better tool for testing digital circuits.
Logic probes also help you solve the problem of poor connections. If you have a loose wire, for example, the audible tone from the probe breaks up or crackles. You get this kind of response from the probe because it canít get a steady, reliable signal. When you hear a weak or unsteady tone, fix the connection and try it again.
Signals that are too fast (even for Superman)
Being versatile little gadgets, most logic probes can also identify a circuit where the signal is rapidly changing. This rapid signal change happens quite often in digital circuits. Figure 10-3 shows an illustration of such a changing signal, called a square wave. This digital signal changes, or pulses, between low and high. How fast it changes depends on the circuit. In some circuits, a signal changes millions of times per second.
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210 Part IV: Getting Your Hands Dirty
Figure 10-3:
A typical square wave has its highs and lows.
Although the logic probe canít tell you how fast a signal is pulsing, for most tests you simply need to know whether the signal is pulsing at all. If you expect the signal to be pulsing, but it isnít, then you know you have a problem somewhere.
When you do need to determine the rate of pulsing, or even what the signal waveform looks like, use an oscilloscope, which you can read about in more detail in the section ďScoping Out the Oscilloscope,Ē later in this chapter. The logic probe is a simple tool to use, and itís great for the kinds of jobs that itís designed for. But in-depth digital analysis isnít one of them.
Why don't all circuits like logic probes?
Believe it or not, some electronic circuits don't like certain pieces of test equipment. Most test gear, including the multimeter and oscilloscope, draws very little current from the circuit that you're testing. Their makers design these testing tools this way so that the tools themselves don't influence the reading. Obviously, it does no good to test a circuit if the testing tool changes the behavior of that circuit. You can't get a reliable result.
Logic probes not only draw power from the circuit, they can load down the signal line that you're testing. Some digital signals are fairly weak. The additional load of the logic probe may cause the signal to drop in voltage to a point where you can't get an accurate reading.
Although this situation doesn't come up all that often, it's a good example of why you need to be somewhat familiar with the circuit that you're testing. Just know that poking the probe into unknown territory may yield unpredictable results.
Be sure to read the manual or instruction booklet that comes with your logic probe for additional pointers, cautions, caveats, warnings, and operating tips. Though many logic probes are similar in design, slight differences can influence the types of circuits that a particular probe best works with.
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 211
Know thy circuit
To get the most use out of a logic probe, you need a wiring diagram, schematic, or service notes for the circuit that youíre testing. This information helps you better determine the source when you discover problems.
You also need to have some documentation for the circuit handy because you have to be careful about where you put the logic probe. A logic probe receives its power from the circuit that youíre testing. To use the probe, you must first connect its power leads to the positive and ground connections of the circuit that youíre troubleshooting. Youíre not supposed to operate most logic probes at more than 15 volts, so you have to know where to tap into the circuit for the power. If you connect the probe to a spot with a high voltage, you run the risk of permanently damaging the probe, the circuit youíre testing, or both. If you donít know the voltage level of a particular circuit, first test it with a multimeter.
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