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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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Testing transistors
You can use a digital or analog multimeter to test most bipolar transistors. The test doesn’t give you conclusive results, but it does provide a useful method of finding out if you have a defective transistor.
Bipolar transistors are essentially two diodes in one package, as you can see in Figure 9-17. You can therefore test the transistor by using the same methodology that we describe in the section “Testing diodes,” earlier in this chapter.
Follow these steps (which assume that your multimeter has a diode-check feature) to determine if the component is good or bad:
1. Set the meter to the diode-check setting.
2. Connect the red and black leads to the terminals of the transistor.
3. Take the reading and note the result. Refer to Table 9-3 for the results you should look for when testing a good transistor.
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206 Part IV: Getting Your Hands Dirty
COLLECTOR
A
BASE-
Figure 9-17:
Testing a bipolar transistor.
EMITTER
NPN TRANSISTOR
?
EMITTER
EQUIVALENT DIODE CIRCUIT
Table 9-3 Bipolar Transistor Readings
Junction Test Reading
Base-emitter (BE) junction Conduction in one direction only
Base-collector (BC) junction Conduction in one direction only
Collector-emitter (CE) junction No conduction in either direction
Testing with a multimeter can permanently damage some types of transistors, especially the FET (field effect transistor) type! Use this test with bipolar transistors only. Data books show these types of transistors with terminals marked as base, emitter, and collector. Schematic diagrams show the bipolar PNP and NPN resistors with either of the symbols shown here. If you’re not sure whether you have a bipolar transistor, look it up in a data sheet before testing. You can find data sheets on the Internet by doing a Google or Yahoo search for the component you’re interested in. Try searching by: “2n2222 datasheet”.
If your multimeter is equipped with a transistor-checking feature, use that feature rather than the method that we give you here. Consult the manual that came with your meter for the exact procedure because it varies from one model to another.
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Chapter 10
Getting Down with Logic Probes and Oscilloscopes
In This Chapter
^ Examining what a logic probe is and what it does ^ Riding signal waves with oscilloscopes ^ Knowing when to use, and not use, an oscilloscope ^ Testing basics with an oscilloscope
f
˙ n Chapter 9, we talk about how to use a multimeter to test for all sorts of glitches and gotchas in your electronic circuits. Your meter is the most important tool on your workbench, but don’t think it’s the only thing that you can use to test your electronics stuff. If you’re really, really serious about electronics, you may want to get several other testing tools for your workbench.
In this chapter, we tell you about two handy test tools that you can use to make yourself a more effective electronics troubleshooter. These tools are the logic probe and the oscilloscope. Neither of the tools is a “must have,” so don’t rush out and buy them this afternoon. But if you start working on intermediate and advanced electronics, you may find these guys handy. Consider adding these tools to your workbench after you gain a bit of experience.
The Search for Spock: Using a Logic Probe
You use a logic probe (a fairly inexpensive tool), like the one in Figure 10-1, to test digital circuits. Specifically, the probe can tell you whether a signal is high or low. In digital electronics, zero volts, or very close to it, is a low signal. Any voltage other than zero means that you have a high signal. When you generate a signal that alternates between high and low very quickly you call it pulsing. Logic probes are great at detecting pulsing.
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208 Part IV: Getting Your Hands Dirty
Figure 10-1:
The logic probe is useful when troubleshooting digital circuits.
With few exceptions, logic circuits operate at 12 volts or less, with 5 volts as the most common. The components in the circuit define the voltage.
As you work with a logic probe keep the following “logical” tidbits in mind:
You may see a low signal indicated by a logical 0 (zero) and a high signal indicated by a logical 1 (one). Just about every digital circuit or computer only allows the two states of 0 and 1.
The term “logic” comes from how you combine these two states, 0 and 1, to create useful information. For example, an AND logic gate analyzes two input signals. The output of the AND gate is 1 (high) if, and only if, both inputs are 1. There are various other logic gates, including NAND, OR, NOR, and XOR. We introduce the most common of these logic gates in Chapter 1 and provide more detail in Chapter 5.
Sound, lights, action!
Although you can use a multimeter to test a digital circuit, you can use a logic probe a lot more easily. With a meter, you have to keep an eye on the readout and determine if the reading indicates low or high voltage.
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