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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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4. Set up your multimeter to measure resistance.
If your multimeter doesn't have auto ranging, set it to a fairly high range such as 200K ohms or higher.
5. Strap the two test probes together at their insulating handles, using a rubber band.
Be sure that the metal parts of the probes don't touch.
6. Dip the probes into the glass of distilled water. Note the reading and set the range downward, if you don't get a good reading.
7. Now dip the probes into the glass of tap water. Again, note the reading.
In grade school, you may have learned that water conducts electricity. Actually, that statement isn't entirely correct. Pure water is an insulator; the minerals in the water conduct electricity. Distilled water has little mineral content, so it has a very high resistance. Depending on where you live, your tap water may contain a lot of salts and minerals, and these additions make the water more conductive. These impurities lead to water with a lower resistance that therefore better conducts electricity.
My own tests show that distilled water has a resistance of about 140KQ; the tap water's resistance came out at about 40KQ. Your own tests may give you higher or lower measurements for both kinds of water because of the differences in water quality and the distance between the probes.
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Chapter 9: Making Friends with Your Multimeter 191
As we talk about in the section ďOkay, So What Exactly Can You Do with a Multimeter?Ē earlier in this chapter, many digital meters have a continuity feature that sounds a tone when the circuit youíre testing reaches zero ohms. However, donít use the continuity setting for zero-adjusting the meter. The tone may sound when the meter reads a few ohms, so it doesnít give you the accuracy that you need. Recalibrate the multimeter using the Ohms setting, and not the Continuity setting, to ensure proper operation.
If you donít get any response at all from the meter when you touch the test probes together, recheck the dial setting of the meter. Nothing happens if you have the meter set to register AC or DC voltage or current. If you make sure that the meter has the right settings and it still doesnít respond, you may have faulty test leads. If necessary, repair or replace any bad test leads with a new set.
After you check the meter out, you can select the desired function (ohms, AC, DC, or current) and range and apply the probes to the circuit under test.
Five Basic Tests That You Can Make with Your Multimeter
Okay, get your meter turned on and all set up, and youíre ready to make some tests. In the following sections, you learn how to conduct five of the most common tests using a multimeter.
Testing voltage
Is your circuit getting the proper voltage? You can find out with your multimeter. You conduct voltage tests with the circuit under power. You can test the voltage at almost any point in a circuit, not just the battery connections. The procedure is simple and involves connecting the black test lead to ground, and the red test lead to a test point in the circuit that you want to check.
To perform this test
1. Set up the meter as described in the earlier section ďSetting Up the Meter.Ē
2. Attach the black lead of the meter to the ground connection of the circuit.
3. Attach the red lead of the meter to the point in the circuit that you want to measure.
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192 Part IV: Getting Your Hands Dirty
Figure 9-9:
Two types of voltage tests performed on a 555 timer circuit.
Figure 9-9 shows an example of a multimeter testing a couple of points in a simple 555 integrated circuit (IC) timer. The top image shows the meter measuring the voltage that powers the entire circuit, and the bottom image shows the meter measuring the voltage at the output of the 555 IC. Because the output of the 555 IC is an on-or-off voltage, the reading on the multimeter alternates between zero volts and five volts.
Signals generated by circuits may change so rapidly that you canít adequately test them by using a multimeter. The multimeter canít react to the change in voltage fast enough. The proper gear for testing fast-changing signals are the logic probe and the oscilloscope. You can read about both tools in more detail in Chapter 10.
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Chapter 9: Making Friends with Your Multimeter 193
Testing current
As with voltage tests, you make current tests using a multimeter while you have the circuit under power. To take the basic approach, you need to connect the meter in the circuit in series with the positive supply voltage so that the meter registers the current passing through the circuit. This measurement tells you how much overall current the entire circuit draws. But remember that many digital meters are limited to testing current draw of 200 milliamps or less. Be careful: donít test higher current if your meter isnít equipped to do so.
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