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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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Zero-set control: On analog meters without an automatic zero feature, designers provide a rotating knob so you can adjust the needle to 0 (zero) ohms before use. Some digital meters have a button that, when you press it, sets the meter to zero.
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182 Part IV: Getting Your Hands Dirty
Figure 9-3:
Your multimeter may not look like this one, but odds are yours has similar features.
Figure 9-4:
Most meters provide additional input sockets for testing capacitors and
transistors.
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Chapter 9: Making Friends with Your Multimeter 183
Accuracy, resolution, and sensitivity
The accuracy of a multimeter represents the maximum amount of error that occurs when it takes a measurement. For example, the multimeter may be accurate to 2,000 volts, Ī0.8 percent. A 0.8-percent error with the types of voltages used in DC-operated circuits ó typically 5 to 12 volts DC ó measures only about 0.096 volts. For hobby electronics projects, you donít need a more precise level of accuracy. As you compare the accuracy of multimeters, bear in mind that just about every model of meter gives the hobbyist the results that he or she needs.
Digital meters have another type of rating, this one more commonly called resolution. The number of digits in the display determines the smallest change that the meter can register. Most digital meters designed for hobbyists have 332 digits, so they can display a value as small as 0.001 (the half digit appears as a 1 on the far left of the display). The hobbyistís meter canít accurately represent anything less than 0.001. For most hobby-level electronics projects, you donít need to worry about this.
Resolution in digital multimeters is also a function of analog-to-digital converter (ADC) electronics. An ADC converts an analog signal to a digital one. Many consumer-grade multimeters use a 12-bit ADC. Without getting into all the technical mumbo-jumbo, a 12-bit ADC can take any analog signal and convert it into 4,096 discrete steps. (These discrete steps are necessary because of the way digital circuits work. In the digital world there canít be any in-between or ďsort ofĒ values.) Meter manufacturers select an ADC with a resolution that works with the number of display digits on the device. A 332 digit digital readout displays the values of a 12-bit ADC just about right.
Along the lines of accuracy and resolution, you need to consider the specification for sensitivity. This phrase means the smallest value that a meter can meaningfully detect when you use it under normal conditions.
^ Quality digital multimeters sport a maximum sensitivity of about 1 microvolt (AC or DC); thatís one millionth of a volt. The lower the value, the better the sensitivity.
^ Quality analog multimeters offer a maximum sensitivity of about 20,000 ohms per volt, typically shown as 20KD/V. The higher the ohms value, the better the sensitivity.
The Well-stocked multimeter
The typical multimeter doesnít come with a lot of accessories, but you need to have a few. We cover the necessities in the following sections.
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184 Part IV: Getting Your Hands Dirty
Manual
The manual for your multimeter may be just a single sheet of paper with little more than a picture, or you may get a small booklet with step-by-step instructions. Either way, be sure to at least browse through the manual. It contains important safety precautions, as well as a run-down of features and specifications for that meter model.
Test leads
The test leads included with most inexpensive multimeters arenít of the highest quality, so you may want to purchase a better set. You may want to get the type with coiled leads because they stretch out to several feet, yet recoil to a manageable length when not in use. Figure 9-5 shows some examples of coiled leads.
Figure 9-5:
Coiled leads stretch out during use but shrink back to normal size for storage.
Standard leads with their pointed metal probes work fine for most routine testing, but some measurements may require the use of a clip lead. These leads have a spring-loaded clip on the end; you can clip the lead in place so that your hands are free to do other things. The clips are insulated to prevent touching the metal against another part of the circuit.
If your multimeter doesnít come with clip leads, you can buy some clip-on attachments that fit over regular test leads.
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Chapter 9: Making Friends with Your Multimeter
185
Extra fuse
Most multimeters use an internal fuse to protect themselves against excessive voltage or current. The better multimeters come with a spare fuse. If yours doesnít have a spare, purchase one when you buy the meter. That way, you have it available when you need it.
Some meter fuses are specially made, and replacements can cost you a bundle. You may want to check the price of replacement fuses before you purchase the meter!
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