Electronics All-in-One For Dummies. Doug Lowe

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Chapter 8

      Measuring Circuits with a Multimeter

      IN THIS CHAPTER

       Familiarizing yourself with a multimeter

       Measuring current, voltage, and resistance

       Looking at your first electronic equation: Ohm’s Law

      Wouldn’t it be great if every circuit you ever built worked right the first time you built it? You’d quickly develop a reputation as an electronic genius and in no time at all you’d be the president of Intel.

      But in the real world, a circuit doesn’t always work right the first time. When it doesn’t, you can scratch your head, stare at it, put a dead chicken in a bag and swing it over your head, or you can pull out your test equipment and analyze the circuit to find out what went wrong.

      In this chapter, you learn how to use one of the electronic guru’s favorite tools: the multimeter. Learn how to use it well. It will be your trusty companion throughout your electronic journeys.

Looking at Multimeters

You know those late-night commercials that try to sell you those amazing kitchen gadgets that are like a combination blender, juicer, food processor, mixer, and snow cone maker all in one? A multimeter is kind of like one of those crazy kitchen gadgets, except that, unlike the crazy kitchen gadgets, a multimeter really can do all the things it claims it can do. And, unlike the crazy kitchen gadgets, the things a multimeter can do turn out to be genuinely useful.

      Along with a good soldering iron, a good multimeter is the most important item in your toolbox. Learn how to use it, and your electronic exploits will be much more fruitful.

Figure 8-1 shows a simple, inexpensive multimeter. This one can be purchased from almost any hardware store for under $20. If you shop around, however, you can often find a basic multimeter for under $10 and sometimes for as little as $5.

      FIGURE 8-1: You can buy a basic multimeter like this one for under $20.

      Of course, you can also spend much more, but if you’re just getting started, an inexpensive multimeter is fine. Eventually, you’ll want to invest a little more money in a better-quality multimeter.

      The multimeter shown in Figure 8-1 is a digital multimeter, which displays its values using a digital display that shows the actual numbers for the measurements being taken. The alternative to a digital multimeter is an analog multimeter, which shows its readings by moving a needle across a printed scale. To determine the value of a measurement, you simply read the scale behind the needle.

Figure 8-2 shows a typical analog multimeter. This one happens to be one of my favorites. Even though it’s old enough to be called vintage, it’s still an excellent and accurate meter. One of the benefits of spending a little more to buy quality equipment is that the equipment will give you many years — sometimes decades — of reliable service.

      FIGURE 8-2: An analog multimeter.

For the most precise measurements, you can pick up a lab-quality bench-top multimeter such as the one shown in Figure 8-3. These meters usually cost $100 or more but offer additional features such as data logging, USB or network connectivity, and a wide array of test leads useful for different circumstances.

      The following paragraphs describe the various parts that make up a typical multimeter:

       Display or meter: Indicates the value of the measurement being taken. In a digital multimeter, the display is a number that indicates the amperage (current), voltage, or resistance being measured. In an analog meter, the current, voltage, or resistance is indicated by a needle that moves across a printed scale. To read the value, you look straight down at the needle and read the scale printed behind it.FIGURE 8-3: A bench-top multimeter.

       Selector: Most multimeters — digital or analog — have a dial that you can turn to tell the meter what you want to measure. The various settings СКАЧАТЬ