CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [142]
Figure 10-4 Outlet voltages
You can use a multimeter—often also referred to as a volt-ohm meter (VOM) or digital multimeter (DMM)—to measure a number of aspects of electrical current. A multimeter consists of two probes, an analog or digital meter, and a dial to set the type of test you want to perform. Refer to Figure 10-5 to become familiar with the components of the multimeter.
Figure 10-5 Digital multimeter
Note that some multimeters use symbols rather than letters to describe AC and DC settings. The V with the solid line above a dashed line, for example, in Figure 10-6, refers to direct current. The V ~ stands for alternating current.
Figure 10-6 Multimeter featuring DC and AC symbols
Every multimeter offers at least four types of electrical tests: continuity, resistance, AC voltage (VAC), and DC voltage (VDC). Continuity tests whether electrons can flow from one end of a wire to the other end. If so, you have continuity; if not, you don’t. You can use this setting to determine if a fuse is good or to check for breaks in wires. If your multimeter doesn’t have a continuity tester (many cheaper multimeters do not), you can use the resistance tester. A broken wire or fuse will show infinite resistance, while a good wire or fuse will show no resistance. Testing AC and DC voltages is a matter of making sure the measured voltage is what it should be.
Using a Multimeter to Test AC Outlets
Every competent technician knows how to use a multimeter. To set up the meter for measuring AC, follow these steps:
1. Move the selector switch to the AC V (usually red). If multiple settings are available, put it into the first scale higher than 120 V (usually 200 V). Auto-range meters set their own range; they don’t need any selection except AC V.
2. Place the black lead in the common (–) hole. If the black lead is permanently attached, ignore this step.
3. Place the red lead in the V-Ohm-A (+) hole. If the red lead is permanently attached, ignore this step.
Once you have the meter set up for AC, go through the process of testing the various wires on an AC socket. Just don’t put your fingers on the metal parts of the leads when you stick them into the socket! Follow these steps:
1. Put either lead in hot, the other in neutral. You should read 110 to 120 V AC.
2. Put either lead in hot, the other in ground. You should read 110 to 120 V AC.
3. Put either lead in neutral, the other in ground. You should read 0 V AC.
If any of these readings is different from what is described here, it’s time to call an electrician.
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NOTE Many devices in the computing world use an AC adapter rather than an internal power supply. Even though it sits outside a device, an AC adapter converts AC current to DC, just like a power supply. Unlike power supplies, AC adapters are rarely interchangeable. Although manufacturers of different devices often use the same kind of plug on the end of the AC adapter cable, these adapters are not necessarily interchangeable. In other words, just because you can plug an AC adapter from your friend’s laptop into your laptop does not mean it’s going to work. You need to make sure that three things match before you plug an AC adapter into a device: voltage, amperage, and polarity. If they don’t match, don’t plug it in!
Using Special Equipment to Test AC Voltage
A number of good AC-only testing devices are available. With these devices, you can test all voltages for an AC outlet by simply inserting them into the outlet. Be sure to test all of the outlets the computer system uses: power supply, external devices, and monitor. Although convenient, these devices aren’t