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CompTIA A+ Certification All-in-One Exam Guide

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aren’t as accurate as a multimeter. My favorite tester is a seemingly simple

tool available from a number of manufacturers (see Figure 7-9). This handy

device provides three light-emitting diodes (LEDs) that describe everything

that can go wrong with a plug.

Figure 7-9 Circuit tester

AC Adapters

Many computing devices 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 internal 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 either the voltage or

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