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CompTIA A+ Certification All-in-One Exam Guide

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Figure 7-39 Testing one of the 5-V DC connections

Do not panic if your power supply puts out slightly more or less voltage

than its nominal value. The voltages supplied by most PC power supplies can

safely vary by as much as ±10 percent of their stated values. This means that

the 12.0-V line can vary from roughly 10.8 to 13.2 V without exceeding the

tolerance of the various systems in the PC. The 5.0- and 3.3-V lines offer

similar tolerances.

Be sure to test every connection on the power supply—that means every

connection on your main power as well as every Molex and mini. Because all

voltages are between –20 and +20 VDC, simply set the multimeter to the 20-

V DC setting for everything. If the power supply fails to provide power,

dispose of it properly and get a new one—even if you’re a component expert

and a whiz with a soldering iron. Don’t waste your time or your company’s

time; the price of new power supplies makes replacement the obvious way to

go.

No Motherboard

Power supplies will not start unless they’re connected to a motherboard, so

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