Rosewill Computer Case 422 Informações Técnicas descarregar pdf (Página 51)

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Chapter 4 • Disassembly and Power

Basic Electronics Overview

A technician needs to know a few basic electronic terms and concepts when testing compo-

nents. The best place to start is with electricity. There are two types of electricity: AC and DC.

The electricity provided by a wall outlet is AC (alternating current), and the type of electricity

used by computer components is DC (direct current). Devices such as radios, TVs, and toast-

ers use AC power. Low-voltage DC power is used for a computer’s internal components and

anything powered by batteries. A computer’s power supply converts AC electricity from the

wall outlet to DC for the internal components. Electricity involves electrons flowing through a

conductor, similar to the way that water runs through a pipe. With AC, the electrons flow alter-

nately in both directions. With DC, the electrons flow in one direction only.

Electronics Terms

Voltage, current, power, and resistance are terms commonly used in the computer industry.

Voltage, which is a measure of the pressure pushing electrons through a circuit, is measured

in volts. A power supply’s output is measured in volts. Power supplies typically put out +3.3

volts, +5 volts, +12 volts, and –12 volts. You will commonly see these voltages shown in power

supply documentation as +5V or +12V. Another designation is +5VSB. This is for the com-

puter’s standby power. This power is always provided, even when the computer is powered

off. This supplied voltage is why you have to unplug a

computer when working inside it.

The term volts is also used to describe voltage from

a wall outlet. Wall outlet voltage is normally 120VAC

(120 volts AC). Exercises at the end of the chapter

explain how to take both AC and DC voltage readings.

Figure 4.16 shows a photograph of a multimeter being

used to take a DC voltage reading on the power con-

nectors coming from a power supply. When the meter

leads are inserted correctly, the voltage level shown is

of the correct polarity.

Figure 4.16 DC voltage reading

Polarity is important only when

measuring DC voltage

When a technician measures the voltage coming

out of a power supply, the black meter lead (which

is negative) connects to the black wire from the

power supply (which is ground). The red meter lead

connects to either the +5 or +12 volt wires from the

power supply.

Tech

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Rosewill Computer Case 422 Informações Técnicas Página 51