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Power Supply Overview

137

Disassembly

and Power

Tech

Tip

Use the right fuse or lose

Never replace a fuse with one that has a higher amperage rating. You could destroy

electronic circuits or cause a fire by allowing too much current to be passed by the fuse,

defeating the fuse’s purpose.

Take a fuse out of the circuit before testing it. A good fuse has a meter reading of 0 ohms

(or close to that reading). A blown fuse shows a meter reading of infinite ohms. Refer to the

section on resistance and Figure 4.18. An exercise at the end of this chapter demonstrates how

to check a fuse.

A technician needs to be familiar with basic electronics terms and checks. Table 4.1 con-

solidates this information.

Table 4.1 Basic electronics terms

Term Value Usage

Voltage Volts Checking AC voltage on a wall outlet (typically 120VAC). Checking the

DC output voltage from a power supply (typically +/– 12, +3.3, and +/– 5

VDC).

Current Amps

(amperes)

Each device needs a certain amount of current to operate. A power

supply is rated for total current in amps for each voltage level (such as

24 amps for 5-volt power and 50 amps for 12-volt power).

Resistance Ohms Resistance is the amount of opposition to electric current. Resistance is

used to check continuity on cables and fuses. A cable that shows little

or no resistance has no breaks in it. A good fuse shows no resistance. If

a cable has a break in it or if a fuse is bad, the resistance is infinite.

Wattage

(power)

Watts Watts is a measure of power and is derived by multiplying amps

by volts. Power supply output is measured in watts. Also, A UPS

(uninterruptible power supply) is rated in volt-amps. The size of UPS to

purchase depends on how many devices will plug in to it.

Power Supply Overview

A power supply is an essential component within a computer; no internal computer device

works without it. The power supply converts AC to DC, distributes lower-voltage DC power to

components throughout the computer, and provides cooling through the use of a fan located

inside the power supply. The AC voltage a power supply accepts is normally either 100 to 120

volts or 200 to 240 volts. Some dual-voltage power supplies can accept either. This type of

power supply can have a selector switch on the back or can automatically detect the input

voltage level. The power supply is sometimes a source of unusual problems. The effects of the

problems can range from those not noticed by the user to those that shut down the system.

There are two basic types of power supplies: switching and linear. A computer uses a

switching power supply. It provides efficient power to

all the computer’s internal components (and possibly

to some external ones, such as USB devices). It also

generates minimum heat, comes in small sizes, and is

cheaper than linear power supplies. A switching power

supply requires a load (something attached to it) in

order to operate properly. With today’s power supplies,

a motherboard is usually a sufficient load, but a techni-

cian should always check the power supply specifica-

tions to be sure.

Powering on a power supply without

anything attached could damage the

power supply

Do not power on a power supply without connecting

to the motherboard and possibly a device such as

an optical drive or hard drive. An ATX power supply

usually requires a motherboard connection at a

minimum.

Tech

Tip

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