Rosewill Computer Case 422 Technické informace stáhnout pdf (Strana 52)

Electronics Terms

135

Disassembly

and Power

The reading on the meter could be the opposite of what it should be if the meter’s leads

are reversed. Since electrons flow from one area where there are many of them (negative polar-

ity) to an area where there are few electrons (positive polarity), polarity shows which way an

electric current will flow. Polarity is the condition of being positive or negative with respect to

some reference point. Polarity is not important when measuring AC. Figure 4.17 shows rules to

observe when working with meters.

1. Select AC or DC on the meter (some meters automatically select AC or DC).

2. Select the appropriate voltage range (0-10V, 0-100V, etc).

The meter can be damaged if you measure a high voltage in

a low range (but not the reverse). Use the highest range for

unknown voltages.

3. Touch only the insulated parts of the meter probes.

Insulated

parts

Meter probes

Figure 4.17 Meter rules

Monitors and power supplies can have dangerous voltage levels. Monitors can have up to

35,000 volts going to the back of the CRT. Note that flat-panel displays and mobile device dis-

plays use low DC voltage and AC voltage, but not at the voltage levels of CRTs. 120 volts AC is

present inside the power supply. Power supplies and monitors have capacitors inside them. A

capacitor is a component that holds a charge even after the computer is turned off. Capacitors

inside a monitor can hold a charge for several hours after the monitor has been powered off.

Current is measured in amps (amperes), which is the number of electrons going

through a circuit every second. In the water pipe analogy, voltage is the amount of pres-

sure applied to force the water through the pipe, and current is the amount of water flow-

ing. Every device needs a certain amount of current

to operate. A power supply is rated for the amount

of total current (in amps) it can supply at each

voltage level. For example, a power supply could be

rated at 20 amps for the 5-volt level and 8 amps for

the 12-volt level.

Power is measured in watts, which is a mea-

surement of how much work is being done. It is determined by multiplying volts by amps.

Power supplies are described as providing a maximum number of watts. This is the sum of

all outputs: For example, 5 volts × 20 amps (100 watts) plus 12V 8 amps (96 watts) equals

196 watts. An exercise at the end of the chapter

explains how current and power relate to a techni-

cian’s job.

Resistance is measured in ohms, which is the

amount of opposition to current in an electronic

circuit. The resistance range on a meter can be

used to check continuity or check whether a fuse

is good. A

continuity check is used to determine

whether a wire has a break in it. A conductor

(wire) in a cable or a good fuse will have very low resistance to electricity (close to zero

ohms). A broken wire or a bad fuse will have a very high resistance (millions of ohms,

sometimes shown as infinite ohms, or OL). For example, a cable is normally made up of

several wires that go from one connector to another. If you measure the continuity from

Do not work inside a CRT monitor unless

you have special training

Monitors require high-voltage meters and special

precautions.

Tech

Tip

Current is what kills people when

an electrical shock is received

Voltage determines how much current flows

through the body. A high-current and low-voltage

situation is the most dangerous.

Tech

Tip

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