CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [141]
Many years ago your electrical supply used fuses instead of circuit breakers. Fuses are small devices with a tiny filament designed to break if subjected to too much current. Unfortunately, fuses had to be replaced every time they blew, making circuit breakers much more preferable. Even though you no longer see fuses in a building’s electrical circuits, many electrical devices—such as a PC’s power supply—often still use fuses for their own internal protection.
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EXAM TIP An electrical outlet must have a ground wire to be suitable for PC use.
Electricity comes in two flavors: direct current (DC), in which the electrons flow in one direction around a continuous circuit, and alternating current (AC), in which the flow of electrons alternates direction back and forth in a circuit (see Figure 10-2). Most electronic devices use DC power, but all power companies supply AC power because AC travels long distances much more efficiently than DC.
Figure 10-2 Diagrams showing DC and AC flow of electrons
Essentials
Powering the PC
Your PC uses DC voltage, so some conversion process must take place before the PC can use AC power from the power company. The power supply in a computer converts high-voltage AC power from the wall socket to low-voltage DC. The first step in powering the PC, therefore, is to get and maintain a good supply of AC power. Second, you need a power supply to convert AC to the proper voltage and amperage of DC power for the motherboard and peripherals. Finally, you need to control the byproduct of electricity use, namely heat. Let’s look at the specifics of powering the PC.
Supplying AC
Every PC power supply must have standard AC power from the power company, supplied steadily rather than in fits and spurts, and protection against accidental blurps in the supply. The power supply connects to the power cord (and thus to an electrical outlet) via a standard IEC-320 connector. In the United States, standard AC comes in somewhere between 110 and 120 volts, often written as -115 VAC (volts of alternating current). The rest of the world uses 220–240 VAC, so most power supplies have a little switch in the back so you can use them anywhere. These power supplies with voltage-selection switches are referred to as fixed-input. Power supplies that you do not have to manually switch for different voltages are known as auto-switching. Figure 10-3 shows the back of a power supply. Note the three components, from top to bottom: the hard on/off switch, the 115/230 switch, and the IEC-320 connector.
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CAUTION Flipping the AC switch on the back of a power supply can wreak all kinds of havoc on a PC. Moving the switch to ~230 V in the United States makes for a great practical joke (as long as the PC is off when you do it)—the PC might try to boot up but probably won’t get far. You don’t risk damaging anything by running at half the AC the power supply is expecting. In countries that run ~230 standard, on the other hand, firing up the PC with the AC switch set t2115 can cause the power supply to die a horrid, smoking death. Watch that switch!
Figure 10-3 Back of fixed-input power supply, showing typical switches and power connection
Before plugging anything into an AC outlet, take a moment to test the outlet first by using a multimeter or a device designed exclusively to test outlets. Failure to test AC outlets properly can result in inoperable or destroyed equipment, as well as possible electrocution. The IEC-320 plug has three holes, called hot, neutral, and ground. These names describe the function of the wires that connect to them behind the wall plate. The hot wire carries electrical voltage, much like a pipe that delivers water. The neutral wire carries no voltage, but instead acts like a water drain, completing the circuit by returning electricity to the local source, normally