CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [149]
No power supply can turn 100 percent of the AC power coming from the power company into DC current, so all power supplies provide less power to the system than the wattage advertised on the box. ATX12V 2.0 standards require a power supply to be at least 70 percent efficient, but you can find power supplies with better than 80 percent efficiency. More efficiency can tell you how many watts the system puts out to the PC in actual use. Plus, the added efficiency means the power supply uses less power, saving you money.
One common argument these days is that people buy power supplies that provide far more wattage than a system needs and therefore waste power. This is untrue. A power supply provides only the amount of power your system needs. If you put a 1000-watt power supply (yes, they really exist) into a system that needs only 250 watts, that big power supply will put out only 250 watts to the system. So buying an efficient, higher-wattage power supply gives you two benefits. First, running a power supply at less than 100 percent load lets it live longer. Second, you’ll have plenty of extra power when adding new components.
As a general recommendation for a new system, use at least a 500-watt power supply. This is a common wattage and gives you plenty of extra power for booting as well as for whatever other components you might add to the system in the future.
Don’t cut the specifications too tightly for power supplies. All power supplies produce less wattage over time, simply because of wear and tear on the internal components. If you build a system that runs with only a few watts of extra power available from the power supply initially, that system will most likely start causing problems within a year or less. Do yourself or your clients a favor and get a power supply that has more wattage than you need.
Installing, Maintaining, and Troubleshooting Power Supplies
Although installing, maintaining, and troubleshooting power supplies take a little less math than selecting the proper power supply for a system, they remain essential skills for any tech. Installing takes but a moment, and maintaining is almost as simple, but troubleshooting can cause headaches. Let’s take a look.
Installing
The typical power supply connects to the PC with four standard computer screws, mounted in the back of the case (Figure 10-27). Unscrew the four screws and the power supply lifts out easily (Figure 10-28). Insert a new power supply that fits the case and attach it by using the same four screws.
Handling ATX power supplies requires special consideration. Understand that an ATX power supply never turns off. As long as that power supply stays connected to a power outlet, the power supply will continue to supply 5 volts to the motherboard. Always unplug an ATX system before you do any work! For years, techs bickered about the merits of leaving a PC plugged in or unplugged while you serviced it. ATX settled this issue forever. Many ATX power supplies provide a real on/off switch on the back of the PSU (see Figure 10-29). If you really need the system shut down with no power to the motherboard, use this switch.
Figure 10-27 Mounting screws for power supply
Figure 10-28 Removing power supply from system unit
When working on an ATX system, you may find using the power button inconvenient because you’re not using a case or you haven’t bothered to plug the power button’s leads into the motherboard.