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Replacing or Upgrading a Power Supply

Power supplies are rated in watts. Today’s typical computers have power supplies with ratings ranging from 250 to 500 watts, although powerful computers, such as network servers or higher-end gaming systems, can have power supplies rated 600 watts or higher. Each device inside a computer uses a certain amount of power, and the power supply must provide enough to run all the devices. The power each device or adapter requires is usually defined in the documentation for the device or adapter or on the manufacturer’s website. The computer uses the wattage needed, not the total capacity of a power supply. The efficiency (more AC is converted to DC) is what changes the electricity bill.

Some power supplies are listed as being dual or triple (or tri) rail. A .dual-rail power supply has two +12V output lines. A triple-rail power supply simply has three +12V output lines for devices. Keep in mind that most manufacturers do not have two or more independent 12V sources; they all derive from the same 12V source but have independent output lines. Figure 4.30 shows how the +12V rails might be used.

Figure 4.30

Figure 4.30. 12V rails

Power supplies can be auto-switching or have a fixed input. An auto-switching power supply monitors the incoming voltage from the wall outlet and automatically switches itself accordingly. Auto-switching power supplies accept voltages from 100 to 240VAC at 50 to 60Hz. These power supplies are popular in mobile devices and are great for international travel. A power supply might also allow adjusting the input value by manually selecting the value through a voltage selector switch on the power supply. A fixed-input power supply is rated for a specific voltage and frequency for a country, such as 120VAC 60Hz for the United States.

Some people are interested in exactly how much power their system is consuming. Every device in a computer consumes power, and each device could use one or more different voltage levels (+5V, –5V, +12V, –12V, +3.3V). A power supply has a maximum amperage for each voltage level (for example, 30 amps at +5 volts and 41 amps at +12V). To determine the maximum power being used, in watts, multiply the amps and volts. If you add all the maximum power levels, the amount will be greater than the power supply’s rating. This means that you cannot use the maximum power at every single voltage level (but since the –5V and –12V are not used very often, normally this is not a problem).

In order to determine the power being consumed, you must research every device to determine how much current it uses at a specific voltage level. Internet power calculators are available to help with this task. Table 4.6 lists sample computer components’ power requirements.

Table 4.6. Sample computer component power requirements


Power consumption

Motherboard (without processor)

5 to 150W


10 to 140W

Floppy drive


PATA hard drive

3 to 30W

SATA hard drive

2 to 15W

Optical drive

10 to 30W

Non-video adapter

4 to 25W

AGP video adapter

20 to 50W

PCIe video card with one power connector


PCIe video card with two power connectors


Extra fan


RAM stick


Different physical sizes of power supplies are available. When replacing a power supply, purchasing a power supply for a new computer, or upgrading a power supply, verify that the power supply will fit in the computer case. Also, verify that the power supply produces enough power for the installed devices and for future upgrades. Do not forget to check that the on/off switch on the new power supply is in a location that fits in the computer case.

When purchasing a new power brick for a laptop or battery for a mobile device, ensure that it has the same specifications as the one from the manufacturer. Less expensive models might not provide the same quality as approved models. Ensure that the replacement has a power jack that does not wiggle when it is inserted into the device. Ensure that a laptop power brick has the appropriate DC voltage required by the laptop. Current (amperage) should be equal to or more than the original power brick.

Power management on both laptops and desktops is important. Most computer components are available as energy-efficient items. ENERGY STAR is a joint effort by the U.S. EPA (Environmental Protection Agency) and Department of Energy to provide device standards and ratings that easily identify products (including computer components) that are energy efficient. Many computers today are on more than they are off, and settings such as power options, CPU throttling, and some advanced BIOS settings affect power settings. A technician must be aware of all these options and be willing to offer advice such as turn the computer off when finished working on it; set the power management option to allow work to be performed at an affordable cost; disable options not being used, such as wireless capabilities when wired networking is functioning; be aware of monitor costs (CRT-type monitors take the most energy, followed by plasma displays and then LCD or flat-panel technology); and purchase energy-efficient parts and computers.

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