Electricity and the Power Supply

 < Day Day Up > 



The flow of electrons is known as electricity. When electricity flows in only one direction, it is called Direct Current (DC). When electricity flows in two directions or in a bi-directional fashion, it is called Alternating Current (AC). To understand the flow of electricity through a computer system and troubleshoot electrical issues in a computer system, you should be familiar with the following electrical terms:

  • Current is the amount of electricity moving across a wire. Current is measured in milliamperes or amperes (amps).

  • Resistance is a measure of how much an object resists or holds back the flow of current. When electrical resistance is increased, the amount of current is decreased. Resistance is measured in ohms.

  • Voltage is a measure of the pressure on electrons as they are being pushed through a medium. Voltage is measured in volts.

  • Wattage is the amount of work that electrical current is capable of performing. Wattage is measured in watts. You should be very concerned about wattage and its effects when changing or repairing a power supply. A common practice is to simply replace a power supply that is defective.

The main function of your computer's power supply is to convert AC to DC. Current that enters the power supply from an electrical outlet in the wall is typically at 110V or 115V AC.

The power supply converts AC to the +5V, -5V, +12V, or -12V DC current that the motherboard and its components require. A useful tool to test power (voltage) coming from the power supply and going to the motherboard is a digital multimeter. The wires that extend from the typical power supply have different colors, and each represents a different voltage: red = +5V, white = -5V, yellow = +12V, and blue = -12V. Older motherboard form factors (e.g., AT) accept the P8 and P9 Molex-type connectors from the power supply. These connectors plug into the motherboard side by side. When plugging the P8 and P9 connectors into the AT motherboard, you must remember to keep the black ground wires next to each other. If you don't, you might cause electrical damage to the board. The ATX form factor introduced a single ‘keyed' power connector that eliminated the risk of plugging the P8 and P9 connectors into the wrong power sockets on the motherboard (see Figure 2.3).

click to expand
Figure 2.3: P9 and P8 power supply connector wiring.

If you are troubleshooting a ‘dead' computer, first verify that there is electricity coming from the AC wall outlet. Next, use a digital multimeter to measure the voltage going from the power supply to the motherboard. There are fuses in a computer system that can also be tested with a multimeter. A good fuse measures a resistance of zero ohms. If the fuse is bad, the multimeter registers a resistance of infinity ohms. If your system continuously reboots on its own, it may not be receiving enough power from the power supply.

Uninterruptible Power Supply

To protect your computer and data, your computer should be connected to an Uninterruptible Power Supply (UPS), also known as a battery backup. If it is not connected to a UPS, your computer may be subject to a power surge. If your computer screen is flickering, you may be experiencing simple power sag. The UPS (if properly maintained) provides power to the computer in the event of a power failure; it is not meant to be a long-term power-providing solution. There are three types of UPS: online, standby, and line-interactive. In an online UPS, the battery is contained in a circuit. In a standby UPS, the battery is not contained in the circuit. A line-interactive UPS has the best characteristics of the online and standby UPSs.

A laser printer should never be plugged into a UPS. A laser printer draws a large amount of electricity. It should be placed on its own electrical circuit, separate from the system unit and other electrical devices.

Surge Suppressors

A surge suppressor is a device used to protect electronic equipment, such as computer systems, printers, modems, and monitors, from transient voltage or ‘spikes' that occur in the flow of electricity. In America, the average or ‘effective' AC power voltage is around 110V to 120V. Spikes, which can be caused by lightning or other electrical conditions, can cause electrical surges of hundreds or even thousands of volts. A surge suppressor or ‘protector' can stop AC voltage from going above or below a certain amount of voltage. A typical surge protector has several outlets that equipment can be plugged into, a main power switch, and a three-pronged plug that plugs into a power outlet.



 < Day Day Up > 



A+ Complete Study Guide, Third Edition (220-301 and 220-302)
The A+ Exams Guide: (Exam 220-301), (Exam 220-302) (TestTakers Guides)
ISBN: 1584503076
EAN: 2147483647
Year: 2003
Pages: 237

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net