Lesson 2: Power-Supply Problems

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Power-supply problems can come from both internal and external sources. Failure of a power supply can be caused by failure of components within the computer (internal), but the most common failures come from the power source itself (external). In this lesson, we look at common problems associated with power supplies and what you, as a technician, can do about them.

After this lesson, you will be able to:

  • Determine the types of problems that can be caused by power supplies.
  • Know when to check and when to replace a power supply.
  • Plan how to protect your system from external power-supply problems.
Estimated lesson time: 10 minutes

Power Failures

Power supplies are affected by the quality of the local power source. Common power delivery problems such as spikes, surges, sags, brownouts, and blackouts affect the stability and operation of the main power supply and are passed on to the computer. While most users don't notice sudden changes in the quality of electrical power, computers and other sensitive electronics do and while we can't fully control these problems, there are a few things we can do to protect our equipment and data and ensure a reasonably clean electrical supply.

Problem Description
Surges These are brief (and sometimes catastrophic) increases in the voltage source (very high voltage for a very short time). They can originate with the power source (the local power company), but most often are due to lightning strikes.
Spikes Spikes are very short overvoltage conditions. Spikes are measured in nanoseconds, while a surge is measured in milliseconds.
Sags These are brief decreases of voltage at the power source.
Brownouts If a sag lasts longer than one second, it is called a brownout. The overloading of a primary power source can cause brownouts. Some brownouts are "scheduled" by power companies to prevent overloading of circuits and potential catastrophic failure of the system.
Blackout This is a complete power failure, which can be caused by equipment failure (local or regional) or accidental cutting of power cables. When the power returns after a blackout, there is a power spike and danger of a power surge.

Power-Protection Devices

Surge suppressors are devices used to filter out the effects of voltage spikes and surges that are present in commercial power sources and smooth out power variations. They are available from local computer dealers and superstores. A good surge suppressor will protect your system from most problems, but if you purchase an economy model, it might not work when you need it most. Keep in mind that almost nothing will shield your hardware from a very close lightning strike.

Most power strips with surge protection have a red indicator light. If the light goes out, this means that the unit is not providing protection. Most power strip/surge protectors should be replaced every year or so. If the light starts flashing before then, the power strip is failing and should be replaced.

When evaluating the quality of surge suppressors, look for performance certification. At a minimum, it should have an Underwriters Laboratory (UL) listing and power ratings. A high-quality unit will also provide protection for phone/fax/modem and network connections. These units protect up to a point; however, for complete protection from power fluctuations and outages, the uninterruptible power supply (UPS) is recommended.

A UPS is an inline battery backup. When properly installed between a computer and the wall outlet, a UPS device protects the computer from surges and acts as a battery when the power dips or fails. It also provides a warning that the power is out of specification (above or below acceptable levels). Many models can also interact with the computer and initiate a safe shutdown in the event of a complete power failure. They do this by means of software that runs in the background and is set in action by a signal through one of the computer's COM ports when the power goes down.

The amount of time that a UPS device can keep a system running is determined by battery capacity and the power demands of the equipment connected to it. A more powerful UPS device will need its own line and circuit breaker. One of the principal power drains is the monitor. To keep a system online as long as possible during a power failure, turn off the monitor immediately after the failure commences.

When recommending a UPS, take into consideration how much protection is needed as well as the importance of peace of mind to the user. The VA rating (voltage x amps = watts) must be sufficient to supply the computer and all its peripherals with power for long enough to safely shut down the system. The easiest way to calculate this number is to total the power rating (watts) for all pieces of equipment that are to be connected to the UPS, as shown in the following table.

Device Power Rating (Watts) Connected to UPS Power Required
Computer 200-350 Yes 250
Monitor 80-100 Yes 80
External modem 5.5 No 0
External backup drive 50 Yes 50
Total 330-500 380

Never plug a laser printer into a UPS unless the UPS is specifically rated to support that type of device. Laser printers can easily require more power than an underspecified UPS is able to provide; the printer, the UPS, and the computer could all be endangered if the printer is connected to the UPS power source.

Power-Supply Problems

The most easily recognized problem is a complete failure of the power supply. This is easy to detect because in the event of a failure, the computer will not begin to boot up (no lights, no sound). If there is apparently no power, be sure to check the power source and the plug at both ends: the outlet and the computer.

If you are experiencing intermittent failures such as memory loss, memory corruption, or unexplained system crashes, don't rule out the power supply. It is often the culprit. Fortunately, it is easy to check and replace.

Good power supplies have line-conditioning circuits, but these might not be sufficient enough in locations where the power source has substantial quality flaws. If you have problems with several systems, or if a second power supply still does not fix a related complaint, add a UPS with good line-conditioning features.

Lesson Summary

The following points summarize the main elements of this lesson:

  • Power-supply problems can be caused by component failures within the power supply or from the power source.
  • Two devices protect against external power problems—surge suppressors and UPSs.

Microsoft Corporation - A+ Certification Training Kit
Microsoft Corporation - A+ Certification Training Kit
Year: 2000
Pages: 127

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