This lesson presents basic information about power supplies for computer systems. We take a look at the different variations of power supplies, how to identify the proper type, and how to connect it to the computer (both motherboard and related devices). We also examine the safety considerations related to working with power supplies. As a certified computer technician, you will often be called on to troubleshoot, identify, and replace power supplies.
Power supply problems can come from both internal and external sources. Component failure within a computer can cause a power supply to fail, but the most common failures come externally from the power source itself. 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
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. Although most users don't notice sudden changes in the quality of electrical power, computers and other sensitive electronics are affected. Although we can't fully control these problems, there are a few things we can do, noted in the following table, 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, whereas a surge is measured in milliseconds. |
Sags | These are brief decreases of voltage at the power source. |
Brownouts | If a sag lasts longer than 1 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. |
Blackouts | A blackout 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 the danger of a power surge. |
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.
NOTE
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. These types of surge suppressors need to be replaced every year or so. If the indicator light starts flashing before then, it means the power strip is failing and should be replaced immediately.
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, an 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 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 using software that runs in the background and sends 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 considering a UPS, take into account how much protection is needed, as well as the importance of peace of mind to the user. The VA rating (voltage × amps = watts) must be sufficient to supply the computer and all its peripherals with power for enough time to safely shut down the system. The easiest way to calculate this number is to add 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 | 335.5–505.5 | — | 380 |
CAUTION
Never plug a laser printer into a UPS unless the UPS is specifically rated to support that type of device. Laser printers often require more power than a UPS is able to provide, potentially placing the printer, the UPS, and the computer at risk.
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 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 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.
Most power grids in the United States provide current that is far from ideal for sensitive electronic components. Line-conditioning hardware added in the chain just before the current reaches the machine adds a much needed level of protection from spikes (very fast jumps in power levels), surges (longer ones), and drops. All can cause transient problems with operations. These are often erroneously blamed on the operating system or software.
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.
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