Electricity is the lifeblood of your PC. It courses through its motherboard and CPU in miles of circuits that can be narrower than 1/100th the width of a human hair. To run smoothly, your PC needs a constant flow of precisely monitored electricity pumped through these tiny channels. Even small fluctuations can lead to lost data or system crashes. Accordingly, you need to know how to regulate your PC's power supply.

First, the basics. Your PC survives on a diet of direct current, the kind produced by a typical flashlight battery. DC flows with a uniform force, or voltage, much like the flow of water through the pipes in your house.

The voltage of electricity coming from a wall outlet, on the other hand, continually fluctuates. This alternating current changes from highest voltage to lowest, then back to highest, 60 times (or cycles) per second. Your PC's power supply must convert the wall socket's fluctuating AC to the uniform DC your system needs.

The most common power-related PC problems come from errant voltages in your wall outlet's AC. When a fallen tree brings down a power line or lighting strikes a little too close for comfort, the resulting voltage spike -- in thousands of volts -- can flash from your wall outlet and fry the sensitive circuits in your PC.

Your best defence against a voltage spike is a good surge protector. Expect to pay at least $60, and beware of cheap power strips that claim they have surge protection but really don't.

Another important feature to look for: an indicator light that shows the unit is still working. Many low and moderately priced surge protectors use metal-oxide varistors, which can lose their capability to protect against excess current after a single high-voltage or multiple moderate-voltage surges, and the indicator is the only way to tell if the device is still functioning. If you're using a cheap surge protector with no indicator light, it's wise to replace the unit once every 12 to 18 months -- or more frequently if you live in an area prone to lightning or other electrical disturbances.

Some surge protectors also feature a resettable circuit breaker, which lets you correct small interruptions with a quick flip of a switch instead of having to replace a fuse. Surge protectors with this feature often cost more but are worth it.

When protecting your PC against voltage spikes, don't forget about your modem's phone lines; they're just as susceptible. Many good surge protectors these days come with one or two RJ-11 telephone connectors. You can also buy such a connector separately.

Too little voltage can be just as harmful to your computer as too much. Brownouts and blackouts can shut down your system improperly, damaging files or causing data loss. The solution is an uninterruptible power supply -- basically a bunch of batteries -- that will keep your system running long enough for you to save important files and shut down properly.

Power supply and demand

The power supply is the heart of your PC. Though it requires almost no maintenance -- except the occasional check for excessive dust around the fan's air intake vents -- it can still fail. Keep alert for the following warning signs:

Smoke: It sounds obvious, but if you smell smoke, turn off your system and see if the smell subsides. Then turn it on and see if it returns. If it does, open up the PC's case and investigate.

No fan noise: If your power supply's fan is quiet, your power supply may need replacing. Turn off your system immediately to avoid any damage from overheating.

Intermittent memory error messages: Your PC's RAM is very intolerant of power fluctuations. If you consistently get the same error messages at the same memory locations, you most likely have a problem with your RAM. Erratic error messages, on the other hand, may be due to a problem with your power supply.

Blank screen with noisy computer: If you get a blank screen but can still hear your hard disk and fans running, your power supply may be on the fritz. (Of course, first make sure that your monitor hasn't just slipped into power-saving mode.) The power supply sends a constant signal to the delicate motherboard indicating that power output is normal. When an abnormality occurs, the signal stops and the motherboard shuts down in an act of self-preservation.

Problems at start-up: If you notice any unusual behaviour or problems -- such as memory errors or a distorted video display -- appearing at start-up but not on subsequent "warm" restarts, you may have a power supply problem.

If you conclude that you do need a new power supply, be sure to get the right one for your PC. The two most popular types are ATX and AT, corresponding to the PC's motherboard form factor. To meet design needs, some PC manufacturers use a non-standard power supply, so always check your system documentation before buying a replacement.

And make sure the new unit can handle your PC's power demands. Capacity ratings for power supplies are measured in watts, or maximum energy output. Most PCs have a rating of 180 to 300 watts, with 200- to 250-watt power supplies. You should find your power supply's rating written on the outside of the unit's case.

If you're planning to add any new devices to your PC, consider buying a power supply with a higher rating than the one that originally came with your system, since additional cards and drives can cause an overload.

Every last drop

All notebook and laptop PCs come with power-saving modes that shut down various combinations of devices to extend battery life. You can select modes from the CMOS setup program, keyboard, or even Windows. Most systems today offer power-saving modes through at least partial support of the Advanced Power Management standard.

The five APM modes of operation range from full-on to full-off, with three states of hibernation in between. Standby allows for the quickest return to work by keeping current data in RAM and shutting down only a few components. Suspend shuts down more components, and Sleep actually writes the current data to hard disk and cuts off power to RAM. Unfortunately, because different vendors implement APM in different ways, conflicts and incompatibilities can occur. For example, many APM systems allow settings to be selected in both CMOS and software. So if your settings under the Power Management icon (in Control Panel) don't work, your Power Management may be conflicting with settings in the CMOS setup program. Other settings that fix APM incompatibilities can be found in Device Manager: Select Start*Settings*Control Panel, double-click System, select the Device Manager tab, scroll down to and double-click System devices, double-click Advanced Power Management Support, and select the Settings tab.

Many notebooks -- and desktops -- now come with the successor to APM, the Advanced Configuration and Power Interface. ACPI allows a SoftOff option, which lets your PC, even in sleep mode, respond to events like incoming faxes or quickly get up and running.

ACPI has gotten off to a rocky start, however, and conflicts abound. To discover if your PC has ACPI, look under "System devices" in Device Manager. You should see several listings with ACPI in the title (as in FIGURE 1). If you know that your new PC supports ACPI, but you don't have it installed, you can manually install it (see article Q195218 in Microsoft's Knowledge Base.Support.microsoft.com). But check with your manufacturer first -- there's probably a good reason it's not there.

FIGURE 1