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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 1010:  Safety and the Environment
      9  III  The Three Demons
           9  EMI

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Dealing with EMI

Note that when a PC has been certified to the limitation of EMI production, it is done with the case cover on. This both protects the PC from EMI as well as preventing EMI from the PC interfering with other devices. Tracking down EMI issues can be incredibly challenging. Your authors have found a very simple and inexpensive device to assist in tracking down EMI challenges. That device is a compass.

Figure 101: A compass makes a great “quick and dirty” EMI detector.


Avoiding EMI

Use dedicated circuits. Avoid plugging in a PC to a circuit that is powering devices with motors, such as refrigerators or air conditioners.

Physically isolate the PC from radios, televisions, appliances, fluorescent lights, and stereo speakers that have not been shielded.

Use an Uninterruptible Power Supply (UPS), line conditioner, or at the very least a surge suppressor.

Refer back to the diagram on how a power supply works in Chapter 0000. Notice that the capacitor filters out the last bits of a wave after the bridge rectifier.

Now lets think about a power surge. The transformer will simply step down the excess voltage by a proportional amount.

The rectifier will convert this to too much voltage with all positive waves.

From there, the capacitor will become saturated, passing on an excessive amount of voltage in positive pulses, which are fed to all the parts of your PC. In other words, zap, your PC is dead.

Surge Protection

A surge protector can remove excessive voltages as well as RFI. Without a power source, a surge protector cannot protect against a power drop or under voltage condition known as power sag.

Stay Grounded

Always plug computer equipment into a three prong grounded outlet.

[spacer]UL 1449

When looking at surge suppressors, be sure it carries the Underwriters Laboratories (UL) rating 1449.ULtests for 1449 consists of Grades A, B and C, Classes 1, 2 and 3, and Modes 1 and 2. Within this classification, the best rating is A-1-1.

[spacer]Dedicated Circuits

Looking for an AC outlet that is orange color can Identify dedicated circuits for AC power. The triangle on an AC outlet indicates dedicated ground (see
Figure 102).

Figure 102: Isolated Ground is identified by the triangle on the outlet.


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CertiGuide to A+ (Core Hardware) ( on
Version 1.0 - Version Date: December 6, 2004

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