EMI – Engines

Last Updated on 05/26/2009 by dboth

Gasoline and other fossil fuel engines can cause EMI problems. The ignition systems on older automobiles and still found on all small engines such as those used for lawn mowers and similar equipment produce significant amounts of radiated EMI from the sparks in the spark plugs and old-fashioned distributors.

The Lawn Mower

This is another of my favorite illustrations of some of the strange EMI problems I have encountered during my career.

One day quite a number of years ago I was working at home one Saturday and had connected via modem to the IBM PC Company BBS where I was responding to questions and comments from users. At fairly frequent intervals my computer would lock up.

The Explanation

After some sleuthing I determined that the neighbor was mowing his lawn and every time he got close to our property line the computer would fail. I deduced that the gasoline mower was producing electrical noise from the ignition system. This noise was being induced into the outer shield of the TV cable which was the only cable on that side of the house. But my computer was not connected to the cable system; in fact, this was before cable companies got into the Internet access business. It seems that the noise was transmitted along the cable to a place where the TV and telephone company cables ran parallel to each other. The signal was induced from the TV cable into the telephone cable and was transmitted along the telephone cable into the modem and from there to the computer.

The Fix

The cause of this problem was that the TV and telephone cables were improperly grounded where they entered the house at nearly the same place. After they came out and provided corrected grounding to a copper stake, the noise was grounded out and the symptoms never occurred again.

Entry Paths

As with all radiated EMI sources access can be through openings in the computer case or from cables connected to the computer which have the electrical noise induced into them. Access points can be caused by running the computer without the side of the case in order to facilitate easy access for various reasons and cover plates left off of the bus slots in the back of the computer. EMI can infiltrate a computer even through a cover that is merely set in place and not fastened down with the appropriate screws.

Cables connected at one end to the computer but left dangling at the other end while a peripheral is disconnected can act as antennae to capture signals and draw them into the computer. And sometimes even correctly installed cables can be the pathway for EMI into your computer for noise which has been induced or radiated into them.

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