UltraCAD Design, Inc



Going In and Going Out Electrons

Sometimes we have too much spare time on our hands! When we drafted this letter and sent it to Pete, he used it in the "Back Page" of the September, 1997, issue of Printed Circuit Design. It received one 'hot' reply from a reader who took exception to our descriptions of how electronic components work, and this reader went on to give us his version of how they really work! We like our descriptions better!



Every designer should participate in the IPC's Designer Council e-mail forum. There are some good discussions that occur there, and it is a potential source for good information. To subscribe, send an e-mail to DesignerCouncil-request@ipc.org with no message but the word "subscribe" (no quotes) as the subject.

Topics discussed range from design guidelines to requests for help, to requests for specifications, to occasional job opportunities. Sometimes there are unexpected comments there, however. One person (who shall remain unidentified) commented during a discussion about routing traces near power and ground planes, "I thought a plane was a plane. Are 'going in' electrons different than 'going out' electrons?"

That got me thinking.

Of course "going in" electrons are different from "going out" electrons. They each are very aggressive and rush in and fight each other at every opportunity. You have seen that if you have ever accidentally shorted the positive terminal of a car battery (going out electrons) to the negative terminal (going in electrons.) Any time the two are allowed to mix uncontrollably, sparks tend to fly. That is really apparent if you accidentally short one side of an electrical outlet in you home to the other one (DON'T try this!). The reason there is such a violent reaction is that the electrons there are very confused --- and very angry. The electrons at one side think they are going-out electrons, then the rules get changed and they becoming going in electrons. Then the rules get changed again and they become going out electrons. This happens 60 times each second. You'd be confused and angry, too, if someone kept changing the rules on you 60 times each second!

Most designers have, at some point in their career, accidentally and inadvertently connected a trace between the power and ground plane on the board they were working on. The circuit almost never works when this happens. The reason is the going in and going out electrons all rush to each other and begin fighting each other, and there aren't enough electrons left to run the circuit. The more they fight, the hotter the action gets; often hot enough to start heating up a part of the board.

It is very important to control these critters and not let them fight too aggressively.

Long ago, when we first began to use electricity, it became necessary to design a set of circuit components to keep control of things. That's why Mr. Ohm, Farad, and Henry, respectively, created the resistor, capacitor, and inductor. A resistor controls the flow of the electrons so that not too many of them can mix at any one time. It does this by opening a sort of gate for a fraction of a second. The time is inversely related to the value of the resistor. For example, a 10 ohm resistor has a gate that opens for just one-tenth of a second; a 100 ohm resistor opens for a hundredth of a second, and so on. A half-ohm resistor opens for a full second, but the gate is twice as large. A zero ohm resistor is not possible. You can verify this by punching one divided by zero into your calculator or spreadsheet and see what happens.

A capacitor is formed by taking two, fairly large conducting sheets and separating them by a VERY thin insulating sheet. The insulating sheet prevents the electrons from mixing, but not from seeing each other. The going in and going out electrons rush in on the opposite conducting sheets and face off across the insulator. The result is not unlike two dogs facing off on opposite sides of a glass patio door.

The electrons are really excited when they can see the other electrons on the other side of the insulator, and so when a circuit has a need for some electrons, they are really ready to go. That's why capacitors work so well in bypass situations; they deliver highly excited and charged electrons to a circuit really quickly.

The inductor was originally created to tease electrons. The original inductor was a tightly wound coil. When electrons went through the coil they got dizzy and had a tendency to keep going even after they passed through the coil. The effect is similar to what happens when you close your eyes and turn around in a circle several times. It turns out electrons really like inductors. When the see an inductance they scream and yell almost like people do when they get on a roller coaster. In fact, that's what is measured during FCC compliance testing --- the noise created by excited electrons when the see an inductance.

You are probably aware that the Coriolis force is clockwise in the Northern Hemisphere and counter clockwise in the Southern Hemisphere. The Coriolis force works on electrons just like it works on everything else. That's why it is good practice to wind inductive coils in a clockwise manner in the Northern Hemisphere and a counter clockwise manner in the Southern Hemisphere. Inductors not wound this way will still work, but efficiencies are improved if this rule is followed.

So there you have it. Going in and going out electrons are certainly different and many people have worked very hard through the years devising ways to control them.

Click on logo to return to UltraCAD's Home Page.