EFAR- Electronics For Ham Radio-session one

We start with the schematic. The schematic is a road map of sorts. The difference between a roadmap and a schematic is that roads are for automobiles, while the electronic paths on a schematic are for electrons. We could extend this analogy further by saying that like highways, circuits have intersections, speed limits, and things to go through.

The analogy of electrons as vehicles breaks down a bit however, when we consider that there are two different kinds of electric current, DC and AC. DC current flows in one direction, while AC is constantly changing direction, first one way, then reversing course and going the other many times a second.

The important thing to realize is that AC and DC currents are not affected in the same way by all obstacles the find along the way. In some cases AC will pass through some with ease, while DC will be stopped in its tracks. This leads to an interesting situation where AC and DC travelers can take totally different paths through the same circuit. Knowing the rules of the road for each is critical.

We are going to start our study of electronics with a review of components. We will discuss the major electronic components in detail, and then assemble them into meaningful subgroups. Whenever possible we will do hands on demonstrations of component and combinations of components response to both AC and DC currents. Since these demonstrations will require measurement we will also demonstrate the proper use of test and measuring devices such as multi-meters, oscilloscopes, signal generators, and the like.

You cannot avoid some math when practicing electronics. That said, we will try to keep things as simple as possible, and when necessary, deal with needed math for electronics within this series of lectures, as necessary. Because in electronics we have to deal with extremely large and extremely small numbers, we will review scientific notation early on. This math knowledge, and the ability to solve very simple equations should be sufficient for the kinds of calculations we will do.

We start with the capacitor. A capacitor is a device for storing electrical charge. It consists of two electrical conductors, separated from each other by either air or some non-conducting material. This non-conducting material has a name. It is called a dielectric. The dielectric has a great impact on the amount of charge a capacitor can hold, and also its voltage rating. If we put more voltage across a capacitor than it was designed for we will punch through this dielectric and damage the capacitor. Some capacitors can be charged either way. That is, they can be reversed in a circuit. However, other capacitors are one way only. We say that these are polarized. Reverse them in a circuit, and you are likely to damage them. An electrolytic capacitor is an example of a polarized capacitor.

Think of a capacitor as a storage vessel for charge under pressure. Physics books will tell you that capacitance is equal to the charge divided by the voltage across the capacitor.

The formula for capacitance is quite simple:

C=Q/V

whre C is the capacitance in farads, Q is the charge measured in coulombs, and V is the voltage measured in volts.

So what? Well, for starters, we can see that the capacitance, or the ability of a capacitor to hold charge, Q, for a given voltage, V, increases with Q. In plain English, bigger capacitors hold bigger charges.

The mathematical formulas for real capacitance can get very very complicated, depending on the geometry of the capacitor. There is one geometry, however, that is quite simple to work with. This is the case of the simple parallel plate capacitor.

The formula for capacitance in pico-farads is:

Cpf = .225K(Area in square inches)/d plate separation in inches)

K is a constant. It is called the dielectric constant, and depends on what fills the sapce between the plates. For air, the value is around 1.0. You don’t memorize K. You look it up in a book!

A is the area of overlap between the two parallel plates in square inches, and d is the separation between the plates in inches. For wax paper K is 2.2 Values of K for other materials are published in Electronics and Physics books.

During session one, Don demonstrated how to build a simple capacitor from a twisted pair of wires. Remember, a capacitor is essentially two conductors separated by an insulator, and the twisted wire pair satisfies this requirement. For nostalgia buffs, this capacitor made from a twisted pair of wires has a name. It is called a gimmick, and found use in some early tube radios.


At the next session we will continue to study the capacitor, and begin to learn how it behaves when connected to DC and AC current sources. The difference is remarkable.