by Norris Chambers
You have learned about direct current and alternating current, commonly called DC and AC. In DC the current always flows in the same direction,
where in AC it continuously reverses in direction at a specific frequency. In magnetic fields, you learned that current flowing in a coil causes a magnetic field to be developed around it. If it has an iron core, the core becomes magnetized.
When AC is applied to the coil, the magnetic field builds up and collapses, then builds up with an opposite polarity each time your current reverses. So when AC current is flowing in the coil with an iron core, the field is pulsating >back and forth in accordance with the current reversals. You also learned that when a coil is around or in a magnetic field, as the field changes a current is induced in the windings. Therefore, if you wind another coil around the one you are connecting to AC, a similar voltage is induced in the second coil. This voltage is determined by the turns ratio of the coils. For example, if the first coil that you are passing the current through has 100 turns, and if the second coil has 50 turns, then the voltage in the second coil will be one half of that applied. On the other hand, if you have twice as many turns in the 2nd coil, the voltage will be twice as much as that applied. The first coil is called the "primary" and the 2nd the "secondary." An arrangement like this is called a "transformer" because it transforms voltage from one level to another. Now, when you hear them talking about a transformer, you will know what it is. The transformers on power line poles work just like this. They have a primary coil connected to the high voltage on the main line and a secondary coil with fewer turns that transforms the voltage to 120 volts for your household appliances. It is changing polarity 60 times a second, so it is called 60 cycle AC.
Another interesting thing is the action of a "condenser." Newer books refer to it as a "capacitor." But just as a flower would smell the same by any other name, a condenser works the same as a capacitor. A condenser is nothing more than two plates made of conducting material separated by a thin insulator. The insulator is usually plastic, air or mica. The larger the plate, the larger the "capacity" of the condenser. The capacity refers to the number of electrons it can hold, as it is nothing more than a storage area for electrons. Here's how it works: connect the negative terminal ofyour battery to one plate and the positive terminal to the other plate. When you do this the electrons from the negative side will rush onto the plate in an attempt to reach the positive pole. Because the plate connected to the positive attracts the electrons, they pile up against the insulator trying to reach it. But they can't pass through the insulator. You can disconnect your battery and the electrons stay on the negative plate. The condenser is said to be "charged." It is holding electricity. The larger the plates, the more electrons it will hold. The size of a condenser is measured in "farads." One farad is a very large condenser, so the term microfarad is more commonly used. A microfarad is one millionth of a farad. Condensers are also rated by working voltage and peak voltage. The working voltage is the voltage which can be safely applied without arcing across the insulator. The peak voltage is the maximum voltage that can be applied without insulation breakdown.
In the old tube circuits, the working voltage was usually from 200 to 800 volts. In modern solid state circuits it is usually from 6 volts to 50 volts.
When you apply AC voltage to a condenser, it acts differently. It is said to pass AC and block DC. On an AC cycle, when the applied voltage is negative, the plates charge negative and positive, as in DC. Then, when you change polarity to positive, electrons are drawn from the voltage source to the other plate. As your voltage rocks back and forth between >negative and positive, the current flow into one plate and then into the other, giving the effect of an AC current flow. So a condenser passes AC and blocks DC. The larger the condenser, the more current it will conduct. Also, when you increase the frequency of your polarity reversals, it will conduct more. So the higher the frequency, the more current a condenser will conduct.
Now you know what a condenser is and how it works with DC and AC. A coil works just the opposite when you apply AC. The higher the frequency, the more resistance it offers to current flow. The resistance that a coil offers to AC is called "reactance." There is much more theory that has to do with tuned circuits of condenser and coil combinations and how they can be tuned to different frequencies. That is how tuned circuits work to separate your radio or TV stations. This theory is adequately explained in many books and we will not go into it here.
A condenser is shown on a schematic diagram as two vertical lines separated by a small space with a connecting wire on each line. Thinkof the vertical lines as plates and the horizontal lines as the connecting wires. A transformer is shown by a line representing an input voltage coiled several times and returning to its source, and a second coil close to it with similar output wires. If the coil has an iron core, it is shown by several straight lines between the coils.
A resistor is shown by a zig-zag line like this --/\/\/\/\--. If it is shown with an arrow drawn through it diagonally, it means that the resistance can be varied, like with a knob. That would indicate a volume control, or any other kind of resistive control. It is sometimes called a "rheostat." With a variable resistor connected between your battery and a light bulb, you can make the bulb dimmer or brighter by changing it.
A resistor usually acts the same way with either AC or DC current flowing through it. A condenser will pass AC but block DC. A coil will offer more resistance to AC than DC.
The unit of measure for measuring the amount of resistance is called the OHM. One million ohms is called a megohm. The resistance offered to an AC current by a coil is called "inductive reactance" or "impedance." Impedance is usually the total resistive effect of all the components in the circuit. The resistance offered to an AC current by a capacitor is called "capacitive reactance," or sometimes impedance if used with a coil and resistance. These are also measured in "OHMS."
As the frequency of AC applied to a coil increases, (remember, frequency is the number of times per second that the polarity reverses) the impedance, or difficulty of the electron flow, increases. A capacitor is just the opposite. As the AC frequency increases, the impedance (or capacitive reactance) decreases, and the current flow becomes less difficult. Remember these characteristics, because the operation of "tuned circuit" depends on them.
You have probably absorbed enough hard theory for now. Try to remember these terms and actions. We will get away from this how these principles are used in the operation of microphones, speakers and some other electronic components. After that we will discuss "rectifiers" and their uses. Much later, if you and I both last that long, we will see how simple it is to learn about transistors and solid state devices.
And remember always that learning electronics is FUN - if it isn't, just don't bother with it and watch TV.
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Lesson Three