Reading Assignment

Our TextRead Chapter 3 pages 3-6 to 3-17 in your text before you continue.

If you have questions after reading the assignment, consult the recommended reading.

To make an electronic device (like a radio) do something useful (like a receiver), we need to control and manipulate the flow of current. There are a number of different electronic components that we use to do this.


ResistorsThe function of the resistor is to restrict (limit) the flow of current through it. Another use of a resistor is whenever you need to produce a voltage drop in a circuit. This drop can then be utilized for some other purpose, noting the voltage will differ depending on the load. If the load varies in resistance, so will the voltage drop.

Variable resistors can be used to vary volume in a radio. By the position of the variable resistor a certain voltage or potential is picked off. When used this way they are sometimes referred to as a potentiometer.Resistors

Variable resistors can also be used to vary the current in a circuit, say to a lamp so as to vary its brilliance. When used to vary the current they are called rheostats.

The values of resistance are ohms. The letter that resistors are referred to is 'R'.


CapacitorsIn a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric. You can easily make a capacitor from two pieces of aluminum foil and a piece of paper.

In reality the dielectric can be any non-conductive substance. Usually Mica, ceramic, cellulose, porcelain, Mylar, Teflon and even air are some of the non-conductive materials used. The two conductors hold equal and opposite charges on their facing surfaces, and the dielectric develops an electric field.

Capacitors Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass, in filter networks, for smoothing the output of power supplies, in the resonant circuits that tune radios to particular frequencies, and for many other purposes.

In other words capacitors store energy in an electric field.

Capacitors pass the effect of AC and block DC Voltage.

Capacitance is rated in Farads. The letter that refers to a capacitor is 'C'.

Inductors L

An inductor is usually constructed as a coil of conducting material, typically copper wire, wrapped around a core either of air or of ferromagnetic or ferrimagnetic material. If a current is flowing through the coiled wire, a magnetic field is created. An inductor (also choke, coil or reactor) is a passive two-terminal electrical component that stores energy in its magnetic field. Inductance is measured in Henries, The letter that refers to an inductor is 'L'.

An inductor appears to be a short circuit to DC however it opposes the flow of AC.

InductorsAn inductor is usually constructed as a coil of conducting material, typically copper wire, wrapped around a core either of air or of ferromagnetic or ferrimagnetic material. Core materials with a higher permeability than air increase the magnetic field and confine it closely to the inductor, thereby increasing the inductance.

Low frequency inductors are constructed like transformers, with cores of electrical steel laminated to prevent eddy currents. 'Soft' ferrites are widely used for cores above audio frequencies, since they do not cause the large energy losses at high frequencies that ordinary iron alloys do. Inductors come in many shapes. Most are constructed as enamel coated wire (magnet wire) wrapped around a ferrite bobbin with wire exposed on the outside. Some inductors have an adjustable core, which enables changing of the inductance. Inductors used to block very high frequencies are sometimes made by stringing a ferrite cylinder or bead on a wire.

TransformerOne of the common uses of coils (inductors) is as a power transformer. Windings are wrapped around two sides of a soft iron core. When the changing (alternating current) magnetic lines of flux created by current running through the primary winding cut the coil windings of the secondary winding, a current is induced (created) in the secondary coil.

The voltage induced into the secondary is the same ratio as the winding ratio. In other words if there are 16 windings in the primary and 8 windings in the secondary, a 2:1 ratio in voltage occurs. Figure M6-6 is a 'step-down transformer and can take 24 VAC (volts AC) down to 12 VAC. Note the only way this transformer can work is if AC voltage is used. DC would not have changing lines of flux, therefore they would not grow and collapse causing an induction of voltage in the secondary coils.

Relay Another common usage of a coil is as an electromagnet relay. A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a path for magnetic flux, a movable iron armature, and one or more sets of contacts. The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the contacts and when current energizes coil the contacts close.

It is called a relay because the action of the first circuit 'relays' the action of the second circuit. This way a small voltage can control a large one or with more contacts several circuits can be controlled.


Inductors and capacitors can interact to perform other functions of great importance to electronics.

We have discussed DC resistance but capacitors and inductors also exhibit resistance but it is called reactance. Reactance is AC resistance. Reactance varies with the frequency applied to the circuit, unlike resistors which remain the same.

Capacitive Reactance decreases as the frequency rises, while Inductive Reactance increases a frequency raises.

Impedance is the combination of capacitive & inductive reactances and resistance It is referred to in ohms and represented by the letter 'Z'.

In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequency. At this frequency, even small periodic driving forces can produce large amplitude oscillations, because the system stores energy.

Resonance Resonance of a circuit involving capacitors and inductors occurs because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, and then the discharging capacitor provides an electric current that builds the magnetic field in the inductor. This process is repeated continually. An analogy is a mechanical pendulum.

Take a close look at Fig 6-8. It is an 'Active GIF', which means it acts like a mini-movie. It will illustrate this process.

Resonance Electrical resonance occurs when the reactance of an inductor balances the reactance of a capacitor at a given frequency. In other words the capacitive reactance cancells out the inductive reactance and all that remains is the resistance.

In such a resonant circuit where it is in series resonance, the current will be maximum and offering minimum impedance. In parallel resonant circuits the opposite is true.

In a series circuit at resonance, frequencies away from resonance see an ever increasing impedance. For a parallel circuit at resonance, frequencies away from resonance see an ever decreasing impedance.

There are many applications for this circuit. They are used in many different types of oscillator circuit. Another important application is for tuning, such as in radio receivers or television sets, where they are used to select a narrow range of frequencies from the ambient radio waves. In this role the circuit is often referred to as a tuned circuit. An RLC (Resistance, Inductor, Capacitor) circuit can be used as a band-pass filter, band-stop filter, low-pass filter or high-pass filter. The tuning application, for instance, is an example of band-pass filtering.

This is probably a lot more than you need to know at this point. Again, I feel sometimes an incomplete explanation of a concept just muddies the water of understanding. Just understanding the interaction of the capacitor and inductor resulting in an oscillation or ringing is a very important point and will lead to better understanding the mystery of electronics.


DiodesA semiconductor diode is a crystalline piece of semiconductor material connected to two electrical terminals. The most common function of a diode is to allow an electric current to pass in one direction (called the forward direction), while blocking current in the opposite direction (the reverse direction). Thus, the diode can be thought of as an electronic version of a check valve or rectifier. Thus a diode can change AC voltage into pulsating DC voltage. This unidirectional behavior is called rectification, and is used to convert alternating current to direct current. It can also extract modulation from radio signals in radio receivers.

There are special purpose diodes that perform many different functions, to regulate voltage (Zener diodes), to protect circuits from high voltage surges (avalanche diodes), to electronically tune radio and TV receivers (varactor diodes), to generate radio frequency oscillations (tunnel diodes, Gunn diodes, IMPATT diodes), and to produce light (light emitting diodes).

The two ends of a diode are called the anode and the cathode. The cathode end is marked with a stripe usually, but can be marked like the drawings.

Bi-Polar Transistors

A Bi-Polar transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals (an emitter, a base, and a collector) for connection to an external circuit.

Xistors The bipolar junction transistor (BJT), is formed of either a thin layer of p-type semiconductor sandwiched between two n-type semiconductors (an n-p-n transistor), or a thin layer of n-type semiconductor sandwiched between two p-type semiconductors (a p-n-p transistor). This construction produces two p-n junctions: a base-emitter junction and a base-collector junction, separated by a thin region of semiconductor known as the base region.

A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, a transistor can amplify a signal. This ability is referred to as gain. The input can also turn the conduction completely off thereby acting like a switch.

FET Transistors

FETThe field-effect transistor (FET), sometimes called a unipolar transistor, The terminals of the FET are named source, gate & drain. In a FET, the drain-to-source current flows via a conducting channel that connects the source region to the drain region.

The conductivity is varied by the electric field that is produced when a voltage is applied between the gate and source terminals; hence the current flowing between the drain and source is controlled by the voltage applied between the gate and source. As the gate-source voltage is increased, the drain-source current increases greatly. As a result amplification is obtained (gain).

The input resistance of the FET is much higher and they generate a lower noise level than the bipolar transistor making it advantageous for use as the first stage Radio Frequency amplifer in receivers. They are also more temperature stable.

Integrated Circuits

An integrated circuit (IC), sometimes called a chip or microchip, is a semiconductor wafer on which thousands or millions of tiny resistors, capacitors, and transistors are fabricated. An IC can function as an amplifier, oscillator, timer, counter, computer memory, or microprocessor.

Integrated circuits are used in virtually all electronic equipment today and have revolutionized the world of electronics. Computers, mobile phones, and other digital appliances are now inextricable parts of the structure of modern societies, made possible by the low cost of production of integrated circuits.

ICs are in virtually all ham radios. They make possible the multi-mode transceiver and the small hand-held radios of today. Not far in the past many radios you could take with you were so big they were referred to as portables rather than HTs (hand-held transceivers). They were big, bulky, heavy and used large batteries to supply the needed power to run them.

Fuses & Circuit Breakers

Fuses are a one time circuit protection device. You have all heard of a fuse blowing, well they do actually blow or break open a little wire inside the fuse itself.

Fuses cannot be reused. They come in different types such as slo-blow and fast-acting. Each time power is applied the little wire flexes and after a while they will break the wire. Many mobiles use a slo-blow fuse in this service. You do sacrifice some quick response to an actual overload. Don't replace a fast-acting fuse with a slow-blow.

Fuses are rated in amperes by the maximum current they can carry without blowing. It is a bad idea to replace a blown fuse with one of a higher amperage. You risk damaging the equipment or maybe even causing smoke and fire.

Set me share an old technician joke with you: Electronics is magic. It is done with smoke and mirrors. In fact, the smoke is what makes them work. You can prove this because whenever you let the smoke out of an electronic device it no longer works!

A Circuit Breaker acts like a fuse but can be reset. They work on heating a bi-metal trip mechanism that 'trips' the circuit breaker open. A breaker that trips on a regular basis should be replace because they do wear out. Alternately, you could check the current draw to be sure there is not a problem, but just a weak circuit breaker.

Circuit Symbols

We have already seen most of the circuit symbols as we have gone along talking about the individual components. But here is a picture of common schematic symbols used in schematics, just to put them all in one place for a little review.

Pay some attention to the switch schematic symbols. You just might see a test question on one of those on your exam.

Switches act pretty much just like the schematic shows them: Single Pole Single Throw = a single circuit that is either open or closed; Double Pole Double Throw is two separate circuits that switch between two choices; and so on.


Schematic Diagrams

Hams, Technicians & Engineers need a kind of road map to show how components are connected in a circuit. These road maps are called schematics.

SymbolSchematics do not show the actual physical layout of a circuit. It only shows how the components are connected together using schematic symbols.(Good Test Question!)

Many times appliances will have a wiring diagram which goes into more inter-wiring detail than a schematic. It pays more attention to the actual wires that connect devices together, so that people that are not electronic technicians can trace the wiring and troubleshoot the appliance.

I mention wiring diagrams because most of us learning to be a ham may be called or call ourselves to look at the washing machine etc. You will usually see the wiring diagram and schematic next to each other.

The schematic is usually used to figure how the unit functions.


Again the best way to learn & retain formulas is to do some problems and learn from them. So, lets do some practice.

You can take the quiz as often as you wish.


In this module you have been introduced Electronic Components & Circuit Schematics

Kind of the nuts and bolts of Ham Radio. Lots of information as always.

Recommended reading

First Steps in Ham Radio - a series of articles intended to help you get started.

ARRL Technical Information Service - numerous links and articles on a variety of technical topics

Understanding Basic Electronics - introduction to the electronics of radio

View the complete Technician Question Pool in Chapter 11 of your Ham Radio License Manual.

Review the Term Glossary in Chapter 10 of your Ham Radio License Manual

Practice Exams are available at and there are links to many other test practice sites also.