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Practical Electronics
A Self-Teaching Guide
By Ralph Morrison
John Wiley & Sons
Copyright © 2003
Ralph Morrison
All right reserved.
ISBN: 0-471-26406-7
Chapter One
Resistors, Capacitors,
and Voltage
Objectives
In this chapter you will learn:
two definitions of electronics (and how to tell which one is intended)
how to study electronics using the Learning Circuits
the equipment you will need for the Learning Circuits
the characteristics of two basic components used in most electronic
equipment-the resistor and the capacitor
What Is Electronics?
The word electronics has two different, though closely related, meanings.
This can be confusing, but you will find you can quite easily tell which
meaning is intended. In the first definition, electronics is "the study of
voltage and current waveforms that vary in time." When this meaning
is intended, the word electronics is singular, and in a sentence it is used
with a singular verb. For example, one would say, "Electronics is the
study of voltage and current waveforms."
Electronics can also refer to electrical devices created to perform
specific tasks, such as amplifying an electrical signal, sending or receiving
radiation, or any one of hundreds of different functions. If this is the
senseintended, the word electronics is plural and is followed by a plural
verb. One would say something like, "The electronics onboard an airplane
are very sophisticated."
You will be able to tell whether electronics refers to the study or to
the devices by observing the way the word is used in a sentence and the
context in which it is used. In ordinary conversation, the second sense of
the word is used most often. If you walk into almost any large department
store, you will find a section of the store called electronics. It's the section
where you can buy DVDs, CD players, and so on. Clearly, the word refers
to electronic devices. In this book, on the other hand, the word electronics
refers most of the time to the study of voltage and current waveforms.
That is why this section is called "What Is Electronics?" and not "What
Are Electronics?"!
The Learning Circuits
Throughout this book you will find experiments in electronics you can
do yourself. They are called Learning Circuits, and they have been
designed to give you a hands-on sense of the way electronic circuits
work. A circuit is a group of interconnected electronic components.
They perform such tasks as amplification, waveform generation, filtering,
signal sensing, signal switching, logic, radiation, and electromagnetic
field detection.
Don't worry if you don't know what these functions are. By the
time you finish this book you will be familiar with all of them. They are
the functions that make up radios, VCRs, stereo amplifiers, telephones,
and all the other electronic devices we use. You will not be able to
design these electronic products when you finish this book (that
requires more advanced study), but you will have a much better appreciation
of how they work, and you will be well prepared to take the
next step toward learning to design them yourself.
In this chapter there are 8 Learning Circuits. These first experiments
will show you different ways of connecting two basic electrical
components: resistors and capacitors. Before you create your first circuit,
however, you need some equipment and some understanding of
how to use it.
First, you need some means of measuring and observing changing
waveforms. The Learning Circuits include pictures of changing voltage
waveforms you can expect to see, and equations describing them. Pictures
and equations are helpful, of course, but there is no substitute for seeing
the voltages in real time, and for this you need some equipment. The two
main tools used in electronics to make observations are the waveform generator
(also called a function generator or signal generator) and the oscilloscope.
So, should you dive in and immediately purchase these two pieces
of equipment? That will be your decision, but be sure to read appendix
I, "Preparing to Use the Learning Circuits," first. As you'll see, the
equipment is costly. Before making the purchases, you might want to go
through at least a chapter or two of this book, studying the Learning
Circuits and the drawings that accompany them. You can certainly
learn a great deal this way. Then, if you find you are still excited about
electronics, you can look for some used equipment and start making
your own observations.
For the first few Learning Circuits, you can use another piece of
equipment called a multimeter. As the name implies, this is a multiuse
measuring device that can function as a voltmeter, ohmmeter, or
ammeter. Multimeters are not very expensive, and they can measure ac
and dc volts, dc current, and resistance. What they cannot do is show
waveforms-for that an oscilloscope is needed.
Circuits also need a source of power, but using utility power from a
wall plug poses a safety hazard. To resolve this difficulty, all of the ac
sources in the book make use of an ac adapter. An adapter is an Underwriter's
approved transformer that supplies a source of low-voltage ac
power. (Underwriter's Laboratory is a testing organization that approves
electrical hardware for use by consumers.)
Connecting circuit components together requires some tinned bus
wire, some insulated wire, some solder, and a soldering iron. Simple circuits
can be connected using clip leads or test leads. A test lead is an
insulated wire that has mechanical clips (alligator clips) on each end. In
some circuits you can make a connection simply by twisting leads
together. You can also purchase a circuit board that has a grid of holes
so that tie pins can be pressed into the holes, and you can solder components
to these pins. (Just be sure not to cut the leads short until you
are certain they are resting in their final location.) But when circuits
become more complicated, soldering works best. You will find a step-by-step
description of soldering in appendix 1.
You will also need some basic tools, which you may already own: a
pair of needle-nosed pliers and wire cutters.
Finally, you will need a workstation to do the Learning Circuits,
which does not need to be more than a few feet of counter space near
a wall outlet. Ideally this should be a place where you can leave your
equipment out and available while you experiment with the various
Learning Circuits.
The Waveform Generator
A waveform (function) generator is a piece of electronic test equipment
used to generate a repetitive changing voltage, or a voltage waveform
that repeats itself over and over. The voltage waveforms that can be
selected are sine, square, or triangle. The lowest settable frequency is
often around 0.1 Hz (see chapter 8). The highest sine wave frequency is
often 10 MHz. The voltage amplitude is often limited to 10 V peak or
20 V peak-to peak.
Each of these waveforms has its own particular use. Sine waves are
used to test the response of circuits. A sine wave is sinusoidal in character.
A sine wave is often referred to as a sinusoid. See Figure 1.1. This
waveform is used because the currents and voltages in a linear circuit are
all sine waves. Square waves are valuable because they provide information
about circuit behavior not easily seen with sine waves. A square
wave voltage can transition symmetrically around 0 volts or transition
from 0 to a peak voltage level once per cycle. The transition time or rise
time should be short compared to the time of one cycle. This makes it
difficult to generate a 2-MHz square wave, as the transition times should
be around 5.0 ns. Shorter transition times raise the cost. Triangular waves
are useful because the voltage slopes are constant. However, triangular
waves are not generally used in testing. These three waveforms are
shown in Figure 1.1. For more on these concepts, see chapter 8.
An output cable is usually supplied with the waveform generator
(which can also be called a signal generator or a function generator). The
cable can have alligator clips on the end so that it can be connected to
various points in a circuit. The outer conductor of the cable is called the
zero reference conductor of the signal. It is also called a shield, a ground, or
the common conductor. (The word ground is often used to mean "earth"
but this is not always the intended meaning.) It connects to the ground
or common of the circuit you are testing. In some generators this shield
is connected to the safety or green wire of the power conductor. For
most testing, you should remove this connection link or strap. The circuit
you are testing may already be connected to ground. If this is the
case, then two connections to ground can be troublesome. This can be
checked by measuring a low resistance from the common output lead
to the third pin on the power cord.
The Oscilloscope
An oscilloscope is a piece of electronic test equipment used to observe
circuit behavior in real time. The oscilloscope generates a picture of the
changing signal patterns. All of the waveforms shown in the figures in
this book can be observed in real life by applying a signal generator to a
circuit and then observing the waveforms with an oscilloscope. The
vertical scale on an oscilloscope displays voltage, and the horizontal axis
displays time.
The operation of a basic oscilloscope is simple. A dot moves (transitions)
linearly across a viewing screen from left to right. When it
reaches the right edge of the screen it immediately returns to the left
side. A single crossing is called a "sweep." If the sweep frequency is set
to 1 kHz, the dot moves across the screen in 1 millisecond (ms). In the
first sweep, the time goes from 0 to 1 ms. The dot returns to the start
and traces the same path for the second millisecond, and so on.
If the voltage probe is connected to a 1-kHz sine wave voltage, a single
sine wave will be displayed on the screen. If the sine wave frequency
is 2 kHz, then two full sine waves will be displayed. When the dot makes
many sweeps per second, the screen pattern appears stationary.
You can observe the oscilloscope display in slow motion by observing
a 1-Hz sine wave from a function generator, with the sweep frequency
on the oscilloscope set to 1 Hz. At this slow rate you will be
able to see the dot move across the screen, writing a sine wave pattern
over and over. A sine wave voltage display is shown in Figure 1.2.
It is worthwhile spending a little time with the oscilloscope and the
function generator before you get started on the Learning Circuits. I
can't tell you exactly how to work the controls, as there are many different
designs. You will have to hook up the oscilloscope to the function
generator and play with the dials until it becomes clear. Don't
worry, you can't hurt yourself or the equipment, so go ahead and
experiment.
An oscilloscope has one or two input probes. The probe tip is
designed to connect to points in a circuit. The grounding clip on the
probe is usually connected to the zero reference or ground of the circuit.
At its other end (inside the oscilloscope) the grounding clip connects
to the oscilloscope frame and to the power safety or green wire
(the third plug in a three-pronged electrical plug). This connection to
ground is required by the National Electrical Code.
A problem arises when you have both an oscilloscope and a function
generator (or in more complicated circuits, multiple devices) all
connected to one circuit. If two or more devices are connected to the
power safety, you have multiple grounds. This is not desirable. For this
reason, when you do have several devices connected to your circuit, use
a "cheater plug" (a two-pronged plug) for all but one of your devices.
In the Learning Circuits you will be using an adapter plug, which provides
an additional level of isolation and safety.
Voltages
Throughout this book, the figures generally include a reference to a
voltage source. The voltage source can be either ac or dc (see chapter 8).
Voltage may come from a waveform generator, a battery, or the power
utility.
The symbol used for a voltage source is either the letter V in a
circle or the symbol for a battery. A lowercase v refers to a changing
voltage. The polarity (plus or minus) of a dc voltage will always be
indicated.
We will assume that the voltage source can supply the current
demanded by the circuit without changing voltage. This is referred to as
an ideal voltage source. In actuality voltage does change with load, but
assuming an ideal source simplifies the discussion.
If the voltage is a step function or square wave, it will be clearly
stated in the text.
Resistors and Capacitors
We are now ready to begin using our first electronic components, resistors
and capacitors. These two components are found in most electronic
equipment because they do very basic and important jobs needed in
every circuit.
Resistors are the most common electrical component (see chapter
8). They are used to limit the flow of current in a circuit. By comparison
a conductor offers very little opposition to current flow. There are
many types and sizes of resistors. In electronics, resistors are apt to be
small cylinders that are about a half-inch long. This is the circuit symbol
for a resistor:
[ILLUSTRATION OMITTED]
Capacitors are the second most common component. Their basic
function is to store electrical field energy. This field energy requires
electric charge on the plates of the capacitor. (See chapter 8 for discussion
of capacitors and electric charge.) The ratio of voltage to
charge is called capacitance. Since it takes time to store energy, capacitors
can be used to control frequency response, provide filtering
action, provide timing, and store energy in power supplies. Capacitors
are found in almost every circuit design. The circuit symbol for a
capacitor is
In the next sections we will be examining the way resistors and
capacitors respond to various voltage waveforms. You will recall that a
waveform generator produces sine waves, square waves, and triangle
waves. Sine wave voltages are the only waveform that keep the same
shape in any combination of resistors and capacitors.
One way to study resistors and capacitors is to apply a "step function"
to the circuit. A step function is a voltage that changes from one
value to another. In many cases a low-frequency square wave can be
used as a step function. Digital circuits make extensive use of square
waves and step functions.
A common source of dc voltage is the battery. Batteries can be placed
in series to increase the dc voltage. If two 9-V batteries are placed in
series, the total voltage is 18 V. This series arrangement is shown in Figure
1.3.
If one of the batteries is reversed in polarity, the voltages subtract.
Continues...
Excerpted from Practical Electronics
by Ralph Morrison
Copyright © 2003 by Ralph Morrison .
Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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