Silicon Chip's Guide to
Discrete Electronic Devices
With this first issue of "Silicon Chip"
magazine, we begin a series of articles
on discrete electronic components. These
embrace virtually all electronic
components except for integrated
circuits. And if you want to understand
how integrated circuits work, it is
essential to have a prior understanding
of discrete components.
Electronic circuit components
can be divided into two broad
categories: discrete and integrated.
Discrete components are those
which usually only have two or
three terminals and provide only
88
SILICON CHIP
one circuit function within their
package.
Integrated components are these
days mostly integrated circuits.
These usually contain thousands or
even millions of internal circuit
components which together may
provide many functions.
Discrete components can be
regarded as the basic building
blocks for all electronic circuits.
They are individually soldered or
otherwise connected into circuit
and they are made in an almost
countless variety of types and
values.
Passive & Active Devices
Within the broad range of
discrete devices there is a further
division into Passive and Active
devices. These terms need defining
although as we will ultimately see,
the dividing line between active and
passive devices is a blurred one.
According to The New Penguin
Dictionary of Electronics an active
device is one which "introduces
gain or has a directional function".
More generally, an active device
Left: all the components in this
photograph are discrete devices
except for the integrated circuit.
Most discrete devices have only two
or three connections, but some (such
as valves) have many more. Our
series will look at the majority of
devices.
can be one which amplifies, detects
or otherwise changes the shape of a
signal waveform. Within that
definition we can include transistors, diodes and a whole host of
semiconductor devices.
Similarly, passive devices are
those which do not amplify or
change the shape of a signal
waveform. They include resistors,
capacitors, inductors and variations and combinations of these
devices.
Active & Non-linear
Where the distinction between
active and passive devices becomes
blurred is with the terms linear and
non-linear. Most passive components are said to be linear which
means that their output is more or
less similar, or in direct proportion,
to their input. Under that definition
most active components can be said
to be non-linear.
The tricky part comes when considering components such as thermistors which are a special type of
resistor. Thermistors are certainly
not linear but they are not active
either. Fuses, another type of
resistor, are passive but also highly
non-linear. Incandescent lamps (the
familiar light globes), which are
resistive components, are again
highly non-linear, because their
light output and resistance is not at
all proportional to the amount of
current fed through them.
The more you look into electronic
components the more you will find
that none of them can be simply
defined as active or passive, linear
or non-linear. Bear with us though,
because even though the distinctions are blurred, the scene will
become a lot less confusing as you
read on.
For the moment then, let us
blithely assume that everything is
black and white and carry on with
the thought that there really are
these two broad divisions of
discrete electronic devices. In fact,
TABLE 1: ACTIVE AND PASSIVE DEVICES
Active
Diodes Transistors
Silicon controlled rectifiers
Triacs
Light emitters
Light detectors
Trigger devices
Hall effect devices
Thermionic devices
Gas discharge devices
Cathode ray tubes
X-ray tubes
Camera tubes
Magnetrons
Travelling wave tubes
let us list a whole gamut of components under those two broad
headings, as shown in Table 1.
We could go on but we have probably already started arguments
among the academics as to whether
some of those devices listed above
are legitimate or whether they are
merely variants of others. For example, relays and solenoids might
be regarded as special types of inductors, while light dependant
resistors, thermistors and strain
gauges might all be regarded as
variants of the broad category of
resistors.
Well so be it. We have started a
few arguments. But we have also
left out many devices which are
crucial to electronics such as the
many different types of
transducers, motors, power
sources, and peripheral components such as switches, connectors and heatsinks.
While we are in the process of
making definitions, let us comment
on the word discrete. Some people
take this to mean small and low
power, especially when applied to
semiconductor components such as
diodes and transistors, which tend
to be very small. It is true that most
discrete electronic components are
very small and there is a continuing
trend to make them much smaller.
But in using the word discrete to
describe the whole gamut of electronic devices, we are embracing
some very large and high power
components indeed. These can be
the very large transformers,
Passive
Resistors Capacitors
Inductors
Transformers
Relays
Solenoids
Light dependant resistors
Strain gauges
Piezoelectric devices
Quartz crystals
Thermistors
Ferroelectric components
Incandescent lamps
Fuses
Antennas
capacitors, reactors, rectifiers and
circuit breakers used in electrical
power distribution. And let us not
forget the very large valves used in
radio and television transmitters
and the even larger antennas used
to broadcast the signal.
In fact, we could point to specific
examples of all the classes of components listed above which are very
large and rated for very high
power, as well as examples at the
other extreme which are highly
miniaturised and operate at very
low powers. The smallest of these
are called surface mount components and these are quite difficult to handle, simply because
they are so small. They are
specifically designed to be handled
and assembled into electronic
equipment by machine.
Finally, we made the comment
above that most discrete components have only two or three connecting terminals and this is indeed
true. But it is possible to point to
devices which have many terminals, even though they truly come
under the heading of discrete
devices. Some examples of these
are · valves, transformers and
cathode ray tubes, all of which can
have six or many more terminals.
This is just another case where the
general rule can be shown not to
apply universally.
Here then is the end of the beginning. In the next chapter we shall
look at the most simple and perhaps
the most commonplace discrete
component, the resistor.
~
NOVEMBER1987
89
