Selection and Testing of Electronic Components for LM

Fig. 6
Humidity test
High air humidity Is one of the most serious adverse environmental
conditions to which electrical components
can be exposed. The dampness can affect the outside of
the components by corroding metal surfaces and reduce
the Insulation between the leads. It can also seep Into the
components and In so doing impair their caracterlstlcs or
cause total breakdown.
Humidity testing Is carried out in an climatic chamber,
where the air humidity and temperature are either
held constant or varied cyclically with time. The components
can either just be stored In the chamber or they
can also be connected to an electrical voltage source
during the humidity tests
Fig. 7
Programming equipment
In a certain type of semiconductor memories, designated
PROM (Programmable Read Only Memories), the contents
of the memory cells are fed In with the aid of special programming
equipment A metallic connection is thereby
burnt off electrically in the cells that are to be programmed.
The memory program can be fed in in different ways:
manually via a push-button set, from a punched tape or
with the aid of a previously programmed memory (master).
The equipment also checks that the programming Is correct
uses infra-red lights as the transfer
medium between a light-emitting diode
and, for example, a photo transistor.
MICRO CIRCUITS
Micro circuits are built up of a number
of interworking semiconductor elements
and can integrate a number of
analogue functions, digital functions or
memory functions.
Analogue micro circuits
In analogue circuits the voltages on the
inputs and outputs can vary continuously
over certain ranges and are thus
not limited to fixed levels. In modern exchange
systems these circuits are used
as
— voltage regulators for power units
— sensors of voltage levels
— interface circuits between different
subsystems
— operational amplifiers in MFC filters
etc.
Digital micro circuits
These circuits are predominant among
the micro circuits. Digital circuits carry
out logic operations by means of digital
signals, i.e. voltages on the inputs and
outputs that take up values close to
fixed levels. Many of these circuits belong
to specific so-called circuit
families with a certain type of logic element
and in other respects designed so
that they can interwork in systems.
In addition to these circuit families a
number of digital circuits are used that
do not belong to any particular family.
These can be divided into a number of
groups according to function, such as
registers, adders, arithmetic circuits,
counters, data switches, coders and decoders.
DTL circuits (Diode-Transistor-Logic)
constituted one of the first families in
micro circuit technique. They are no
longer used when designing new
equipments.
TTL circuits (Transistor-Transistor-
Logic) are faster than DTL circuits and
have gradually become the predominant
type.
A large number of circuits are available
on the market in several different variants.
Certain series (e.g. 74S and 74LS)
have integrated so-called Schottky
diodes, whereby the circuits have been
made faster.
CMOS (Complementary-Metal-Oxide-
Silicon) circuits operate within wide voltage
limits and have low power consumption,
but they are not particularly
fast.
Memories constitute an important
group. They can be in the form of random
access memories, where the con-