Lesson 2/Learning Event 1
a. Factors. Some of the factors that must be known to the technical
controller before he attempts to patch telegraph circuits include the
following: Battery source, speed of transmission, classification of
circuit, mode of operation, normal value of current, direction of current,
characteristics of the circuits, and type of code transmitted.
b. Battery Source.
The term battery in telegraph (teletypewriter)
communication refers to the DC power source for circuit current.
Most
teletypewriter equipment as well as most telegraph carrier terminals can
furnish power to the circuit. However, for simplicity of operation, only
one batter source should be used in each telegraph circuit.
Normal
polarity in telegraph circuits is negative battery for mark.
In neutral
circuits positive battery is grounded, while in polar circuits both
positive and negative polarities are made available to the relay contact
points, with the space contact being positive. Ground return to the polar
circuit is made to the center point of a voltage-dividing network placed
across the power supply, with the power supply "floating"; that is,
neither side of the power supply is ground.
c. Amount of Current.
When two DC telegraph circuits are to be
connected, they must be electrically compatible both as to value of
current and its direction. Assume, for example, that one loop connects to
a terminal using 30 ma, and a second loop connects to a teletypewriter
which requires 60 ma for normal operation.
If these two loops are
patched, the current will be too high for the terminal. If the current is
readjusted to satisfy the terminal, it will be too small for
teletypewriter operation.
One way to solve this problem is to use an
isolating relay. The line winding of the relay is connected to the line
or loop carrying the DC signals, while the contact points of the relay are
connected to the keyed circuit.
In this way there is no metallic
connection between the two circuits; they are electrically isolated.
Notice that the amount of current is the important factor, not the amount
of voltage applied. The minimum requirement of voltage is the value that
is needed to force the correct amount of current through the resistance of
the circuit. Excessive resistance in the circuit may restrict the current
from reaching its proper value.
d. Direction of Current. The direction of current is important in a
circuit which contains a polar relay, because the direction of current
will determine the direction of armature movement between mark and space
contracts. If the direction of current is reversed, marks will appear as
spaces and spaced will appear as marks.
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