Figure 2-17.Schematic diagram.
The connection diagram shows all the internal and
external connections. The circuitry can be traced more
easily than on the wiring diagram. The components are
still shown in their relative positions. This diagram can
be used to help you connect all the wiring and trace any
part of the circuit. The connection diagram is a valuable
This type of diagram is often
found inside the access cover of a piece of equipment.
The schematic or elementary diagram (fig. 2-17) is
a drawing that shows the electrical connections and
functions of a specific circuit arrangement. It facilitates
tracing the circuit and its functions without regard to the
physical size, shape, or relative position of the
component device or parts. The schematic diagram,
like the connection diagram, makes use of symbols
instead of pictures. Figure 2-17 shows, by a schematic
diagram, the same motor control system shown in
figures 2-14, 2-15, and 2-16. This diagram is laid out in
a way that makes the operation of the components easy
to understand. This type of schematic diagram with the
components laid out in a line is sometimes called a one-
line or single-line diagram.
Most schematic diagrams are more complicated
than the one shown in figure 2-17. The more
complicated ones can be broken down into one-line
diagrams, circuit by circuit. You can draw (or freehand
sketch) your own one-line diagram by tracing only one
circuit, component by component, through a
multicircuit schematic, using the symbols in figure 2-6.
Circuits "A" and "B" in figure 2-18 show only the
control circuit from figure 2-16 laid out in one-line
form. From these simple circuits, it is easy to see that as
soon as the start button is pushed, the M coil
(operating coil of the motor controller) will be
energized. The operating coil is now held closed
through the "M" contacts.
Your own freehand sketches can help you
understand other types of diagrams as well as the
schematic. You may vary these sketches to suit your
needs. You may draw a one-line diagram, using
symbols, from a wiring diagram, an isometric diagram,
or a connection diagram, as long as all the necessary
details are there for you to convert to lines and symbols.
Figure 2-18.One-line diagram of a motor control circuit.