Just For Starters
by Mark Balch
Basics For Beginners
Just For Starters
Reading Schematic Diagrams
People communicate their
ideas via written text, yet it is
true that a picture is worth a
thousand words. Electrical circuits
are commonly documented in
graphical form to describe their
specific components and connections.
These pictorial representations are
termed “schematic diagrams”
because they explain functionality
rather than serving as a photograph
of a circuit board. Reading and
interpreting schematic diagrams is
an important skill for anyone who
wants to work with electronic circuits.
There is no single set of rules
governing how schematic diagrams
are created. There are almost as
many styles as there are engineers.
A common stylistic difference is
how each type of component (e.g.,
resistor or transistor) is drawn.
Fortunately, schematic diagrams
tend to share many common
attributes.
Once you’ve learned the basics
and have seen a few different styles,
you can usually decipher a new style
with relative ease. Every so often,
however, we all come across a
mystery. Unraveling that mystery
may be done by its context or
through descriptive text that may
accompany the schematic diagram.
This article provides a quick
overview of how schematic diagrams
are commonly drawn.
and poor printing/scanning processes
can lead to confusion as to whether
or not intersecting wires are really
meant to connect. Some schematic
diagrams clarify this by placing a
“bump” at the intersection of two
non-connecting wires.
Power and Ground
Wires
Wires are a basic element of
schematic diagrams — they attach to
and connect every electrical
component. A wire is usually drawn
as a straight line with neat 45° or 90°
bends. Some diagrams may have
curved wires, but many curved wires
can start to look like spaghetti very
quickly. Figure 1 shows a variety of
wire representations connecting
generic components that are
represented as rectangles.
A small, filled circle or dot
indicates a connection between two
intersecting wires. Intersecting wires
without a dot do not have an electrical
connection. Small
intersection dots
Figure 1. Wire Representations.
Figure 2. Power Nodes.
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Electrical circuits require power to
operate and each schematic diagram
shows how its components are
powered. There are many styles for
representing power nodes, some of
which are shown in Figure 2. Arrows
are most common and they point up
or down, depending on whether the
voltage level is positive or negative.
Tee and inverted-tee symbols are
sometimes used in place of arrows.
Some people may draw circles.
Regardless of the symbol used, power
nodes are often labeled with their
voltage (e.g., + 5 V, - 12 V, etc.) or with
a variable (e.g., +V, -VPOWER, etc.).
Explicit ground nodes are found
in most electrical circuits, although
some circuits may refer to ground as
a variable, such as GND. Ground
nodes are drawn in
varying styles, but
almost always point
downward with multiple
small lines, as shown in
Figure 3. Some circuits
contain multiple ground
nodes (e.g., earth and
signal ground) and each
ground is distinguished
by using a different
symbol. There is no universal standard (though
there are standards in
MAY 2004
