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Chapter 3
Electrical Circuits and PLCs
Electrical Symbols and Diagrams • Standard
Electrical Symbols • PLC Programming Symbols •
Pictorial Drawings • Wiring Diagrams • Line (Ladder)
Diagrams • Logic Functions • AND Circuit Logic •
OR Circuit Logic • NOT Circuit Logic • NOR Circuit
Logic • NAND Circuit Logic • Electrical Wiring
Methods • Direct Hardwiring • Hardwiring Using
Terminal Strips • PLC Wiring
Chapter 3 — Electrical Circuits and PLCs
Switch symbols are
drawn to provide a visual
indication of how each
switch operates in a
circuit.
Chapter 3 — Electrical Circuits and PLCs
Any normally open (NO) switch that is manually
operated has the operator of the switch drawn above
the terminals, and any normally closed (NC) switch
that is manually operated has the operator of the
switch drawn below the terminals.
Chapter 3 — Electrical Circuits and PLCs
Electrical switches
are drawn in normal
condition or activated
condition on
electrical prints.
Chapter 3 — Electrical Circuits and PLCs
Switches are drawn on prints in the operating
condition the switch is most likely to be in at any point
in time.
Chapter 3 — Electrical Circuits and PLCs
Not understanding how
manual, mechanical, and
automatic switch symbols
are drawn makes circuit
understanding and
troubleshooting difficult.
Chapter 3 — Electrical Circuits and PLCs
Solid-state switches start and stop the flow of
electricity in a circuit without the use of moving parts.
Solid-state switch symbols show the actual switch
symbol in a diamond shape.
Chapter 3 — Electrical Circuits and PLCs
Programming symbols
for input devices are
drawn as generic
normally open or
normally closed
contacts, and all output
components are drawn
as parentheses.
Chapter 3 — Electrical Circuits and PLCs
Special symbols are used in PLC
programming when a PLC has
advance control functions.
Chapter 3 — Electrical Circuits and PLCs
Pictorial drawings are used to
provide a visual picture of
devices, components, and
wiring.
Chapter 3 — Electrical Circuits and PLCs
Wiring diagrams
show the connection
of all devices and
components to a
PLC.
Chapter 3 — Electrical Circuits and PLCs
Thermocouples are
connected to analog PLC
input terminals because
thermocouples produce a
voltage proportional to the
measured temperature.
Chapter 3 — Electrical Circuits and PLCs
Line diagrams use
standard electrical
symbols to indicate what
types of input devices
and output components
are being used in a
circuit.
Chapter 3 — Electrical Circuits and PLCs
Standard line diagrams
and PLC programming
diagrams are similar,
although different
symbols are used to
represent the same
device or component.
Chapter 3 — Electrical Circuits and PLCs
All electrical control
circuits that use any
type of switches to
control loads are
comprised of
switching logic
functions.
Chapter 3 — Electrical Circuits and PLCs
All input devices of an AND logic
circuit must be activated in order
to energize the output
component.
Chapter 3 — Electrical Circuits and PLCs
Only one input of an OR logic
circuit is activated in order to
energize the output component.
Chapter 3 — Electrical Circuits and PLCs
The input of a NOT logic
circuit must be deactivated in
order to energize the output
component.
Chapter 3 — Electrical Circuits and PLCs
Only one input of a NOR logic
circuit must be deactivated in
order to energize the output
component.
Chapter 3 — Electrical Circuits and PLCs
All input devices of a NAND logic
circuit must be deactivated in
order to energize the output
component.
Chapter 3 — Electrical Circuits and PLCs
In direct hardwired
circuits, the power circuit
and control circuit are
wired point-to-point.
Chapter 3 — Electrical Circuits and PLCs
To add forward rotation
indicator lamps and reverse
rotation indicator lamps to a
direct hardwired circuit is
difficult because the exact
connection points for the
lamps must be found.
Chapter 3 — Electrical Circuits and PLCs
To make modifications to
direct hardwired circuits
typically requires the
removal and/or addition
of circuit wiring.
Chapter 3 — Electrical Circuits and PLCs
Hardwiring using terminal
strips dramatically
simplifies circuit
troubleshooting.
Chapter 3 — Electrical Circuits and PLCs
Modifications are easier
with terminal strips
because most, if not all,
of the wires required to
make a change are
disconnected and
reconnected at the
terminal strip.
Chapter 3 — Electrical Circuits and PLCs
When using a PLC to
control a circuit, all input
devices are wired to the
input module of the PLC,
and all output
components are wired to
the output module of the
PLC.