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Chapter 2
Electrical Principles and PLCs
Programmable Logic Controllers and Electrical
Principles • Voltage • Current • Resistance • Series
Circuits • Parallel Circuits
Chapter 2 — Electrical Principles and PLCs
Basic rules must be
followed when installing
output devices to PLCs.
Output devices cannot be
installed in series from
PLC output terminals.
Chapter 2 — Electrical Principles and PLCs
When all PLC system
hardware is functioning
but the system is not
operating correctly, the
problem may be with the
software, such as an
incorrect program,
incorrect symbols, or the
program not being keyed
correctly into the PLC.
Chapter 2 — Electrical Principles and PLCs
A PLC hardware
problem is caused by
problems with input
devices, output
devices, wiring, or a
short or open
somewhere in the
system.
Chapter 2 — Electrical Principles and PLCs
The voltage supplied to
portable PLCs can come
from portable generators,
batteries, or solar panels.
Chapter 2 — Electrical Principles and PLCs
All points in a
DC voltage
circuit have
polarity.
Chapter 2 — Electrical Principles and PLCs
AC voltages are
produced by generators,
which create singlephase or three-phase
sine waves when rotated.
Chapter 2 — Electrical Principles and PLCs
PLCs are powered by
single-phase AC but can
control both single-phase
or three-phase AC loads.
Chapter 2 — Electrical Principles and PLCs
Low, medium, and high
voltages are used in
commercial and industrial
PLC applications.
Chapter 2 — Electrical Principles and PLCs
PLCs have power
supply voltage ratings
that can be a fixed
voltage rating, a
voltage range, or a
dual voltage rating.
Chapter 2 — Electrical Principles and PLCs
PLC input terminals are available
with DC, AC, or DC/AC ratings
that typically use low voltages
such as 12 VDC or 24 VDC.
Chapter 2 — Electrical Principles and PLCs
The output terminal
switches of a PLC are
either mechanical
switches or solid-state
switches.
Chapter 2 — Electrical Principles and PLCs
Typical PLC input circuitry
draws about 5 mA to 20 mA
of current.
Chapter 2 — Electrical Principles and PLCs
A two-wire input switch circuit
uses mechanical (pushbutton) or
solid-state (photoelectric and
proximity) switches, and a threewire input switch circuit uses
solid-state switches.
Chapter 2 — Electrical Principles and PLCs
Switching devices that use
NPN transistors as switching
elements are called current
sinking, negative switching, or
NPN devices.
Chapter 2 — Electrical Principles and PLCs
Switching devices that use PNP
transistors as the switching
elements are called current
sourcing, positive switching, or
PNP devices.
Chapter 2 — Electrical Principles and PLCs
The output sections of
PLCs use transistors,
triacs, and mechanical or
solid-state relays for
switching.
Chapter 2 — Electrical Principles and PLCs
Copper (Cu) is a better
conductor than aluminum
(Al) because copper can
carry more current for a
given size (AWG).
Chapter 2 — Electrical Principles and PLCs
Series circuits have two or more devices or
components connected so there is only one flow path
for current to take.
Chapter 2 — Electrical Principles and PLCs
PLC communications cables connected in series
(daisy-chained) are used to connect individual PLC
processors.
Chapter 2 — Electrical Principles and PLCs
The total resistance of a series circuit increases when
loads are added in series and decreases when loads
are removed.
Chapter 2 — Electrical Principles and PLCs
Electron flow in a series circuit is the same
everywhere in the circuit.
Chapter 2 — Electrical Principles and PLCs
Voltage drop is the
amount of voltage
consumed by a
device or
component as
current passes
through it.
Chapter 2 — Electrical Principles and PLCs
Parallel circuits have two or more devices or
components connected so that there is more than
one flow path for current to take.
Chapter 2 — Electrical Principles and PLCs
Total resistance of a parallel circuit decreases when
loads are added in parallel and increases when loads
are removed.
Chapter 2 — Electrical Principles and PLCs
Total current in a
parallel circuit equals
the sum of the current
through all loads in the
parallel circuit or leg.
Chapter 2 — Electrical Principles and PLCs
The voltage drop
across individual loads
remains the same
when parallel loads
are added or removed.