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Chapter 11
Troubleshooting PLC Hardware
PLC Hardware Problems • Power Supply Problems •
Troubleshooting PLC Input Sections or Modules •
Troubleshooting PLC Output Sections or Modules
Chapter 11 — Troubleshooting PLC Hardware
PLCs are designed for use with all
common supply voltages, such as
230 V, 208 V, 115 V, and 24 V.
Chapter 11 — Troubleshooting PLC Hardware
When problems occur with
PLC power supplies, input
and/or output modules, or
any system components, a
check of the supplied
power should be
performed over time using
a test instrument with a
recording function.
Chapter 11 — Troubleshooting PLC Hardware
Short circuits typically occur
when conductor insulation is
damaged, which allows a
current-carrying conductor to
come in contact with any
grounded noncurrentcarrying metal parts. An
overload is an overcurrent
condition that occurs when
circuit current exceeds
normal PLC operating
current and/or designed
circuit current.
Chapter 11 — Troubleshooting PLC Hardware
Voltage measurements
can be used to test fuses
that are connected to a
circuit.
Chapter 11 — Troubleshooting PLC Hardware
Resistance measurements are used to test fuses that
have been removed from a circuit.
Chapter 11 — Troubleshooting PLC Hardware
Circuit breakers perform the same function as fuses
and are basically tested the same way.
Chapter 11 — Troubleshooting PLC Hardware
Control transformers are typically
used to step down 115 VAC,
208 V, 230 V, or 460 V to 12 VAC
or 24 VAC for control circuit use
in a PLC enclosure.
Chapter 11 — Troubleshooting PLC Hardware
When the power rating of
a transformer is
exceeded by placing too
great a load on the
transformer, the voltage
on the secondary side
will start to decrease and
circuit problems will
develop.
Chapter 11 — Troubleshooting PLC Hardware
PLCs can have a single
fixed voltage rating or a
dual voltage rating. Dual
voltage rated PLCs
(115/230 VAC) have a
selector switch (or
movable links) to set the
PLC to one of the
possible voltage ratings.
Chapter 11 — Troubleshooting PLC Hardware
Output power supplies of
PLCs are typically used
to supply voltage to input
devices connected to the
PLCs.
Chapter 11 — Troubleshooting PLC Hardware
The listed current rating
of a power supply must
be accepted as the
maximum amount of
current that can safely be
provided from a PLC
output power supply.
Chapter 11 — Troubleshooting PLC Hardware
When an input signal is
sent to a PLC, the PLC
conditions, filters, and
optically isolates the
signal.
Chapter 11 — Troubleshooting PLC Hardware
When troubleshooting PLC input sections,
programming diagrams and manufacturer
troubleshooting charts indicate how input devices are
connected and which output components are being
controlled.
Chapter 11 — Troubleshooting PLC Hardware
Testing input modules
requires that power
supplies and input
devices be tested and
that status lights of input
modules and symbols on
computer monitors be
checked.
Chapter 11 — Troubleshooting PLC Hardware
All input devices and the PLC program must operate
correctly in order for an automated circuit to operate
properly.
Chapter 11 — Troubleshooting PLC Hardware
Proper heat sinking and cooling
is required with solid-state
devices to eliminate any
potential heat problems.
Chapter 11 — Troubleshooting PLC Hardware
The current that flows
through an “open” solidstate switch is called
leakage current. Leakage
current can turn on the
input circuitry of a PLC,
affecting system
operation.
Chapter 11 — Troubleshooting PLC Hardware
Testing the leakage
current of solid-state
devices requires that the
OFF state leakage
current measurement be
compared to the
specified minimum
operating current of the
PLC input module.
Chapter 11 — Troubleshooting PLC Hardware
Signals from the CPU of
a PLC are sent through
logic circuits to output
section status lights and
opto-isolation circuits
before being sent to
output terminals.
Chapter 11 — Troubleshooting PLC Hardware
When troubleshooting PLC output sections,
programming diagrams and manufacturer
troubleshooting charts help the
troubleshooter/technician understand how the input
devices are connected and which output components
are being controlled.
Chapter 11 — Troubleshooting PLC Hardware
Testing output modules
requires that power
supplies and output
components be tested
and that status lights of
output modules and
symbols on computer
monitors be checked.
Chapter 11 — Troubleshooting PLC Hardware
All output components and the PLC program must
operate correctly in order for an automated system to
operate properly.