Chapter 11 Troubleshooting PLC Hardware

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Chapter 11Troubleshooting PLC Hardware

• PLC Hardware Problems Power Supply Problems
  Troubleshooting PLC Input Sections or Modules
  Troubleshooting PLC Output Sections or Modules

2

• PLCs are designed for use with all common supply
  voltages, such as 230 V, 208 V, 115 V, and 24 V.

3

• 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.

4

• Short circuits typically occur when conductor
  insulation is damaged, which allows a
  current-carrying conductor to come in contact
  with any grounded noncurrent-carrying metal
  parts. An overload is an overcurrent condition
  that occurs when circuit current exceeds normal
  PLC operating current and/or designed circuit
  current.

5

• Voltage measurements can be used to test fuses
  that are connected to a circuit.

6

• Resistance measurements are used to test fuses
  that have been removed from a circuit.

7

• Circuit breakers perform the same function as
  fuses and are basically tested the same way.

8

• 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.

9

• 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.

10

• 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.

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• Output power supplies of PLCs are typically used
  to supply voltage to input devices connected to
  the PLCs.

12

• 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.

13

• When an input signal is sent to a PLC, the PLC
  conditions, filters, and optically isolates the
  signal.

14

• When troubleshooting PLC input sections,
  programming diagrams and manufacturer
  troubleshooting charts indicate how input devices
  are connected and which output components are
  being controlled.

15

• 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.

16

• All input devices and the PLC program must
  operate correctly in order for an automated
  circuit to operate properly.

17

• Proper heat sinking and cooling is required with
  solid-state devices to eliminate any potential
  heat problems.

18

• The current that flows through an open
  solid-state switch is called leakage current.
  Leakage current can turn on the input circuitry
  of a PLC, affecting system operation.

19

• 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.

20

• 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.

21

• 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.

22

• 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.

23

• All output components and the PLC program must
  operate correctly in order for an automated
  system to operate properly.