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Chapter 11 Troubleshooting PLC Hardware

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Title: Chapter 11 Troubleshooting PLC Hardware


1
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.

11
  • 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.
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