Chapter 2 Electrical Principles and PLCs Programmable Logic

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Chapter 2Electrical Principles and PLCs

• Programmable Logic Controllers and Electrical
  Principles Voltage Current Resistance
  Series Circuits Parallel Circuits

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• Basic rules must be followed when installing
  output devices to PLCs. Output devices cannot be
  installed in series from PLC output terminals.

3

• 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.

4

• A PLC hardware problem is caused by problems with
  input devices, output devices, wiring, or a short
  or open somewhere in the system.

5

• The voltage supplied to portable PLCs can come
  from portable generators, batteries, or solar
  panels.

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• All points in a DC voltage circuit have polarity.

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• AC voltages are produced by generators, which
  create single-phase or three-phase sine waves
  when rotated.

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• PLCs are powered by single-phase AC but can
  control both single-phase or three-phase AC loads.

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• Low, medium, and high voltages are used in
  commercial and industrial PLC applications.

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• PLCs have power supply voltage ratings that can
  be a fixed voltage rating, a voltage range, or a
  dual voltage rating.

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• PLC input terminals are available with DC, AC, or
  DC/AC ratings that typically use low voltages
  such as 12 VDC or 24 VDC.

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• The output terminal switches of a PLC are either
  mechanical switches or solid-state switches.

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• Typical PLC input circuitry draws about 5 mA to
  20 mA of current.

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• A two-wire input switch circuit uses mechanical
  (pushbutton) or solid-state (photoelectric and
  proximity) switches, and a three-wire input
  switch circuit uses solid-state switches.

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• Switching devices that use NPN transistors as
  switching elements are called current sinking,
  negative switching, or NPN devices.

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• Switching devices that use PNP transistors as the
  switching elements are called current sourcing,
  positive switching, or PNP devices.

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• The output sections of PLCs use transistors,
  triacs, and mechanical or solid-state relays for
  switching.

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• Copper (Cu) is a better conductor than aluminum
  (Al) because copper can carry more current for a
  given size (AWG).

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• Series circuits have two or more devices or
  components connected so there is only one flow
  path for current to take.

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• PLC communications cables connected in series
  (daisy-chained) are used to connect individual
  PLC processors.

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• The total resistance of a series circuit
  increases when loads are added in series and
  decreases when loads are removed.

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• Electron flow in a series circuit is the same
  everywhere in the circuit.

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• Voltage drop is the amount of voltage consumed by
  a device or component as current passes through
  it.

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• Parallel circuits have two or more devices or
  components connected so that there is more than
  one flow path for current to take.

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• Total resistance of a parallel circuit decreases
  when loads are added in parallel and increases
  when loads are removed.

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• Total current in a parallel circuit equals the
  sum of the current through all loads in the
  parallel circuit or leg.

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• The voltage drop across individual loads remains
  the same when parallel loads are added or removed.