Relay Ladder Logic Examples

1
Relay Ladder Logic Examples

• Digital Logic

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Three Wire Control

• Goal
• Push a button to start a motor.
• The motor runs until another push button stops
  it.
• Motor will not restart automatically after a
  power outage.

3
Push Button Station
4
Maintaining Circuit Operation

• In order to energize the relay the following
  conditions must be met
• Stop not pushed AND start pushed.
• OR
• stop not pushed AND R contact closed.
• AND OR represent COMBINATIONAL logic.

5
Multiple Start/Stop
start 2
L1
L2
start 1
M
O.L.
M
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Control Functions

• Either start 1 OR start 2 will energize the coil.
  
• Start functions are in parallel.
• Parallel both ends in common.
• Either stop 1 OR stop 2 will de-energize the
  coil.
• Stop functions are in series.
• Series one after another.

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Example Anxiety Level Alarm

• Define Problem
• Green light one exam coming.
• Yellow light two exams coming.
• Red light three exams coming.
• Only one light should be on at a time.

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List Outputs

• Green Light
• Yellow Light
• Red Light

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List Inputs

• Digital switch(D) , closed when a test is
  announced. (N.O.)
• Fabrication switch, closed when a test is
  announced. (N.O.)
• Math switch, closed when a test is announced.
  (N.O.)

10
Internal Logic Relays

• Since each input switch will be used several
  times, each switch will control a relay with
  multiple contacts that can be either normally
  open or normally closed.

11
Draw Rails and Outputs
12
Determine inputs to Lights
DFMFDMMDF
DFMDMFMFD
DFM
13
Finished Ladder
14
Typical Air ConditionerControl Side
load circuit
on
fan switch
auto
thermostat
FR fan relay CC compressor contactor
15
Air Conditioner Operation

• for fan motor to go on
• manual switch must be closed
• OR auto switch AND thermostat

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Drawn as RLL
thermostat
on
TR
auto
FR fan relay CC compressor contactor
TR
17
Interlocked Latching Coils
Push top start button. Coil L will energize.
When the top button is released, the armature
will be held in place by a lever. Push bottom
button. The lever holding the armature will
release and the armature moves to its unlatched
position.
start
start
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Solenoids

• Takes sustained current to hold solenoid in
  place.
• Used for mechanical switching of.
• Valves.
• Relays.