Relay Ladder Logic

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Relay Ladder Logic

• Digital Logic
• ELET 1133

2
Ladder Diagram

• A systematic sketch showing the electrical
  operation of a control system.
• A ladder diagram can show discrete devices,
  switches, relays, motors.
• Or a ladder diagram can be a computerized means
  of handling control operations.

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Relay Ladder Logic

• Describes the use of discrete devices.
• The format for drawing the relay ladder diagram
  is not as strict as the format of a computer
  implemented ladder logic diagram.
• Thus different manufacturers use different
  symbols.
• You must understand context to interpret ladder
  diagrams.

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RLL Symbols

• Distribute symbols handout.
• We will go through these quickly since you have
  the handout.
• Memorization of all of these is not required.

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Manual Switches
SPST single pole, single throw
SPDT single pole, double throw
DPST double pole, single throw. Both switch
together.
DPDT double pole, double throw.
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Relay Symbols
Coil, designated R
Normally Open Contact, NO, associated with coil R
Normally Closed Contact, NC, associated with
coil R
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Momentary Pushbuttons
Normally Open. Downward force will close. Will
spring back to open when force is removed.
Normally Closed. Downward force will open. Will
spring back to closed when force is removed.
NC NO or break-make pushbutton
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Thermostats
When temperature cools, element contracts closing
contacts.
Heating Thermostat
When temperature warms, element expands closing
contacts.
Cooling Thermostat
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Float Switch
NC. Opens when fluid level rises.
NO. Closes when fluid level rises.
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Limit Switches
Normally Open. Object pushing from underneath
will close it.
Normally open, held closed.
Normally Closed. Object pushing from underneath
will open it.
Normally Closed, held open. Object pushing from
underneath holds it open.
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Proximity Sensors
NO and NC Like limit switch, but activate without
physical contact. These use light, inductive or
capacitive sensing.
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Pressure Switches
Normally Open. Pressure from underneath will
close it.
Normally Closed. Pressure from underneath will
open it.
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Indicator Lamps
Letter indicates color. Rred. Wwhite.
W
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Interlocked Latching Coils
Once energized, coil L is held mechanically until
coil U (unlatch) is energized releasing the
holding mechanism. No power is needed to hold the
armature.
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Timing Contacts
TR
On Delay
NCTO Normally Closed Timed Open
NOTC Normally Open Timed Closed
Off Delay
NCTC Normally Closed Timed Close
NOTO Normally Open Timed Open
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Relay Symbol
A single coil may have multiple contacts.
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Ladder Diagram

• Every electric device must have a power supply.
• Usually the power supply is a fixed voltage
  between two lines.
• Power supply could be
• ac house current
• a battery

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Ladder Rails

• A load connected between the two prongs will be
  powered when plugged in.

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Power Rails

• Previously drew the control circuit like this.
• If there were two relay coils connected it would
  be draw thusly

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Power Rails

• The upright part of the ladder.
• Label the rails
• dc plus or ground
• ac line or neutral

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Control Rungs

• Horizontal members of ladder.

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General Ladder Logic Format

• Loads are drawn to the right.
• There is one load per rung.
• Contacts are drawn to the left.
• There may be many contacts in a rung.
• All devices are drawn in their NORMAL condition.

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Simple Example

• AC light switch.
• Rails.
• Control switch to left.
• Bulb as load to right.

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Relay Control Example

• Goal
• Use a relay to turn on a high wattage flood
  light with a miniature switch.
• The switch cannot pass the current required for
  the light.

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Relay Control Example

• Close manual switch.
• Energize Relay.
• Close contacts.
• Light lamp.

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Add Labels
ac line
ac neutral
Miniature Switch
Relay
W
R
Flood Light
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General Ladder Procedure

• List the required outputs (loads).
• Determine which outputs require relay control.
• List the required inputs.
• Draw all outputs connected to the right side
  rail.
• Fill in the controls needed for each rung.