Ladder-logic Tutorial

Relay Ladder Logic

Relay Ladder Logic Control ►

39.1

Normally Open Schematics

Logic control is used with relatively simple ON/OFF systems - like pneumatics 120 VAC PB-1

CR-1

Limit Switch

Momentary Contact Pushbutton

Pressure Switch

LS-1 CR-1A

Sol. A

LS-1

CR-1B

Pneumatic System

SOL-A

Relay Ladder Logic (RLL) Control

Normally Closed Schematics

Limit Switch

Momentary Contact Pushbutton

Manual Switch

Pressure Switch

Contacts

Control Relay - Not Activated NC contact

Manual Switch

Contacts

Output Schematics

Control Relay Coil

Solenoid Coil

Annunciator (Horn)

Lamp

Control Relay - Activated

Current can flow thru NC and Common

NC contact

No current can flow thru NC and Common

Common

Common

NO contact

NO contact

current flowing in coil

no current through coil

"control relay" activated

"control relay" not activated

No current can flow thru NO and Common

Vpower 

Current can flow thru NO and Common

Vpower 

No current thru CR

Current thru CR

Relay Ladder Logic

Why is it called “Logic Control?”  IF (PB-1 is pressed) AND (LS-2 is activated) THEN (SOL-2 will be turned on) PB-1

LS-2

IF there is continuity across the inputs

 “OR” Example IF (PB-1 is NOT pressed) OR (LS-1 is activated) THEN (CR-6 will be turned on)

SOL-2

CR-6

PB-1

LS-1

THEN current will flow through the output

 “One Shot” - Single Stroke ►

39.2

 “One Shot” - Step #1

Pressing the pushbutton PBPB-1 will cause the cylinder to extend and retract one time

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Pressing the momentary contact pushbutton PBPB-1 energizes the control relay CR -1

120 VAC PB-1

Sol. A

LS-1

CR-1A

CR-1

LS-1

CR-1B

Pneumatic System

120 VAC PB-1

Sol. A

LS-1

SOL-A

CR-1A

LS-1

CR-1B

SOL-A

“Ladder” Logic Control

 “One Shot” - Step #2

 “One Shot” - Step #3

 After control relay CR -1 energizes, normally open contacts CR -1A and CR -1B activate

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Control relay CR -1 is now energized by a 2nd path, solenoid SOLSOL- A  A also activates

120 VAC PB-1

Sol. A

CR-1

LS-1

CR-1A

CR-1B

120 VAC CR-1

LS-1

PB-1

Sol. A

SOL-A

LS-1

CR-1A

CR-1B

CR-1

LS-1

SOL-A

Relay Ladder Logic

 “One Shot” - Step #4 ►

39.3

 “One Shot” - Step #5

PBPB-1 is released, but control relay CR CR -1 is still energized by the 2nd path (“hold” circuit)

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Solenoid A shifts the valve spool to the right, and the cylinder begins to extend

120 VAC PB-1

Sol. A

LS-1

CR-1A

120 VAC CR-1

LS-1

CR-1B

PB-1

Sol. A

SOL-A

Sol. A

LS-1

CR-1A

►

SOL-A

With control relay CR -1 dede-activated, the contacts CR -1A and CR -1B return to their normally open state 120 VAC

CR-1

LS-1

CR-1B

LS-1

 “One Shot” - Step #7

Cylinder activates the normally closed limit switch switch LSLS-1, which “kills” the hold circuit for control relay CR -1 120 VAC PB-1

CR-1A

CR-1B

 “One Shot” - Step #6 ►

LS-1

CR-1

PB-1

Sol. A

LS-1

SOL-A

CR-1A

CR-1

LS-1

CR-1B

SOL-A

Solenoid SOL-A is no longer energized

 “One Shot” - Step #8 ►

 “One Shot” - Step #9

CR -1B is now open, SOLSOL- A  A is dede-activated, spring returns valve to default state

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Cylinder begins to retract, and “rolls off” of  LSLS-1, which returns to its N.C. N.C. state

120 VAC PB-1

Sol. A

LS-1

CR-1A

CR-1B

120 VAC CR-1

LS-1

PB-1

Sol. A

SOL-A

LS-1

CR-1A

CR-1B

CR-1

LS-1

SOL-A