Latching circuit (seal-in logic) in PLC Programming

Introduction

A latching circuit keeps a machine or device running even after you release the start button. It helps control machines safely and easily.

Starting a motor that should keep running until you press stop.

Turning on a light that stays on until you switch it off.

Controlling a pump that must stay active after a button press.

Keeping a conveyor belt moving without holding the start button.

Automating a process that needs to stay on until manually stopped.

Syntax

PLC Programming

(Start_Button OR Output_Memory) AND NOT Stop_Button -> Output_Memory

The Output_Memory is a memory bit that remembers the output state.

The circuit uses the output itself to keep the output ON after start.

Examples

Basic ladder logic showing start button (normally open), stop button (normally closed), and output coil with seal-in contact.

PLC Programming

Start PB --| |----+----( ) Output
Stop PB  --|/|    |
Output    ---------+

Simple boolean logic equivalent of the latching circuit.

PLC Programming

IF (Start_Button OR Output) AND NOT Stop_Button THEN Output = TRUE ELSE Output = FALSE

Sample Program

This program simulates a latching circuit in PLC Structured Text. It shows how the output stays ON after pressing start until stop is pressed.

PLC Programming

(* PLC Structured Text Example for Latching Circuit *)
VAR
  Start_Button : BOOL := FALSE;
  Stop_Button : BOOL := TRUE;
  Output : BOOL := FALSE;
END_VAR

(* Simulation loop for 5 cycles *)
FOR i := 1 TO 5 DO
  (* Simulate pressing start on cycle 2 *)
  IF i = 2 THEN
    Start_Button := TRUE;
  ELSE
    Start_Button := FALSE;
  END_IF;

  (* Simulate pressing stop button on cycle 4 *)
  IF i = 4 THEN
    Stop_Button := FALSE;
  ELSE
    Stop_Button := TRUE;
  END_IF;

  (* Latching logic *)
  Output := (Start_Button OR Output) AND Stop_Button;

  (* Print output state *)
  IF Output THEN
    WriteLn(CONCAT('Cycle ', INT_TO_STRING(i), ': Output is ON'));
  ELSE
    WriteLn(CONCAT('Cycle ', INT_TO_STRING(i), ': Output is OFF'));
  END_IF;
END_FOR;

Important Notes

The stop button is usually wired as normally closed to break the circuit when pressed.

The output memory (seal-in contact) keeps the output ON after start is released.

Always test latching circuits carefully to avoid machines running unexpectedly.

Summary

A latching circuit keeps a device ON after pressing start until stop is pressed.

It uses the output itself to hold the ON state, called seal-in logic.

Common in motor control and automation for safe and easy operation.

Practice

1. What is the main purpose of a latching circuit (seal-in logic) in PLC programming?

easy

A. To keep a device ON after pressing start until stop is pressed

B. To immediately turn off a device when start is pressed

C. To reset all outputs when the PLC powers on

D. To count the number of times a button is pressed

Solution

1. Step 1: Understand the function of latching circuits

   Latching circuits keep an output ON even after the start input is released.

2. Step 2: Identify the stop input role

   The stop input breaks the latch, turning the output OFF safely.

3. Final Answer:

   To keep a device ON after pressing start until stop is pressed -> Option A

4. Quick Check:

   Latching circuit = Keeps device ON until stopped [OK]

Hint: Remember: latch holds output ON until stop breaks it [OK]

Common Mistakes:

• Thinking latch turns device OFF immediately after start
• Confusing latch with reset logic
• Ignoring the stop input function

2. Which of the following is the correct ladder logic rung for a latching circuit using a start pushbutton Start, stop pushbutton Stop, and output coil Motor?

easy

A. |---[Stop]---[Start]---(Motor)---|
| +----(Motor)---------|

B. |---[Start]---[Stop]---(Motor)---|
| | |
| +----(Motor)---------|

C. |---[Stop]---[Start]---(Motor)---|
| | |
| +----(Motor)---------|

D. |---[Start]---[Stop]---(Motor)---|
| +----(Motor)---------|

Solution

1. Step 1: Check the order of stop and start contacts

   Stop is normally closed and should be first to allow breaking the circuit; start is normally open and second.

2. Step 2: Confirm the seal-in contact

   The output coil Motor is used as a contact in parallel with Start to latch the circuit.

3. Final Answer:

   Stop first, then Start, with Motor contact in parallel to Start -> Option C

4. Quick Check:

   Stop before Start, Motor seals in [OK]

Hint: Stop contact always first; Motor seals in after Start [OK]

Common Mistakes:

• Placing Start before Stop contact
• Missing Motor contact in parallel
• Confusing normally open and normally closed contacts

3. Given this ladder logic snippet:

|---[Stop]---[Start]---(Motor)---|
|          |                   |
|          +----(Motor)---------|

If the Start button is pressed and released, then the Stop button is pressed, what is the state of Motor output?

medium

A. Motor toggles ON and OFF with each Start press

B. Motor remains ON after Start is released and turns OFF when Stop is pressed

C. Motor never turns ON because Stop is first

D. Motor turns ON only while Start is pressed and OFF immediately after release

Solution

1. Step 1: Analyze pressing Start

   Pressing Start energizes Motor coil, which closes Motor contact in parallel, latching the circuit ON.

2. Step 2: Analyze pressing Stop

   Pressing Stop opens the circuit, breaking the latch and turning Motor OFF.

3. Final Answer:

   Motor stays ON after Start release, OFF when Stop pressed -> Option B

4. Quick Check:

   Latching holds Motor ON until Stop breaks it [OK]

Hint: Start latches ON, Stop breaks latch OFF [OK]

Common Mistakes:

• Thinking Motor turns OFF immediately after Start release
• Ignoring Stop button effect
• Confusing latch with toggle

4. Identify the error in this ladder logic for a latching circuit:

|---[Start]---[Stop]---(Motor)---|
|          |                   |
|          +----(Motor)---------|

medium

A. Start contact should be normally closed

B. Motor coil is missing from the rung

C. Motor contact is not in parallel with Stop

D. Start and Stop contacts are in wrong order; Stop should be first

Solution

1. Step 1: Check contact order

   Stop contact should be first and normally closed to allow breaking the circuit safely.

2. Step 2: Confirm Start contact position

   Start contact should be after Stop and normally open to energize Motor.

3. Final Answer:

   Start and Stop contacts are in wrong order; Stop should be first -> Option D

4. Quick Check:

   Stop first, then Start for correct latch [OK]

Hint: Stop contact always first to break circuit safely [OK]

Common Mistakes:

• Placing Start before Stop
• Confusing normally open/closed contacts
• Missing Motor contact in parallel

5. You want to control a motor with a latching circuit but add a safety feature: the motor should only latch ON if a safety switch Safety is closed. Which ladder logic modification correctly adds this condition?

hard

A. Add Safety contact in series before Start and Motor contacts

B. Add Safety contact in parallel with Start

C. Add Safety contact in parallel with Stop

D. Add Safety contact in series after Motor coil

Solution

1. Step 1: Understand safety switch role

   Safety must be closed to allow motor start; it should be in series to block power if open.

2. Step 2: Place Safety contact correctly

   Putting Safety in series before Start and Motor ensures motor cannot latch if Safety is open.

3. Final Answer:

   Add Safety contact in series before Start and Motor contacts -> Option A

4. Quick Check:

   Safety in series blocks motor if open [OK]

Hint: Series Safety contact blocks motor if open [OK]

Common Mistakes:

• Placing Safety in parallel, bypassing it
• Putting Safety after Motor coil, ineffective
• Confusing series and parallel placement