Latching circuit (seal-in logic) in PLC Programming - Mini Project: Build & Apply

Latching Circuit (Seal-in Logic) in PLC Programming

📖 Scenario: You are working with a simple machine control system. The machine should start when a Start button is pressed and keep running even if the Start button is released. The machine should stop when a Stop button is pressed. This is called a latching circuit or seal-in logic.

🎯 Goal: Build a latching circuit using PLC ladder logic instructions. You will create variables for the Start and Stop buttons, a coil for the Motor, and write the logic to keep the motor running after the start button is released until the stop button is pressed.

📋 What You'll Learn

Create boolean variables for Start, Stop, and Motor

Create a variable SealIn to hold the latched state

Write the seal-in logic using a boolean expression

Output the state of the Motor variable

💡 Why This Matters

🌍 Real World

Latching circuits are used in industrial machines to keep motors or devices running after a start button is released, improving safety and control.

💼 Career

Understanding seal-in logic is essential for PLC programmers and automation engineers who design reliable control systems.

1

Create input and output variables

Create three boolean variables called Start, Stop, and Motor. Initialize Start and Stop to False, and Motor to False.

PLC Programming

# Create boolean variables Start, Stop, and Motor initialized to False
# Your code here

Hint

Use simple assignments like Start = False to create the variables.

2

Create the seal-in variable

Create a boolean variable called SealIn and initialize it to False. This variable will hold the latched state of the motor.

PLC Programming

Start = False
Stop = False
Motor = False
# Create a boolean variable SealIn initialized to False
# Your code here

Hint

Just add SealIn = False below the other variables.

3

Write the seal-in logic

Write the seal-in logic using a boolean expression. Update SealIn to be True if Start is True or if SealIn is already True, but only if Stop is False. Use this expression: SealIn = (Start or SealIn) and not Stop. Then set Motor equal to SealIn.

PLC Programming

Start = False
Stop = False
Motor = False
SealIn = False
# Write seal-in logic to update SealIn and Motor
# Your code here

Hint

Use the exact expression SealIn = (Start or SealIn) and not Stop and then Motor = SealIn.

4

Display the motor state

Print the state of the Motor variable using print(f"Motor is {'ON' if Motor else 'OFF'}").

PLC Programming

Start = False
Stop = False
Motor = False
SealIn = False
SealIn = (Start or SealIn) and not Stop
Motor = SealIn
# Print the motor state
# Your code here

Hint

Use the print statement exactly as shown to display if the motor is ON or OFF.

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