Latching circuit (seal-in logic) in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a basic seal-in logic in ladder diagram

Given a seal-in circuit where a start pushbutton (I0.0) energizes a coil (Q0.0) and the coil's own contact seals itself in, what is the output Q0.0 after the start button is pressed and released?

PLC Programming

(* Ladder logic description: *)
(* I0.0 --| |----+----( Q0.0 )
               |
               +----| | (Q0.0) *)

A Q0.0 toggles ON and OFF repeatedly

B Q0.0 turns OFF immediately after releasing I0.0

C Q0.0 remains ON after releasing I0.0

D Q0.0 never turns ON

💡 Hint

Think about how the coil contact keeps the circuit closed after the start button is released.

The coil's own contact (seal-in contact) keeps the circuit closed, so once the start button energizes the coil, the coil contact maintains power to itself even after the start button is released.

🧠 Conceptual

intermediate

Purpose of a seal-in contact in a latching circuit

What is the main purpose of including a seal-in (latching) contact in a PLC ladder logic circuit?

A To keep the output energized after the input is released

B To immediately turn off the output when input is released

C To prevent the output from ever turning ON

D To reset the PLC program automatically

💡 Hint

Think about what happens when the start button is released but the output remains ON.

The seal-in contact keeps the output coil energized even after the input (start button) is released, effectively latching the output ON.

🔧 Debug

advanced

Identify the error in this seal-in logic snippet

This ladder logic snippet is intended to latch output Q0.1 using input I0.1 as start and I0.2 as stop. What is the error causing Q0.1 to never latch?

PLC Programming

(* Ladder logic: *)
(* I0.1 --| |----+----( Q0.1 )
                 |
                 +----| | (I0.2) ----| | (Q0.1) *)

A The seal-in contact is missing from the circuit

B The stop input contact is normally open instead of normally closed

C The stop input contact is normally closed but placed in series incorrectly

D The start input contact is normally closed instead of normally open

💡 Hint

Stop buttons are usually normally closed contacts to break the circuit when pressed.

The stop input should be a normally closed contact (|/|) so that pressing it opens the circuit and stops the output. If it is normally open, the circuit never completes to latch the output.

🚀 Application

advanced

Design a seal-in circuit with start and stop inputs

Which ladder logic snippet correctly implements a seal-in circuit with start input I0.3, stop input I0.4, and output coil Q0.2?

A

I0.3 --| |----+----|/| (I0.4) ----( Q0.2 )
               |
               +----| | (Q0.2)

B

I0.3 --| |----+----| | (I0.4) ----( Q0.2 )
               |
               +----|/| (Q0.2)

C

I0.3 --|/|----+----|/| (I0.4) ----( Q0.2 )
               |
               +----| | (Q0.2)

D

I0.3 --| |----+----| | (I0.4) ----( Q0.2 )
               |
               +----| | (Q0.2)

💡 Hint

Remember stop input should be normally closed and start input normally open.

Option A correctly uses a normally open start input, a normally closed stop input, and a seal-in contact from the output coil to latch the output.

🧠 Conceptual

expert

Effect of power loss on seal-in circuits

What happens to the output coil state in a seal-in circuit when the PLC loses power and then power is restored?

A The output coil remains ON due to the seal-in contact memory

B The output coil state depends on the last input state before power loss

C The output coil toggles ON and OFF automatically on power restore

D The output coil resets to OFF because seal-in state is not retained without power

💡 Hint

Consider if the coil and contacts are physical or virtual and what happens when power is lost.

Seal-in circuits rely on power to maintain the coil energized. When power is lost, the coil de-energizes and the output resets to 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