Latching circuit (seal-in logic) in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a latching circuit (seal-in logic) in PLC programming?

A latching circuit is a control logic that keeps an output ON even after the input that started it is turned OFF. It 'seals in' the output until a reset or stop command is given.

beginner

Why do we use seal-in logic in automation?

Seal-in logic helps keep machines or processes running continuously without needing to hold the start button. It improves safety and convenience by maintaining the output state until stopped.

intermediate

In a seal-in circuit, what role does the output contact play?

The output contact acts like a memory. When the output turns ON, its contact closes and keeps the circuit energized even if the start button is released.

beginner

How do you stop a latched output in a seal-in circuit?

You use a stop input or reset command that breaks the circuit, opening the seal-in path and turning the output OFF.

intermediate

Show a simple ladder logic example of a seal-in circuit.

Start Button (NO) ----+----( ) Output Coil | Output Contact (NO) ----+

What does a seal-in circuit do in PLC programming?

A Turns output ON only while start input is pressed

B Keeps output ON after start input is released

C Turns output OFF immediately after start input is pressed

D Prevents output from turning ON

Seal-in logic keeps the output ON even after the start input is released by using the output's own contact to maintain the circuit.

Which component in a seal-in circuit acts like memory to keep the output ON?

A Output contact

B Stop button

C Power supply

D Start button

The output contact closes when the output is ON, keeping the circuit energized and acting like memory.

How do you turn OFF a latched output in a seal-in circuit?

A Press the start button again

B Disconnect the power supply

C Use a stop or reset input to break the circuit

D Wait for the output to turn off automatically

A stop or reset input breaks the seal-in path, turning the output OFF.

What happens if the output contact in a seal-in circuit fails to close?

A The circuit will short-circuit

B The output will latch ON

C The output will turn OFF immediately

D The output will not stay ON after start is released

If the output contact does not close, the circuit cannot maintain power to the output after the start button is released.

Which of these is a real-life example of seal-in logic?

A Pressing a light switch once to turn the light ON until switched OFF

B Pressing a button that only works while held down

C Holding a doorbell button to ring the bell

D A timer that turns off automatically

Pressing a light switch once to keep the light ON until switched OFF is like seal-in logic, where the output stays ON after the input is released.

Explain how a latching circuit (seal-in logic) works in simple terms.

Think about how a machine stays running after you press start and only stops when you press stop.

Describe a real-world situation where seal-in logic is useful and why.

Imagine a machine or light that should stay ON without holding a button.

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