Output coil in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an output coil in PLC programming?

An output coil is a command in PLC programming that controls an output device, like a light or motor, turning it ON or OFF based on the logic.

beginner

How does an output coil behave when the input condition is TRUE?

When the input condition is TRUE, the output coil energizes, which means it activates the connected output device.

beginner

In ladder logic, what symbol usually represents an output coil?

An output coil is usually shown as a pair of parentheses with a label inside, like ( ) or (Q0.0), representing the output address.

beginner

Can an output coil be used to turn OFF a device? How?

Yes, if the input condition to the output coil is FALSE, the coil de-energizes, turning OFF the connected device.

beginner

Why is the output coil important in automation?

The output coil is important because it links the PLC's decision (logic) to real-world actions by controlling devices like motors, lights, or valves.

What does an output coil do in a PLC program?

A Reads input signals from sensors

B Controls an output device based on logic

C Stores data temporarily

D Performs mathematical calculations

An output coil controls an output device like a motor or light depending on the logic conditions.

In ladder logic, how is an output coil usually represented?

A A pair of parentheses with a label inside

B A rectangle with input lines

C A triangle pointing right

D A circle with a cross inside

Output coils are shown as parentheses with the output address inside, like (Q0.0).

What happens to the output coil when the input condition is FALSE?

A The coil de-energizes

B The coil energizes

C The coil stores data

D The coil sends a signal to input

When input is FALSE, the output coil de-energizes, turning OFF the connected device.

Which of these is controlled by an output coil?

A A sensor input

B A programming instruction

C A motor or light

D A memory register

Output coils control devices like motors or lights, not inputs or memory.

Why do we use output coils in automation?

A To read sensor data

B To perform calculations

C To store program code

D To activate devices based on logic

Output coils activate devices like motors or lights based on the PLC's logic decisions.

Explain what an output coil is and how it works in a PLC program.

Think about how the PLC turns devices ON or OFF.

Describe the symbol used for an output coil in ladder logic and what it represents.

Look for the shape that looks like ( ) in ladder diagrams.

Practice

1. What is the main purpose of an output coil in PLC programming?

easy

A. To control devices like lights or motors by turning them ON or OFF

B. To read input signals from sensors

C. To store data temporarily

D. To perform mathematical calculations

Solution

1. Step 1: Understand the role of output coils

   Output coils are used to control physical devices by turning them ON or OFF based on program logic.

2. Step 2: Differentiate from other PLC elements

   Input signals are read by input instructions, data storage is done by registers, and calculations are done by math instructions, not output coils.

3. Final Answer:

   To control devices like lights or motors by turning them ON or OFF -> Option A

4. Quick Check:

   Output coil controls devices = B [OK]

Hint: Output coils always control devices ON/OFF [OK]

Common Mistakes:

• Confusing output coils with input instructions
• Thinking output coils store data
• Assuming output coils perform calculations

2. Which of the following is the correct syntax to define an output coil named Motor in ladder logic?

easy

A. ( ) Motor

B. COIL Motor

C. OUT Motor

D. SET Motor

Solution

1. Step 1: Recognize ladder logic output coil symbol

   In ladder logic, output coils are represented by parentheses around the coil name, like ( ) Motor.

2. Step 2: Compare other options

   OUT and SET are instructions but not standard coil syntax; COIL is not a typical keyword.

3. Final Answer:

   ( ) Motor -> Option A

4. Quick Check:

   Output coil uses parentheses = A [OK]

Hint: Output coils use parentheses in ladder logic [OK]

Common Mistakes:

• Using keywords like OUT or SET instead of parentheses
• Confusing coil syntax with input instructions
• Omitting parentheses around coil name

3. Given the ladder logic snippet:

( ) Light
--[ ] Switch

What happens when the Switch input is ON?

medium

A. Nothing changes

B. The Light output coil turns ON

C. The Switch input turns OFF

D. The Light output coil turns OFF

Solution

1. Step 1: Understand the ladder logic flow

   The normally open contact [ ] Switch controls the output coil ( ) Light. When Switch is ON (closed), current flows to Light coil.

2. Step 2: Determine output coil state

   Since Switch is ON, the Light coil energizes and turns ON the Light output.

3. Final Answer:

   The Light output coil turns ON -> Option B

4. Quick Check:

   Switch ON energizes Light coil = D [OK]

Hint: Input ON energizes output coil ON [OK]

Common Mistakes:

• Thinking output coil turns OFF when input is ON
• Confusing input and output roles
• Assuming no change occurs

4. Identify the error in this ladder logic snippet:

( ) Motor
--[ ] Start
--[/] Stop

where [/] is a normally closed contact. The Motor never stays ON even when Start is pressed and Stop is released.

medium

A. The output coil syntax is incorrect

B. The Stop contact should be normally open, not normally closed

C. The contacts are not connected in parallel

D. The Start contact is wired incorrectly

Solution

1. Step 1: Analyze contact wiring logic

   The Start and Stop contacts are in series, so both must be ON for Motor coil to energize.

2. Step 2: Understand typical start/stop control

   Usually, Start and Stop contacts are wired in parallel or with a latch to maintain Motor ON after Start is pressed.

3. Step 3: Identify wiring issue

   Because contacts are in series without latching, Motor never stays ON; wiring should be parallel or use latch logic.

4. Final Answer:

   The contacts are not connected in parallel -> Option C

5. Quick Check:

   Start/Stop need parallel or latch wiring = C [OK]

Hint: Start/Stop usually wired parallel or latched [OK]

Common Mistakes:

• Assuming output coil syntax is wrong
• Changing Stop contact type incorrectly
• Not recognizing wiring arrangement

5. You want to control a valve using an output coil named Valve. The valve should turn ON only if both Pressure_OK and Temperature_OK inputs are ON. Which ladder logic snippet correctly implements this?

hard

A. ( ) Valve
--[/] Pressure_OK
--[ ] Temperature_OK

B. ( ) Valve
--[ ] Pressure_OK
--[/] Temperature_OK

C. ( ) Valve
--[ ] Pressure_OK
--| | Temperature_OK

D. ( ) Valve
--[ ] Pressure_OK
--[ ] Temperature_OK

Solution

1. Step 1: Understand AND condition in ladder logic

   To turn Valve ON only if both inputs are ON, the contacts must be in series, both normally open.

2. Step 2: Check each option

   ( ) Valve
   --[ ] Pressure_OK
   --[ ] Temperature_OK
   

   has both [ ] Pressure_OK and [ ] Temperature_OK in series, correct for AND logic.

   ( ) Valve
   --[ ] Pressure_OK
   --[/] Temperature_OK
   

   uses normally closed for Temperature_OK, wrong.

   ( ) Valve
   --[ ] Pressure_OK
   --| | Temperature_OK
   

   uses invalid symbol | |.

   ( ) Valve
   --[/] Pressure_OK
   --[ ] Temperature_OK
   

   uses normally closed for Pressure_OK, wrong.

3. Final Answer:

   with both normally open contacts in series -> Option D

4. Quick Check:

   Both inputs ON in series = Valve ON [OK]

Hint: AND logic uses series normally open contacts [OK]

Common Mistakes:

• Using normally closed contacts incorrectly
• Confusing AND with OR logic
• Using invalid contact symbols