Digital output wiring (relay, transistor, triac) in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a digital output for a relay coil.

PLC Programming

RelayCoil := [1];

Drag options to blanks, or click blank then click option'

ABOOL

BTRUE

CINT

DFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using data types like BOOL or INT instead of a value.

Setting the output to FALSE which turns the relay off.

The relay coil is energized by setting the output to TRUE.

2fill in blank

medium

Complete the code to activate a transistor output using a variable.

PLC Programming

TransistorOutput := [1];

Drag options to blanks, or click blank then click option'

A0

BFALSE

C5

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using numeric values like 0 or 5 instead of boolean TRUE.

Setting the output to FALSE which keeps it off.

Setting the transistor output to TRUE energizes it.

3fill in blank

hard

Fix the error in the code to correctly energize a triac output.

PLC Programming

TriacOutput := [1];

Drag options to blanks, or click blank then click option'

A0

BFALSE

CTRUE

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using numeric values like 1 or 0 which may not be valid in this context.

Setting the output to FALSE which deactivates the triac.

The triac output must be set to TRUE to energize it properly.

4fill in blank

hard

Fill both blanks to create a condition that energizes the relay only when the input is ON and the safety is enabled.

PLC Programming

IF InputSignal = [1] AND SafetyEnabled = [2] THEN
  RelayCoil := TRUE;
ELSE
  RelayCoil := FALSE;
END_IF;

Drag options to blanks, or click blank then click option'

ATRUE

BFALSE

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using FALSE or numeric values instead of TRUE.

Setting one condition to FALSE which prevents relay activation.

The relay energizes only if both InputSignal and SafetyEnabled are TRUE.

5fill in blank

hard

Fill all three blanks to create a logic that energizes the transistor output when the start button is pressed, the system is ready, and no error is present.

PLC Programming

IF StartButton = [1] AND SystemReady = [2] AND ErrorDetected = [3] THEN
  TransistorOutput := TRUE;
ELSE
  TransistorOutput := FALSE;
END_IF;

Drag options to blanks, or click blank then click option'

AFALSE

BTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Setting ErrorDetected to TRUE which prevents output activation.

Using FALSE for StartButton or SystemReady which stops the output.

The transistor output energizes only if StartButton and SystemReady are TRUE, and ErrorDetected is FALSE.

Practice

1. Which digital output device is best suited for switching high power AC loads safely in a PLC system?

easy

A. Relay

B. Transistor

C. Triac

D. Diode

Solution

1. Step 1: Understand device types for AC loads

   Relays and triacs can switch AC loads, but relays provide galvanic isolation and are safer for high power AC loads.

2. Step 2: Identify the best device for high power AC

   Relays are commonly used for switching high power AC loads safely due to their isolation and robustness.

3. Final Answer:

   Relay -> Option A

4. Quick Check:

   High power AC load control = Relay [OK]

Hint: Relays provide isolation for high power AC loads [OK]

Common Mistakes:

• Confusing transistor with triac for AC switching
• Thinking diode can switch loads
• Assuming triac is always best for high power AC

2. Which of the following is the correct symbol for a transistor output in a PLC wiring diagram?

easy

A. --|>|-- (diode symbol)

B. --( )-- (relay coil symbol)

C. --|>|--| (transistor symbol)

D. --/\/\-- (resistor symbol)

Solution

1. Step 1: Identify common PLC output symbols

   Relay coils are shown as ( ), diodes as |>|, and transistors combine diode and line.

2. Step 2: Match transistor symbol

   The transistor symbol in PLC wiring often shows a diode with a line indicating switching action.

3. Final Answer:

   --|>|--| (transistor symbol) -> Option C

4. Quick Check:

   Transistor symbol = --|>|--| [OK]

Hint: Transistor symbol includes diode plus line [OK]

Common Mistakes:

• Choosing relay coil symbol for transistor
• Confusing diode symbol with transistor
• Selecting resistor symbol incorrectly

3. Consider this PLC ladder logic snippet controlling a relay output:

--[ ]--( )--
 I0.0  Q0.0

What happens when input I0.0 is ON?

medium

A. Output Q0.0 turns off

B. Output Q0.0 energizes the relay coil

C. Input I0.0 is ignored

D. PLC enters error state

Solution

1. Step 1: Understand ladder logic symbols

   The normally open contact [ ] represents input I0.0; the coil ( ) represents output Q0.0.

2. Step 2: Trace signal flow when I0.0 is ON

   When I0.0 is ON (closed), current flows through contact to energize coil Q0.0.

3. Final Answer:

   Output Q0.0 energizes the relay coil -> Option B

4. Quick Check:

   I0.0 ON energizes Q0.0 [OK]

Hint: Contact ON energizes coil output [OK]

Common Mistakes:

• Thinking output turns off when input is ON
• Ignoring input state effect
• Assuming error without cause

4. A PLC output transistor is wired to a DC motor but the motor does not start when the output is ON. Which is the most likely wiring error?

medium

A. Output transistor connected without common ground

B. Output wired to AC load instead of DC

C. Missing flyback diode across the motor

D. Using a relay instead of transistor

Solution

1. Step 1: Check transistor wiring requirements

   Transistor outputs need a common ground reference with the load for current to flow.

2. Step 2: Identify wiring error preventing motor start

   If common ground is missing, the circuit is incomplete, so motor won't start despite output ON.

3. Final Answer:

   Output transistor connected without common ground -> Option A

4. Quick Check:

   Missing common ground stops DC motor [OK]

Hint: Transistor outputs need common ground [OK]

Common Mistakes:

• Ignoring common ground wiring
• Confusing flyback diode absence with motor start failure
• Thinking relay use causes motor not to start

5. You need to control a 230V AC heater using a PLC digital output. Which wiring method ensures safe operation and why?

Options:
A) Use a relay output with a snubber circuit
B) Use a transistor output directly
C) Use a triac output without any protection
D) Use a relay output without any additional components

hard

A. Use a relay output without any additional components

B. Use a transistor output directly

C. Use a triac output without any protection

D. Use a relay output with a snubber circuit

Solution

1. Step 1: Identify suitable device for 230V AC heater

   Relays and triacs can switch AC loads; transistors cannot switch AC directly.

2. Step 2: Consider protection for inductive or resistive AC loads

   Snubber circuits protect relay contacts from voltage spikes and arcing, improving safety and reliability.

3. Final Answer:

   Use a relay output with a snubber circuit -> Option D

4. Quick Check:

   Relay + snubber = safe AC heater control [OK]

Hint: Add snubber with relay for AC load protection [OK]

Common Mistakes:

• Using transistor directly on AC load
• Ignoring protection on triac or relay
• Assuming relay alone is always safe