Digital output wiring (relay, transistor, triac) in PLC Programming - Step-by-Step Execution

Concept Flow - Digital output wiring (relay, transistor, triac)

PLC Digital Output Signal

↓

Relay

↓

Switch Load

↓

Load Powered ON/OFF

The PLC sends a digital signal to one of three output devices (relay, transistor, triac), which then switch the connected load ON or OFF.

Execution Sample

PLC Programming

IF OutputSignal THEN
  Relay Coil Energized
  Load ON
ELSE
  Relay Coil De-energized
  Load OFF
END_IF

This code energizes a relay coil when the output signal is ON, switching the load ON; otherwise, it switches the load OFF.

Execution Table

Step	OutputSignal	Relay Coil	Load State	Action
1	FALSE	OFF	OFF	No energizing, load remains OFF
2	TRUE	ON	ON	Relay coil energized, load switched ON
3	TRUE	ON	ON	Maintains energized coil, load ON
4	FALSE	OFF	OFF	Relay coil de-energized, load switched OFF

💡 OutputSignal FALSE at step 4 stops energizing relay, turning load OFF

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
OutputSignal	FALSE	FALSE	TRUE	TRUE	FALSE
Relay Coil	OFF	OFF	ON	ON	OFF
Load State	OFF	OFF	ON	ON	OFF

Key Moments - 3 Insights

Why does the load turn ON only when the relay coil is energized?

Because the relay coil acts as a switch; when energized (see execution_table step 2), it closes the circuit to power the load.

What happens if OutputSignal stays TRUE for multiple steps?

The relay coil remains energized and the load stays ON continuously (see steps 2 and 3 in execution_table).

Why is the load OFF when OutputSignal is FALSE?

Because the relay coil is de-energized, opening the circuit and cutting power to the load (see steps 1 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the Load State at step 3?

A Unknown

B OFF

C ON

D Flashing

💡 Hint

Check the 'Load State' column in execution_table row for step 3.

At which step does the relay coil first energize?

A Step 1

B Step 2

C Step 4

D Never

💡 Hint

Look at the 'Relay Coil' column in execution_table to find when it changes from OFF to ON.

If OutputSignal never becomes TRUE, what will be the Load State throughout?

A Always OFF

B Alternates ON and OFF

C Always ON

D Unknown

💡 Hint

Refer to variable_tracker for OutputSignal and Load State values when OutputSignal is FALSE.

Concept Snapshot

Digital output wiring connects PLC signals to devices:
- Relay: uses coil to switch load ON/OFF
- Transistor: switches DC loads quickly
- Triac: switches AC loads
PLC output TRUE energizes device, powering load
Output FALSE de-energizes device, load OFF

Full Transcript

This visual execution shows how a PLC digital output controls a relay to switch a load ON or OFF. When the OutputSignal is TRUE, the relay coil energizes, closing the circuit and powering the load. When FALSE, the coil de-energizes, opening the circuit and turning the load OFF. The execution table traces these steps, showing variable states and actions. Key moments clarify why the load depends on the relay coil state and what happens if the signal stays TRUE or FALSE. The quiz tests understanding of load state changes and relay coil energizing steps.

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