What Is Output Coil in Ladder Logic: Simple Explanation and Example

In ladder logic, an output coil is a symbol that represents a device or action controlled by the program, like turning on a motor or light. It acts like a switch that gets energized when the conditions before it are true, activating the connected output.

How It Works

Think of an output coil in ladder logic like a light bulb controlled by a switch. When the switches (conditions) before the bulb are all on (true), the bulb lights up. Similarly, the output coil turns on when the conditions in the ladder rung are met.

In ladder diagrams, the output coil is drawn on the right side of the rung and it changes state based on the logic on the left side. If the logic is true, the coil energizes, which means it sends power to the connected device, like a motor or a valve.

Example

This example shows a simple ladder logic rung where a start button controls a motor through an output coil.

pascal

(* Ladder logic example in structured text style for clarity *)
VAR
  StartButton : BOOL := TRUE; (* Simulate button pressed *)
  Motor : BOOL := FALSE;     (* Motor output coil state *)
END_VAR

(* Logic: If StartButton is TRUE, Motor coil energizes *)
IF StartButton THEN
  Motor := TRUE;
ELSE
  Motor := FALSE;
END_IF;

Output

Motor = TRUE

When to Use

Use output coils whenever you want to control a physical device or an action in your PLC program. For example, turning on a conveyor belt motor, opening a valve, or switching on a light.

Output coils are essential in automation to translate logical decisions into real-world actions. They respond to inputs and conditions, making machines work as intended.

Key Points

• An output coil represents a controlled device or action in ladder logic.
• It energizes (turns on) when the rung conditions are true.
• Output coils connect the program logic to real-world outputs like motors or lights.
• They are placed at the right end of a ladder rung.

Key Takeaways

An output coil activates a device when ladder logic conditions are true.

It acts like a switch controlling real-world outputs in PLC programs.

Output coils are essential for turning logical decisions into actions.

They are always placed at the end of a ladder rung.

Use output coils to control motors, lights, valves, and other devices.