Normally Open Contact in Ladder Logic: Definition and Usage

A normally open (NO) contact in ladder logic is a symbol representing a switch that is open (off) when inactive and closes (turns on) when the input condition is true. It allows current flow only when the controlling input is active, simulating a real-world push button or switch that completes a circuit when pressed.

How It Works

Think of a normally open contact like a door that is usually closed and blocks passage. When you press a button or activate a sensor, the door opens, allowing electricity to flow through the circuit. In ladder logic, this contact stays open by default, so no current passes through until the input condition turns true.

This means the circuit connected after this contact only activates when the contact closes. It’s similar to a light switch that is off until you flip it on, completing the circuit and lighting the bulb.

Example

This example shows a simple ladder logic rung with a normally open contact controlling a motor output. The motor runs only when the start button is pressed.

ladder_logic

START_BUTTON  ----[ ]----( MOTOR )

// Explanation:
// [ ] is the normally open contact representing the start button
// ( MOTOR ) is the output coil that turns on the motor

Output

When START_BUTTON is pressed (true), the normally open contact closes, energizing MOTOR output.

When to Use

Normally open contacts are used when you want a device or process to start only after a specific action, like pressing a button or triggering a sensor. They are common in start-stop motor controls, safety interlocks, and simple on/off switches.

For example, a conveyor belt motor might start only when a start button is pressed (NO contact closes), and stop when a stop button is pressed (using a normally closed contact).

Key Points

• A normally open contact is open (off) when inactive and closes (on) when activated.
• It simulates a switch that completes a circuit only when pressed or triggered.
• Used to control outputs that should activate only on specific input conditions.
• Common in start buttons, sensors, and control logic requiring activation signals.

Key Takeaways

A normally open contact closes to allow current flow only when its input condition is true.

It acts like a push button that completes a circuit when pressed.

Use normally open contacts to start devices or processes on demand.

They are fundamental in controlling outputs in ladder logic programs.