Complete Guide to PLC On-Delay Timer (TON) with Ladder Logic Examples

What are PLC Timers?

PLC timers are software-based components that mimic the functionality of traditional timing relays. They allow a PLC to introduce time-based delays or intervals into control logic. Timers are essential for sequencing operations, controlling motor start-up, and implementing safety mechanisms. Instead of relying on external hardware, PLCs utilize internal memory and processing power to emulate timer functions, providing greater flexibility and programmability.

PLC timer instructions come in various forms, each designed for specific timing applications. The most common types include:

• On-Delay Timers (TON)
• Off-Delay Timers (TOF)
• Retentive On-Delay Timers (RTO)

Why On-Delay Timers are Used

On-delay timers are widely used due to their ability to introduce a delay before an output is activated after an input is turned on. This functionality is critical in numerous industrial applications:

• Motor Delay: Staggering the start-up of multiple motors to prevent excessive inrush current and overloading the power supply.
• Sequencing: Coordinating a series of operations in a specific order, such as activating different stages of a production process.
• Safety: Implementing safety protocols that require a delay before a machine starts up after a safety gate is closed.

By using PLC timer functions, engineers can create more sophisticated and reliable control systems.

What is an On-Delay Timer (TON)?

The On-Delay Timer (TON) is a fundamental timer instruction in PLC programming. In simple terms, it waits for a specified time period after its input becomes true before activating its output. Imagine a light switch: when you flip it (input ON), the light doesn't immediately turn on (output ON). Instead, there's a programmed delay.

Symbol Representation in Ladder Logic

The TON timer is typically represented in ladder logic by a rectangular box with the following elements:

• Timer Tag: A unique name assigned to the timer instance (e.g., Timer1).
• Preset Value (PT): The desired delay time (e.g., 5 seconds).
• Accumulated Value (ACC): The current elapsed time since the input became true.
• Enable Bit (EN): A bit that is true when the timer is enabled (input is true).
• Done Bit (DN): A bit that is true when the accumulated value equals or exceeds the preset value.

How TON Works

The PLC TON instruction works in a straightforward manner:

1. Input Turns ON: When the input condition to the TON timer becomes true (e.g., a sensor activates), the timer starts counting.
2. Timer Starts Counting: The accumulated value (ACC) begins to increment from zero.
3. Output Activates After Preset Time: Once the accumulated value (ACC) reaches or exceeds the preset value (PT), the Done bit (DN) is set to true, activating the timer's output.
4. Input Turns OFF: When the input condition goes false, the accumulated value (ACC) resets to zero, and the Done bit (DN) goes false.

Truth Table

Timing Diagram

A timing diagram provides a visual representation of the TON timer's operation, illustrating the relationship between the input, accumulated time, and output.

Ladder Logic Example

Let's consider a practical example: starting a motor 5 seconds after pressing a start button. This can be easily implemented using a TON timer in ladder logic.

// Ladder Logic Rung
            //---| Start Button |---(TON Timer)---(Motor Output)---

            // Timer Configuration
            // Timer Tag: MotorStartTimer
            // Preset Value (PT): 5 seconds

In this example, when the "Start Button" is pressed (input becomes true), the TON timer "MotorStartTimer" starts counting. After 5 seconds, the timer's Done bit (DN) becomes true, energizing the "Motor Output."

Practical Applications

The on-delay timer PLC example can be seen across many industrial applications:

• Conveyor Belt Delayed Start: Ensuring a conveyor belt starts a few seconds after a safety interlock is released, allowing personnel to clear the area.
• Preventing Motor Inrush Current: Staggering the start-up of multiple motors using TON timers to reduce the overall inrush current on the power supply.
• Sequential Process Control: Coordinating different stages of a manufacturing process, such as activating a filling valve after a specific delay to ensure proper tank levels.

TON vs TOF (On vs Off Delay)

It's important to understand the difference between TON and TOF (Off-Delay) timers. While TON timers delay the activation of an output after an input is turned on, TOF timers delay the deactivation of an output after an input is turned off. The TOF timer keeps the output energized for a specified time after the input goes false. This is useful in applications where you need to maintain an output for a certain duration after an event occurs.

Conclusion

The PLC on-delay timer (TON) is a powerful and versatile tool for implementing time-based control in industrial automation systems. Understanding its operation, ladder logic implementation, and practical applications is essential for any PLC programmer. By mastering the TON timer, you can create more efficient, reliable, and safe automated processes. Now it's your turn! Try experimenting with TON timers in simulation software like RSLogix, TIA Portal, or CX-Programmer to solidify your understanding and explore its capabilities firsthand.