Allen Bradley PLC Timers: A Beginner’s Guide

Allen Bradley PLC Timers: A Beginner’s Guide

Allen Bradley PLCs are widely used in industrial automation, and timers are an essential part of many PLC programs. Timers allow you to control the duration of events, which is critical for tasks like controlling motor speeds, managing conveyor systems, and triggering alarms.

This guide will introduce you to Allen Bradley PLC timers, covering the basics of how they work and how to use them in your PLC programs. We’ll discuss different types of timers, their parameters, and provide examples to illustrate their applications.

Understanding PLC Timers

A PLC timer is a function that measures time. It’s like a stopwatch that starts counting when a specific condition is met and stops when another condition is met. The time measured is then used to control other parts of your PLC program.

Types of Allen Bradley PLC Timers

Allen Bradley PLCs offer several types of timers, each designed for specific applications. Here are some common types:

• TON (Timer On Delay): This timer starts counting when its input becomes true and continues counting until its input becomes false. The timer output remains false until the preset time has elapsed.
• TOF (Timer Off Delay): This timer starts counting when its input becomes false and continues counting until its input becomes true. The timer output remains true until the preset time has elapsed.
• RTO (Retentive Timer On Delay): This timer is similar to a TON timer, but it retains its accumulated time even when its input becomes false. The output remains false until the preset time has elapsed.
• RTOF (Retentive Timer Off Delay): This timer is similar to a TOF timer, but it retains its accumulated time even when its input becomes true. The output remains true until the preset time has elapsed.

Timer Parameters

Allen Bradley PLC timers have several parameters that you can configure to control their behavior. These parameters include:

• Preset Time: This parameter defines the duration of the timer. It’s the time that the timer will count for before its output changes state.
• Input: This parameter determines when the timer starts counting. It’s typically a contact or a bit that becomes true or false to trigger the timer.
• Output: This parameter represents the timer’s output. It’s typically a contact or a bit that changes state after the preset time has elapsed.

Using Timers in PLC Programs

Here’s a simple example of how to use a TON timer in a PLC program:

Let’s say you want to control a motor that should be turned on for 10 seconds when a push button is pressed. You can use a TON timer to achieve this:

1. Create a TON timer in your PLC program.
2. Set the preset time to 10 seconds.
3. Connect the push button to the timer’s input.
4. Connect the timer’s output to the motor’s control relay.

When the push button is pressed, the timer’s input becomes true, and the timer starts counting. After 10 seconds, the timer’s output becomes true, energizing the motor’s control relay, and turning the motor on. When the push button is released, the timer’s input becomes false, stopping the timer. The motor will remain on until the preset time has elapsed.

Conclusion

Timers are essential components in Allen Bradley PLC programs, enabling you to control the duration of events and automate various industrial processes. By understanding the different types of timers and their parameters, you can effectively implement them in your PLC programs to achieve desired results.

This guide provided a basic introduction to Allen Bradley PLC timers. For more advanced applications, you can explore additional timer functions and techniques. Practice using timers in your PLC programs to gain experience and confidence in implementing them for various automation tasks.