PLC Timers Explained for Beginners: TON, TOF and TP

Table of Contents

• Introduction
• What Is a PLC Timer?
• Why Are Timers Used in PLCs?
• TON Timer
• TON Timer Example
• TOF Timer
• TOF Timer Example
• TP Timer
• TP Timer Example
• Common Timer Parameters
• Practical Example: Motor Delay Control
• Common Beginner Mistakes
• How to Troubleshoot PLC Timers
• How to Practice PLC Timers
• Conclusion

Introduction

PLC timers are important instructions used in industrial automation to create time-based control operations.

Timers allow a PLC to wait for a specific amount of time before turning an output ON or OFF.

They are commonly used in motor control, conveyor systems, pumps, fans, lighting systems, machine sequences, and many other industrial applications.

If you are learning PLC programming, understanding timers is an important step after learning basic Ladder Logic.

In this article, we will learn the basic working of PLC timers and understand TON, TOF, and TP timers with simple examples.

What Is a PLC Timer?

A PLC timer is a programming instruction that measures time according to the logic and conditions defined in the PLC program.

For example, suppose you want a motor to start five seconds after pressing a Start button.

The PLC can use a timer to create this delay.

The basic sequence is:

Start Button → Timer → 5 Seconds → Motor ON

The exact timer operation and available parameters depend on the PLC manufacturer and programming software.

Why Are Timers Used in PLCs?

Timers are used whenever an industrial process requires a time delay or a timed operation.

Common applications include:

• Motor start delay
• Motor stop delay
• Conveyor sequencing
• Lighting control
• Pump control
• Fan control
• Machine start-up sequences
• Alarm delays
• Valve control
• Automatic machine operations

Timers make it possible to control machines according to time instead of relying only on input signals.

TON Timer

TON means Timer ON Delay.

A TON timer waits for a specified amount of time after its input condition becomes true.

After the preset time has elapsed, the timer output becomes active.

The basic concept is:

Input ON → Wait for Preset Time → Timer Output ON

For example, if the preset time is five seconds, the timer output will become active after the input condition remains true for five seconds.

TON Timer Example

Consider a simple motor-control application.

When the Start push button is pressed, the PLC activates a TON timer.

The timer has a preset time of five seconds.

The PLC waits for five seconds.

After the five seconds have elapsed, the timer output becomes true and the motor output is activated.

The sequence is:

Press Start → TON Timer Starts → Wait 5 Seconds → Motor ON

This type of timer is useful when a machine needs to wait before starting the next operation.

TOF Timer

TOF means Timer OFF Delay.

A TOF timer is commonly used when an output needs to remain active for a specific amount of time after its input condition becomes false.

The basic concept is:

Input OFF → Timer Continues → Preset Time Ends → Output OFF

For example, a cooling fan may need to continue running for 30 seconds after a machine is switched OFF.

A TOF timer can be used to create this type of delay.

TOF Timer Example

Consider a machine cooling fan.

When the machine is running, the fan is ON.

When the machine stops, the fan should continue running for another 30 seconds.

A TOF timer can be used to provide this delay.

The sequence is:

Machine Running → Fan ON

Machine Stops → TOF Timer Starts → Wait 30 Seconds → Fan OFF

This can help provide additional cooling time for equipment.

TP Timer

TP is commonly known as a pulse timer or monostable timer.

When the timer is triggered, its output remains active for a specified period of time.

The basic concept is:

Trigger → Output ON → Preset Time → Output OFF

For example, a machine may need to activate a solenoid valve for exactly three seconds whenever a sensor detects a product.

A TP timer can be used for this type of application, depending on the PLC platform.

TP Timer Example

Consider a product marking machine.

When a product is detected, the PLC triggers the timer.

The timer output turns ON for three seconds.

The output can activate a marking device during this period.

The sequence is:

Product Detected → TP Timer Triggered → Output ON for 3 Seconds → Output OFF

This type of operation is useful when an output must remain ON for a fixed period.

Common Timer Parameters

Different PLC platforms may use different timer parameters and names, but several concepts are common.

Preset Time

The preset time is the amount of time the timer is programmed to measure.

For example:

Preset Time = 5 seconds

Elapsed Time

Elapsed time represents how much time has already passed while the timer is operating.

For example, if a timer has a five-second preset and three seconds have passed, the elapsed time is three seconds.

Timer Output

The timer output indicates whether the timer has completed its required operation or whether its timed output is currently active.

The exact behavior depends on the timer type and PLC platform.

Practical Example: Motor Delay Control

Let's consider a simple conveyor system with two motors.

Motor 1 starts first.

Motor 2 should start five seconds after Motor 1 starts.

A TON timer can be used to create the required delay.

The sequence can be:

Start Button → Motor 1 ON → TON Timer Starts → Wait 5 Seconds → Motor 2 ON

This type of sequencing is commonly used in conveyor systems and industrial machines.

The timer allows the PLC to control the order in which equipment starts.

Common Beginner Mistakes

Beginners can make several mistakes when working with PLC timers.

One common mistake is selecting the wrong timer type for the required operation.

Another mistake is entering an incorrect preset time or misunderstanding the time unit used by the PLC software.

It is also important to understand what happens when the timer input becomes false.

Always check the timer documentation for the specific PLC you are using because timer behavior can vary between manufacturers and programming environments.

How to Troubleshoot PLC Timers

If a timer is not working as expected, check the following:

• Check whether the timer input condition is becoming true.
• Check the timer preset value.
• Check the timer elapsed value.
• Check whether the timer output changes state.
• Check the PLC program logic around the timer.
• Check the PLC scan and diagnostic information.
• Check whether the output device is working correctly.

Monitoring the timer status online in the PLC programming software can make troubleshooting much easier.

How to Practice PLC Timers

The best way to understand timers is to create small practical programs.

Start with simple exercises such as:

• Turn ON a lamp after five seconds.
• Turn OFF a fan five seconds after a machine stops.
• Start two motors with a time delay.
• Operate a solenoid valve for three seconds.
• Create a conveyor sequence using multiple timers.
• Create an automatic pump start delay.
• Create an alarm delay circuit.

After practicing these examples, you can combine timers with counters, sensors, motors, and other PLC instructions.

Conclusion

PLC timers are essential for creating time-based operations in industrial automation.

TON timers are commonly used for ON-delay operations, TOF timers are used for OFF-delay operations, and TP timers can be used for fixed-duration pulse operations, depending on the PLC platform.

Understanding these timer concepts will help you create more advanced PLC programs and machine sequences.

In the next article, we can learn about PLC counters and understand CTU, CTD, and practical counting applications.

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Learn PLC timers for beginners, including TON, TOF and TP timers. Understand timer operation, preset time, practical motor-control examples, troubleshooting and industrial automation applications.

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