Lesson#12 ON Delay and OFF Delay Timers in Ladder Logic

Overview

In Ladder Logic, ON Delay and OFF Delay timers are essential components for creating time-dependent control systems. These timers help you introduce delays in turning on or off devices after a certain condition is met. Both timers use TON (ON delay timer) and TOF (OFF delay timer) functions, but they operate differently in terms of when the delay starts and how they react to input changes.

Let’s break them down:

1. ON Delay Timer (TON)

Function: The ON Delay Timer (TON) will turn the output ON after a specific time delay once the input condition is TRUE.
Usage: The timer starts counting once the input is activated, and after the set delay time, it activates the output.

How it Works:

When the input turns ON (True), the timer starts counting.
Once the time delay is over, the output is activated.
If the input turns OFF before the delay is completed, the timer resets, and the counting restarts when the input turns ON again.

Ladder Logic Example (ON Delay):

ladder

|----[ I0.0 ]----(TON)----( Q0.0 )  |
|                    T1             |
|----[ T1.Q ]------------------------|
|              (Timer ON)           |

Parameters for `TON`:

IN: Input condition that triggers the timer.
Q: Output that is activated after the delay.
PT: Pre-set time (in seconds, milliseconds, etc.).
ET: Elapsed time that is counted after the timer starts.
Q (Timer output): Activated once the time has passed.

Example:

Input (I0.0) goes high.
Timer T1 starts counting.
Q0.0 (output) is activated after a specified time (e.g., 5 seconds) and stays ON until the input turns OFF.

2. OFF Delay Timer (TOF)

Function: The OFF Delay Timer (TOF) will turn the output OFF after a specific time delay, once the input condition is turned OFF.
Usage: The timer starts counting when the input is deactivated (goes OFF), and after the set delay time, it turns off the output.

How it Works:

When the input turns OFF, the timer starts counting.
After the delay time has passed, the output is turned OFF.
If the input turns ON before the timer finishes, the timer resets, and the delay is interrupted.

Ladder Logic Example (OFF Delay):

ladder
|----[ I0.0 ]----(TOF)----( Q0.0 )  |
|                    T2             |
|----[ T2.Q ]------------------------|
|              (Timer OFF)          |

Parameters for `TOF`:

IN: Input condition that triggers the timer.
Q: Output that is deactivated after the delay.
PT: Pre-set time (in seconds, milliseconds, etc.).
ET: Elapsed time that is counted after the timer starts.
Q (Timer output): Deactivated once the time has passed.

Example:

Input (I0.0) goes high.
Q0.0 remains ON while the input is high.
Input (I0.0) goes low.
Timer T2 starts counting.
Q0.0 turns OFF after the specified time (e.g., 5 seconds).

3. ON Delay vs OFF Delay - Key Differences

Aspect	ON Delay (TON)	OFF Delay (TOF)
Activation	Starts when input turns ON.	Starts when input turns OFF.
Output	Output turns ON after delay when input is ON.	Output turns OFF after delay when input is OFF.
Reset Behavior	Resets if input turns OFF before delay time is complete.	Resets if input turns ON before delay time is complete.
Common Use	Turn devices ON after a delay (e.g., motor start delay).	Hold devices ON for a specific time after input turns OFF.

4. Real-Life Examples

ON Delay Example (TON):
Motor Start: A motor needs to start after a delay. When you press a start button, it triggers a relay to close. However, you want to introduce a delay of 5 seconds before the motor actually starts.
```
ladder

|----[ Start Button ]----(TON)----( Motor Start )  |
|                         T1                        |
|----[ T1.Q ]----------------------------------------|
```
- The relay will only energize the motor after 5 seconds if the button stays pressed.
OFF Delay Example (TOF):
Motor Stop: A motor should remain ON for 10 seconds even after the stop button is pressed. After the 10-second delay, the motor will stop.
```
ladder

|----[ Motor Running ]----(TOF)----( Motor Stop )  |
|                        T2                       |
|----[ T2.Q ]--------------------------------------|
```
- If the motor is running and the stop button is pressed, the motor will continue to run for 10 seconds before it finally stops.

5. Combining ON Delay and OFF Delay

You can also combine both types of timers in a single ladder logic to control complex timing sequences, such as:

ON delay to start a process after a certain delay.
OFF delay to keep the process running for a while even if the input condition goes away.

Combined Example:

ladder
|----[ Start Button ]----(TON)----( Motor Start )  |
|                         T1                        |
|----[ Stop Button ]----(TOF)----( Motor Stop )    |
|                         T2                        |

Motor Start uses an ON delay, while Motor Stop uses an OFF delay.

6. Simatic Manager Implementation

In Siemens SIMATIC Manager (S7), ON and OFF delay timers can be implemented using TON and TOF instructions in the Ladder Diagram.

Example for TON (ON Delay Timer) in SIMATIC Manager:

Use the TON instruction to create a delay when the input condition is met.
Connect the input and output with the TON block.

ladder

Network 1: ON delay
|----[ I0.0 ]----(TON)----( Q0.0 )  |
|                    T1             |

Example for TOF (OFF Delay Timer) in SIMATIC Manager:

ladder

Network 2: OFF delay
|----[ I0.1 ]----(TOF)----( Q0.1 )  |
|                    T2             |

In these examples:

T1 and T2 are timer blocks used to create the delay.
Q0.0 and Q0.1 represent the outputs triggered by the ON or OFF delays.

Conclusion

ON and OFF delay timers are crucial for creating controlled delays in automation systems, such as waiting before starting an output or maintaining an output for a while after the input condition changes. They are commonly used in industrial systems for applications like motor control, pump operation, and other time-dependent processes.

Lessons : 0
Level : intermediate