PLC Delay‑on‑Start and Delay‑on‑Stop Programming Guide - PLC Provider Industrial Automation Supplier PLC HMI Drives

Table of Contents

Introduction

For many beginners learning PLC programming, timers represent one of the first major hurdles to overcome.

Delay‑on‑Start and Delay‑on‑Stop are two of the most fundamental and frequently‑used control logics in industrial automation. A wide variety of real‑world equipment relies on these functions, including blowers, water pumps, conveyor belts and many other types of machinery.

This complete guide walks you through both delay‑on‑start and delay‑on‑stop PLC programming examples. We will cover ladder‑logic diagrams alongside timing‑sequence diagrams, so even total beginners can understand and implement these functions in practical projects.

1. What is Delay‑on‑Start (Power‑on Delay)?

Delay‑on‑Start means: after receiving a start trigger signal, the load does not activate immediately. The system waits for a preset time period, and only turns on the output after the timer finishes counting.

Working Principle

When you press the start button, the input signal triggers the timer to begin timing. Once the preset timing value is reached, the timer’s contact closes, which energizes the output and activates the connected load.

Practical Example: Delay‑on‑Start for an Industrial Blower

Control requirement: When the start button X0 is pressed, the blower will not start right away. The blower output Y0 will turn on automatically after a 5‑second delay.

Input: X0 = Physical start push‑button
Timer: T0, preset value K50

Background knowledge for Mitsubishi FX‑series PLC: The base time unit for this timer type is 100 ms.K50 = 50 × 100 ms = 5 seconds

Output: Y0 = Blower fan motor

Detailed Ladder‑Logic Explanation

Normally‑open contact X0 drives the T0 TON (Timer‑ON) instruction with preset value K50
Normally‑open contact of timer T0 drives output coil Y0

Execution sequence:

Press start button → X0 contact closes. Timer T0 begins timing. At this moment, output Y0 remains de‑energized and the blower stays off.
After the 5‑second timing period completes → the normally‑open contact of T0 closes.
Closed T0 contact energizes Y0 coil, and the blower begins operation.

Timing‑Sequence Diagram Breakdown

X0 (Start trigger signal): Remains continuously ON during the delay period.
T0 Timer: Starts timing immediately after X0 turns ON. Its contact only changes state after the full 5‑second delay has elapsed.
Y0 Output: Goes high‑level (energized) only after the timer finishes counting, which physically starts the blower equipment.

Important note: This basic program has a limitation. If you release the X0 push‑button during the 5‑second waiting window, the timer will reset immediately, and the blower will never start.If you want one‑push‑button activation without holding down the button, add a self‑locking latch circuit using the Y0 output contact.

2. What is Delay‑on‑Stop (Power‑off Delay)?

Delay‑on‑Stop is also known as delay‑off, delayed power‑down or inertial shutdown. When a stop signal is triggered, the load does not power off instantly. It keeps running for a predefined duration before the output finally cuts off.

Working Principle

When the start signal turns ON, the load runs directly. When the stop condition occurs, the load stays active while a timer starts counting. Once the timer completes its preset time, the output contact opens and the equipment shuts down.

Practical Example: Delay‑on‑Stop for a Water Pump

Control requirement: Press start button X1, water pump Y1 starts running immediately. When X1 is released, the water pump does not stop right away. It keeps working and shuts down after a 3‑second delay.

Input: X1 = Start push‑button
Timer: T1, TOF (Timer‑OFF) power‑off delay timer, preset value K30

K30 = 30 × 100 ms = 3 seconds

Output: Y1 = Water‑pump motor

Detailed Ladder‑Logic Explanation

Normally‑open contact X1 drives TOF timer T1 with preset value K30
Normally‑open contact of timer T1 drives output coil Y1

Execution sequence:

Press X1, X1 contact closes → TOF timer T1 is energized instantly, T1 contact closes → water‑pump output Y1 starts immediately.
Release X1, X1 contact opens. The water pump does not stop. Timer T1 starts the 3‑second power‑off delay countdown.
After the 3‑second timing finishes, T1 contact opens → Y1 coil de‑energizes and the water pump stops.

Timing‑Sequence Diagram Breakdown

X1 Start signal: Turns ON when pressed, and turns OFF when released.
T1 Timer: The 3‑second delay countdown starts exactly when X1 changes from ON to OFF.
Y1 Water‑pump output: Maintains high‑level status for an additional 3 seconds after X1 switches off, then cuts power.

Critical reminder: The TOF power‑off delay timer instruction is not supported by every PLC hardware platform.Mitsubishi FX‑U and FX3U series do not have a native TOF instruction. If your PLC lacks TOF support, you can implement delay‑on‑stop function by combining TON power‑on timers together with latch‑self‑lock logic and normally‑closed contacts.

3. Delay‑on‑Start vs Delay‑on‑Stop: Quick Comparison Table

4. Real‑World Extended Application Examples

These practical scenarios help deepen your understanding of delay timer logic.

Case 1: Conveyor‑Belt Delay‑on‑Start

For safety purposes on production‑line machinery, after operators press the main start button, the conveyor belt does not move instantly. An 8‑second delay gives workers time to step away from moving mechanical parts.

Logic summary: X0 start input triggers TON timer with preset K80 (equals 8 seconds). After timing completes, Y0 conveyor‑belt output activates.

Case 2: Cooling‑Fan Delay‑on‑Stop

After motor‑driven machinery stops its main operation, the cooling fan cannot be turned off immediately. The fan keeps spinning for 10 extra seconds to dissipate residual heat and protect the motor hardware from overheating damage.

Logic summary: Fan runs while main equipment is active. When the main‑unit stop signal arrives, the fan continues operating for 10 more seconds before powering off, which is a classic delay‑on‑stop implementation.

5. Common Pitfalls for New PLC Programmers

Pitfall 1: Delay‑on‑Start timer resets when you release the button

The basic delay‑on‑start program will reset timing progress if you let go of the start push‑button before time is up.If you want single‑button one‑shot triggering without needing to hold the physical button down, add a self‑locking latch circuit using your output contact. Otherwise your delayed startup will fail whenever you release the input trigger mid‑count.

Pitfall 2: Cannot find TOF power‑off‑delay instruction

Mitsubishi FX family PLCs do not natively support TOF instructions. Directly writing TOF in your program will trigger compile errors. For these PLC models, you must build delay‑on‑stop functions manually using TON power‑on‑delay timers paired with self‑lock circuits and normally‑closed contacts.

6. Final Summary

Delay‑on‑Start and Delay‑on‑Stop represent the foundational usage modes for PLC timers. These building blocks are essential prerequisites before you learn more complex automation workflows and sequential control programming.

Easy‑to‑remember rule of thumb:

Delay‑on‑Start: Signal comes in, turn the load ON later.Delay‑on‑Stop: Signal drops away, turn the load OFF later.

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