PLC timers are essential components of programmable logic controllers used in industrial automation. They enable the control of time-based events and sequences in automated systems. Here's an overview of the basics of PLC timers and the types of timers commonly used:

1. Basics of PLC Timers:

• Function: PLC timers are used to control the timing of events, delays, and sequences within a control system. They allow users to initiate, monitor, and control actions based on specific time intervals or conditions.

• Timing Modes: PLC timers operate in various timing modes, including on-delay (TON), off-delay (TOF), and retentive timers. Each timing mode serves a specific purpose and is used based on the requirements of the control application.

• Timing Resolution: Timers in PLCs typically have a fixed timing resolution, which determines the smallest time increment that the timer can measure or count. Common timing resolutions include milliseconds (ms), microseconds (μs), or even seconds (s), depending on the PLC model and configuration.

2. Types of PLC Timers:

• On-Delay Timer (TON):

  • Function: An on-delay timer (TON) activates its output after a preset time delay once the input condition becomes true.
  • Operation: When the input condition turns on, the timer starts counting. The output is activated only after the preset time delay has elapsed while the input condition remains true.
  • Example: Used in applications where a delay is needed before an action is initiated, such as conveyor belt start-up sequences.

• Off-Delay Timer (TOF):

  • Function: An off-delay timer (TOF) deactivates its output after a preset time delay once the input condition becomes false.
  • Operation: When the input condition turns off, the timer starts counting. The output remains active for the preset time delay after the input condition becomes false, and then deactivates.
  • Example: Used in applications where a delay is needed before stopping an action after the input condition changes, such as motor cooling-down sequences.

• Retentive Timer:

  • Function: A retentive timer maintains its accumulated time value even when the input condition changes or power is lost. It continues counting from where it left off when the input condition returns true.
  • Operation: Similar to on-delay or off-delay timers, but the accumulated time value is retained in memory until reset or cleared, ensuring accurate timing across power cycles or input changes.
  • Example: Used in applications where accumulated time needs to be preserved, such as batch processing or totalizing functions.

• Pulse Timer:

  • Function: A pulse timer generates a fixed-duration output pulse when triggered by an input condition. The output pulse duration is independent of the input condition's duration.
  • Operation: The output pulse is generated immediately after the input condition becomes true, with a duration set by the timer's preset value.
  • Example: Used in applications requiring a precise, fixed-duration output pulse, such as triggering solenoid valves or activating alarm signals.

These are some of the basic types of PLC timers commonly used in industrial automation. Understanding their functions and operation is essential for designing and implementing effective control strategies in PLC-based systems.