PLC Ladder logic – Timers

A timer is a PLC instruction measuring the amount of time elapsed following an event. Timer instructions come in two basic types: on-delay timers and off-delay timers. Both “on-delay” and “off-delay” timer instructions have single inputs triggering the timed function.

The accuracy and repeatability of the timer are extremely high because the PLC processor generates delays. A timer starts counting at time-based intervals and continues until the accumulated value equals the preset value. When the accumulated value is equal to the preset time, the output will be energized. Then the timer sets the output.

There are mainly two types of timers: On delay timer, Off-delay timer


On delay timer:

An “on-delay” timer activates an output only when the input has been active for a minimum amount of time.

  • The timer starts operating, when the rung is turned ON.
  • When the rung is on, the timer starts counting until the preset value is equal to the accumulated value.
  • The timer starts counting when the rung is turned ON, the counting of accumulated value will be shown at the ET rung on the timer logic.
  • The preset value is given at the timer box, it can be changed.
  • Whenever the rung is ON, the ET bit starts counting until the count equal the preset value.

For example, To start the conveyor belt motor, the operator must press and hold the “Start” pushbutton for 10 seconds, during which time the siren sounds, warning people to clear away from the conveyor belt that is about to start. Only after this 10-second start delay does the motor actually start (and latch “on”):

OFF delay timer:

This timer instruction differs from the on-delay type in that the timing function begins as soon as the instruction is deactivated, not when it is activated.

  • The off-delay timer works normally for logic true. When the rung is turned ON the system works as the same.
  • But when the rungs turn false, which means when the rung is false the output will no get turn immediately.
  • The timer starts counting, until when the accumulated value is equal to the preset value.
  • When the switch is turned OFF, the instrument continues it state for a period given at the ET line. And turn off the instrument after a delay.

An application for an off-delay timer is a cooling fan motor control for a large industrial engine. In this system, the PLC starts an electric cooling fan as soon as the engine is detected as rotating, and keeps that fan running for two minutes following the engine’s shut-down to dissipate residual heat.


Instrumentation Engineer

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