PLC timers are instructions that provide the same functions as on-delay and off-delay mechanical and electronic timing relays. A PLC timer provides a preset delay to the control actions.
In general, there are three types of PLC timer delays, ON-delay timer, OFF-delay timer and retentive timer on.
The terms represented in the timer block in the PLC are a Preset value which means the delay period of the timer, an Accumulated value which is the current delay of the timer.
A timer begins the counting on time-based intervals and continues until the accumulated value equals the preset value. When the accumulated value equals the preset time the output will be energized. Then the timer sets the output.
TON timer or ON delay timer
An ON delay timer is used where we need a time delay before the time delay before an instruction becomes true.
A representation TON timer is shown above, which contains,
Timer number: The timer file name
Time base: which is shown in seconds,
Preset value: Numeric valve set as the delay required to the timer.
Accumalated value: The values are counting is displayed from zero. Value becomes zero whenever the timer is reset
The timer starts operating when the rung condition becomes true. The timer delay starts counting when the rung condition starts to accumulate.
When the Preset value becomes equal to the accumulated value, the output is made true.
The timed output becomes true sometime after the timer rung becomes true; hence, the timer is said to have an on-delay.
The length of the delay can be adjusted by setting the preset value.
TOFF timer or OFF delay timer:
A TOFF timer will keep the output energized for a preset time after the rung signal has gone false.
The TOFF timer will have all the contents as in the TON timer, with the similar function.
When the rung timer is true, the output will be true without any delay. When the rung signal becomes false the timer starts operating.
The timer starts accumulating times when the rung condition becomes true, until the accumulated value becomes equal to the Preset value.
The output turns off when the output will turn false when the accumulated value equals the preset value.
A retentive timer is used when you want to retain accumulated time value through the power loss or the change in the rung state.
A retentive timer accumulates time whenever the device receives power, and it maintains the current time should power be removed from the device
Loss of power to the timer after reaching its preset value does not affect the state of the contacts. The retentive timer must be intentionally reset with a separate signal for the accumulated time to be reset
An example logic is shown above.
When the push bottom PB1 is pressed, the timer starts working and the reading starts accumulating.
When we push the PB1 button then rung become false and the timer stops working. Consider we are switching ON the push after a time, the timer starts counting from the previous value before the timer is stopped.
We have to add another switch PB2 to reset the values in the timer