Timer in PLC

Timer is a application which uses to delay our time of operation. There are four types of timer we are going to study.

	On- Delay	Off- Delay
Retentive	RTO	RTF
Non Retentive	TON	TOF

           

An on delay timer will wait for set time after a line of ladder logic has been true before turning on but it will turn off immediately. For eg. a car. If you turn the key in ignition and the car does not start quickly. Or an on delay timer allows an oven to reach temperature before starting production.

An off delay timer will turn on immediately when a line of ladder logic is true but it will delay before turning off. For eg. A car. If you turn key to stop engine it takes time to stop. Or An off delay timer can keep cooling fans for a set after the oven been turned off.

A retentive timer will sum all of the on or off time for a timer, even if the timer never finished.

A non retentive timer will start timing the delay from zero each time. Typical applications for retentive timers include tracking the time before maintenance is needed. A non retentive timer can be used for start button to give a short delay before conveyor begins moving.

In short we know that retentive timer works after supply voltage is off (no input). But Non retentive timer works only if it has supply voltage.

On delay timer works to delay for turning on where as Off delay timer works to delay for turning off.

 I also want to remind you about Siemens PLC timers as shown below…

Timer instruction: TON (On delay timer), TOFF (OFF delay timer)

Timer: - Time Base                   Number

              1ms                            T32, T96

              10ms                          T33-T36, T97-T100

             100ms                         T37-T63, T101-T255

If you want to delay for 5 sec by T32 or T 96 timer then you should use timer in Siemens software and write value “T32  5000” and it seems as below

TON            T32                   5000

If you want to delay for 5 sec by T33-T36 or T 97-T100 Timer then you should use timer in Siemens software and write value “T98 500” and it seems as below

TON            T98                500

If you want to delay for 5 sec by T37-T63, T101-T255 Timer then you should use timer in Siemens software and write value “T110 50” and it seems as below

TON            T110            50

The below figure shows the program in which two timers are given the same input X0 (i0.0) and two outputs are taken Y0 (Q0.0) and Y1 (Q0.1). Both outputs lighten at same time but as we take T32, we use value 5000 where as for T101 we have to use only 50.

By using timer we can turn on or turn off many machines at our desired time automatically. We are using 50 men to operate 100 machines then now with a singe plc 5 men can complete the entire task. This is how we can save our money time and gets efficient output.