PLC · Lesson 5 of 10

Timers

In classic control every timer was a part mounted in the panel. In a PLC you have hundreds of timers inside the program, and you change their time by typing a number. Here you will understand the three types TON, TOF and TP on a live chart, and write a complete star-delta program.

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1The timer in Ladder

In the simulator a timer is a block in the output column like a coil. It has an address (T0, T1…) and a time PT (preset time). Inside it there is a time counter ET (elapsed time). When it finishes, its bit T0 becomes 1, and you can use it in any rung with an NO or NC contact.

The first rung runs the timer; the second uses its bit (done).

2TON, TOF and TP on a chart

Choose the type, then press and release IN for different times (or use the switch), and watch the timing chart. PT here is two seconds.

Live timing chart

Timer	What it does	Example
TON	On-delay	A conveyor starts after a warning horn; confirming a sensor is steady (debounce)
TOF	Off-delay	A cooling fan keeps running after the oven turns off; a stair light
TP	Fixed-length pulse	A solenoid gives a half-second push; a one-minute bell

3Exercises: TON and TOF

Simulator: timers

4Star-delta with a timer

The same circuit as star-delta in the Classic Control course: 3 contactors. KM1 is the main contactor, KM3 the star (the motor starts in star to reduce the starting current), and after a delay KM2 the delta. The important part: star and delta must never be on together, or there is a short circuit.

On site: the star time is set according to the load (usually 5–10 seconds) until the motor is close to its speed. Many people add a short pause (50–100 milliseconds) between switching star off and switching delta on, so one contactor has really opened before the other closes.

Simulator: star-delta

5Common faults on site

Common program faults:

The timer never finishes: its condition drops for a moment every now and then (a flickering sensor), so the TON resets to zero each time.
Wrong time base: in some brands PT is in 100ms units, so K50 = 5 seconds, not 50. Read the manual.
The same timer number in two places: T0 is used in two rungs for two different things → strange behaviour. One timer for one job.
Star and delta were on together for a moment: there is no NC interlock between them in the program or in the wiring.

?Lesson questions

Answer at least 5 of 6 correctly to mark the lesson done.

Timer names and shapes differ: in TIA Portal an IEC_Timer block with IN, PT, Q and ET; in Allen-Bradley a TON instruction with Preset, Accum and the DN bit; in Delta and Mitsubishi TMR or OUT T0 K50 (in 100ms units). The simulator here is a generic trainer with IEC / Siemens style addresses: the exact addresses and instructions differ from one brand to another, and the manual of your PLC is the reference. Your progress is saved on this device only.
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