Timer in PLC and its types

• A PLC timer is an electrical system component used in ladder logic programming. Timers are devices that count time divisions.
• A PLC timer is a program that controls and operates a device for a set period. We can use the timer to do any certain activities for a fixed period.
• A timer is one of the most important and helpful entities. With the help of a PLC programming timer order, you can build up a time-based action. Each PLC has a unique set of timer functions.
• The timer instruction is used to provide programming logic and to decide when to switch on or off the circuit. It contains two types of contacts: normally open (NO) and typically closed (NC).
• On connection or instant disconnection, the timer causes a delay in both PLC programming and relay boards.
• Simply stated, when the input is turned on, the timer begins to run and keeps track of the time. The timer activates its output when this time exceeds the programmed time.
• In the programming of the Ladder Diagram (LD) PLC, you could specify the timer from milliseconds (ms) to hours (hr).

Here are some of the key terms to know about timers in PLCs:

1. Input and Output Modules-

In the PLC system, there are several input (I) and output (O) modules. They serve as a link between the CPU and programmable devices. Input Module refers to the component that interacts with the input signal. It is necessary to connect input devices such as different types of switches. Output Module refers to the component that interacts with the output signal. The output module is necessary for connecting output devices, such as electric applications.

2. Power Supply Module-

The power module supplies electricity to the timing circuit, which allows it to function properly. It can be connected to either an ac (120, 230 V AC) or a dc voltage source (like 5,12, 24 V DC).

3. Internal Timer Circuit-

The set and reset functions are performed by the timer circuit. The timer will provide the momentary input pulse for the set and reset operation if the auxiliary power source is turned on.

4. Timer Digital Display-

The set and elapsed timing values are displayed on the digital timer. Values can be shown in a few milliseconds for automated purposes (ms). This will make tracking your automation system simple.

Different types of PLC Timer:

1. On Delay Timer- Timers are the most commonly used timer in electric circuits. You may be familiar with the term “on delay,” which means “delayed on.” It means that the timer will not update the contacts until the preset time has passed. These timers are used in Star-delta starter, capacitive load starter etc.

On-Delay Timer — PLC Program

As you can see in the diagram, the input supply is delivered, but there is no output until the predetermined time has passed. The contact changeover occurs when the timer reaches the preset time. Such Timers are called on delay timers.

2. Off Delay Timer (TOFF)- You may recognize the term “off-delay = delayed off” from the word itself. This means that even if the timer’s input power is turned off, the timer continues to give contacts to the exiting circuit.

Off-delay timer- PLC Program

As shown in the diagram, the input signal is turned off, but the contact remains closed. Such timers are called off delay timer. These off delay timers are used in motor cooling systems.

3. Retentive On/Off Timer (RTO)- The RTO’s primary function is to hold or store the set (accumulated) time. RTO is used when the rung status changes, there is a power outage, or there is a system disruption.

Retentive Timer — PLC Program

Applications of Timer:

Here are some of the most common timer applications in a PLC automation setting:

1. Use for the delay action.

2. It’s used to start and stop operations according to the user’s instruction.

3. The RTO timer is useful for storing or recording intermediate time values.

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