A PLC timer is a crucial component in ladder logic programming that counts time divisions to control devices for set periods. There are various types of timers including on delay, off delay, and retentive on/off timers, each serving unique functions in automation systems. Key applications of timers include facilitating delay actions and managing start/stop operations based on user instructions.

1. Timer in PLC and its
Types
Certified PLC Programmer
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2. What is Timer in PLC?
• 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.
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3. • 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.
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4. • In the programming of the Ladder Diagram
(LD) PLC, you could specify the timer from
milliseconds (ms) to hours (hr).
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5. 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.
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6. 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.
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7. 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.
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8. 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.
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9. 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.
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10. 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.
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.
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11. 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.
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12. 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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