The time delay operation is an essential building block for industrial control systems. Dedicated relays are available for Time-On delay (TON) and Time-Off delay (TOFF). Multifunction relays are also available supporting operating modes such as TON, TOFF, interval, pulse, single shot, blinking, latching, and alternating.
In this engineering brief, we will construct the equivalent of a TOFF relay using a TON relay. While you should generally purchase the purpose-built TOFF relay, there are important lessons in building physical circuits with an eye toward optimization. The pattern is also applicable to the Programmable Logic controller (PLC) Ladder Logic (LL). Here it is common to program using the TON construct to do a task after an event has occurred. In many respects, the TON operation is easier to implement in the same way that positive logic (active high) is easier to use than negative logic (active low). We will demonstrate the operation using the Finder relays and timer pictured in Figure 1.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Relay Logic
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 09 Mar 2026
Figure 1: The TOFF function is reproduced using a TON timer. The timer module is installed in the socket of the relay on the left. Observe that both a normally closed and a normally open pushbutton contact are necessary.
Select a TON Relay
Can you find the time delay module in Figure 1?
Recall that industrial relays often include several components including the relay, matching socket, and a snubber module to tame the flyback voltage (inductive kick) generated when the relay is turned off. Close inspection of Figure 1 reveals that the snubber modules are different. The module on the right is a traditional diode while the one on the left is a timer, as described in this post. The DIP switches are just visible above the socket’s A1 and A2 coil connections. The blue time adjustment screw is also visible on the top of the module.
Design Criteria
There are several different ways to use a TON timer to perform the TOFF function. The featured solution is shown in Figure 2. The most important design constraint is power consumption. Ideally, the system will idle in a zero-power state with all relays deenergized. The additional circuit complexity is secondary to the power consumption.
Figure 2: Ladder logic for one possible implementation for developing the TOFF function using a TON timer relay. Note that both relays are turned off when they are in the idle state.
Circuit Operation
The circuit in Figure 2 is best described as a state machine with 3 states including:
-
Idle with no power to either relay.
-
Active run when the operator holds the pushbutton.
-
Counting down when the pushbutton is released. For this example, the time delay is set to 5 seconds.
In this circuit Control relay (CR1) performs the TOFF function. It immediately activates when the run pushbutton is pressed. It deactivates 5 seconds after the run pushbutton is released.
The latch shown in rungs 1 and 2 is critical to the circuit operation. Observe that the latch is formed when the normally open contacts of the run pushbutton are closed. The latch is broken when the normally open contacts of time delay relay (TON1) open.
The timeout operation in 3 rung begins when CR1 is latched and the user releases the run pushbutton. When the 5 seconds have elapsed, TON1 activates. This releases the CR1 latch which then deactivates both CR1 and TON1 returning the system to the idle (no power state).
Parting Thoughts
This exercise provides a glimpse into the design of relay-based ladder logic. While this is a good learning exercise, in practice you should select a purpose built device such as the Omron H3CR-H8L AC/DC24 S timer shown in Figure 3. It features a large hand dial to quickly set the time delay. The relay also features an octal socket for quick replacement.
Speaking of learning, please take the time to answer the questions and critical thinking questions located at the end of this note.
Best wishes,
APDahlen
Figure 3: Image of the Omron H3CR-H8L AC/DC24 S TOFF relay featuring an octal socket and a large hand dial to set the delay time.
Continue Exploring Industrial Control Systems
If this discussion was helpful, you may also want to explore:
DigiKey Navigation
- Full Catalog: Industrial Control & Automation
Questions
The following questions will help reinforce the content of the article.
-
Describe the operation of a relay-based latch circuit. Be sure to describe the initiation and holding sections.
-
Sketch the operation of the ideal TON and TOFF relays as a function of time. Assume a pushbutton is used to drive the coil.
-
True / False: A 3PDT time delay relay may be constructed using an octal socket.
-
True / False: The function of a pushbutton’s contact block is often encoded in its color or the color of the plunger tab.
-
True / False: For industrial 24 VDC circuits, blue indicates positive and white with blue stripe is reserved for the ungrounded return.
-
Describe the Figure 2 circuit operation if the rung 2 wire prevented CR1 from latching.
-
Describe the Figure 2 circuit operation if the run pushbutton was broken such that the normally open contacts never opened.
-
Modify Figure 2 to accommodate a purpose-built TOFF relay.
Critical Thinking Questions
These critical thinking questions expand the article’s content allowing you to develop a big picture understanding the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.
-
How many unique components are used for the power distribution block on the left hand of Figure 1?
-
Redesign the circuit shown in Figure 2 to eliminate the run pushbutton’s normally closed contact block.
-
Physical relays can be challenging especially if you are thinking like a PLC programmer. People often get into trouble when they forget that real world relays take time to activate. Examine Figure 2 and determine if a race condition is present to both the turn on and turn off cycle. Hint: Consider the operation of a TON relay.
-
The octal socket remains a popular connection for larger industrial relays. As a general statement, are DPDT octal relays interchangeable. Assuming it is functionally appropriate, can we readily swap a conventional for a time delay relay?
About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.