Guide to Constructing a Time Off (TOFF) delay using a Time On (TON) Delay Relay

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 the construct of 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.

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:

  1. Idle with no power to either relay.

  2. Active run when the operator holds the pushbutton.

  3. 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

Return to the Industrial Control and Automation Index.

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.

About this author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.

Highlighted Experience

Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 150 unique posts and provided an additional 500 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.

A collection of Dahlen’s industrial control and automation articles may be found in this index page.

Connect with Aaron Dahlen on LinkedIn.

Questions

The following questions will help reinforce the content of the article.

  1. Describe the operation of a relay-based latch circuit. Be sure to describe the initiation and holding sections.

  2. Sketch the operation of the ideal TON and TOFF relays as a function of time. Assume a pushbutton is used to drive the coil.

  3. True / False: A 3PDT time delay relay may be constructed using an octal socket.

  4. True / False: The function of a pushbutton’s contact block is often encoded in its color or the color of the plunger tab.

  5. True / False: For industrial 24 VDC circuits, blue indicates positive and white with blue stripe is reserved for the ungrounded return.

  6. Describe the Figure 2 circuit operation if the rung 2 wire prevented CR1 from latching.

  7. Describe the Figure 2 circuit operation if the run pushbutton was broken such that the normally open contacts never opened.

  8. 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.

  1. How many unique components are used for the power distribution block on the left hand of Figure 1?

  2. Redesign the circuit shown in Figure 2 to eliminate the run pushbutton’s normally closed contact block.

  3. 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.

  4. 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?