When it comes to timing relays, two of the most common—and commonly misunderstood—types are the Delay on Make (DOM) and Delay on Break (DOB) relays. Both are designed to control when a relay activates or deactivates, but they serve distinct purposes depending on the application. In this guide, we’ll explain the key differences between DOM and DOB relays, give practical examples of where each is used, and highlight some of the reliable options available from Functional Devices.
Answers at a Glance
Q: What is a Delay on Make (DOM) relay?
A: A DOM relay delays the activation of its output after the control signal is applied—perfect for preventing short cycling of motors or fans.
Q: What is a Delay on Break (DOB) relay?
A: A DOB relay delays deactivation of its output after the control signal is removed, making it ideal for timed shutdowns using a momentary switch or control input.
Q: Can a DOM relay be used to prevent a motor from rapidly cycling on and off?
A: Yes! A DOM relay holds off activating a motor after it stops, preventing quick restarts and protecting equipment from wear.
Q: How does a DOB relay work with a pushbutton switch?
A: When the pushbutton is pressed, the relay stays activated; when released, the DOB relay starts its countdown before deactivating—useful for timed operations.
Q: Does Functional Devices offer both DOM and DOB relays?
A: Absolutely! Functional Devices manufactures a variety of DOM and DOB models to suit different voltage and control needs.
More About DOM Relays
Relays with the DOM functionality may also be referred to as "On Delay" or "Delay on Operate." Even though the names may differ, the functionality is the same:
- Upon application of the input signal, the time delay begins.
- At the end of the time delay, the relay coil is energized.
- The input signal must be removed to reset the time delay and de-energize the relay coil.
Functional Devices DOM Relays
One possible use of a DOM relay is to ensure that a motor is not short-cycled. That means the motor would be prevented from starting and stopping in a very short amount of time. A real-world example of this is the control of one of the big fans that blow down on you in an entry to a store or other facility. It's not wise to allow the fan to turn on and off as soon as the door is opened and then closed. With a DOM relay, the fan could turn on as soon as the door opens. Then, when the door closes, the time delay begins and the fan motor continues to run for a short time in case another customer walks in shortly after the door closes.
Relays with the DOB functionality may also be referred to as "Off Delay" or "Delay on Release." The names may be different, but they function the same:
- Apply the input power to activate the timing "brain" (needs to be a constant power source).
- Upon status change of the input trigger, the relay coil is energized.
- The timing countdown begins when the input trigger is broken*
- After the time is exhausted, the relay coil is de-energized.
*Any application of the trigger input during the countdown will reset the time delay countdown and the relay coil remains energized
Functional Devices DOB Relays
One very common application of a DOB relay is to activate a load for a set amount of time with the press of a normally open momentary pushbutton switch. As soon as the pushbutton is pressed, the relay coil will de-energize. Continue to hold the pushbutton and the coil will stay de-energized. As soon as you take your finger off the pushbutton switch, the time delay begins. After the set time delay, the relay coil is energized and is ready for the next command.
Reach Out for Help With DOM or DOB Relays
Whether you need to delay the start or extend the operation of your equipment, choosing the right timing relay makes all the difference. DOM and DOB relays each have unique roles in control systems, and understanding their functions will help you select the best option for your application. And if you’re still not sure, our Support Team is always ready to help you pick the perfect RIB® relay for the job! Feel free to call in or email!
About Functional Devices, Inc.
Confidence and Peace of Mind in Every Box
At Functional Devices, Inc., we have been designing and manufacturing high-quality electronic components in the USA since 1969. Our mission is simple: to enhance lives in buildings and beyond by delivering reliable, innovative solutions for the building automation industry. Our product lineup includes RIB® relays, current sensors, power controls, power supplies, transformers, lighting controls, and more—each designed to meet the highest industry standards.
We test 100% of our assembled products, ensuring an exceptionally low failure rate—less than 1 in 16,000 in the field. This commitment to excellence means you get products that perform reliably, reducing downtime and increasing efficiency. With every Functional Devices product, you receive more than just a component—you gain confidence and peace of mind knowing your systems are built on trusted, field-proven technology.