When it comes to a delay relay, there’s typically a request for two different versions: the Delay on Make (DOM) and the Delay on Break (DOB). We have met that demand for the two different delay relays, as we manufacture and supply a variety of options in the delay relay category. However, which is the best option? Perhaps a better question to ask here is what’s the difference between DOM and DOB? Let’s try to answer that and give some real-world examples of applications where each could be used.
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.
I hope this helps to clear up the differences between DOM and DOB relays, so that you may make the best decision for your next controls project. As always, feel free to call in or email our Support Team if you would like a second opinion or want to talk about anything RIB®!