What Time Delay Relay Do I Need?

December 10, 2019
Tech Tuesday

We often get technical support calls asking about Time Delay Relays, but many people are unsure of the type they need:

  • Delay-on-make (DOM) or
  • Delay-on-break (DOB)

Let’s look at a simple scenario to better understand the difference between DOM and DOB.

Suppose you have some security floodlights that you want to turn on for 20 minutes when an unsavory character triggers a nearby motion sensor. What would happen if a DOM relay were being used?

With a DOM relay, the motion sensor would have to be triggered for 20 minutes before the lights turned on and would turn off immediately once the trigger is gone. Under this setup, the lights don’t turn on and the unsavory character gets away. However, if a DOB relay were being used, the results would be quite different.

With a DOB relay, the motion sensor only needs to be triggered for a brief time before the lights turn on. Once the trigger is gone, the lights will remain on for the 20 minutes, and the unsavory character gets caught.

If you have an application that involves timing take a look at the following products:

As always, if you have any questions or need help selecting the right relay, be sure to contact us at tech support.


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About the Author

Scott Gardner is an Electrical Engineer from Indianapolis. He has a BS EE obtained from Rose-Hulman Institute of Technology in 2018. As an engineer at Functional Devices, he is involved with the design, manufacturing, and technical support of our products.