Using a Dry Contact Relay with an Irrigation System

June 16, 2020
Tech Tuesday

As summer approaches, it is a good time to consider ways to improve your irrigation system. We have all driven by homes or businesses that were watering their property in the middle of a downpour. I usually just shake my head as I imagine them spraying money all over the place, that promptly runs into a storm drain. While the wasted money and water is obvious, it could be even more severe. In some states, it is illegal to water while it is raining, and rain sensors are required on irrigation systems.

In a recent technical support email, someone inquired about using one of our Dry Contact Input RIB® Relays in their irrigation system. The customer had a simple setup that used a timer to control a pump for the purposes of watering plants, and they were trying to devise a way to prevent the pump from running if it were raining outside. They proposed using an inexpensive, dry contact rain sensor with one of our relays to accomplish this.

Many irrigation controllers have a terminal to accept a rain sensor for this purpose. However, this user needed to cut power to the pump, and a typical rain sensor is not capable of handling that much load. That was where the Dry Contact Input Relay came into play. It can accept a dry contact sensor or mechanical contact of almost any type, and the isolated relay contacts can handle loads of up to 30 Amps or 3 HP (model dependent).

For this particular application, the RIB01BDC was selected, because it required 120 Vac, just like the pump. The normally closed (N/C) rain sensor would connect to the dry contact input wires and the normally open (N/O) relay contact would be wired in series with the timer relay, to break the 120 Vac Hot connection to the pump (see THIS general diagram).

In a setup like this, the N/C rain sensor contact would keep the input to the RIB® closed, if it did not detect rain. With the dry contact input closed, the N/O relay contact would be closed and allow the timer to control the pump based on its schedule. If rain were detected by the sensor, it would open the input to the RIB01BDC, and the relay’s N/O contact would open. This would prevent the pump from starting or turn it off if it were already running, regardless of the state of the timer relay.

We also offer some of our Dry Contact Input relays with an override switch built into the enclosure. I recommended the user consider one of these models, as it would:

  • aid in troubleshooting the system,
  • provide a means to manually override the relay ON if the sensor failed,
  • and provide a means to manually override the relay OFF for maintenance purposes.

If you have a system like the one described above, and you want to save money and water while avoiding the sneers of your neighbors, consider upgrading your system with a RIB® relay.


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About the Author

Henry Smith is a design engineer at Functional Devices, Inc. He has a BS EET obtained from Purdue University in 2014 and a lifelong interest in electronics. As an engineer at Functional Devices, he gets to provide Technical Support to our customers, from distributors to specifying engineers and installers.

Henry enjoys providing tech support, as it allows our company to assist at every level of our product’s lifecycle and exposes us to interesting and unique applications. While not every technical question is unique, even answering a simple question or providing someone with the information in a timely manner can go a long way to helping him or her meet a deadline.