Logical AND & OR Function with a RIB®

July 24, 2018
Tech Tuesday

In automation and control, there are many applications that require a Logical AND or Logical OR function. Let’s say a load, like a fan or light, needs to be turned on if two things occur. That would be a Logical AND function. The Logical OR would be a scenario where the load needs to be turned on if one thing or the other occurs. Since the Normally Open (N/O) contact of a relay can be thought of as a Boolean value of 1 (contact closed) or 0 (contact opened), almost any Logical function can be achieved with relays. This blog will discuss the simplest of those using the RIBU2C.

The RIBU2C contains two separate, isolated relays, each with two coil inputs (120 Vac and 24 Vac/dc) and a SPDT relay output; it is essentially two RIBU1Cs in a single enclosure. If it is wired like Figure 1, the Load will not be energized unless SW1 AND SW2 are closed. Switches are used in the diagrams for simplicity, but any sensor or control contact rated for the coil voltage and current would suffice.


Figure 2 illustrates how the RIBU2C would be wired for an OR function. If either SW1 OR SW2 closes, the Load will be energized. Both diagrams are only using two inputs and two outputs, because that is what the RIBU2C has. Either can be expanded, by adding more RIBs, to include as many inputs and outputs as needed, and the same principles apply. The AND function requires the contacts to be wired in series, and the OR function requires the contacts to be wired parallel. There are two other RIB products that would be great for this application as well.


The RIBH2C could be used in place of the RIBU2C in the above diagrams. The RIBH2C is the same as the RIBU2C, except its High Voltage coil input is 208-277 Vac instead of 120 Vac. Also, the RIBU2S2 would work great for these applications. It has two isolated, SPST-NO outputs with an override switch for each. Just remember that the contacts are wired in series or parallel, so care must be taken to flip the override switch(es) appropriately to turn the Load ON or OFF. Always be sure to verify Load ratings are within the specifications of the devices that are controlling them, and contact us if you are not sure.

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About the Author

Henry Smith is a 34-year-old 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.