Beginner's Guide to Selecting an Industrial Control Relay

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DigiKey offers a wide selection of relays suitable for use in the industrial control environment. This brief will help you select a group of relay components that work together. We will also include a few tips along the way.

Presentation of a representative control relay

In this note we will use the Phoenix Contact RIFLINE family as a representative example. Like many of DigiKey’s DIN mounted relays, it features a socket mounted relay, an optional surge suppression assembly, mechanical assemblies, markers, and various jumpers to facilitate grouping with other relays of the same family. These jumpers will not be addressed in this note.

The device as pictured below is configured for operation in a 24 VDC system using a SPDT mechanical relay. Other options are available including relays with 12 VDC, 48 VDC, 60 VDC, 24 VAC, 120 VAC, 230 VAC, SPDT, DPDT, and even solid-state relays. Appropriate surge suppressors are available for each coil voltage.

The example as pictured below consists of the following components:

Necessity of the surge suppression diode

In a DC system, a “freewheeling diode” is placed in parallel with the relay’s coil. This prevents the high voltage spike that naturally occurs when the relay is turned off. This reduces the stress / damage to sensitive electronics. For example, this diode is mandatory component for Programmable Logic Controllers (PLC) that feature solid state output contacts.

Tech Tip: If while troubleshooting a PLC, you notice a damaged output pin. Stop! Inspect the surge suppression diodes installed on associated relay or contactor. A failed-short or a failed-open diode could damage the PLC output contact. For a 24 VDC system you could substitute a general purpose 1N4002 diode until the appropriate module is ready for installation.

The surge suppressor as pictured below requires additional consideration. Notice that the picture shows two diodes. One is the surge suppression diode, the other is a LED with accompanying current limiting resistor. Note the +A1 and -A2 contact designators. These are the relay and socket designation. They are just visible on the left side of the socket as pictured above. It is vital that the positive terminal of the power source be connected to A1.

Personally, when I work in a control panel this feels awkward as it breaks the left to right flow in my thinking. For many control relays, the positive A1 terminal is on the right-hand side. The A2 return terminal is on the left. It doesn’t take long to remember this important aspect of relay polarity as a mistake could trip the circuit protection or damage a PLC output pin.

Tech Tip: Control relays such as the one in this note may be used as “interposing” relays. They serve as an interface allowing a 24 VDC PLC with solid state outputs to control larger high-current loads or loads with different voltages. The interposing relay can also be a safety feature allowing the high voltage to be segregated from the control panel’s 24 VDC control logic.

DigiKey selection tools

The selection process can be daunting for the beginner. DigiKey does have a few tools to make this easier; typically located at the end of a product’s page. As shown below, you can often locate parts using the “Associated Product” or the “You May Also Be Interested In” feature. In this particular example, we find that the relay components featured in this post are available in kit form. You may also find information on the manufacturer’s pages.


Selecting a control relay for your industrial application can be challenging. These relays are generally offered as a family of related components as featured in this post with components offered individually or as a kit. Careful attention to specifications is required as there are many options such as coil voltage to consider. Finally, the selection of surge suppression devices may dictate the polarity of the relay connections.

We look forward to hearing from you as you design, build, troubleshoot, and repair your industrial control systems. Your feedback is invaluable to us and our readers. Please share your comments and question about this material on this page or back on DigiKey’s primary TechForum page.

Best Wishes,


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