An obvious definition is that the DIN rail mounted relay is a relay designed to be physically installed on a DIN rail as shown in Figure 1. While this definition is certainly true, we should seek a more nuanced answer that will help you identify and purchase an appropriate relay for your given application.
Figure 1: A collection of DIN mount relays installed on a 35 mm section of “top hat” 35 mm rail.
Note that each device in Figure 1 includes a plug-in relay, DIN rail socket, LED status indicator, and flyback (free-wheel) diodes.
Tech Tip: Relays were once the primary control method for industrial equipment. Today, they serve a lesser role as many of the process control and logic functions have been taken over by the Programmable Logic Controller (PLC). We are most likely to encounter the DIN rail relay as an interface between the PLC and field devices such as sensors and actuators. Another common application is to serve as an interposing relay. As described in this article, the interposing relay is used:
to increase the current handling capabilities of a device such as a Programmable Logic Controller (PLC).
to change voltages such as when a 24 VDC PLC output is required to drive a contactor featuring a 120 or 277 VAC coil.
for improved safety via panel segregation. This application provides exclusive 24 VDC (blue wires) on the face of a PLC with higher AC voltages located elsewhere in a control panel. This deliberate segregation is a troubleshooting aid as a clean panel layout with dedicated space for each type of device is easier to troubleshoot.
to increase the speed at which a contactor opens thereby minimizing arc and prolonging life.
What is a Din Rail?
A DIN rail is like the blueprint for a child’s plastic building blocks except designed for industrial control and automation. It’s a standard promulgated by Deutsches Institut für Normung (DIN). Just like our building block analogs, the standard describes a variety of mounting platforms and associated components that are designed to seamlessly work together. The modular components are designed for use in low voltage (24 VDC to about 480 VAC) control panels.
One of the most popular options is the “top hat” 35 mm DIN rail. It’s a stretch of the imagination but you can imagine the channel cross section in the shape of a fedora or straight-brim top hat.
What is a relay?
By definition, a relay is an electromechanical switch featuring a coil, armature, and at least one set of electrical contacts. The contacts will change state when the coil is activated. This is a broad definition that includes assemblies of all shapes and sizes. It has nothing to do with DIN rail.
Tech Tip: As a manufacturer it’s in your best interest to make adaptable components. This strategy allows your products to be used in a wide variety of setting, thus increasing sales. This applies to relays, and perhaps more importantly, relay accessories such as sockets – more specifically to the DIN mount sockets for relays. Consequently, we need to think about relays and other industrial components in terms of interrelated family members.
Defining the DIN rail relay
The DIN rail relay is not defined by the attributes of the relay itself. Instead, the definition includes the attributes of the device family. This extends to relay, DIN rail socket, and associated relay accessories.
Tech Tip: There is a close relationship between the terms relay and contactor. One way to describe this technology is to recognize that relays are general purpose devices with a variety of normally open and normally closed contacts.
The contactor fails our previous definition of relay, as it is purpose-built device designed to activate large loads. A contactor’ large high-power contacts would therefore be type normally open. If we are strict with our definitions, the contactor and motor starter would not be considered DIN rail relays. These larger devices are DIN rail mounted, although the largest family members are bolted to the chassis to support size and weight.
A representative family of DIN rail relays
As an example, consider the plug-in Weidmüller RCL424024 DPDT relay as shown in Figure 2. It is a member of Weidmüller’s TERMSERIES family. As a control panel designer, you are free to select relay and accessories with a wide variety of coil voltages, polarity plus inductive protection circuits, status indicators, number / type of contacts, contact materials, test button, and wire connection methods. Video 1 provides a short look at the relay family.
The DPDT relay shown in Figure 2 becomes a DIN rail relay when installed in an appropriate socket as shown in Figure 3.
Figure 2: Close up picture of the Weidmüller RCL424024 DPDT relay.
Figure 3: Picture of the Weidmüller RCL424024 mounted in a DIN mount relay socket featuring a LED indicator, polarity steering, and flyback (free-wheel) diodes.
Video 1: Introduction the Weidmüller TERMSERIES family.
Conclusion
This engineering brief reminds us that definitions are important. This is especially true as we search for components such as the DIN rail relay. We find that DigiKey has a large selection of suitable components. These components are usually grouped into families by each manufacturer with a wide range of options designed to provide the greatest flexibility at the lowest cost. The family often include coil voltage options along with LED indicators and flyback protection for the selected coil including both AC vs DC protection. We also determined that contactors and other high-power devices such as motor starters are excluded from this category.
For additional content such as this article please visit the Industrial Control and Automation Index. Please leave your comments and questions in the space below. Picture of your control panels featuring DIN relays are especially welcomed. Even better, share a story about your experience troubleshooting or designing a panel.
Best Wishes,
APDahlen
About the author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey