Polarity is reinforced using the +24 VDC industrial control relays shown in Figure 1. This is necessary because students often make polarity reversal mistakes when they connect the relay socket’s A1 and A2 terminals. Students must be able to quickly identify the self-induced no-operation failure.
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This content is a natural continuation of the earlier electrical nodes exercise.
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Refer to Polarity and Flyback Protection in a 24 VDC Industrial Control Relay for a complete description of the flyback and series protection diodes.
Learning Objectives
- The relay coil itself does not care about polarity, however the protection diodes attached to it do.
- Always refer to the datasheet for the relay and associated socket to determine the correct polarity and presence or absence of diodes.
- Some industrial relay sockets appear to be missing diode modules. This is normal when the diodes are integrated into the relays themselves.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Relay Logic → Fundamental
Learning Pathway: Industrial Automation Technician → Fundamentals
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 29 Jul 2026
Figure 1: Featured DIN-rail-mounted relays used in this exercise.
Industrial Control Relay Terminology
Let’s define our terms to prevent confusion. Refer to Figure 2 which identifies the various components.
- Relay: This is the removable ice cube relay that contains the coil and contacts. It is a mechanical device subject to wear. Consequently, it is socketed.
- Relay socket: The DIN-rail-mounted base that hosts the plug-in relay and provides wire terminals for control panel wiring.
- Relay assembly: This includes the complete combination of plug-in relay, DIN-rail-mounted socket, and any surge suppression mechanism.
- Protection diodes: Many OEMs include diodes to suppress the high voltage surge associated with a relay turn-off event. These diodes may be integrated into the relay itself, integrated into the relay socket (Figure 3), or added as a module that attaches to the relay socket (Figure 4).
Figure 2: Image of an industrial control relay assembly.
Polarity Introduction for DC Control Relays
Relays present an immediate contradiction:
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The underlying physics do not depend on polarity. When energized, the coil “pulls” on the armature. Refer to this article for an advanced description of the magnetic field in a relay.
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Industrial control relays rarely appear in isolation. They are often integrated into larger systems that includes a flyback suppression diode and a steering diode.
Together we can state that a conventional DC relay is not polarity sensitive. Yet, many assembled industrial control relays are polarity sensitive.
Tech Tip: The relay A1 and A2 terminal distinction is fundamental to troubleshooting. Every person reading this article needs to recognize the failure modes. Even the most experienced technicians occasionally swap the A1 and A2 wire connections.
On a related note, the A1 terminal is often on the right-hand side of the relay socket. This breaks the natural left-to-right signal flow, adding to the probability that the connections will be reversed.
Figure 3: The cover has been removed from this relay socket to reveal the series and flyback diodes.
Figure 4: Schematic of the 2900939 plug-in module showing the polarity steering diode and the flyback diode.
Required Materials
The required materials are outlined in an introductory article which identifies components for Outfitting an Industrial Automation Lab. Continue to use the DC distribution block and node-based wire markers as introduced in the previous labs.
Tech Tip: You are correct to question the wisdom of applying reverse polarity to a relay bypassed with a flyback diode. Normally, this is considered a destructive test as the flyback diode will be forward-biased and conduct an uncontrolled current. This could destroy the diode or power supply if appropriate current limiting or fuse protection is not included.
This precaution does not apply to the control relays pictured in Figure 1.
Industrial controls components are generally designed with the assumption that reverse current will be inadvertently applied. Many manufacturers include a steering diode to prevent the high current condition. From a perceived reliability perspective, it is far better to have a circuit that does not function with reverse polarity than one that self-destructs. In one case, the user thinks in terms of operator error. In the other they remember the OEM’s “self-destructing” components; never mind the user’s error in both cases. So it goes.
Relay Polarity Experiment
Perform the following experiment to characterize relay polarity. Use the Phoenix Contact 2903334 DPDT relay assembly which includes a removable surge suppression diode as described in this article.
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With the surge-suppression diode module installed, connect +24 VDC to terminal A1 and RET to A2. Verify that the relay energizes when the power supply is turned on.
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With the suppression diode module installed, connect +24 VDC to terminal A2 and RET to A1. Verify that the relay does not energize when the power supply is turned on.
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Remove the suppression diode module. Note that this closes contacts within the relay socket, thereby bypassing the suppression diode module. The A1 and A2 terminals are now connected directly to the relay’s coil.
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Repeat steps 1 and 2 to verify that the relay activates with both polarities.
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Repeat steps 1 and 2 of the experiment with the Weidmüller 2576190000 DPDT relay. Note that the flyback and steering diodes are integral to the plug-in relay. The results should be identical to those obtained in Steps 1 and 2.
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Retrieve the datasheet information for the Phoenix Contact 2903370 SPST and the Phoenix Contact 2903308 4PDT relays. For each assembly, determine if the diodes are present in either the socket or the relay.
Questions
The following questions will help reinforce the content of the article.
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What is polarity?
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Why is a free-wheeling diode often included across a relay coil?
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As a rule, should the +24 VDC blue colored wire be connected to the A1 terminal? Be sure to refer back to the document describing the labs recommend wire colors.
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What markings or visual clues are used to determine if a relay or associated socket has an integral flyback diode? Ensure your answer applies to all of the relays included in the kit.
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What happens when reverse polarity is applied to a relay that includes flyback protection? Hint: there are two answers depending on the presence or absence of the steering diode.
Critical Thinking Questions
These critical thinking questions expand the article’s content allowing you to develop a big-picture understanding of the material and its relationship to adjacent topics. They are often open-ended, require research, and are best answered in essay form.
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Describe the magnetic field in a DC relay and the impact of polarity.
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Can a multimeter be used to determine the polarity of relay assembly? Hint: The plug-in relay itself may be removed so that the socket and relay may be tested independently.
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Estimate the magnitude of the transient voltage when a relay coil is turned off. Provide estimates for relay coils with and without the free-wheeling diode.
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How does the flyback diode work, and what is it protecting. Hint: think in system terms with an emphasis on the devices that control the relay coil.
About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.