Industrial control components are designed to operate as a system. A classic division is the 24 VDC ecosystem that includes components such as Programmable Logic Controllers (PLC), panel lamps, solenoids, motor starters, and control relays. Components in the 24 VDC class are designed for seamless integration. For example, the PLC’s Input Output (I/O) is constructed for 24 VDC operation. The field devices are also designed for 24 VDC operation, allowing direct connection to the PLC. The mainstay control relay acts as an interface between the PLC and a high-powered device. An example is an interposing application, where a control relay is used to couple a PLC to a larger motor starter.
This engineering brief focuses on the DIN mount Phoenix Contact 2903334 as shown in Figure 1. This is a general-purpose Double Pole Double Throw (DPDT) relay with a DIN socket. DigiKey sells the relay as a set that includes the 2900931 socket, 2961192 relay, 2900939 plug-in module, and 2900953 retaining bracket. Markers such as the white 0811972 are available separately.
Figure 1: Image of the Phoenix Contact 2903334 DPDT control relay.
Many industrial relays are polarity dependent
“Why doesn’t my control relay activate?”
It’s a common expression of frustration heard in introductory PLC classes across the world. It’s a learning moment when students discover the polarity dependence of the 24 VDC industrial control relay. In most cases, +24 VDC is applied to A1 and return (ground) to terminal A2. If the connections are reversed, the relay will not operate.
This polarity dependence is associated with the plug-in module as shown in Figure 2.
Figure 2: Image of the 2900939 plug-in module with Metal Electrode Leadless Face (MELF) diodes.
The purpose of the steering and the flyback diode
The industrial control relay is designed to interface with other equipment. Most of these relays include protective circuitry to mitigate the problems associated with flyback voltage. Recall that an unprotected relay will develop a high-voltage spike when the relay is turned off. This inductor-like property is caused by the coil’s collapsing magnetic field.
The classic solution is to add a diode across the coil. When the relay turns off, this diode will safely conduct the flyback energy. Instead of a hundred-volt spike, we see the 0.7 VDC associated with the forward biased diode.
There are five general approaches to integrating a flyback diode:
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External: With this option, the end user is responsible for selecting and installing an external diode.
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Integrated: Some relays are constructed with the flyback diode integrated within the relay package. This may or may not include an indicator LED as a troubleshooting aid.
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Module: The external plug-in module is a common surge suppression option. A representative example is shown in Figure 1 with a close-up of the module in Figure 2. A key consideration is the reusability of the relay socket. For instance, the featured Phoenix Contact socket may be used with a wide variety of AC and DC control relays. Plug-in relays and plug-in modules are selected based on the application requirements. This family-based design provides low cost and maximum flexibility. The featured relay is part of the Phoenix Contacts RIFLINE family.
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PLC integration: Many PLCs and control modules are supplied with semiconductor output modules. Nearly all incorporate some flyback protection. Be sure to consult the datasheet to determine the nature of this flyback protection. It is prudent to use this built-in protection as a second line of defense. The primary protection is provided using one of the three previously defined methods. When we discover a PLC with a failed output, we should immediately verify the flyback diode of the associated relay.
Tech Tip: PLCs and industrial control modules with relay output do not strictly require flyback suppression. However, their contacts will be repeatedly exposed to an arc. Also, you may experience problems with Electromagnetic Interference (EMI) radiating from the relay and associated wires. In larger control systems, we may inadvertently construct antenna-like structures. Just like Marconi’s original spark experiments, we may end up with energy in undesirable locations such as analog circuitry sharing the same wire duct. There have been reports of this stray energy causing a step-direction motor to advance. Given the unintended consequences, along with difficulty of troubleshooting intermittent problems, it is best to constrain the flyback energy to a small loop located near the relay.
Tech Tip: This conversation about flyback voltage is complicated by the time it takes to open a relay. Please see [this article] for a full exploration of relationship between flyback voltage suppression and contactor opening time as applied to a small three-phase motor starter.
Polarity considerations for industrial control relays
Figure 3 presents the schematic for the featured Phoenix Contact relay. There are two diodes including:
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the flyback diode in parallel with the relay’s coil.
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the polarity protection diode installed in series with the +24 VDC A1 input terminal
An indicator LED is also integrated into the plug-in module. This is highly desirable troubleshooting aid that immediately allows the service technician to identify an active relay.
Figure 3: Schematic of the 2900939 plug-in module showing the polarity steering diode and the flyback diode.
Close inspection of Figure 3 highlights the importance of polarity. The critical factor is the direction of the flyback diode. The diode must be reverse biased while the relay is turned on (“pointing” in the direction of the DC power supply’s positive terminal). This is critical as it prevents the diode from shorting out the source when the relay is active. In this “pointing up” position, the diode serves its job to commutate the coil’s flyback voltage. The indicator LED also has a well-known polarity dependence.
Since the polarity of the flyback diode and LED is important, the engineers at Phoenix Contact have added a series polarity protection diode. This does not prevent the installer or technician from incorrectly installing a relay. However, it does prevent damage to the PLC or other sensitive devices from the inadvertent short circuit.
Tech Tip: Without the polarity diode, an improperly connected control relay would appear as a short circuit with uncontrolled current flowing through the flyback suppression diode. This is highly undesirable as it could damage the associated PLC output driver especially those with sensitive semiconductor outputs.
Before departing, let’s turn our attention to the relay socket as shown in Figure 4. Here we see the socket that accepts the optional plug-in surge suppression module. The term optional implies that the socket will function without the plug-in module. From Figure 2, we see that the module is simply a double-sided Printed Circuit Board (PCB). The plug-in module socket shown in Figure 4 is designed to accept this PCB so that the polarity protection diode is in series between the A1 input terminal and the relay’s coil.
To better visualize the relay socket’s physical construction, consider the toothpick shown in Figure 4. If we were to push on the exposed metal, we would break the connection between the relay coil and the input terminal. We can now visualize the operation as the plug-in module is slipped into position between the opposing fingers.
Figure 4: Close up image of the Phoenix Contact relay socket showing the socket connection for the plug-in relay.
Tech Tip: For the professors reading this post, the relay socket connection shown in Figure 4 is an excellent place to insert a problem. A small wedge or piece of clear tape would render the system inoperable. Students are unlikely to directly discover your test problem. Instead, they focus on proper troubleshooting techniques.
Parting thoughts
This was a fun exploration of technology. At first glance the Phoenix Contact relay appears very simple. Yet, as we peel back the layers, we discover the clever technology that makes this relay simple to use.
Personally, I appreciate the inclusion of the series-connected polarity diode, as I’ve miswired my share of circuits. Far better to have a relay that will not energize when improperly connected than a relay that destroys the driver.
Best wishes,
APDahlen
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About this 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.