A flyback diode across the coil of a large contactor can reduce contact wear in the smaller driving relay. However, this solution may degrade overall reliability by shifting the wear to the driven contactor. This topic is part of a larger design consideration involving interposing relays and the impact they have on equipment reliability. It is applicable to both stand-alone control relays and relay-based PLC outputs.
Figure 1 shows a control relay’s contact wear which is the focus of this engineering brief. We must be careful in the 24 VDC environment as protecting the control relay contacts can slow the release of the larger contactor. When misapplied, we protect the inexpensive control relay at the expense of the larger and considerably more expensive contactor. This also explains why some equipment designers prefer to use PLCs with solid-state outputs that then drive interposing relays. Doing so moves the point of wear outside of the PLC to where it is easier to replace.
Key Takeaways
- A misplaced focus on control relay contact wear could cause damage to the large contactor, where increased wear matters more.
- Slower-opening contacts in the three-phase motor starter can extend arc time, accelerating wear in the larger contactor’s contacts.
- Follow the contactor OEM’s recommendation rather than using a generic 1N4002 diode clamp. This will provide a snappy contactor opening to preserve contact life.
- Don’t inadvertently add a 1N4002-like clamp. This requires careful study of the PLC’s output drivers.
- The interposing relay is a replaceable component that can move control contact wear outside of the PLC.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Relay Logic → System-Level Applications
Difficulty: 
Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 08 Jul 2026
Figure 1: DPDT relay contacts, the one on the right shows wear.
Setup for Contact Wear Demonstration
A contact wear demonstration was conducted using the equipment shown in Figure 2. A small PLC is used to toggle a Finder 60-12-9-024-0040 relay. This control relay is used as an interposing relay to drive a pair of Siemens 3RT2015-1BB42 contactors.
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The PLC was programmed to toggle the interposing relay with one second on and one second off.
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The contactors were driven independently via the two halves of the interposing relay.
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The test was run for a day resulting in approximately 43,200 on/off cycles.
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The Siemens 3RT2916-1BB00 was removed from one of the contactors. Note that the surge suppressor is prominently seen on the right but not the left contactor in Figure 2.
Tech Tip: This test is very hard on the equipment, as relays, and especially large contactors, are not designed to “blink” like a hazard lamp. For reference, the number of relay cycles in this experiment is approximately equal to a machine that was cycled 5 times a day, every day for roughly 25 years. To be blunt, we have compressed a lifetime of relay cycles into a single day. The contact wear shown in Figure 1 is normal and to be expected given the equipment abuse outlined in this article.
Figure 2: Equipment used to measure contact wear in the Finder control relay.
Results of the Demonstration
With respect to Figure 1, the protected contact (left) looks very good while the unprotected contact (right) is worn. The worn contact was exposed to high-voltage inductive spikes as the associated contactor’s surge suppression socketed surge-suppression device was removed. While the equipment was running, we could see the inductive nature of the contactor’s coil; a small arc appeared every time the control relay’s contacts opened.
These small arcs are responsible for the contact pitting. While the arcs are small, they are still hot. Each arc vaporizes a minuscule amount of contact material. Yet, 43,200 cycles later, there is a noticeable difference between the surge-suppressor protected and the unprotected contact.
A Misapplied Resolution of Control Relay Contact Wear
We are tempted to add a 1N4002 diode clamp across each contactor coil. We wrongly suppose that protecting the control relay’s contacts will preserve the equipment. While that is true for the control relay itself, we may have sacrificed the performance and life of the larger and more expensive contactor. You can read more about the opening speed of a contactor in one of my previous articles. We can summarize the article by noting that a contactor such as the featured Siemens 3RT2015 will take about 100 ms to open if a 1N4002 diode is across the coil; there will be no spark, but it takes considerable time to dissipate the contactor’s inductive energy before the armature will relax. By contrast, an arc dissipates the inductive energy as a hot plasma. The contactor will open between 5 and 10 times faster as demonstrated in the earlier test. The disadvantage is the wear of the control relay contacts and a radio-frequency spike that could potentially interfere with other equipment as described in this article. That article points out that strong EMI is associated with the arc. This can be strong enough to cause a step/direction motor drive to advance.
Mitigation of Control Relay Contact Wear
Follow the contactor OEM’s recommendation:
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Use the contactor’s recommended surge suppression. Rarely do we see simple 1N4002-like diodes. Instead, we see things like varistors, TVS diodes, or Zener diodes that allow the discharge voltage to rise to a much higher value than the diode drop (approximately 0.7 VDC) associated with the simple diode.
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Don’t inadvertently choke the high-voltage swing into an SSR or PLC. Many solid-state devices include 1N4002-like clamps that prevent the rise of the coil’s flyback voltage. Instead, use a conventional mechanical interposing relay. This will allow the higher flyback voltage to quickly dissipate the inductive energy.
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Be prepared to replace the control relay after a few decades. In this experiment we compressed decades’ worth of cycling into a single day. The control relay’s contacts were pitted but there was plenty of life remaining.
Tech Tip: Some engineers will not use PLCs with relay-based outputs to avoid mechanical wear inside the PLC. Instead they will use solid-state PLC output modules plus interposing relays. This is a design decision that looks decades into the future. It assumes that control relays are plentiful and inexpensive relative to the PLC or PLC output modules.
Parting Thoughts
These are subtle but very important design decisions. The primary lesson is that the elementary advice of adding a 1N4002-like clamp is too simplistic. The previous interposing relay article was concerned about reduced opening speed in the larger three-phase contactor. This article strongly suggests that we allow at least a portion of the inductive spike to reach the associated control relay. The best approach is to recognize that most 24 VDC three-phase contactors are equipped with a surge suppression device that allows the flyback voltage to increase well above the simple diode drop.
Don’t eat the current in a simple 1N4002 diode. Instead, let the flyback voltage breathe to dissipate the energy outside of the coil.
That leads us to the topic of burnishing contacts. That’s a hot topic that we may address another day.
Sincerely,
APDahlen
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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.