When to Use a PCB-Mounted Relay Socket Instead of Soldering the Relay Directly to the PCB

Automation and Control Industrial Automation and Control
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The decision depends on which parts are most likely to fail over the decades-long life of the machine.

Decide: Should relay replacement be a simple relay swap or an expensive PCB replacement.

Key Takeaways

  • Sockets improve serviceability.
  • Sockets are not a universal solution for environments with high vibration or when conformal coating is used.
  • The same socket may host different relay types including electromechanical, DC SSR, or AC SSR.
  • Design the PCB around a socket, then match the relay variant to the given load.
  • The thin gold contact plating will be vaporized by the plasma associated with an inductive load.

This article is part of the DigiKey Field Guide for Industrial Automation

Location: Understand It → Industrializing Microcontroller-Based Designs
Difficulty: :gear: Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 07 Apr 2026

Think in Terms of Relay Families

Don’t focus on a singular relay. Instead, think of relays in terms of families. By this definition, “family” is more than just the coil voltage. It also includes the sockets, electromechanical relays, and Solid-State Relays (SSR) that operate with either AC or DC loads.

We have several options for incorporating relays into our designs including:

  • direct PCB mount
  • plug-in relay on a PCB-mounted socket
  • off-board mount, such as DIN rail

Hazards for Relays

  • Electromechanical Relays have a limited lifespan. As an example, consider an electromechanical power relay rated for 60,000 cycles at rated load. This number seems large until we consider the age of industrial and commercial equipment. Assuming a 20 year lifespan, we are down to 3,000 cycles per year, or simply 8 cycles per day. This estimate ignores the contact wear associated with highly inductive loads.

  • Solid-State Relays have a clear advantage when we consider mechanical wear. However, they have their own failure modes. Voltage spikes are a hazard, especially for AC-rated SSRs interfacing with line-powered equipment. There is also a risk of damage from an accidental short circuit.

Real World Implications

The initial cost of the relay socket is justified when we consider the total equipment lifecycle cost.

The relay is the interface between your PCB and the field devices. It shields the sensitive microcontroller logic from load side transients and faults. When failure occurs, a socketed relay is a simple fix, while a soldered relay requires PCB replacement or repair. This often demands the overhead of an expensive parts inventory, extended equipment outages waiting for a replacement PCB, or in-house technicians who can perform PCB-level repairs.

Reputation is also mixed into the serviceability conversation. Consider the sticky reputational difference between a five-minute relay swap and a machine that was down for a week awaiting parts.

Tech Tip: Study the PCB design of the PLC. The PLC manufacturers have already solved the hard problems of the logic-to-field isolation challenges.

Advantages of Direct-Soldered Relays

This article would be incomplete without mentioning the advantages of a direct-soldered relay:

  • The direct-mount PCB relay has the lowest upfront cost especially when the socket cost is similar to the cost of the relay itself.

  • Elimination of multiple unsoldered connections. The socket itself is a point of failure. Even a simple SPDT changeover relay socket adds five additional connection points. These connections are spring-maintained and may corrode or weaken over time.

  • Conformal coating may be applied over the board and relay to increase system life.

  • Vibration is better controlled using direct-mounted relays. Here, the relay is closer to the PCB which reduces the length of the lever arm upon which vibration operates. This is especially important as the coil, metal core, and yoke are relatively heavy components.

System Example from Industrial Controls

As a closely related example, consider the PLC. They are available with either semiconductor or relay outputs. Some equipment designers reject the relay output and instead use a solid-state PLCs with external DIN rail mounted relays. They justify the added cost by pointing to the cost differential between replacing a small control relay as opposed to an entire PLC.

Representative PCB-Mounted Relay Socket

The Finder 93 series PCB socket as shown in Figure 1 is a representative PCB mounted relay socket. Thinking in terms of family, the Finder 93 series is related to:

  • 34 series electromechanical relays
  • 34 series SSRs
  • 93 series DIN rail sockets and accessories

Note that the Finder example is not unique. DigiKey offers families of relays from several other OEMs. However, do not assume footprint or part interchangeability across OEMs.

Figure 1: Finder 93 series PCB mounted relay socket.
Figure 1: Finder 93 series PCB mounted relay socket.640×640 77.6 KB

Figure 1: Finder 93 series PCB mounted relay socket.

Case Study

Let’s assume a commercial PCB with eight output relays operating from a 24 VDC drive domain. As a first approximation we will assume:

  • Finder 93 series sockets
  • Finder 34 series electromechanical relays each consuming approximately 7.1 mA. Combines, they require a power supply capable of providing 56.8 mA or about 1.4 W.

Potential Drive Side Components

Allowing a generous derating, the 6 W Traco Power TMR 6-2415 DC-to-DC converter would provide 1.6 kV galvanic isolation for the sensitive driver logic. We could consider using the Infineon TLE75080ESDXUMA1 for a modern integrated solution or the classic discrete drive 2N3904 transistor.

Tech Tip: Be careful when selecting DC-to-DC converters as some have a minimal load requirement. This is important for this application as there will be times when all relays are turned off. The Traco supply in this example has a ‘Not Required’ minimal load specification.

Potential Load Side Components

Once the socket is installed there are multiple options from within the Finder 34 series. Here are a few examples for our chosen 24 VDC coil domain:

  • SPDT electromechanical with Silver Nickel (AgNi), Gold (Au) contacts
  • DC SSR with the ability to switch up to 6 A in a 1.5 to 33 VDC system
  • AC SSR with the ability to switch up to 2 A in a 12 to 275 VAC system.

Tech Tip: The gold coating on the AgNi contact provides good electrical continuity for low currents such as dry contact interface in a 5 VDC environment. However, the gold will be vaporized by the plasma associated with an inductive load. An uncoated AgNi contact is all that remains. Swapping a “burned in” relay with a small signal relay produces a latent fault. Without the gold protection the small signal relay’s contacts will corrode and fail to operate when needed.

:books: Continue Exploring Industrial Control Systems

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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.