Relays: Far from Dead

Given the long life of solid state relays and the low cost and wide range of ratings offered by printed circuit (PC) board mount electromechanical relays (EMRs), why would anyone specify a plug-in EMR for a new design? Well, there are still many situations when the plug-in relay represents an excellent solution.

By David C. Thomas, Tyco Electronics February 1, 2008

Given the long life of solid state relays and the low cost and wide range of ratings offered by printed circuit (PC) board mount electromechanical relays (EMRs), why would anyone specify a plug-in EMR for a new design? Well, there are still many situations when the plug-in relay represents an excellent solution. That’s right, the traditional socket mounted, electromechanical relay, often based on designs that are several decades old, is still a highly effective solution to load-switching problems in industrial and commercial applications.

Typically, socket mount EMRs provide excellent isolation, multiple pole configurations, numerous options and easy field replacement all at a relatively low price. Below is a list of frequently asked questions to help you determine when a plug-in EMR may be the best solution for your application.

I use a PLC, why do I need a relay?

Microprocessor-based systems have revolutionized the world of logic control, and they perform switching functions admirably, but their outputs cannot typically handle the heavy loads encountered in many applications. They are ideal for controlling relays that can and do switch those lamps, motors, heaters, solenoids, pumps and other loads. Since the beginning of microprocessor based control systems, relay usage has actually increased. Fewer relays are used for ladder logic as they once were, but many more are used for load switching.

Many times a need arises to modify an existing control system in the field. Perhaps one or more functions must be added, or a circuit must be modified for proper system operation. Plug-in relays and sockets often play key roles in such modifications.

Some control design purists might wince at this thought, believing that reprogramming the system would be the only effective solution; however, in real world applications, system modifications are routinely accomplished in other ways. Adding a properly designed external, relay-based circuit may be a quick, effective solution. Using one or more DIN-rail mountable screw terminal sockets, can be a relatively easy way to add the required relay(s) within the system enclosure.

Why not use a solid state solution?

Solid state relays are great in some, but not all, applications. While solid state relays can offer extremely long life and quiet, fast operation in many applications when properly applied, they are often more expensive than comparably rated electromechanical relays. This cost disadvantage can become much more pronounced if multiple loads need to be switched.

Multi-pole plug-in electromechanical relays are common and typically cost slightly more than single-pole models; however, multi-pole solid state relays often cost significantly more. This is largely due to the relays’ structures. The electromechanical relay can be designed to readily accommodate multiple sets of contacts on a common frame, all controlled by a single coil. The few multi-pole solid state relays that are on the market are usually more similar to multiple, complete solid state relays placed in a common package.

The need for normally closed contacts in a circuit may also limit the use of a solid state solution. Solid state relays offering normally closed contacts are both scarce and costly. On the other hand, normally closed contacts are quite common on plug-in electromechanical relays. Usually, there is not a significant cost difference between normally open and normally closed contacts on electromechanical relays. Additionally, double throw (Form C or changeover) contacts are readily available on electromechanical relays. Another advantage of plug-in electromechanical relays is that a given relay can typically switch both ac and dc loads on its contacts. Conversely, solid state relays must be specifically selected for switching either ac loads or dc loads.

Many solid state relays also require space-consuming heatsinks to achieve their specified ratings, while electromechanical relays do not. And with the leakage current through solid state relays, a load is never completely “off.” As long as an electromechanical relay’s contact-to-contact breakdown voltage is not exceeded, no current flows through its open contacts, and the load can be truly “off.”

Solid state relays are also more susceptible than electromechanical relays to false operation and damage from surges on the line and emitted electrical noise. Additionally, when solid state relays fail, it is typically in the shorted mode (load circuit closed). Conversely, the most common mode of failure for electromechanical relays is open (load circuit open). In some applications, there is no doubt that a solid-state relay is, indeed, the best solution. Electromechanical relays, however, are the appropriate choice for many more applications.

Don’t plug-in relays cost more than PC board relays?

Although a comparably rated PC board relay may be lower in price than a plug-in relay, the initial cost savings means little if the relay must be replaced in the field. Many millions of operations are required of relays in some industrial controls, elevator systems and other equipment. In these instances, a relay may wear out and require replacement.

This is an area where the plug-in relay can really shine. Desoldering a PC board relay and soldering a new relay to the board in the field is impractical in many cases. The process can be time consuming, and a certain level of skill is required in the handling, desoldering, and soldering of a PC board. Sometimes replacing a module containing a PC board on a board is an option, but that module can be more costly than a plug-in relay. Plug-in EMRs are offered in many more multi-pole configurations than are PC board relays. A switching task that might require three single-pole PC board relays might be accomplished with just one three-pole plug-in relay, thus significantly impacting the cost per pole in the application. This can help to offset even the initial cost differential between socket mount and PC board mount relays.

A vast array of plug-in relays, sockets and hold-downs can permit users to quickly and easily remove one relay and insert another. Generally, no tools are required for replacement of the relay. Some sockets mount directly on a chassis, while others mount on DIN rails or PC boards. Users may select sockets with various types of screws, quick-connects, PC terminals or solder lugs for termination. Depending upon the particular relay, socket, mounting position and other application factors such as shock and vibration, some sort of hold down mechanism may be required to secure the relay in the socket. Various types of wire springs, plastic clips and levers are used as hold-downs.

Do plug-in EMRs provide extra features?

Socket-mount relays offer a broad range of features. Of course, they offer many different contact ratings, contact arrangements and coil options, but that is just the beginning. Some models incorporate a push-to-test button, or tab that can be used to manually actuate the contacts, a helpful feature when troubleshooting circuits. This manual actuator incorporates a locking mechanism on some models, allowing the user to lock the contacts in their closed position.

Various types of indicators are also available on plug-in relays. An LED or neon indicator is wired in the coil circuit on some types to provide a visual indication that the coil is being powered. Other types have a brightly colored mechanical flag inside the relay’s case that indicates whether the relay is “on” or “off.”

Plug-in relays are also available with a variety of special function circuits incorporated within their enclosures. Integral diode suppression networks and resistor-capacitor networks are now available on some models. Other types of plug-in relays are offered with external modules that may be plugged into sockets which accommodate both the relay and the module.

Other circuits can be incorporated within the relay’s enclosure to create a time delay relay, voltage monitor, current sensor or other higher-function plug-in device.

Is there room for a socket-mount relay?

Relays and their associated sockets are offered in many different sizes and configurations, and the number continues to grow. Some of the first broadly used plug-in relays were created by simply enclosing small (for their time) chassis mount general purpose relays in a metal can or an enclosure with a clear plastic cover. That was then fitted with a plug like that used on a vacuum tube. Later models of that same relay, measuring about 4 in3(65.5 cm3), are still widely used in industrial controls.

Through the years, relay manufacturers developed even smaller relays, as well as various compact plug-in configurations. As the relays shrank in size, so did the sockets. Today, some of the plug-in relays that are in use have been adapted from products that were originally designed for direct application on PC boards. In some instances sockets have been designed to accept the PC board relay terminals “as is.” In other cases, versions of the relays with strengthened terminals have been designed specifically for socket mounting.

While the concept may be many decades old, many updated plug-in electromechanical relays are now available. Due to their feature set and favorable cost, they remain a viable choice for many applications.

Author Information

David C. Thomas is a product manager at Tyco Electronics.