By Norman Carnt, Technical Manager, Finder plc

Processes, machine control and automation are becoming ever more complex, and as the cost-to-performance ratio for electronics continues to fall, industrial controllers have become even more of a commodity item, offering an amount of ‘control power’ sufficient to handle most applications. However, one considerable drawback of this trend is that controllers invariably provide fixed and inflexible electrical interfaces.

This inflexibility is almost inevitable since no single interface can satisfy the disparate needs of switching both thermocouples and high-power contactor coils. And since the provision of optimised plug-in interfaces is seen as too costly a feature for the standard industrial controller, no ‘easy fix’ exists within the controller. This point, combined with the need for quick and easy replacement of worn or damaged interfaces, has meant the use of external interface relay modules has become the de facto solution.


A number of interface relay module styles are currently on offer to provide the bridge between the industrial controller and the increasing number of real-world control scenarios. At one time the most common style simply incorporated multiple PCB relays on a single board, which was in turn mounted on a 35mm rail. This could be helpful where an enhanced current rating was needed across a block of outputs, but it tended to hinder the tailoring of outputs from channel to channel and was also bulky, taking up considerable panel space.

Today, the preferred solutions are of an ultra-slim single pole module type, particularly those equipped with a replaceable relay element. Here, different versions can be selected channel-by-channel to match the particular requirements of the load, without over or under-specification. The replaceable relay type also offers quick and easy maintenance for those applications where exceptional longevity is paramount.

Development of this approach has led to further benefits in cost and time-saving through innovations such as multi-way jumper links for simple common circuit inter-connection between adjacent interface relay modules and the inclusion of identification labels to aid commissioning and maintenance.

Over time, interface relays have become ‘fully-featured’ ready-made assemblies comprising the relay and socket, with the appropriate coil indication and coil suppression built in. Should it be necessary to switch more than one device from a single controller output, then a multi-pole relay interface module could be used, incurring only a small penalty in panel space.

The Finder way

By identifying a relatively limited number of relay/socket/module combinations, and by giving each combination a single part number, Finder has long provided the design engineer with access to a range of ideally matched components that are assembled and ready to accept wiring – frequently offering the additional benefit of saving inventory, purchasing time and cost.

Despite the popularity and flexibility of these devices, there are nevertheless small but significant differences in the ideal feature set of an interface relay – dependent on whether the device is on the input or the output side of the controller. Until now this aspect of design had not been fully addressed.

A new approach

Finder has introduced a new family of ultra-slim relay interface modules that provides a low-cost, space-saving solution for a host of industrial applications. The 39 Series MasterINTERFACE range incorporates an innovative, replaceable fuse for output circuit protection, plus busbars and terminals that substantially simplify input and output wiring.

Housed in a compact 6.2mm wide package, the 39 Series offers integral coil protection circuitry across the gamut, as well as a secure retention and ejection clip, and terminal screws suitable for blade and crosshead screwdrivers. Five dedicated module types are offered, designed to meet the specific requirements for basic, enhanced, input, output and timer interface functions.

Available with electromechanical or solid state relays for general interfacing applications, the MasterBASIC features common link supply terminal positions, allowing the use of a 16-way jumper link.

The enhanced MasterPLUS module adds the option of a quick connection replaceable output fuse and special leakage current suppression versions to mitigate any detrimental effects caused by residual current in the input circuit.

Both the MasterINPUT and MasterOUTPUT incorporate additional busbar jumper links and terminals necessary for the full connection of 2- or 3-wire input devices and output devices respectively, thus reducing component cost, time and panel space.

The MasterTIMER is a multi-function and multi-range device with DIP-switch selection of 4 time scales and 8 functions, with timer delay adjustment achieved via an easy-to-access rotary knob on the top surface of the module.

A range of accessories is available to support the 39 Series relay interface modules, including red, blue and black 16-way jumper links, and dual purpose separators (1.8mm or 6.2mm) for the visual or protective separation of groups of interfaces operating from different supply voltages, or for the protection of cut ends of jumper links.  The MasterADAPTER permits the easy connection of A1/A2 terminals of up to 8 MasterINTERFACE modules to PLC outputs via a 14-way ribbon cable, plus simple 2-wire power supply connection, providing significant wiring savings.

Further technical data and ordering information for Finder’s 39 Series MasterINTERFACE relay interface modules is available at