1 Finder Plc - Interface relay technology brings greater flexibility

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Interface relay technology brings greater flexibility

21 March 2012

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. Norman Carnt reports.

Figure 1.  The Master Plus relay interface module offers the unique feature of an integral replaceable fuse in the output circuit

This inflexibility is almost inevitable since no single interface can satisfy, for example, 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, for example, 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 very bulky, taking up a lot of panel space.

Today, the preferred solutions are of an ultraslim 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.

Until recently, this range of interface relay modules has comprised seven product types, each with its own key features, where the following design choices are catered for:

-  1, 2, 3 or 4 changeover volt-free contacts.

Figure 2.  The Master Input interface module shown here used with a 3-wire inductive proximity sensor

-  A choice of contact current ratings up to 16A.

-  A choice of contact material; from gold plate for low level high reliability switching, to silver tin oxide for high current inrush applications such as lighting.

- A choice between electromechanical relays for their isolation and simplicity of use characteristics, and solid state relays for their life cycle and switching speed.

All of the interface relays offer 35mm rail mounting, some also with direct chassis mounting.  All feature an integral coil indication LED for instant status recognition, and for simplifying diagnostics, as well as an integral suppression of relay coil back EMF.

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

With this in mind, Finder has designed and recently introduced a new range of relay interface modules called the Master 39 Series.  The progeny of the successful ultraslim 38 Series which addressed the mainstream market for single pole devices, this latest series significantly incorporates innovative terminal arrangements that make interfacing input and output devices especially easy and cost effective, with the unique feature of an integral replaceable fuse in the output circuit.

‘Master’ product variants within the range are targeted at general use (‘Basic’, ‘Plus’ and ‘Timer’), and then specifically at output interfacing (‘Output’) and input interfacing (‘Input’):

The Master Basic relay interface module is without the output fuse or additional common terminals. In this sense it is not specifically oriented one way or the other and can perform both input and output functions, albeit the user may have to provide additional termination points.

The Master Plus relay interface module (Figure 1) is one step up from the Master Basic, the main difference being an optional fuse in the output circuit.  It also has a wider input voltage range than the Master Basic, and leakage current suppression versions for 125 and 230v AC are also available.

Figure 3. The Master Timer adds a manually adjustable multifunction timer to either an input or output of a PLC/controller

The Master Output interface module is designed specifically for easy and efficient connection of multiple loads, such as electromagnetic valves or similar.  The PLC, or similar controller, switches the interface relay coil in the usual way.  The supply live for the load circuits is distributed to all modules via a simple jumper link inserted into terminal 11 (terminal 13 for SSR version).  The switched live for each load is available at terminal 14 of each module and the supply return is distributed to all modules via another jumper link, and is then available to wire to each load at terminal BB of each module. As a result, the wiring and termination of each load is both easy and efficient, since the common power connections and the individual switched wires are all terminated locally at the respective interface module.

The Master Input interface module (Figure 2) is designed specifically for easy and efficient connection of multiple input devices, such as conventional mechanical switches or 2 or 3-wire proximity sensors, or similar.  The supply positive for the switches/sensors can be efficiently distributed to all the modules via a jumper link, and is then made available for connection to each input device at terminal BB on each module. 

The supply negative is easily distributed to all the common A2 connections of each interface module relay coil, using the jumper link.  A 3-wire sensor will need to pick up the same supply negative, and connection of the sensor’s negative wire can be made directly to terminal A2 on the respective module  (this connection is not needed for 2-wire switches and sensors).  The switched positive from each switch/sensor is wired to terminal A1, energizing the Interface relay coil. 

Like the Master Output interface module, the wiring and termination of each input switch/sensor is both easy and efficient, as the common power connections and the individual switched wires are all terminated locally, at the respective interface module.

The Master Timer (Figure 3) is primarily intended to add a manually adjustable multifunction timer feature to either an input or output of a PLC/controller.  This can be particularly useful when the local adjustment of a time delay might be required, or where a time delay is later found to be necessary and reprogramming the PLC is not a short or long-term option.  The Master Timer also features the optional output fuse module.

The Master 39 Series offers both electromechanical and solid state relays, and is currently available in screw termination style, although screwless will also be available shortly.  Versions with the option of suppressed coil input leakage current circuitry can be the solution for applications suffering from high leakage current levels in the coil circuits, even when the circuit is supposed to be off.  Long cable runs, for example, can give rise to these induced currents, as can older style triac outputs of PLCs.  These special ‘suppressed current’ interface relay types provide an effective countermeasure.

New generation interface relay modules such as those from the Master 39 Series from Finder offer a more flexible means of meeting the often conflicting needs of today’s more demanding industrial automation environment.  The series provides a simple, ready-assembled selection of relay/socket combinations, with a comprehensive range of advanced operational features, in particular fused output circuits and additional terminals for efficient connection of multiple input and output devices.

Norman Carnt is Technical Manager, Finder plc

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