Electromechanical Relays (EMR's) are the most commonly encountered form of mechanical switching in general purpose applications. The manufacturing processes used to create these relays provides a cost effective solution for many switching applications, but users need to be aware of the limitations just as with any other relay technology.
How they work
An EMR has a moving contact that is operated by a push rod which in turn is moved by a magnetic circuit between two positions. The magnetic circuit may be just a coil, or it may be a coil and a magnet. Even when no intentional magnet is included the magnetic circuit may have an unintended magnet caused by iron or nickel content in the materials.
The contacts use a precious metal to provide a reliable on resistance for the intended application. Different metals have different properties, for example gold has good low signal characteristics but is relatively soft so tends to erode more rapidly both through mechanical wear and hot switch erosion of the contacts. Manufacturers may use composite materials for the contacts to try optimize the characteristics. The relay is designed to provide a wiping motion to try to clean the contact area as a closure operation on the relay is in progress.
EMR's are rarely hermetically sealed, they are often in plastic enclosures that provide a degree of sealing which can be aided by resin sealing materials. The plastic itself can out gas contaminants that effect the relay contacts over time, an affect that can be minimized by using a wiping action on contact closure.
Some EMR's have no sealed enclosure at all for good technical reasons. When a high power contact arc is created a plasma forms which is hard to extinguish in a sealed enclosure, having the contact area open allows the plasma to disperse more quickly and without trapping vapours in the relay. In some cases the relay may be obviously open frame, in others it may have a plastic enclosure that semi seals the relay for cleaning processes during assembly, but then has a "nib" which is broken off to reveal a hole which allows free movement of air.
Higher voltages require higher clearance distances and more mechanical movement of the contact blades, which leads to larger devices and more coil current being required.
Higher currents require larger contact areas on the switches to avoid localized heating of the contact materials, again leading to the need for larger magnetic circuits.
Higher currents also result in the use of contact materials which are harder (to resist the mechanical forces involved) and have higher melting points (to improve hot switch currents).
High current materials (typically anything other than gold) often have a minimum current or voltage that the relay is intended to switch. minimum switching capacity. This minimum signal ensures that any contact contaminants breakdown under the contact stress (for applications just requiring low signal levels reed relays can have both longer life and more reliable contact resistance). The relevance of the minimum load condition is affected by how the relay is used in the application and the minimum load issues are more likely to occur nearer the end of life than when new. In some cases minimum load effects appear as a non linear contact resistance which can appear to be time dependent, in extreme cases the relay contacts may appear open under light loads. Relay manufacturers data on minimum load values are not always representative of what users will see on relays used under the conditions that prevail in their application.
Pickering Interfaces invest considerable effort in the testing of EMR relays using the facilities of our sister company Pickering Electronics who have had long experience of relay proving. So we only fit relays that have been thoroughly life tested - we do not rely on manufacturers data sheets.