Minimum Switch Capacity in EMR Relays
Electro-Mechanical Relays (EMRs) often have a minimum switch capacity and Pickering Interfaces generally publish this information on their switching product data sheets. Here is an explanation of the cause and the effect on users.
EMR versus Reed Relay
The minimum capacity issue is generally confined to relays which do not have a hermetic seal and do not have gold (or similar precious metal) contacts. As reed relays have a hermetic seal around the contacts minimum capacity is not an issue which is commonly seen on reed relays other than very high capacity types.
Minimum Capacity Cause
Relay contacts are generally made from selective use of precious metals. These metals have different characteristics. Gold is usually considered to be a soft metal suited to low level switching that does not tarnish with time. It is soft, so when two contacts come together they tend to make good electrical contact under all signal conditions. The contacts are aided by making sure they "wipe" the contacts as they close, pushing any surface materials away from the contact area. However, under higher signal levels there is an increased risk of welding occurring or of contact erosion (especially hot switching DC where metal transfer is a significant problem) which exposes the underlying metals on the contact. So while gold generally makes a good contact material for low level signals it is a poor material for higher hot switching levels.
Underlying materials or materials which resist contact erosion more effectively than gold are more prone to surface oxidization (or other contaminants such as vapors from plastic cases) which either do not conduct as well or do not conduct at all at low signal levels. At high signal levels there is sufficient energy in the signal to punch through the surface layers and provide a good electrical contact. As a consequence higher power relays have a more variable low power contact resistance—the behavior being dependent on how the relay is used, how many high level switching operations have been performed, the degree of hermetic seal in the package and the pollutants (from outside or from inside the package) that are present.
Consequently the relay may have a minimum capacity for switching, for levels below the this signal the contact resistant may be higher and in some cases even an open circuit. If a relay is used in a benign environment and does not suffer significant contact erosion it may be able to handle low level signals for many years with no problems. If a relay is to be used for both high and low signal levels it may be essential to handle the signal in a different way with composite relays to ensure good operation as, for example, Pickering Interfaces does with the 7-Channel PXI Fault Insertion switch (model 40-194).
Cold switching high capacity relays may result in the relay having higher than expected contact resistance which varies with time (typically falling with time). Pickering Interfaces experience with high capacity relays is that relay vendor specifications are often a l significant simplification of the effect and low level sign resistance variation is different to their claimed specification and much more complex than is first appreciated - often their data sheet contain notes to that effect. Some vendors do not describe the effect at.
Some EMR relays are described as having gold flash contacts, this should not be confused with relays where the prime contact material is gold. Gold flash is present to protect the contacts between manufacture and first use, the flash may not last long before the underlying materials are exposed. New relays may therefore behave differently to used relays. Typically relays with a rating of 2A or less do use gold contacts for much of their life.
Caution on DMM Use
Many DMM's measure resistance with relatively low voltage and low current (1mA is common). This can cause measurement problems on relays with a minimum capacity since the DMM may show a higher resistance than expected, or even an open contact, when in reality the relay is performing to specification. If the user suspects this is the case a few hot signal switches (or even rapid dry switch operations that rely on the wiping action of the contacts to clean them) may eliminate the problem. The issue only arises on relays that have switching on contacts that are not gold (EMR) or are not reed relays.
Pickering interfaces switching systems are not measured with a DMM in manufacturing, they are measured using a cold switch current appropriate to the switching system of up to 1A.
Pickering Interfaces also has two diagnostic test tools, eBIRST and BIRST, that can use higher test currents (up to 30mA) than a DMM. Even so these tools are not used on relays with high switching capacities.