Relay Specs

[haklesup]

If you look closely at the spec sheet for a relay, you might find a spec for Minimum Switching Capacity, which in the case of the one I am looking at is 100uA, 100mV.<p>Surely a closed relay can have smaller currents through and smaller voltage across it (I do this all the time). So I am having trouble understanding the true meaning of this spec. All the other specs (mainly maximum or nominal values) are easily understood.<p>Can someone explain this to me?

[dyarker]

What kind of relay?

[rshayes]

This may be a characteristic of the contact material.<p>Contacts meant for "dry" operation (low voltage, low current) are usually gold plated. Gold does not form films that can interfere with the contact operation. Since gold is an expensive material, this plating is very thin and can be destroyed by relatively small voltages and currents.<p>Contacts meant for higher power operation usually use a silver alloy or other material for the contact. These materials can form thin films of oxides or other compounds, but these films can usually be broken down by moderate voltages. It appears that this is what your minimum specification is trying to address.<p>Sometimes films will form that are thick enough to require mechanical cleaning. I have seen contacts dirty enough that they would not complete a 110 VAC circuit. This was in a piece of equipment that had probably been stored for years.

[MrAl]

Hello there,<p>On most contacts there is some thin layer of
oxide or other film that is of a different
material then the contact metal. This different
material coats the contact metal forming a
semiconductor. Recall that a semiconductor
is not a true conductor under many circumstances.<p>When the two coated contacts first meet, they
dont really touch...only the films on each contact
really touch, but since the film is usually
very thin there is an initial electron punch
through (called A-Fritting). Once this occurs
a secondary reaction (called B-Fritting) occurs
which widens the conduction path and normal
conduction begins.
Since the initial electron flow requires a
certain level of voltage there may be a certain
min voltage specified for the contacts.
Since the secondary current flow requires a
certain min level of current, there may be a
spec for this current level.<p>On the practical side of the story when a relay
is used for say measurements of voltage there are
usually very low currents involved. The relay
would have to be spec'd for low current operation
in order to operate with a reasonably low
contact resistance.<p>In general the contact resistance goes up for
low voltages/currents. This may or may not
be acceptable for a given application.<p>Take care,
Al