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The Omron VX series datasheet mentions on page 6 that "Using a model for ordinary loads to open or close the contact of a micro load circuit may result in faulty contact.", and shows that their micro load models even still require a minimum of 1mA @ 5V. Their D3V datasheet goes further and explicitly marks the low end as being an "inoperable range". But neither datasheet explains why a microswitch would have a problem with a low load condition. Why is this, and what would happen if a microswitch was operated at loads lower than the minimum operating range?

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    \$\begingroup\$ This has been asked several time. Search for "wetting current". TL;DR: The contacts oxygenize and it needs a minium current and voltage to "burn" them free. Gold contacts typically have much lower minimal current. \$\endgroup\$
    – Andy
    Oct 26, 2018 at 6:27
  • \$\begingroup\$ Good question, I think it's related to the contact coating, micro load are gold coated and normal load are silver coated and probably has to do with silver oxidizing and needing a minimum current to break the oxide layer when contacting, the gold contact doesn't have this issue but might in other hand be more mechanically sensitive. \$\endgroup\$
    – Damien
    Oct 26, 2018 at 6:28
  • \$\begingroup\$ Another good resource worth a read: allaboutcircuits.com/textbook/digital/chpt-4/… \$\endgroup\$
    – Andy
    Oct 26, 2018 at 14:31

1 Answer 1

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The switch is available in Gold (Au) and Silver (Ag) alloy.
Gold allows switching of low voltage and current signals, such as measurements.
Silver allows switching of high voltage and current, such as power and coils.

This difference is because the contact material degrades over time and per switching action, and a low voltage might not be able to get any current though the high resistance oxidation layer on the contacts.

This why gold plated contacts exist. (Au)
However, high current arcing destroys the gold plating, which is why Silver Nickel + Gold plating exist (AgNi + Au) to get the best of both.
When using low current, the gold remains intact, but when switching high current the gold is destroyed and the properties of a Silver Nickel contact apply.

See also the Fundamentals of Relay Technology.

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    \$\begingroup\$ And there are AgNI + Au contacts where the gold coating is so thin that it only prevents oxidation while storage and gets mechanically destroyed after the first few switch cycles \$\endgroup\$
    – Andy
    Oct 26, 2018 at 6:32
  • \$\begingroup\$ So, if I understand this correctly, the actual failure mode is an increase in contact resistance over time resulting in the pull-up being too strong relative to the switch contact resistance? Wouldn't this be a violation of the contact resistance spec, or does >1mA of current have some sort of healing effect that removes the oxidation? \$\endgroup\$
    – bdonlan
    Oct 26, 2018 at 7:17
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    \$\begingroup\$ Found the answer in the PDF you linked: "In addition to the constant cleaning effect due to contact friction, this switching spark assumes the function of penetrating non-conductive contamination layers that are formed on the contact surfaces of power contacts." \$\endgroup\$
    – bdonlan
    Oct 26, 2018 at 7:22

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