solved: Can NC silver relay contacts fail by sitting idle too long? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 19, 2025, 12:38:15 pm
content: Suppose there is a SPST NC relay with typical cheap silver alloy contacts.The relay sits for years in the normally closed position without moving - the load being only a few hundred microamps at 24V.Then one day a load demanding several amps from the 24V supply comes along.Could the relay contacts fail to deliver (go high impedance) because they sat idle for too long?The contacts won't experience much cleaning action. I'd worry that oxidation/corrosion (sulfur) might creep between the contacts and ruin them without anyone ever noticing.

author: jwet
author_rank: Frequent Contributor
timestamp: April 19, 2025, 04:06:31 pm
content: This definitely happens. Relays have a spec called "wetting current" which is a minimum current. Small signal relays are often cycled and may have test currents run through them for "cleaning". Gold is the gold standard for relay plating. Platinum too. Silver has good conductivity but is not a great contact surface unless its in a wiping application that is sealed.The underlying materials are often exotic, very hard materials like ruthenium and tungsten. The metallurgy and physics are complicated. COTO makes sexy little signal relays for applications like ATE, they have good app notes.

author: tooki
author_rank: Super Contributor
timestamp: April 19, 2025, 04:58:31 pm
content: Quote from: jwet on April 19, 2025, 04:06:31 pmWhoa whoa hold on…This is absolutely not correct, at least as a blanket statement.Gold is one of the preferred platings for relays and switcheswhich are used for extremely small currents. However, gold is not at all resistant to arcing damage, so it is an exceptionallybadplating for relay and switches used for anything other than tiny currents!!This is why it is critical to know a relay or switch’s real application, so that an appropriate model is selected. There is no plating option that is optimal for all applications.In high-reliability applications, a “sealing current” is kept permanently applied to ensure a relay’s contacts are always kept above the minimum wetting current.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: April 20, 2025, 12:38:53 am
content: Silver does tarnish and with age will make a bad contact.Remember Dual Turntable headshells?Jeff

author: bdunham7
author_rank: Super Contributor
timestamp: April 20, 2025, 01:39:42 am
content: I think your contacts are more likely to go bad while the few microamps are going through it. Putting several amps through the contacts would probably fix them even without cycling the relay. This behavior is why there are certain "dry contact" resistance measurement devices (or ranges on a more conventional instrument) that measure resistance with very small currents and voltages.

author: floobydust
author_rank: Super Contributor
timestamp: April 20, 2025, 01:54:52 am
content: There is both a wetting voltage and wetting current, this is to punch through the oxide layer formed on the contacts. As well, maybe half a dozen different contact metals are used depending on if a signal or power relay etc.I hate silver/sulfur oxides, they can rectify making a diode junction of sorts and you can hear this with audio connections sounding absolutely terrible until you increase power (loudspeaker connections).I would say OP has more than enough voltage and current at the contacts to force punch through any old oxide.