Wetting current for NC pressure switch - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: March 26, 2022, 03:21:13 am
content: Hi,I have a system with a normally closed pressure switch. I have not yet decided the exact model but am wondering about wetting current.On one side you read everywhere that the minimum wetting current and wetting voltages must be applied to clear and prevent any oxide on the contacts which forms if too little current flows. But on some datasheets you also read a spec for "Dry Switching Maximum Recommended Current" which is often around 10mA 24V dc. Like on page 7 of this datasheet:https://www.farnell.com/datasheets/11929.pdfSuggesting that low currents do not form oxide? Or is it metal dependent?So wondering about...:- With a NC switch which remains closed 99% of the product's lifetime (whether the power is on or off) do I still need to worry about providing enough wetting current because the oxide still forms due to non-perfect contact between the contacts? Or is it required only if the switch opens and closes?- Does the "Dry SwitchingMaximumRecommended Current" spec (on that datasheet for example) apply only to noble metals plated contacts? Why Maximum? Does it mean it can work even with micro-amps or is there still a minimum?- with a switch that requires 24V and 100mA wetting current, that wastes 2.4W of power across the pull-up… that is a lot… is it normal? am I missing something? what do you do in low power applications?And why don't normal push button switches specify or mention wetting current?Thank you

author: CaptDon
author_rank: Super Contributor
timestamp: March 26, 2022, 07:06:18 pm
content: Some switches provide a physical wiping action when opening and closing to break up the oxide layer. I had a dry switching problem with relay contacts and had to buy the much more expensive bifurcated noble metal relays. I am unfamiliar with a 'max' dry switching current and would have assumed anything up to the full capacity of the switch would be fine?

author: edpalmer42
author_rank: Super Contributor
timestamp: March 26, 2022, 07:30:37 pm
content: I think that spec. is a typo. If you look elsewhere in the document you'll find "Dry SwitchingMinimumCurrent", which makes sense, but no other references to "Dry SwitchingMaximumCurrent" which doesn't make sense. AFAIK, 'Dry Switching' refers to switching without current so what would 'Dry Switching Maximum Current' mean?Ed

author: Ian.M
author_rank: Super Contributor
timestamp: March 26, 2022, 07:45:37 pm
content: I'd bet it means: If you exceed the specified dry switching current and/or voltage, there may be enough energy to erode the precious metal plating which will rapidly lead to it becoming unsuitable for dry switching. i.e. will subsequently require wetting current/voltage.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 27, 2022, 01:29:40 am
content: Quote from: Ian.M on March 26, 2022, 07:45:37 pmThat seems unlikely since other switches in the document specify 'Dry SwitchingMinimumCurrent'. Not only does this oneNOTlist that spec, but it's the only one that lists 'Dry SwitchingMaximumCurrent'.Ed

author: Ian.M
author_rank: Super Contributor
timestamp: March 27, 2022, 01:36:40 am
content: 'Dry' switching shouldn't *HAVE* a minimum current. If there's a minimum current specified, that's a wetting current so cant be regarded as dry switching.

author: ricko_uk
author_rank: Super Contributor
timestamp: March 27, 2022, 04:28:46 pm
content: Thank you allStill wondering if it is a safety critical application, a NC switch that remains closed 99% of the product's lifetime (whether the power is on or off) do I still need to worry about providing enough wetting current to ensure reliability? Perhaps because the oxide still forms due to microscopic non-perfect contact between the contacts and creeps into the area where there actually is contact and then break it?Also how do I know if a specific switch provides a wiping action that clears the oxide?Thank you