EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: ricko_uk on March 26, 2022, 03:21:13 am
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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.pdf (https://www.farnell.com/datasheets/11929.pdf) Suggesting 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 Switching Maximum Recommended 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 :)
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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?
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I think that spec. is a typo. If you look elsewhere in the document you'll find "Dry Switching Minimum Current", which makes sense, but no other references to "Dry Switching Maximum Current" which doesn't make sense. AFAIK, 'Dry Switching' refers to switching without current so what would 'Dry Switching Maximum Current' mean?
Ed
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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.
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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.
That seems unlikely since other switches in the document specify 'Dry Switching Minimum Current'. Not only does this one NOT list that spec, but it's the only one that lists 'Dry Switching Maximum Current'.
Ed
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'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.
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Thank you all
Still 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 :)