Wetting Current 1

[dho]

wiki says "In electrical engineering, wetting current (sometimes also spelt as whetting current in archaic sources) is the minimum electric current needing to flow through a contact to break through the surface film resistance.[1] The film of oxidation occurs often in areas with high humidity. Providing a sufficient amount of wetting current is a crucial step in designing systems that use delicate switches with small contact pressure as sensor inputs. Failing to do this might result in switches remaining electrically "open" when pressed, due to contact oxidation.".
we have a pair of switch contact. how actually to find the wetting current? the test setup? as I know, there always have resistance between the contacts, there always will be current whatever how small milli, pico, nano ... flowing.
thanks.

 

[IRstuff]

If this is such a problem, why not use sealed switches, particularly if they're not necessarily high current?

To apply wetting current on a routine basis would seem to run the risk of damaging the switch into a permanently ON condition, i.e., the contacts get fused together.

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[MikeHalloran]

I remember having trouble with this issue on CMOS circuitry.

Sealed or not sealed has nothing to do with it; use of the root word 'wet' is misleading.

ISTR that for a Cherry brand gold cross-point switch, the minimum current was specified as 2 mA.
For switches with less noble metals in the contacts, you may need a bit more current; the number _should_ be in the switch specifications, but it's often buried in fine print, and too often absent.



Mike Halloran
Pembroke Pines, FL, USA

 

[MOHR1951]

Years ago this problem was pretty common in aircraft ignition systems.It was called spot welding,you found it on the points on the timing finger.In alot of instances it was do to a faulty capacitor.One pointer to it was a frosted appearance on the matting surfaces of the points themselves,prolonged "wetting" made the problem obvious as they welded themslves together.

 

[MikeHalloran]

Uh, dho, whoever is supplying your contacts should be able to recommend a wetting current.
I'm not aware of a simple reliable test that can give you a number for your circuit; I get the impression it's sort of an experience thing, so work with the contact supplier, who will have to know a little about your circuit in order to make a recommendation.

In our case, our circuitry was mostly CMOS, and in that product we eventually replaced most of the mechanical switches with Hall effect sensors.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Mohr, I believe the problem you are talking about is a completely separate one.

 

[MOHR1951]

Compositepro,what makes you think it is unrelated to wetting currents?

 

[Compositepro]

Contact welding and metal transfer is caused by sparking when contacts open or bounce when closing. The condenser limits the rate of change in current when the contacts open so they can be far enough apart that the inductive voltage kick does not cause a spark. The wetting current has something to do with a very tiny current threshold having to be maintained while the contacts are closed so that a microscopic insulating layer does not form on the contacts. You might want to Google the issue since I'm not very familiar with it, but I have heard of it before. It is somewhat obscure and hence the OP's question.

 

[dho]

thanks to all
the contacts are Honeywell microswitch 12SX31-T.
parker asked to test/measure the wet current of that.
we asked procedure, setup....
parker asked their customer, ..
parker now said to us to measure the contact resistance with our GW Instek GOM 801H.
i sent an email to Honeywell if they have wet current "rating" for this switch.

 

[WKTaylor]

We had major problems in tropical-oceanic US Air Bases with corrosion on electrical component contacts and other mechanical parts [internal springs, pins, etc]... especially circuit breakers... that often failed intermittently or without warning [failed to trip when subject to overcurrent]. We found that 'exercise' was the best medicine: pull and reset every circuit breaker and manual switches several times... especially in remote or rarely opened circuit panels... during org-level maintenance. This prophylactic action wiped the contacts clean... and if a breaker or switch 'felt wrong'... then the tech could take corrective action to investigate/test/replace it ASAP to prevent catastrophe [electrical fire].

In one theater I even had a massive electrical fire causing the loss of TWO [2] aircraft, simply because 1-of-2 parallel 150-amp conductor wires [cables big a my thumb] developed corrosion on ONE of the four silver-plated contact pins. The current, which was supposed to split evenly eventually... split unevenly due to rising resistance... driving most of the current thru only ONE of the TWO cables. The cable overheated so badly it melted/burned the plastic jacket... and eventually ignited a fiberglass enclosed junction box. How did I discover this??? The first aircraft burned-up on the descent to impact/fire: the second aircraft took a swim in the Pacific ocean... which 'froze' the evidence of wiring fire in that circuit. An inspection of a few jets in maintenance revealed one cable with evidence of molten plastic [insulation jacket] oozing through the CRES wire-protective jacket. What a mind-boggling find. OH YEAH... that first aircraft mishap [FMS]... the fires occurred on the right side of the cockpit behind the copilot; the second mishap the fire was on the left side of the cockpit behind the pilot. On the right side of the cockpit, ~9.0" above the starter generator relay plastic box cover, was an unprotected 3/8"-dia OXYGEN line: the copilot never had a chance when pressurized O2 suddenly blew across the burning box which turned into a torch that burned thru his seat back and then ignited his nylon parachute. He burned-to-death in his ejection seat. The [left-seat] pilot barely survived the ejection/landing with major burns to his right side and a fire-damaged parachute. In this mishap we uncovered TWO unknown critical failure modes.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[MikeHalloran]

Wow, that 12SX31-T is one itty bitty, delicate switch.
Its low force, bifurcated gold contacts are rated for
10 mA MAX at 28 Vdc, or
1 A MAX at 125 VAC.

Here is a page that leads to multiple applicable documents:

http://sensing.honeywell.com/12SX31-T-Premium-Miniature-and-Subminiature-Switches

I looked through the data sheet and could find no mention of 'wetting current'.
Then I looked at the document about "Low Energy Switching" mentioned under 'Technical Information' on this page:

http://sensing.honeywell.com/literature-handler?defId=9837&name=12SX31-T

, where they again don't talk about wetting current, but do talk about minimum voltages for various contact materials.

I'm sure there's even more technical information in there, somewhere, and some of it may possibly be applicable.

Since we here are not, and should not be, tasked with evaluating the applicability of the specified switch for use in the so far unspecified circuits, it would be logical for you, dho, to do it. You have some reading ahead of you. Have fun.

Switches are one of those products that look simple, but are really not simple at all.




Mike Halloran
Pembroke Pines, FL, USA

 

[kontiki99]

I believe I've seen this described in an IEEE systems journal. From memory, the article described that relay switches had a minimum current rating as well as a maximum rating, based on contact metal and tendency to form surface corrosion.

There are large arrays of small signal relay switches used in large avionics automated test equipment builds. Over time, performance could be very unpredictable.

In their discussion, they claimed by exercising these large arrays of relays at a slightly higher than usual voltage, the unit then operated with fewer intermittent faults that could be attributed to the ATE.

I personally have seen this problem running MD80 display signal generators on an old Boeing ATE (about 12 years ago at Delta). Unit reliability was very poor. ATE results were very erratic. At one point, I asked the techs run the same unit on the ATE 10 times in a row. It passed 6 times failed 4.

Unfortunately, those machines really aren't designed for diagnostic work. They basically prove you reassembled the unit correctly and it will test.

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[dho]

asked "What is the wetting current for this switch ? Thanks."
Honeywell "Thank you for your interest in Honeywell Sensing and Control. Wetting current; Typically wetting currents relate to minimum currents at low voltage. Honeywell does not specify minimum currents for electromechanical switches because the environment can impact the performance of the switch contacts. Recommendations for a silver contact switch is a minimum current of 0.1 amps at voltages less than 30 vdc. If you have any questions or need further assistance, please contact us. "

 

[jedward]

Perhaps you can get around the contact issue by use of mercury whetted relays. I incorporated them in some low level current switching gear in about 1988 and have had no trouble with them operating at current levels from 1 uA to 10 mA. The voltage was below 1 volt but the offset was much below 1 mV.
Using mercury poses some hazards.