Wetting Current
Wetting Current
(OP)
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.
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.





RE: Wetting 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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RE: Wetting Current
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
RE: Wetting Current
RE: 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
RE: Wetting Current
RE: Wetting Current
RE: Wetting Current
RE: Wetting Current
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.
RE: Wetting Current
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
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RE: Wetting Current
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-Mini...
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?de...
, 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
RE: Wetting Current
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.
RE: Wetting Current
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. "
RE: Wetting Current
Using mercury poses some hazards.