Question about long term contact viability

[phil from seattle]

I'm working on a project that needs a specially constructed mechanical switch. HE and optical are out for a number of reasons. We have a good design but are concerned about oxidation/tarnishing of the contacts.  The device won't be exposed to the elements but is not completely sealed. Are there treatments that might prolong the life of the contacts?  The contacts will only see 5V max.  I could decrease that to 3.3V or maybe even 2.5V if that is a significant benefit.  I've heard of dielectric grease, is that a good idea? I'm a little reluctant to put something viscous on the contact surfaces.  We are currently using stainless steel for the contact material.

[coromonadalix]

gold plating is a first for all pcb surfaces,  conformal coating  .. you know the drill

but for a switch design,  can you give us more  specs,   voltages amperage   isolation 1kv 1.6 kv 2.5  4kv ?  industrial only  or else like medical ? long outdoor uses ??

HE  meaning ?

and you can use dielectric grease to protect the contacts,   apply a good dab of it and do your connection

some marine connectors have special grease in them  who work the same way


but   why would you need  some venting ?  you have completly sealed ones,   but their problems  sit on huge temperature changes,  it may or will create humidity in them  ...

[moffy]

Without knowing the environment, a thick layer of hard gold plating is pretty inert, but I guess it's horses for courses, what exactly are you trying to do? Low voltage mechanical switches have their own unique issues.

[thermistor-guy]

I'm working on a project that needs a specially constructed mechanical switch. HE and optical are out for a number of reasons. We have a good design but are concerned about oxidation/tarnishing of the contacts.  ...

I would talk to an expert in switches, relays, or the contacts themselves e.g. these guys:

  https://www.aps-ct.com/blog/the-best-electrical-contact-manufacturer-in-the-u-s/
  https://www.electrical-contact.com/products/electrical-contacts

There may be an off-the-shelf solution that's similar to what you have, in an industry you're not familiar with.

[ConKbot]

If you're using grease, don't use "light bulb grease"or any other silicone based greases. Contact arcing burns the silicone into various silicon oxides, so your contact life will plummet as it insulates itself with glass powder suspended in the grease. If your contacts have self cleaning wiping action, then the grit just erodes your plating.

[phil from seattle]

gold plating is a first for all pcb surfaces,  conformal coating  .. you know the drill

but for a switch design,  can you give us more  specs,   voltages amperage   isolation 1kv 1.6 kv 2.5  4kv ?  industrial only  or else like medical ? long outdoor uses ??

HE  meaning ?

and you can use dielectric grease to protect the contacts,   apply a good dab of it and do your connection

some marine connectors have special grease in them  who work the same way


but   why would you need  some venting ?  you have completly sealed ones,   but their problems  sit on huge temperature changes,  it may or will create humidity in them  ...

Low Isolation requirements.  HE = hall effect.

Don't need venting, fully sealing makes it more expensive. More worried about gradual intrusion of humid air.

No arcing will happen (at least with 5V, I hope not!).

Yes, gold is good but also expensive.  would like to avoid if possible.

The environment is a workshop so may have heat and humidity swings but at least a covered environment.  may have dust though should be sealed against anything bigger than a couple of microns.

[phil from seattle]

Without knowing the environment, a thick layer of hard gold plating is pretty inert, but I guess it's horses for courses, what exactly are you trying to do? Low voltage mechanical switches have their own unique issues.

Gold = expensive. Trying to keep this fairly low cost. It is for a shop measuring tool. What are the specific issues with low voltage switches are you referring to?

[joeqsmith]

I'm working on a project that needs a specially constructed mechanical switch. HE and optical are out for a number of reasons. We have a good design but are concerned about oxidation/tarnishing of the contacts.  The device won't be exposed to the elements but is not completely sealed. Are there treatments that might prolong the life of the contacts?  The contacts will only see 5V max.  I could decrease that to 3.3V or maybe even 2.5V if that is a significant benefit.  I've heard of dielectric grease, is that a good idea? I'm a little reluctant to put something viscous on the contact surfaces.  We are currently using stainless steel for the contact material.

5V at what current?  1000A, 1uA?  What kind of load (resistive, inductive)?  How many make brake cycles required?  Any kind of switching time requirements?  Do you have a preliminary design you can post the drawing for it?   

[tooki]

Without knowing the environment, a thick layer of hard gold plating is pretty inert, but I guess it's horses for courses, what exactly are you trying to do? Low voltage mechanical switches have their own unique issues.

Gold = expensive. Trying to keep this fairly low cost. It is for a shop measuring tool. What are the specific issues with low voltage switches are you referring to?

Switches often rely on the “self-cleaning” action of arcing itself. If you run such a switch well below the voltage and current levels that cause arcing, the contacts will oxidize over time and without the self-cleaning, will foul and become unreliable. For these applications, switches with special plating (very often gold) are needed.*

(In contrast, gold is very susceptible to arcing damage, so using a gold plated switch on high voltages will damage it quickly.)

Please share a lot more information about the application so we aren’t tiptoeing around in the dark trying to figure out what to ask you.


*As an example, the microswitches used in many modern wireless computer mice are special switch models because they use such low voltages. This becomes a problem when replacing worn-out switches, because these special switch models are not normal stock items at the distributors. You can easily find switches that are mechanically compatible, but their contacts will foul much sooner.)

[edpalmer42]

Magnetic reed switches are harder to source - particularly if you have special mechanical requirements - but the contacts are sealed in an inert atmosphere and will last almost forever in a properly-designed circuit.

Without knowing the environment, a thick layer of hard gold plating is pretty inert, but I guess it's horses for courses, what exactly are you trying to do? Low voltage mechanical switches have their own unique issues.

Gold = expensive. Trying to keep this fairly low cost. It is for a shop measuring tool. What are the specific issues with low voltage switches are you referring to?

Switches often rely on the “self-cleaning” action of arcing itself. If you run such a switch well below the voltage and current levels that cause arcing, the contacts will oxidize over time and without the self-cleaning, will foul and become unreliable. For these applications, switches with special plating (very often gold) are needed.*

I've never heard of a switch or relay that uses arcing to clean the contacts, but I'm not saying it doesn't exist.  Many (most?) switches or relays use a mechanical wiping motion to clean the contact and cut through any dirt or oxide that may be present.  Even with that, AFAIK every switch/relay has a minimum voltage/current that must be present to ensure reliable operation of the contact.

One time, a co-worker was cursing a widely-used circuit that was showing intermittent failures.  I looked at the schematic and questioned him about a particular relay contact.  The contact wasn't gold plated and the circuit didn't provide enough current to ensure reliable operation.  When this was pointed out to the manufacturer, they shipped out boxes of new relays with gold-plated contacts to solve the problem.  Luckily, that relay was socketed so the upgrade was relatively painless.

Ed

[phil from seattle]

I'm working on a project that needs a specially constructed mechanical switch. HE and optical are out for a number of reasons. We have a good design but are concerned about oxidation/tarnishing of the contacts.  The device won't be exposed to the elements but is not completely sealed. Are there treatments that might prolong the life of the contacts?  The contacts will only see 5V max.  I could decrease that to 3.3V or maybe even 2.5V if that is a significant benefit.  I've heard of dielectric grease, is that a good idea? I'm a little reluctant to put something viscous on the contact surfaces.  We are currently using stainless steel for the contact material.

5V at what current?  1000A, 1uA?  What kind of load (resistive, inductive)?  How many make brake cycles required?  Any kind of switching time requirements?  Do you have a preliminary design you can post the drawing for it?

0.5mA.  Could be adjusted, if needed.  Input to a high impedance schmitt trigger IC.  life time cycles in the low 10K range. Life time is expected to be 10+years.  Switch time is not critical.  It is for a DIY kit that a user assembles.  The current design consists of a shorting arm that contacts between two metal balls. Normally closed operation.

Sorry, not trying to play cute.  Just don't want to share all the details at this point. I do appreciate the questions and thoughts.

[duak]

I don't know of any successful low voltage, low current switches made with stainless contacts. The whole raison d'etre of SS is that the chromium atoms form a tough oxide layer around themselves and the iron atoms in the lattice that prevents further corrosion.  Any conduction through this oxide layer is either at point contacts or where the voltage applied is high enough to push through the oxide. AFAIK you will need at least 15 VDC and current on the order of 10 mA to reliably do the job. Note that any corrosion will increase these values.  I remember seeing a paper on this on one of the relay manufacturers' website. The paper also talked about a minimum energy to clear the contacts.

It might be possible to make something that meets the specs if the contacts mechanically wiped or slid past each other that removed the oxide to form a gas tight contact. A flat spring as one contact against a small ball could do this if there was some lateral movement.

It might be possible to use the anti-corrosion compound that is used between aluminum conductors and terminals in electrical wiring.  T&B makes something but I'm blanking on the name.

Cheers,

[Ian.M]

Sounds like some sort of precision Z probe.  I'd bet you are trying to sense the exact position where the shorting arm is deflected away from the contact balls.   

As Duak has pointed out, Stainless steel relies on its chromium oxide surface film for its corrosion resistance.  You *will* have to break through this film to get a reliable contact. 

[phil from seattle]

Thanks guys!

[Someone]

I've never heard of a switch or relay that uses arcing to clean the contacts, but I'm not saying it doesn't exist.

Various mechanical switches and relays specify a minimum switching current:
https://en.wikipedia.org/wiki/Wetting_current
https://www.nkkswitches.com/pdf/ElectricalRatings.pdf
Pretty common when you are trying to get very high actuation life.

[joeqsmith]

I've never heard of a switch or relay that uses arcing to clean the contacts, but I'm not saying it doesn't exist.

Various mechanical switches and relays specify a minimum switching current:
...
Pretty common when you are trying to get very high actuation life.

True.  It was common practice to use a small amount of current in automotive mechanical switches to get the life out of them.   Some of those were rated in the the millions of cycles.   The last fully custom switch I worked on I think was 100k cycles at 20A resistive load.   I did a fair amount of testing before going into production.  I think it's been out in the wild for six or so years now with no failures.   

My advice, test the crap out of it!!

[edpalmer42]

I've never heard of a switch or relay that uses arcing to clean the contacts, but I'm not saying it doesn't exist.

Various mechanical switches and relays specify a minimum switching current:
https://en.wikipedia.org/wiki/Wetting_current
https://www.nkkswitches.com/pdf/ElectricalRatings.pdf
Pretty common when you are trying to get very high actuation life.

I see - mostly a matter of definitions.  To me, arcing is a high energy phenomena that, over time, will damage the contact and must be suppressed to obtain long contact life.  I would never think of an arc as something that would clean a contact.

Ed

[jonpaul]

R&D at Bell Labs/Western Electric on telephone contacts yielded the self wiping, bifurcated designed rated for 40 YEARS of continous  operation in copper based telephone equipment, CO etc.

See the "dry contact" rated relays commenshally available.

Check old BSTJ papers on relay contacts.

Jon

[SiliconWizard]

Also, don't confuse the minimum wetting current with arcing.

[Spar59]

I am no expert on switch design but have encountered many designs during my time as a substation commissionming engineer.

My own experience suggests the best types are those with contact wipe as they are effectively self cleaning, those where the contacts simply move directly together until they touch suffer from higher contact resistance over time due to oxidation which the contact action does not remove.  However thin layers of oxide will arc through and connection will be restored (we are talking about a micro-arc here not a big flash) this does however require a minimum voltage and current, these are sometimes stated in switch specification sheets, so a contact may read open circuit with a multimeter but be fine at 24V with a few milliamps.

Gold plating is obviously ideal but has the downside in that gold is realtively soft and may erode over many switch operations, so definitely ideal for a switch that is operated extremely infrequently but pointless for a switch that operates regularly unless the environmental conditions favour rapid oxidation.

Contact grease may be detrimental for circuits operating at low voltage and current, if the contacts are not a wiping type.