Question about long term contact viability - Page 1
Forum section: electronics / projects

author: phil from seattle
author_rank: Super Contributor
timestamp: December 29, 2023, 11:49:26 pm
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 30, 2023, 01:20:39 am
content: gold plating is a first for all pcb surfaces, conformal coating .. you know the drillbut 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 connectionsome marine connectors have special grease in them who work the same waybut 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 ...

author: moffy
author_rank: Super Contributor
timestamp: December 30, 2023, 04:32:15 am
content: 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.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: January 01, 2024, 11:48:17 am
content: Quote from: phil from seattle on December 29, 2023, 11:49:26 pmI 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-contactsThere may be an off-the-shelf solution that's similar to what you have, in an industry you're not familiar with.

author: ConKbot
author_rank: Super Contributor
timestamp: January 01, 2024, 02:21:04 pm
content: 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.

author: phil from seattle
author_rank: Super Contributor
timestamp: January 01, 2024, 06:05:55 pm
content: Quote from: coromonadalix on December 30, 2023, 01:20:39 amLow 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.

author: phil from seattle
author_rank: Super Contributor
timestamp: January 01, 2024, 06:07:57 pm
content: Quote from: moffy on December 30, 2023, 04:32:15 amGold = 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?

author: joeqsmith
author_rank: Super Contributor
timestamp: January 01, 2024, 06:19:40 pm
content: Quote from: phil from seattle on December 29, 2023, 11:49:26 pm5V 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?

author: tooki
author_rank: Super Contributor
timestamp: January 01, 2024, 06:28:53 pm
content: Quote from: phil from seattle on January 01, 2024, 06:07:57 pmSwitches 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.)

author: edpalmer42
author_rank: Super Contributor
timestamp: January 01, 2024, 07:59:02 pm
content: 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.Quote from: tooki on January 01, 2024, 06:28:53 pmI'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

author: phil from seattle
author_rank: Super Contributor
timestamp: January 01, 2024, 09:06:06 pm
content: Quote from: joeqsmith on January 01, 2024, 06:19:40 pm0.5mA. 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.

author: duak
author_rank: Super Contributor
timestamp: January 01, 2024, 11:12:36 pm
content: 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,

author: Ian.M
author_rank: Super Contributor
timestamp: January 01, 2024, 11:30:30 pm
content: 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.

author: phil from seattle
author_rank: Super Contributor
timestamp: January 01, 2024, 11:32:51 pm
content: Thanks guys!

author: Someone
author_rank: Super Contributor
timestamp: January 02, 2024, 12:19:11 am
content: Quote from: edpalmer42 on January 01, 2024, 07:59:02 pmVarious mechanical switches and relays specify a minimum switching current:https://en.wikipedia.org/wiki/Wetting_currenthttps://www.nkkswitches.com/pdf/ElectricalRatings.pdfPretty common when you are trying to get very high actuation life.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 02, 2024, 04:09:27 am
content: Quote from: Someone on January 02, 2024, 12:19:11 amTrue. 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!!

author: edpalmer42
author_rank: Super Contributor
timestamp: January 02, 2024, 04:50:06 am
content: Quote from: Someone on January 02, 2024, 12:19:11 amI 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

author: jonpaul
author_rank: Super Contributor
timestamp: January 02, 2024, 07:41:38 pm
content: 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

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 02, 2024, 11:59:44 pm
content: Also, don't confuse the minimum wetting current with arcing.

author: Spar59
author_rank: Contributor
timestamp: January 03, 2024, 12:39:08 am
content: 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.