Brass contacts?

[dave11282]

Hi all,

we use a design where we have open face brass contacts for level switching control in a shaft, we now want to make our product suitable for external use,

my concern is the possible reaction of this material and rain water, the brass switch plates/plunger carry 24v dc,

are there any possible electrolysis effects too?

thanks

Dave

 

[itsmoked]

More info please;

DC huh?

What kind of shaft?
Switching rate?
Full submersion?
Only rain or hose water too?
Any gases present?

 

[dave11282]

dc = direct current
switch rate = milliAmps
not full submersion
only rain water
is a lift shaft
no gases

 

[itsmoked]

dave11282;

I don't see any problem. 24V will fry off any oxides that form and the brass-to-brass contacts should not plate one to the other. If the water is dirty you could always end up with non conductive material being carried onto the contact points then causing a problem. You might need to consider this ..or not. Perhaps orientation needs to be considered

 

[dave11282]

itsmoked,

thanks, cant find any real adverse info from web searches for brass, my only thought is similar to yours, where the plunger/plate could beacome tarnished over time in an external environment, and possibly create a high impedance contact

 

[Skogsgurra]

Agree with you both. I have found that some extra energy spending can keep contacts a lot cleaner in such an environment. In one case (also 24 V DC), I added an extra resistor to keep current at about 200 mA in order to clean the contacts. Not so good in these days of energy conservation, but these contacts were normally open and closed only once in a while. So total watt-hours over the years will be low...

Gunnar Englund

http://www.gke.org

 

[SolderingGunslinger]

Have you considered silver contacts? Silver Oxide is a better conductor than metallic silver, so some oxidation is a good thing.

I remain,

The Old Soldering Gunslinger

 

[nbucska]

I would consider using a reed switch with moving permanent magnet .

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[dpc]

I might be a little concerned if this was an application where the contacts stayed open for years and years, then had to work reliably when closed. But if the contacts close once in a while, you're probablyh OK.

Silver or silver-plating would probably be more better.

In the electric utility world, we find 125 V dc very effective at punching through any crud that might build up on seldom-used contacts .

 

[itsmoked]

There is also a design technique where you make sure the contacts actually scrub each other. They translate back and forth as it were, which mechanically scrubs them. This might also reduce their life from 10 million closures to 1 million but eliminate the contamination problem. Anyway you look at it, 1 million elevator transits is a long time.

The reed switch is a good idea too! But I suspect you are switching a relay or something that takes too much current.

 

[dave11282]

dpc
"I might be a little concerned if this was an application where the contacts stayed open for years and years, then had to work reliably when closed. But if the contacts close once in a while, you're probablyh OK. "

this is my concern, the 'contacts' are open, because it is an inhouse design for level control,

 

[itsmoked]

Why not the standard commercial tube with doughnut float.. They work very well. Very reliable in horrible environments..