Wetting current

There are books written on electrical contacts and their design for various conditions. Unless the current is very small gold tends to weld. Rhodium is better in such cases.

Joe Gwinn

We have a similar issue now. We will sell a general-purpose relay module as part of a modular test system, and we are concerned that once a relay is used to switch power, it may not be good for signals any more.

That may be urban legend. It would be awkward to test.

Aren't you actually interested in the existence of practical contact?

Any voltage detected indicates no contact, ie failure, even if it's supposed to be closed. Ensuring contact shouldn't be the job of the contact monitor - it's the job of the application designer.

Your measuring method shouldn't degrade the degree of isolation expected when the contacts are open, however. So high z sensor with lots of common-mode range.

RL

Do not wire the capacitor directly across the contact - add a series resistor else the peak current could weld delicate contacts. Even tens of pF capacitance can weld reeds. 10-50mA peak should be safe.

Too small a test current could also be problematic. Keep the DC current above 100uA. also too low an open circuit voltage can be insufficient to break down oxides, should be above 3-5V. Min switching of 5V 100uA are figures often quoted in switch/relay datasheets.

Old time telegraph/telephone books from 19th cent-early 20th often had information on wetting as folk were just discovering this stuff then.

piglet

Definitely not an urban legend. I have read a warning about gold plated contacts in some manufacturer's datasheet, possibly Tyco?

piglet

I wonder if mercury-wetted contacts are still available?

Are reed relays available in nitrogen-filled enclodures? This might help avoid contamination and oxidation of the contacts.

I'm pretty sure all reeds are either inert gas filled or for HV types, evacuated, so it would be a good solution.

Got a link to a data sheet or appnote? It would be nice to find a general-purpose relay contact material. Maybe not down to thermocouples, but amps to mA maybe.

Reed relays can weld themselves with zero contact current. They are expensive and flakey.

ISTR that somebody makes gold-over-rhodium contacts, so that they're good for dry circuits as well as power.

Of course as you say, once you blow the gold off it's not a dry-contact relay anymore.

Cheers

Phil Hobbs

Pickering make a wide range of reed relays and provide a lot of advice on how to choose and use them.

John

They made an RTD simulator with a heap of reed relays and resistors. It was a horror. We do it electronically, which avoids all sorts of interesting effects.

My experience is not very extensive but does cover things close to both extremes you seem to be looking at. In the netMCA-3 signals (gain, switches opamp stages/feedbacks, fairly wideband so the currents are not unthinkably low) get switched by latching DPDT relays (no longer in production and alas, our stocks are getting low - replaceable but no drop-in) and these work for years without causing any issues at 14 bit overall precision. At the other extreme I have relays switching on/off ventilator heaters here so they maintain a constant temp. About 2kW each, 220V AC. The relays last for 1-2 winters and then begin to stays stuck, not too badly welded as a kick to the housing unwelds them for some time (usually has to be done during the night when you wake up and notice it...). I'd definitely not trust these relays for signal switching even after a very short use with the ovens. (Obviously the relay types are quite different, the power ones have been different over time, same behaviour though).

Dimiter

====================================================== Dimiter Popoff, TGI

What's some weirdness you see with reed relay logic?

I think I saw it SDS Relais-now Panasonic or Omron but am away from my bookshelf. I think it is also in AoE X-chapters.

See page 4 of this from Phoenix Contact:

piglet

This Axicom databook looks helpful on different contact material properties (circa pages 13-20):

piglet

In the RTD case, horrible transient values when a value is changed. like spiking to 500 deg C for some milliseconds, going from 48 to 49. Engine control computers don't like that.

Reeds have huge thermoelectrics too. Lots of intermetallic splices with a heater roughly in the middle.

I discovered reed twang some years ago. The reed wiggles like a struck bell when it's energized, very complex waveforms lasting many milliseconds. In the coil mag field, that makes biggish voltages.

Just make them all the same and spec them differently, with different markings on the terminals.

You could even make a whole assortment of models, with half the channels spec'd for power, or 25% or 10%.

I seen many examples listed, but no real accurate numbers