Detecting switch closure
About 43 yrs ago, I needed to ensure 96 power relays were operating correctly from remote control. They were in a big box for remote power control on my SCADA control system attached to two long umbilical cables to a remote device with lots of batteries, power supplies and research instruments. 

I needed "reliable" relay feedback , so chose (Potter&amp; Brumfield)  P&amp;B 15A and 25A relays with extra low current "Sense" contacts for feedback to a digital MUX for supervisory verification.

There was a problem with the closed  sense contacts appearing open = Fault. It wasn't contact bounce.
 It was contact "rust" or silver oxide on brand new relays.   

I used a 10k pullup for (NO) normally open contacts to detect ="1" but "0"= 1.5mA closed contact to TTL,  going into a TTL logic board,  would sometimes not work.   My fast investigation revealed the contacts were not gold plated an oversight of mine as all modern contacts today &lt;2A must be gold plated but back then, not so.  So the TTL current could not satisfy the 10% typical wetting current needed to burn OFF the oxide.   I was just a fairly new engineer out of Uni. ,so this was a learning experience.

I ran a test program sequence using an HP multiprogrammer and HP2825 computer  to activate each Relay in sequence, but wait for feedback verification then activate the next. It sounded like a machine gun that was jamming....   I'd hit the box and some relays would be sensed closed by jarring a few microns and it would continue then freeze.  These were from a high quality relay company P&amp;B but the silver oxide is no good for low current switching and non-gold plating was an issue for logic currents. 

I fixed it with 22uF Tantalum e-caps across each sense contact and the 5V pullup would charge up the cap and when closed, the cap. discharge arc would burn off the oxide without excessive wear with a 100mΩ ESR and 5V/0.1>=50A pulse in less than 2us. A 5V arc @ ~1V/um means the contacts must be within 5um of oxide thickness of making contact to arc then burn off in microseconds.

Then it sounded like a well-oiled machine. rat-a-tat-tat







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