Been looking into sealing current for some contacts.
With little to go on, I'm swagging 20mA for a goal.
But I'm curious why a cap might not also be part of the answer. My thinking is inrush current would do as well as continuous current in cleaning things up. But find little if anything on the problem, and nothing on using caps.
Comments?
What contacts? Small gold-plated contacts work fine at microvolts/nanoamps.
Mercury-wetted reed switches are even better, but you do have to mount them with 15 degrees of vertical.
They last ten times longer than regular reeds, the contact resistance is a bit lower, and a lot more stable from one closure to the next.
Any of the noble metals can be pretty good for low current applications.
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** Wiki mentions using snubber caps.Telephone loops use a whetting current of about 20mA, plus special alloys for the contacts.
.... Phil
Thanks Phil, I was going to ask the OP for a link. He could still add one if it's better.
Tangential question; How much longer is my (the) telephone land line going to exist?
'Exist' is for a long time, but say coverage down by 10-20%, from peak.
George H.
Mercury-wetted reed switches work even better, though you do have to mount them within 15 degrees of the vertical.
Contact resistance is a bit less than you get with noble-metal tipped reeds, you get ten times the operating life-time, and the contact resistance is quite a lot more stable from one contact episode to the next.
Thermocouple voltages are always a problem with reed switches - it's hard to get them down to microvolt levels.
What's a land-line? We have phones thru-out the house, but no "land-line"... just tied to the outside world via the net and the cell.
(and, unfortunately, tested for 911 multiple times in the past few years... without a hitch). ...Jim Thompson
I've still got a rotary phone in the shop. :^) But I expect it to go away sometime soon.
George H.
I still have one too somewhere. I always meant to hook it up again as a novelty but so far have never got around to it.
A relay driving a capacitor can really "seal" things up - as in welding the contacts together...
"Work fine"?? Gold-plated contacts are recommended for "dry" circuits like that.
You forgot to mention BOUNCE...
:
unt them within 15 degrees of the vertical.
eeds, you get ten times the operating life-time, and the contact resistance is quite a lot more stable from one contact episode to the next.
rd to get them down to microvolt levels.
And you didn't bother to point out that mercury-wette4d reeds don't bounce.
That's a bit silly. If you think contact bounce in reed switches is importa nt enough to make a fuss about (and I never found it to present enough of a problem that I had to do something about it) you might at least have taken the time to mention that mercury-wetted reeds don't bounce.
But you are a Baer of very little brain ...
Well, do... or don't... Baer with him...
Baer in mind that the Baer facts in this case indicate that someone's ass may be laid Baer when posting in this Baerly civil newsgroup.
Worse than bounce is twang. After the contacts finally close, the reeds still vibrate in the mag field of the coil, and generate a complex waveform that looks, on a scope, much like the sound of a bell ringing. It can mess up low-level measurement for many milliseconds. Well, if you dare to use reeds for low-level measurement.
Reeds are horrible. You get a huge, sole-sourced, power-hog, bad SPST switch for about 20x the price per contact of a good little telecom-type relay, with horrible thermal errors as a side benefit. The mercury-wets are toxic and position sensitive, and the dry reeds are fragile and unreliable. Both have mag field sensitivities.
Like the penny... costs more than it is worth to make.
Wives are better, and there is a side (or rear) benefit.
Oddly though... they cost *way* more than they are worth to make. (that's what God told me anyway...)
Some things require reed switches.
HV supplies (low power of course) call for expensive HV reed switches in some instances. Not recommended as a best practice if an avoidance can be cheaply engineered in.
A big pain in HV design was setting up a customer with a single supply which he then used in either polarity, and we set up the switchworks for it all by his request. Explicit "changeover only when NOT energized" rules were explicitly given and requirements that he sign off that he was aware that live use would result in failure. I think it was like
6kV. Why he did not want a dual supply setup, I do not remember.He made it about three months before he had to send it back for servicing. Those relays are not cheap.
Is whet the opposite of dhry?
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