Western Electric made, probably, the best, most reliable electronics in the world, and they made almost all of the components themselves. The reeds that they used were no doubt developed and manufactured specifically for telephone system use.
I recall that the reeds were used in a crossbar matrix and were magnetically latching, so that they could matrix-address the coils.
(I had a comp'd subscription to BSTJ when I was a kid, so I sort of remember some of this stuff. They were also playing with a gas-discharge crossbar switch. Then everything went digital.)
Hey, BSTJ is online!
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Boy, that could knock another big chunk out of my nap schedule.
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And the mercury film acts as an energy absorbing damper, and kills the the twang relatively rapidly.
There's one good reason - reed contacts are hermetically sealed, and regular relays aren't. After prolonged exposure to the wrong kind of air the contacts in the relays you fancy show increased contact resistance, as is mentioned in the data sheet.
My relays have gull-wing style leads, I clamped the leads directly to the 16092A spring clip fixture.I have calibrated the fixture up to the spring clips at some past time- I forget when. My calibration file is old (and probably done with a different clamp-clamp separation), so the instrument did read ~minus 200fF with nothing in the clamps, ~plus
750fF with the G6JU installed. The capacitance reading was quite flat up to 500MHz which the fixture is spec'ed for.
I agree there's a discrepancy. Maybe I did something wrong, it was just a quick-and-dirty test as a reaction to your post.
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John Larkin Highland Technology, Inc
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The ones we buy are sealed. They are amazingly reliable in real life.
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John Larkin Highland Technology, Inc
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I have some 1T 0805 surface-mount resistors around here!
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John Larkin Highland Technology, Inc
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That was switching between transverse modes, which is a horse of an entirely different colour. Diodes don't do that anymore, by and large, except for VCSELs. *Mode hopping* is a fairly complicated thermal-electrical-optical instability that makes diode lasers jump back and forth between longitudinal modes at around 100 kHz, especially in the presence of optical feedback. A backreflection of ~ 10**6 is generally enough to set it off.
They turn up all over the place, but of course not very often, because there aren't many that are that good.
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Phil Hobbs
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It follows from the fact that mercury-wetted relays don't bounce. The process that kills the bounce moves the mercury film around more extensively than the process that damps the twang, but in both cases the mercury sticks the contact areas together and has to move around if there's any displacement between the contacts.
One doesn't need any more data than that to understand what's going on.
But not hermetically sealed, if the data sheet is to be believed. Have you read it yet?
That's just Sturgeons Law - there are a lot more mediocre papers published in every field of science than there are good ones.
It's not precisely Darwinian evolution - the changes from paper to paper as you track back through the citations aren't random - but better papers do get read - and cited - more often than mediocre papers.
And different readers are looking for different things - a rubbish paper drawing lunatic conclusions can still provide useful data.
If the entire reed structure is ringing in multiple modes (which they do... it's obvious from the induced waveforms) and the contacts are closed and pushing against one another, and there's a bit of mercury in the gap, I don't see how that bit of mercury is going to suck up all the mechanical energy that's rattling around in the structure.
You are making all this up. I doubt that you have ever observed the electrical noise from reed relay twang. You probably didn't know it existed until I told you about it.
You have time on your hands: do some experiments.
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John Larkin Highland Technology, Inc
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I'm sure that that bit of mercury is not going to suck up all the mechanical energy rattling around in the structure - the vibrations eventually die away die away even in conventional dry reeds, so internal friction in the rest of structure is going to soak up at least some of that energy - but viscous damping is always a handy extra, if you can arrange it.
I haven't. I've always known that reed coils are noisy because you've got ferro-magnetic metal moving in a magnetic field, and changing the magnetic field as as the contacts open and close, but it's less of a problem than the primary source of noise, which is changing the current in the - appreciable - inductance of the relay coil to open or close the reed in the first place.
All that twang does is a funny-shaped tail to the noise.
Getting excited about the twang is straining at a gnat after swallowing a camel.
Unless you actually want to use reed relays for low-level, millisecond-range signal switching. I've done it, and it can be ugly.
Reeds suck.
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John Larkin Highland Technology, Inc
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Low level millisecond-range switching isn't going to be trivial. Switches always inject noise into the circuit being switched at switch- over.
And so do all the alternatives, but for different reasons. What you've posted here suggests that you could have learned a bit more about reed switches before you tried to use them, and could have thought a bit harder about what was going wrong when they weren't doing what you wanted.
Did you got to the trouble getting reeds with rhodium-plated contacts for your low-level switches? Or did you just buy a packaged reed switch from DigiKey and hope for the best?
Wow. Presumably whatever's inside, dry air or nitrogen or whatever, puddles and freezes in a corner somewhere. I'm surprised it doesn't occasionally freeze up the mechanics.
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I read the data sheets and believed them. What I learned later is that they have some nasty problems, including failure rates maybe 1e4 worse than claimed. What I mainly wanted them to do is open and close reliably.
I read the data sheets on the little Fujitsu telecom relays, and learned later that they are true. Nobody would last in the telecom business if their products weren't very, very reliable.
I mostly bought reeds from the manufacturers, supposedly suitable for my applications. Mostly signal switching, but some cold (ie, zero current at open/closure time) switching of liquid helium level probes. Strange, but several of the reed makers are now out of business.
You seem to have deployed low-hundreds of reeds, and I don't know if you would have gotten feedback on their longterm performance. I've used tens of thousands of reed and conventional relays (about 15k of the Fujitsu telecom types so far) and I *do* get longterm feedback about failures. Yet you keep insisting that you know more about this issue than I do.
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John Larkin Highland Technology Inc
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Hmm I can't picture devices with plastics that would work very well at 77k or below in the first place? Unless I am miss understanding something here ?
In refrigerators the cold plate is located in vacuum. If the relay is not hermetically sealed, I'd expect its interior to get pretty much evacuated while the main vacuum can is pumped. If it doesn't, the interior probably cools via the metallic parts, so that the residual gas condenses on the metal surfaces where the surface tension makes the liquid to spread rather uniformly before it freezes. The frost layer cannot be thick, the solid volume is maybe 700 times less than the gas volume. Still, you made a good point.
I find it amazing that the latching mechanism still works there, it's based on what, a permanent magnet?
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