Isolated variable resistor function?

It was. It was the first thing that Joerg though of, and he spelt out why it wasn't a good solution for his problem in his original post.

-- Bill Sloman, Nijmegen

I've seen data sheet claims of 1e9 cycles. In practise, they sometimes stick closed, or make hi-z closures. The physics is bad: no wiping action, possibility of residual magnetism latching them closed, metalurgical bonding sticking contacts closed, magnetic interaction with adjacent relays. I suspect that getting the reeds exactly aligned, as molten glass cools and hardens, is a trickly production challenge.

No, I've mostly used them in signal-level applications, not power switching.

They don't bounce, but they do twang. By "twang" I mean a sustained mechanical ringdown that generates contact voltage in the magnetic field of the coil. Lasts many milliseconds. It's hell in low-level multiplexing.

Uh, mercury is illegal now.

Reeds really suck.

A Fujitsu DPDT is smaller and cheaper and more reliable than a SPST reed.

I recall that reeds have thermals even with zero coil power. Each end of the reed has a thermocouple to the package leads or copper PCB traces, and they are far apart. I recall measuring microvolts from small local PCB thermal gradients. The reed metal seems to have a big thermal potential against copper.

Reeds suck.

We probably buy them a few hundred, sometimes 1000, at a time.

assembly starts around 4:30

-Lasse

Bounces? On reeds? I've never seen that happen. Is there a certain series of bad applas we should all avoid?

I was just going to place these reeds into a design where I have to artificially "press" the buttons of some gizmo and the currents are microamps, so no big relays possible because of a lack of wetting current:

Well, if they had gold contacts maybe a bigger one would work. But reeds are nice because their contacts are naturally encapsulated.

If gold-contacts, which kind?

I thought of those but dismissed them because they become really non-linear past +/-30mV or so.

use

s.

m,

If you've abused the contacts. If the telephone system used them in millions and found them reliable, it would seem that you are doing something wrong.

But one that was solved a long time ago. You can dream up explanations of why your relays failed for reasons that don't involve you screwing up, but don't expect anybody to take you seriously.

So you haven't been deliberately putting a lot of current through them

- that you know about - but if you haven't got your grounding right, and you ground a subsystem whenever you close a relay you may have been doing it accidentally and unintentionally. Sadly, the relay contacts don't care about intentions.

They shouldn't. The film of liquid mercury between the two contacts should damp any mechanical movement between them, and pretty rapidly too. Mercury isn't all that viscous, but the film is pretty thin.

Releasing it into the environment is frowned on. Inside a hermetically sealed reed it's fine.

In unskilled hands.

In your hands. Do you actually know what's inside the package that you buy?

It certainly does. The nickel-iron alloy used make the reed generates something like 50uV/C - IIRR - against copper, It pays to spread a lot of copper ground plane around the area to minimise any thermal gradients.

In unskilled hands.

Got an exact part number? Or a DigiKey stock number? "Fujistu DPDT relay" is a bit unspecific, but since you don't concoct your own parts lists, you may not have noticed.

-- Bill Sloman, Nijmegen

They do have some inductance, 10 nH-ish, but the capacitance across the open contacts is very much lower than in FET analogue switches. I think the ones John uses (FTR-B3 series, as he says above) are pretty nice parts. I'd love to be able to get MEMS relays built on some technology like the TI micromirrors.

The thermal problem with reeds is made much worse by the high coil dissipation--there's no iron in the magnetic circuit, other than the reeds themselves.

Cheers

Phil Hobbs

You are, as usual, being an obnoxious ass, going personal/insulting in a technical discussion, and pretending to be an authority on things you have no experience with.

I've used thens of thousands of reeds and ditto the little Fujitsu-type telecom relays. The telecoms are better in every respect. I won't use reed relays any more.

Next time you design a volume-production product, use reeds. Get back to us on how they work.

Now you are making things up. Why would a thin film of mercury, between closed contacts, damp a mechanical vibration if the reed structure?

I can't use a mercury relay in an ROHS product. They look to be super-expensive anyhow.

Fathead. What have your hands done lately? Added another turn to your oscillator transformer?

I told you they were the FTR-B3G series. Two that we stock are

FUJITSU FTR-B3GB4.5Z-B10 DPDT SMT 30V 1A 4.5V LATCH OMRON G6KU-2F-Y-DC5

FUJITSU FTR-B3GA4.5Z DPDT SMT 30V 1A 4.5V OMRON G6K-2G-Y-DC5

Every non-mercury em relay that I've ever tested has bounced. Reeds are especially bad; they may close in half a millisecond, then blip open in random patterns for a couple of milliseconds until the vibrations settle down.

Typically looks like this:

"Authorized=no"

I'd have to buy the silly thing to see if it's worth reading.

I used to subscribe to RSI. It wasn't worth it. I downloaded maybe 20 interesting papers from the last 30 years or so, but never actually used them for anything.

Thanks, interesting. I have never seen any relay behaving this badly. Bounce, yes, but only 1-2 times. Never on reeds.

Have to measure some that I've got here. But right know half the world seems to dump their switch mode converter designs on me, so no time. Switchers are fun but it's like eating tomatoes all month :-)

Hey, it's infinitely better than sitting around waiting for the phone to ring.

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Or go to a university library near you to read it in the bound journal.

Don't bother buying it. 1968 is a long time ago, and subsequent authors - including me - have adapted the message for people who don't work for one or other national bureau of standards. But it was ground- breaking in its time.

Because you didn't learn enough at university to be in a position to fully understand or exploit them?

-- Bill Sloman, Nijmegen

Thanks for the hint. Since I am doing a design with relays right now I checked. Couldn't find the Fujitsu at Digikey but found the 12V through-hole version (which I need) from Omron:

Two bucks fifty, ouch. 2x of what the reeds cost. If you say it's worth the extra money I'll plop these into the BOM instead of the reeds.

The closest thing I've seen to a useful instruments-journal paper was one by a Danish guy called Lars Bager, back in about 1990. He had this idea to minimize mode hopping in diode lasers by dithering the bias current, detecting the medium frequency noise power from the monitor photodiode, putting that signal into a lock-in, and using an integrating servo in the usual way to find the minimum.

I built that into one of my gizmos back in the day, but never wound up using it because it wasn't reliable enough for ship in a product.

There are papers about instrument construction that are far, far above RSI and JoP E, e.g. most of the collected works of R. V. Jones, but they don't go in the instrument ghettos^H^H^H^H^H^H^H journals.

Cheers

Phil Hobbs

We're paying $1.38 average for the non-latch version, $1.80 for latching. Both surface mount.

$1.22 in hundreds from Arrow at the moment:

Cheers

Phil Hobbs

I've tried that with polarization hops. But same thing, it only worked as long as there was no earthquake within 5000 miles and nobody sneezed. And not when there was a full moon or a black cat crossed the street.

supervisory

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lots

I always found the idea of an advanced degree, or an academic career, to be repulsive. I got a BSEE because that's what engineers are expected to have. I'm a circuit designer. They don't teach that anyhow.

The papers I loaded from RSI and some other journals had to do mostly with tomographic atom probes and delay line detectors and microchannel plates and FTMS. We actually did a fair amount of work in those areas, with minimal commercial success. We were working with scientists who spun out of universities and started companies. Those things tend to go badly.

I can go over to the UC Medical research facility in Mission Bay, not far away. I can access a mess of journals and print articles for 50 cents per sheet. But that's still a hassle. The food over there isn't all that good either. BARTing to UC Berkeley is another possibility, bigger hassle. It's easiest to hire some grad-school grunt, who has student access, to download papers.