Signal relays vs LC2MOS at low levels

On a sunny day (Mon, 16 Sep 2013 13:27:56 -0400) it happened Spehro Pefhany wrote in :

They were useless, would go into latchup if you plugged in any cable, did throw those away, ordered locally from conrad, those worked OK.

I think it's a surface film effect where you can get some kind of ugly metal-oxide semiconductor action until enough current passes to microweld the metal parts together and form a resistive path.

Looking at an SRS 830 Lock-in amplifier front end schematic, and they use similar Hasco relays to what John was mentioning, in an application where the current is set by some leakage and a 10M to ground, so it could be fairly low (~100pA) with a 1 mV level input. There are lots of other examples around, but that's from a mature product.

That particular Chinglish-esque relay datasheet shows no minimum current, and also refers to optional gold-clad silver-palladium contacts, but with no part number variant to cover that option (newer versions of the datasheet show only gold-flash silver-nickel alloy.

My _guess_ is that there is enough wiping action in the contacts closing in a sealed environment that it won't be a problem, provided they _only_ ever switch extremely low currents, but the datasheets from the Japanese suppliers (the only datasheets I take seriously) look to be covering their butts fairly extensively.

My automated ratio transformer has relays, and it goes through a noisy clackety-clack session whenever it's powered, exercising all the relays.

That would be fine for the intended application. Thanks for the info.

That's a good brand. Without further knowlege I would pick Fujitsu-Panasonic/NEC-Omron-Hasco in that order.

The part number you gave has gold-silver-nickel contacts- the low-level variant has 'P' instead of 'Z', but they look to be special-order.

So about 1/3 the data sheet limits. The Fujitsu data sheet has a distribution of operate times.. 1ms for most and 1.5ms for about all, and they might bounce for some hundreds of usec after that.

I got some 3V NS RS-232 drivers that were tetchy as all get out.. latchup if you look at them crossways.

They actually all do, the ones I have measured anyway, goes for ceramics too. If the spec says +/-20%, you would be pretty stupid, financial wise, to add to much material and sell +20% parts. Better to optimize the production and earnings

Cheers

Klaus

currents? (and how that might show up in the circuit.) Do mechanical switch es have the same 'issues'. Or does the sliding /wiping of the contacts hel p keep them clean?

Thanks Spehro, I know next to nothing about metallurgy (Well except that g old is magical.) But it's weird that the underlying materiel would make a d ifference. Gold over some silver alloy... why would the exact silver alloy make a difference? (Yeah, I read some relay app notes that talks about si lver-palladium as 'the best' for low signals.) But one wonders if that's o nly better because it's a better alloy once the gold has been zapped off by a few arcs. I guess I'd look for a contact with a heavy gold layer.. and not some wimpy flash.

Say, I don't suppose there's enough room to put two relays on the critical signal path. (Or would that be worse? Now the dribble of a current is spl it two ways.) Could you arrange it so that you pushed some 'big' (1mA) cur rent through the relay once it was closed? (Clear it's throat, so to speak .)

George H.

The contacts are bifurcated, so you get 2 for the price of 1.

An R + low leakage C through an analog switch could give a > minimum (say 20uA) current for a short time. . Have to think about relative risks with that one.

t gold is magical.) But it's weird that the underlying materiel would make a difference. Gold over some silver alloy... why would the exact silver al loy make a difference? (Yeah, I read some relay app notes that talks about silver-palladium as 'the best' for low signals.) But one wonders if that' s only better because it's a better alloy once the gold has been zapped off by a few arcs. I guess I'd look for a contact with a heavy gold layer.. a nd not some wimpy flash.

al signal path. (Or would that be worse? Now the dribble of a current is split two ways.) Could you arrange it so that you pushed some 'big' (1mA) current through the relay once it was closed? (Clear it's throat, so to sp eak.)

At some point do you think about measureing a bunch? (sounds time consumin g) So what's wrong with analog switches? too much leakage? from where? can you bootstrap it somehow? (I know about as much about analog swtiches as I do about relays.. not much, every time I've used either they've worked fine.)

George H.

Not sure how to get worst-case with testing, maybe run through a solder process and then bake at 125°C+ for a while.

Best regards, Spehro Pefhany

One point which may not be directly relevant in this particular case, but is of more general help when designing with relays:

Make sure they are mounted with the contact gap in the vertical plane.

I know the manufacturers claim they can be mounted any way up, but they give a lot less trouble and last a lot longer when the dirt is able to drop out of the contact gap. This was common knowledge in the 1920s, but seems to have disappeared from the textbooks and engineering courses since then.

On Mon, 16 Sep 2013 17:42:27 GMT in sci.electronics.design, Jan Panteltje wrote,

Will UL certify them with screws?

Bullshit. They degrade over time, till they no longer function as needed.