It seems to me one of the problems you will fight with mechanical switches is oxidation, or corrosion in a more general sense. It is hard (impossible?) to make completely sealed mechanical switches. I am not even sure that gold contacts would not eventually cause problems at such a low level. This is the basic reason for the development of vacuum sealed reed relays and particularly the mercury wetted type.
Now, you only talk about establishing the contact. How about isolation? I am not sure Deoxit is a good enough isolator in your application. It could create leakage currents, particularly after it itself becomes contaminated. Didier KOBB On Sat, Jun 6, 2020 at 2:15 PM ed breya <[email protected]> wrote: > I would like to get any feedback or experience info on whether Deoxit > would help sufficiently to improve mechanical switch contact performance > used in very low (nV) level circuits. I posted this question in the > tekscopes group a while back, but with no response. I was surprised that > there was no info from such a wide-ranging group, but maybe here in > volt-nuts, with narrow focus, it will stand a better chance. Here is > approximately what I wrote, slightly edited, for consideration. > > Then: > > "One of my projects involves boxing up some special transformers and a > switching system, for isolating and dividing down AC signals by large > factors (E-3 to E-6), down as far as the 1 nV RMS region. A fair number > of switching elements are needed to route and select various transformer > output taps to send to the single output connection. There are two > transformers to cover two frequency bands 10 Hz-1 kHz, and 1 kHz-100 > kHz, and each has four outputs for the decade dividing. The switching > elements need to have as low an on resistance (<< 1 ohm) as possible, > and function well at extremely low levels. Since this is an AC system, > I'm not worried about Seebeck effects, just the low level contacting > ability. The primary sides of the transformers are at more normal > levels, so the switches there are not critical. > > I have all sorts of nice low level regular and Hg reed relays that would > do the job, but since the output side is to be isolated, to minimize > interference, it would be much better to do all the switching passively > with mechanical switches. This would avoid needing electricity, and the > proximity problems of having relay coils and capacitance in the low > level environment, and power supply and line noise and ground loops (no > power cord), or messing with battery power. Especially, there would be > no power transformer emissions to worry about. > > I have lots of mechanical switches of all sorts. I'd like to go with a > rotary wafer type to select the transformer taps and frequency ranges. I > have mostly standard Ag plated type contact ones and some Au ones. I can > usually build a switch from pieces for almost any arrangement. > > Another, but quite complicated option is to use low level reeds actuated > by a mechanically driven magnet, but doing that would be a big project > in itself, so scratch that, unless a simple way pops up. Another way is > to make heavy analog switches with big MOSFETs, but that's a lot of > parts, a battery, and issues with capacitance and crosstalk. So, good > old mechanical switching seems the best way to go - if only it can > actually be done. > > The big question then, is how to get a regular, environmentally exposed > contact to be usable in the nV region. Wetting the contacts with Deoxit > is the only practical thing I can picture so far. There is another class > of contact "protectors" based on synthetic oils, but I think I need > chemical action too, so these may not work - I'll have to investigate > that too. > > So anyway, does anyone have experience or knowledge of how Deoxit would > behave for very low level contacts? It may boil down to a "try it and > see" scenario, but it would be nice to have some info in advance." > > Now: > > I have opted for the "try it and see" scenario, and have built it up so > far using mechanical rotary wafer switches. The low-level section will > use all gold plated contacts, with Deoxit G100 applied. Also to be > included, is a "contact cleaning" mode, where some higher DC (couple > volts and mA) can be temporarily applied to all contacts from a built-in > Li cell circuit. Since I don't have enough of the right kinds of Au > plated contact wafer parts available, the higher level input (primary) > side will include some regular Ag plated contact switching too. These > will be chemically cleaned first, then have Deoxit D100 applied. > > I have it mostly mechanically built, almost ready to begin wiring up and > experimenting. This will be ongoing for a while, and I would appreciate > if anyone here does have experience with such low level applications > with exposed mechanical contacts. I know we usually talk about DC here, > while this is an AC application, but the same principles apply. > > Ed > > > > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com and follow the instructions there.
