[Milsurplus] [ARC5] roller inductors

[Bill Carns]

It is always good to have actual “Data”.

 

While this is less than 100 % accurate, it does give an indication of the relative contact resistance of room temperature AG2S, the most common type of Silver Sulfide.

 

I happen to have a water well that has water that contains a relatively high concentration of Sulphur. The silver in the house regularly takes on a distinct dark tarnish. I resist polishing very often so as to not lose the Silver.

 

I have one set of very high quality older solid Silver candlesticks that are severely tarnished with Silver Sulfide.

 

It is extremely difficult to measure the contact resistance of Silver Sulfide due to the mechanical invasiveness of the contacts being used to perform the measurements. So I performed an experiment that would take mechanical disruption off the table.

 

I measured the conduction through two people so that I could put the ohmmeter in-between the two  (each holding one lead in a never changing configuration) and then establishing a baseline contact path around the loop by each person carefully touching the same contact area of the other’s free finger on one hand – thus trying to take a “Standard” measurement of the resistance around that loop. It averaged about 480K ohms.  Pretty predictable actually and not a big function of pressure.

 

I then broke that loop and inserted three different conductors. A flat blade very shiny stainless steel knife, a nickel chromed steel tool surface and then a tin/nickel plated copper HC-6 crystal can. I did this by have each of us touch a flat surface closely spaced but not touching each other. The later (HC-6 plated surface) known to be a very good conductor and have good stable contact resistance.

 

The results were predictable. The Stainless closed loop with the same finger surface area and pressure averaged about 700K ohms, the Nickle Chrome tool surface was about 800K ohms and the polished cleaned Tin/Nickel HC-6 contact loop averaged 520K ohms.

 

So, the best contact resistance of the three surfaces (the HC-6) added about 40K Ohms with a closely controlled (footprint of contact) loop.

 

Now, here is the interesting part.  The same test run at exactly the same time and using the same finger area of contact (and repeatedly compared back and forth) gave the same closed loop reading on the severely tarnished Silver area. About 520K ohms.

 

While this test and experiment is by no means accurate to 26 decimal places, it does tell us some things pretty conclusively. When every precaution is taken to take film disruption out of the contact resistance equation, the surface contact resistance of tarnished Silver – certainly to within less than an order of magnitude - is not significantly worse than that of a normal non-oxidized cleaned nickel tin surface which is also regularly used for contact purposes.

 

This data would indicate that certainly Silver Sulfide is not an insulator and that its practical contact resistance is not significantly different than other commonly used contact systems.

 

I closed out this rather crude experiment by polishing one area of the base of the candlestick and then reran a comparison between the polished area and the original severely tarnished area..  There was NO detectable difference and if anything I saw an interesting slight increase where I had polished the surface and then cleaned it with Isopropyl Alcohol.

 

I submit the above for what it’s worth.  Just some interesting factual albeit crude data.

 

Finally, the system of plating that Collins specified on their switch contacts is Nickel then Silver w/ 5% Nickel. These switches regularly tarnish and turn dark grey/black. The gentleman that ran the internal rework and repair area at Collins Radio for decades is a good friend. He is now still in the business of repairing Collins for the government – among others – and uses only Lab Grade Isopropyl Alcohol to clean switch contacts. He states firmly that you do not have to remove the tarnish.  Anecdotally, there are thousands of 50 year old pieces of Collins gear out there that are still running and that have tarnished Silver switch contacts. When kept clean with normal solvents like Isopropyl Alcohol, they are still working reliably. Some have never been cleaned and still work well.

 

What I garner from all of this is that, kept clean of other contaminants (lubricants become contaminants), Silver plated contacts (and particularly moving contacts) are pretty reliable when kept clean.  Working aggressively (and that is what it takes) to remove all of that tarnish just risks a lot more damage and is not necessary. It also (over time and repeated cleanings) removes more and more Silver.

 

Bill

 

 

From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Bill Carns
Sent: Tuesday, January 26, 2016 1:40 AM
To: 'Brian' <brianclarke01 at optusnet.com.au>
Cc: 'ARC-5 List' <arc5 at mailman.qth.net>; 'Milsurplus' <Milsurplus at mailman.qth.net>
Subject: Re: [Milsurplus] [ARC5] roller inductors

 

:-)  Ah….Now we are into it, and taking on the subtleties – are we?  Hmmmm….  Re the statement: The myth of silver sulfide being a conductor seems to be promulgated ad uendum by the chemically inert / unsophisticated.  Who is inert ??  Not me boy.

 

Yes, to be technically correct, Silver Sulfide is a “semiconductor” meaning it has typically lower conductivity than the usual conductors such as the commonly used copper or silver.. . . . . But that is a relative term and the use of the word “semiconductor” a perhaps not useful here generalization.

 

Silver Sulfide actually is not a “True” semiconductor in the physical sense of the word. A true semiconductor, particularly an intrinsic one, can be a very poor conductor.  This is true of the very low conductivity (Rhos) of intrinsic Germanium or Silicon – those elements that are usually thought of when you say the word “Semiconductor”  - in the physical sense. The extrinsic conductivity of any true semiconductor can be - of course -  put anywhere you want it by selecting the doping level desired.

 

To be more specific, the actual conductivity of Silver Sulfide is substantially more conductive than intrinsic, or even lightly normally doped Silicon or Germanium, and Silver Sulfide is certainly not an insulator - nor does it exhibit unidirectional conductivity characteristics as in a semiconductor junction device.  Silver Sulfide is a good conductor compared to what one would call a “normal” semiconductor. You should note, in fact, Silver Sulfide exhibits both electronic and Ionic conduction and both components are present under almost all normal environmental condition. This dual mode of conduction - in itself - distinguishes Silver Sulfide from the true physical definition of a “Semiconductor” material.

 

So, to try and negate my observation that Silver Sulfide is conductive, by saying that it is really a semiconductor is to jump into relativity land. I repeat my statement, that Silver Sulfide is conductive and therefore – NOT AN INSULTOR. It does, however, raise the contact resistance until molested (see below - and as you pointed out).

 

Some other notes:

 

I quote from a report by Tyco Electronics on the use of Silver in mechanically connected systems where tarnish can occur: “Silver tarnish films: Silver tarnish films can be many colors, anywhere from yellow to tan to blue to black. . . .A silver plated contact surface can appear discolored and still function very well if used correctly in the application. Tarnish films generated on silver finished surfaces exposed to most connector field applications are predominantly covalently bonded semiconducting α silver sulfide (Ag2S) and to a lesser extent, small amounts of insulating and harder to displace silver chloride (AgCl)

 

Notice that nowhere does it refer to Silver Oxide - which rarely forms (or forms extremely slowly) under most common environmental conditions.

 

After spending a career working in the semiconductor industry and using and designing with various metallic and semiconductor systems, I can assure you that Silver Sulfide is “relatively” conductive.  Is it as good a conductor as Silver or Copper ?  -  -  No. But, Silver Sulfide is a very soft film and typically running, or particularly wiping, contacts will overcome the increase in contact resistance that ensues when a Silver conductor or contact area is exposed to Sulphur and forms a Sulfide. Typically it will not, particularly in a running contact system, interfere with RF conduction.


Re your comments about the tarnish remover and how to handle, or “Not handle”, polishing the roller contact area, I could not agree more.

 

Just as an aside, there is a very sad story about an engineer at the National Bureau of standards that saw that some of their calibration standards were exhibiting contact resistance problems. They had Silver plated contacts. He got out his TarnX – seriously – and polished all the contacts. He then measured the standards again and found the problems gone. However, after a very short period of time, the contacts became completely unusable and when examined under a microscope, they were observed to be uniformly etched and completely unusable.  It was “New Standards” time.

 

Bill

 

 

From: Brian [mailto:brianclarke01 at optusnet.com.au] 
Sent: Monday, January 25, 2016 11:16 PM
To: Bill Carns <wcarns at austin.rr.com <mailto:wcarns at austin.rr.com> >; 'Dennis Monticelli' <dennis.monticelli at gmail.com <mailto:dennis.monticelli at gmail.com> >; 'Joe Munson' <prc74b at fuse.net <mailto:prc74b at fuse.net> >
Cc: 'ARC-5 List' <arc5 at mailman.qth.net <mailto:arc5 at mailman.qth.net> >; 'Milsurplus' <Milsurplus at mailman.qth.net <mailto:Milsurplus at mailman.qth.net> >
Subject: Re: [ARC5] [Milsurplus] roller inductors

 

Please look up the electro-chemistry of silver. In particular, check the conductivity of silver sulphide - it is a semi-conductor. The myth of silver sulphide being a conductor seems to be promulgated ad uendum by the chemically inert / unsophisticated.  The removal of tetra-ethyl lead from petrol / gasoline has exacerbated the effect of sulphur oxides in the air where there are motor vehicles. Pure gasoline has no sulphur – the refiners add various compounds to promote their vision of economic glory. Silver oxide is almost as good a conductor as silver, but is of a similar colour. Only an analytical metallurgist or chemist can tell you what your specific tarnish is. The point I make is that the amount and kind of tarnish you get will depend on what’s in the air where your equipment resides.

 

If your preferred ‘tarnish remover’ can convert silver sulphide back to pure silver and carry off the sulphur, what a lucky person you will be. So will your bank manager. Most ‘tarnish removers’ take away the silver sulphide and leave a larger surface area on the remaining silver – increased porosity – thus increasing susceptibility to air-borne tarnishers. If the silver is a 2 um to 20 um plating, then the tarnish remover may remove the silver completely and irreversibly. Silver sulphide is not the only tarnishing compound. However, proper design of silver-to-silver contacts can wipe away enough silver sulphide and other contaminants to redeem appropriate conductivity. Leaving an oil or grease layer on a roller inductor may protect the untouched underlying silver from air-borne attack. But as soon as you traverse the roller over the wound inductor, you reveal the underlying silver, allowing it to tarnish. This effect can be accelerated by traversing when high current is flowing, giving rise to sparking and formation of contamination. So, tune up at very low power!

 

If all you want is a pretty-pretty roller inductor, then by all means polish it till it reflects and then coat it in a transparent varnish – but don’t expect it to work. If, on the other hand, you want your roller inductor to work, then attach a milli-Ohm meter between the roller support axle and one end of the inductor, traverse through the whole range of the inductor – perhaps a few times – observing in detail the resistance vs traverse position. Deal only with places on the inductor or the roller where resistance is unacceptable.

 

There is a faint possibility you will be pleasantly surprised.

 

73 de Brian, VK2GCE.

 

On Tuesday, January 26, 2016 1:14 PM, Bill said:

 

It should be noted regarding these instructions to “Clean off the oxide” with Deoxit on the roller inductor, that silver tarnish is NOT an oxide, but a Sulfide.  Silver Sulfide is CONDUCTIVE and is not the reason that the contacts are bad somewhere.  Using any cleaner may also remove dirt and other contamination and Deoxit is as good as several others at doing this.  I would use the DL100 Deoxit that does not have a small percent lubricant as does the D5 Deoxit.  It is best to not leave any lubricant or anything at all on the contact. 

 

Many use just lab grade Isopropyl Alcohol for a cleaner.  I use both and they both seem to work equally well. 

 

The following post in the thread after the Deoxit advice that suggest that the problem could be, and probably is, related to the follower spring contact on the contact bar is worth pursuing.

 

While polishing the heck out of those roller contact areas may make you feel good, and it sure looks nice, it is a fool’s errand for the following reason.  There is Sulphur in the atmosphere..  Some environments have it more than others.  That is why your mother’s Silver tarnishes. When the Silver Sulfide forms, it uses up (eats up) a monolayer or two of Silver. The Silver Sulfide is more or less self-limiting.  Once a layer of the Sulfide forms, it way slows down the process.  When you shine it up, you remove all the Sulfide – which includes that Silver Plate monolayer – and the process starts all over again – at high speed…  and the Silver plating gets thinner and thinner.  This is not desirable.

 

So, clean all the contaminants off the roller coil with either Deoxit or Isopropyl and then look for the other causes that may be there as suggested.

 

By the way, do not even think about using Silver Polish..  That is a disaster – especially Tarnex or similar acidic processes.

 

Bill

 

 

 

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