Manufacturing custom high-precision low-TC resistors - Page 1
Forum section: electronics / general

author: JoeN
author_rank: Frequent Contributor
timestamp: January 27, 2018, 02:03:32 am
content: Is there a thread for that? I tried (title only) search for make/manufacture/custom and resistor/resistors and I didn't see anything.I remember a thread about a board member that does custom low-TC resistors for metrology applications. Can a normal person manufacture their own resistors? I am talking about really good ones, rather than going to Vishay for $50 a pop. Is that completely impractical?

author: amspire
author_rank: Super Contributor
timestamp: January 27, 2018, 02:39:46 am
content: Quote from: JoeN on January 27, 2018, 02:03:32 amhttps://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg462296/#msg462296QuoteEdwin G. Pettis is the person you are thinking of.Can you make your own resistors? Yes and no. It is not easy sourcing good materials to make the resistors. If you can get Manganin wire, it is solderable so you can make decent resistors, but Manganin does have a significant temperature coefficient. The better resistors are made from Evanohm wire. It can only be welded and it takes a lot of skill to do it right. The process of making zero tempco Evanohm wire is not easy, so if you just went and bought Evanohm wire on the Internet somewhere, chances it would also have a significant tempco and possibly drift.Edwin explains it in great detail in that thread.

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 27, 2018, 07:21:37 am
content: hello,here is an interesting method for your consideration. (see attached file)regards.-zia

author: ramon
author_rank: Regular Contributor
timestamp: January 27, 2018, 12:58:15 pm
content: Quote from: JoeN on January 27, 2018, 02:03:32 amI searched that same thread some months ago, and I think Zhtoor once asked that question.But I remember that most of the replies were negatively biased (something like ... don't even try that ... hard to do ... better to buy one, etc ...).So, if you don't mind, let's make this thread the thread about 'Manufacturing custom high-precision low-TC resistors'. With a possitive attitude. Something like 'it could be done' (... however I will not dare to say that it will be cheap, easy, or how long it will take to make it.)QuoteWell, let's do it. We will not know if we don't try first.But the very first problem you will find is that no single alloy manufacturer will ever sell the wire spools to a normal person.I asked every single evanohm manufacturer and they completely ignored my (low quantity) requests. But I was able to find one evanohm spool (actually Nikrothal LX) from some auction site. The specs were really impressive : 3 ppm (from -55 to 25C) and -1ppm (from 25-150C).I made a simple resistor around a TO3P mica sheet and protected with heatshrink tubing. (The picture shows a very first test version with copper instead of Evanohm that i made when I haven't received yet the evanohm spool)Once I received the evanohm spool, I made the resistor. As I didn't have equipment to properly test the TC of the DIY resistor I asked TiN for help, and he kindly tested the resistor TCR using his HP 3458A and TEC environment. (Thanks again Tin!)And the first result was a complete disaster, around 400 ppm. You can still check in his website how that 1.82K resistor compares to a Vishay Z202 10K:https://xdevs.com/ramon_res_z202_test1/I am working on my second DIY resistor on my spare time. It will continue ...

author: amspire
author_rank: Super Contributor
timestamp: January 27, 2018, 01:38:48 pm
content: Ramon, how did you weld the Nikrothal LX wire to the leads?

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 27, 2018, 01:49:33 pm
content: hello,a simpler and maybe practical way would be to get hold of YR NEOHM resistors like these:-https://www.digikey.com/product-detail/en/te-connectivity-passive-product/YR1B10KCC/A105970CT-ND/3477555and select -ve tempco resistors the absolute value of which would be < 15ppm,now add a compensating copper-wire resistor (ca .+3930ppm, measure your wire spool first),voila, you would have a pretty decent temperature compensated resistor.(long term drift and environmental issues is entirely another matter of course)regards.-zia

author: Kleinstein
author_rank: Super Contributor
timestamp: January 27, 2018, 03:19:09 pm
content: To get a really low TC, one has to look at more than just the linear TC, but also have a low second order TC. With 2 coefficients to watch for it gets increasingly difficult to find suitable ones for compensation, as chances are that the second order TC will be similar in one batch of resistors - so not much chance off compensation, but more like a chance of getting at least good TC matching.

author: guenthert
author_rank: Frequent Contributor
timestamp: January 27, 2018, 03:20:11 pm
content: It's hard to see on the picture, but you don't want to wind the wire like a coil. You intend to make a resistor, not an inductor after all.

author: ramon
author_rank: Regular Contributor
timestamp: January 27, 2018, 03:22:42 pm
content: Quote from: amspire on January 27, 2018, 01:38:48 pmI guess that is the second problem, in the top ten list of issues. And this question arise every time evanohm is discussed.Welding is the way we are supposed to make the resistors. This is the first and only option. And it is like there is no other correct way to do a precision resistor.And after many tens of hours searching for a cheap, reliable and repetable solution to this issue, didn't find any single commercial device with all those three requirements. If money is not an issue then you just can go to Amada Miyachi website, contact sales and pick any of their machines. Actually, I recommend to visit they website anyway. It is an amazing place to find technical information about how a good welding can be done, and after reading some of their documents I ordered a molybdenum rod and molybdenum plate, and also a copper rod for some experimental DIY welding ... (that was before I found a place where I could buy cheap welding electrodes from a well known manufacturer of resistance welding machines. Those already made electrodes are much better suited for a reliable welding for many reasons described in Amada Miyachi technical notes, and user manuals of other welding manufacturers) ...But the fact is ... for a DIY resistor do we really need to weld the wire? Why not just crimp?Yes, why not? Even Mr. Pettis said that could be possible on one of his first posts.

author: ramon
author_rank: Regular Contributor
timestamp: January 27, 2018, 03:36:43 pm
content: Quote from: guenthert on January 27, 2018, 03:20:11 pmFor DC, that might be in the lower 5 issues of the top ten list.I could have changed winding direction in the first and second half, or have used a much more complicated ayrton perry winding (not sure if the wire would have keep on the place I wanted), but I am not sure that the TCR would have changed a lot.TC was what I wanted to test on first place. (If the wire was actually <3 ppm as written on the spool) .

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 27, 2018, 06:11:40 pm
content: Quote from: ramon on January 27, 2018, 03:22:42 pmThe only way I know of to properly weld such dissimilar materials such as copper and Evanohm wire is to use a pulse-arc (pulsed plasma discharge) welder like these:https://sunstonewelders.com/If you can't find a good used welder (they are used in the jewelry and dental industries), then there is the possibility to build your own:https://frikkieg.blogspot.com/2010/11/pulse-arc-welds.htmlA shielding gas (Argon) is used during the weld, and once the arc is started, then a regular CD-type discharge is initiated. The weld energy is controlled either by switching with a high power MOSFET bank, or by allowing the entire capacitor to dump through an SCR (and you would have to control the main capacitor voltage before the weld).Using the method, you can even weld a metal to a ceramic! (>8000oK during the plasma arc discharge-- pretty much melts everything except the tungsten electrode).

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 27, 2018, 06:22:00 pm
content: Quote from: ramon on January 27, 2018, 03:36:43 pmWinding the resistor as you have done on a mica card results in very low inductance, because there is very low cross sectional area in the "coil"-- so no need for strange wire configurations. The edges of the mica card cause the wire to be bent on a very tight radius, and this raises the TCR. To relieve this stress of the wire, you have to heat it in an oven for about 8 hours at 150C, and then plunge it into some alcohol chilled by dry-ice. Repeat this cycle as many times as needed to get the TCR back down (and it will not fully recover anyway-- but you do what you can). I would be more concerned about how the heat-shrink is stressing wire over a temperature range-- this might be a good percentage of your TCR change. Try making a "case" for the resistor with a 3D printer instead-- so that the wire touches nothing except the mica card.

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2018, 06:48:31 pm
content: ESI made precision resistors by winding the wire around a mica square so I wonder how good their temperature coefficient was.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 27, 2018, 06:58:48 pm
content: JoeN: "I remember a thread about a board member that does custom low-TC resistors for metrology applications. Can a normal person manufacture their own resistors? I am talking about really good ones, rather than going to Vishay for $50 a pop. Is that completely impractical?"I presume you are talking about PWW resistors with a TCR of 5PPM/°C or less with high stability, then your answer is not very likely. On its face, it looks like it would be fairly simple, just wind some wire on a form, attach leads and there you have it....doesn't work that way in most cases. Manganin (and its derivatives) are indeed solder-able which makes termination much easier but it also has mechanical handling requirements and stress relief needed in order to produce that under 15 PPM/°C TCR spec. If you really want to play with some relatively low TCR resistors, you could obtain some Cupron alloy which is also solder-able and it doesn't need the pampering that Manganin requires.You did not say how you attached leads to the Evanohm wire, obviously it was not by welding AND you CANNOT weld Evanohm to copper leads, I've said this several times in different threads. However you attached the leads, that is the likely cause of your 400PPM/°C TCR, don't blame the wire.Technically, you can crimp the wire BUT you're not going to get a very good joint, I also said why this wasn't a good idea in the threads, crimping Evanohm to copper leads is going to make for a bad joint with lousy results.You can make reasonably good resistors to experiment with using Cupron or even Manganin but you are not going to get high performance resistors you're chasing after. It is not easy, it takes a lot of experience and knowledge and specialized equipment. If it was so easy, everyone would be making resistance standards. I can tell you this, any ideas you come up with to try and make a homemade precision metrology grade wire wound resistor has already been tried many times over the years and all have failed.It is not a case of negative thinking, it is nothing more than reality, you can only go so far with home made resistors and metrology grade is not one of them. It is much more expensive to set up and make PWW resistors correctly than to just buy good resistors and no you don't need to spend anything like $50 on a resistor to get there.Let's say you acquire all of the equipment and materials you need, you've already found out a 'simple' capacitive discharge welder plus weld head isn't cheap (I'll even give you a clue, second hand ones aren't cheap either), you've decided on how you're going to attempt to put wire on a form with some kind of terminations, you've got everything you think you need. How much have you spent, even on the cheap, to get to this point? Next you're going to start trying to make a resistor, you're going to get frustrated pretty quickly because things aren't going the way you expected. You experiment with different ways, nothing seems to be working very good. At this point, how many PWW resistors could you have bought for all the time and money you've invested in trying to make your own?I encourage you to get some Manganin or Cupron wire and experiment with it on various forms, you will learn from this, have some fun and hopefully realize just how hard it is to make really good resistors. This wire can be soldered to normal terminations without expensive equipment, you won't get 5 PPM/°C TCR but it should be a fun and learning experience without so much frustration.Ramon: No you won't find an alloy supplier that will sell 'small' amounts to anybody, they have their clientele who buys wire is some quantity, in my case, I have minimum line item cost of $550.00, no exceptions. Your best bet to find wire is just to goggle for it, such as 'Evanohm', 'Cupron', or 'Manganin'. There is a company in England, Goodfellow Cambridge Limited, that does sell small quantities but they are expensive.There are differences in PWW resistor construction, it depends on the manufacturer on how they are doing it, but in the end they are all similar, how good the terminations are is the main difference. Winding the wire on a mica form has been used for decades, it can work very well, if done properly. Each way of making a PWW resistor requires different techniques and always has compromises and 'flaws' to some degree, there is no 'perfect' way of making a resistor. The SR-104 comes very close but it entails a whole lot of work and time, that is why it isn't cheap, it isn't easy to make.If you tried to solder Evanohm to a copper lead, all you ended up with is a cold solder joint and lousy TCR, actual welding is the only way and it has to be done just right or it doesn't work well.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 27, 2018, 08:59:35 pm
content: Quote from: Edwin G. Pettis on January 27, 2018, 06:58:48 pmYES youCAN. You cannot "CD Weld" (capacitive discharge welding)-- that won't work, but you CAN "Pulse Arc Weld"-- this is like TIG welding, but with an intense pulse of energy rather than a continuous discharge. A gas plasma is very hot (greater than 8000oK)-- and will melt almost anything.Watch THIS (stainless steel weld characteristic is similar to Evanohm):

author: branadic
author_rank: Super Contributor
timestamp: January 27, 2018, 09:27:29 pm
content: QuoteIs this bare wire or is it an enameled wire? Could only find wires without coating on the bay.-branadic-

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 27, 2018, 11:41:15 pm
content: To Branadic: The spool indicates that it is enameled, if you try to make a resistor using the mica card method, if you're careful, you can use bare wire.To M.M.: I was replying to JoeN, as he did not mention how he connected the wires to leads (he didn't say what his leads were either), since it was very unlikely that he had an arc welder, my reply was geared to the more common CD welders which are more commonly available and just plain soldering. If you have read my other postings you'll know exactly what I've said before (many times) about welding resistors.Specifically to your indicated method; technically you can certainly weld dissimilar metals with an arc welder, that is not in dispute, the problem with your method is that is cannot be successfully used for precision wire wound resistors, the reason being (among others) is that the very high heat of the arc will profoundly and permanently change the TCR of the resistance alloy wire as far as the heat is above ~350°C along the wire for Evanohm. Yes you certainly can weld Evanohm to a copper wire/slug with the arc but you are going to mess up the TCR of the wire where ever it was above ~350°C, this is undisputable as a known characteristic of Evanohm. For that reason, Evanohm wire is not welded directly to the copper lead, there is always an intermediary alloy between them. The intermediary is usually Alloy 180 ribbon which is arc welded into the copper lead and then the Evanohm wire is CD welded to the Alloy 180 (this too is not the best practice).Sorry if I was not totally clear on the point before. Your method would also be quite impracticable for production as well. As a point of interest, my lead assemblies are actually made with a special arc welder setup and that piece of equipment usually goes for around $20,000 new. It isn't just an arc welder, there is a lot more to it in order to to do it right. Your method is actually used to fuse incandescent lamps (tungsten) to wire leads for example, in that case the high heat does not cause problems with TCR as that characteristic is not of primary interest like it is with PWW resistors.

author: amspire
author_rank: Super Contributor
timestamp: January 28, 2018, 12:42:33 am
content: Edwin, could the Evanohm ends be plated with a solderable metal as a method of termination?Just did a quick search and saw a Dale patent for a Bulk Metal Chip resistor and they used an Evanohm Alloy R plate plated with a nickel undercoat and tin/lead plating on top. This patent was for a process where they plate the whole surface of the Evanohm plate and then use a method such as a laser to selectively burn away the plating to expose the resistive element. The aim of this patent was to be able to make surface mount resistors without the need to involve any copper along with the associated copper temp. coefficient.Welding is probably fast for production, but as you point out, probably out of the range of the hobbyist. Electroless nickel plating on the other hand is very easily managed. It is probably hoping too much that it could be that simple. This Dale patent was for a mass manufacturing process for low resistance 25ppm/C resistors and not for precision resistors.https://www.google.com.mx/patents/US5287083

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 28, 2018, 02:03:35 am
content: To amspire:I haven't tried that but I don't see any reason why you couldn't plate Evanohm with nickle...be advised that pure nickle has a TCR of about 6,000PPM/°C so you won't easily get very low TCR even if the wire is under 3 PPM/°C, not counting the tin/lead plating on top of that. That's why they're carrying a 25PPM/°C TCR. Always a trade off in play, nothing is for free unfortunately. I'd give a try just to see what kind of results you get.

author: amspire
author_rank: Super Contributor
timestamp: January 28, 2018, 05:28:44 am
content: Quote from: Edwin G. Pettis on January 28, 2018, 02:03:35 amI thought you would say something like that.Looks like Electroless Nickel will not plate to Evanohm without a Strike - a thin layer of copper plating. So maybe just a thin copper layer is the thing to try.My main purpose in having a go is more to make a precision voltage divider like a Hamon 10:1 voltage divider rather then a standard resistor. If I could make resistors with a decent coefficient and very little short term drift, I will probably also try a Hamon resistance transfer (need 10- 12 equal resistors matching to better then 0.1%). The Hamon resistance transfers can get expensive with commercial resistors as you don't need just one - you need to make 4 different transfer boxes to span the range from 10 ohms to 1Gohm. The 10M to 1Gohm range is fine if it a lower accuracy - I am planning on using some fairly cheap 100M resistors for that job.It still means that if I have to buy precision resistors, I will need either 4 or 10 resistors for the voltage divider and 30 for the resistive transfers. It starts to add up.Richard

author: TiN
author_rank: Super Contributor
timestamp: January 28, 2018, 10:30:39 am
content: The 1.82K resistor in test that Ramon mentioned had ~-10ppm/K, not 400 ppm/K. Total resistance change was -400ppm from temperature delta of 40C.I crimped the wire ends in the short pieces of clean small copper tube, which in turn soldered to test cable going to meter. Whole assembly was in TEC box setup.

author: branadic
author_rank: Super Contributor
timestamp: January 28, 2018, 11:24:21 am
content: I think this single measurement (ramp up from 21°C to 60°C and back) is not representitive for this resistor at all. If you plot resistance over temperature you can see:1. that the resistor is lagging temperature, visible at 60°C2. you have same relaxing during first ramp upThis plots with time on x-axis do hide most of the valuable information. I would suggest repeating the measurement over several cycles to get an idea of the real T.C., hysteresis etc.-branadic-

author: ramon
author_rank: Regular Contributor
timestamp: January 28, 2018, 11:32:19 am
content: The spool I have is not enamelled, it is bare wire. I guess that they use the same standard label for all their wires.If you look again there is nothing written bellow ('Enamel', 'SWG B&S', and 'Net lb' all have empty fields).About using other alloys: Edwin thanks for suggesting Manganin and Cupron, but no. I have no interest to experiment with wires that have higher TC (more than 5 ppm); or other characteristics that made them unusable: like a high thermal EMF against copper, or permanent change/drift if exposed to more than 140C (this is probably the worst issue of all). All this three requirements (TC, EMF, and maximum usable temperature) exclude almost every other alloy: Zeranin, Constantan, Noventin (CuMn25Ni10), etc ...Tin, you made my day ! Did you remember that I 'baked' that resistor for you the same morning we meet on Taipei. Burned by lighter!The very first crappy resistor I made, and I got 10 ppm? Crazy! I am the happy owner of around 5,000 meters of a 10 ppm spool.That means it could be true that the spool I have is actually a 3 ppm wire? And maybe we could that spec soon.To celebrate this moment (and accelerate R&D) I want to make an offer to the eevblog metrology community: I can ship some meters of wire for free, just cost of shipment. I am thinking about sending 4 meters per person, maximum 25 persons (a total quantity of 100 meters). I just only ask that the experiment and results are made public on this thread.Highly desired DIY experts in the fields of micro-crimping and DIY micro-spot-welding machines (but I highly recommend to check the crimping method first, lower cost and great results ;-)Thanks JoeN for starting this thread!

author: TiN
author_rank: Super Contributor
timestamp: January 28, 2018, 11:47:45 am
content: Quote from: branadic on January 28, 2018, 11:24:21 amNot sure I get what you mean here.Quote from: branadic on January 28, 2018, 11:24:21 amPlot over time does not hide anything (in fact it have more data, than just T vs R curve), it's just different representation (which might be harder to glance, granted on that).Quote from: ramonI can understand the enthusiasm, but it's not that simple, as there is no real world value in short-term tempco over few hours, if the resultant resistor not stay stable/have large hysteresis and affected by mechanical reliability of the joint. With same idea, you can have 0 ppm/K wire, but the moment you actually make a resistor and put it into the board/device, it would have very different characteristics, no matter crimped, or welded or soldered. Wire itself (and it's qualities) is just one little variable in the large big formula of final errors.

author: branadic
author_rank: Super Contributor
timestamp: January 28, 2018, 12:15:04 pm
content: QuoteSee picture attached... no change in temperature but change in resistance is a hint for a lack in temperature.QuoteOkay, we can be pettifogging, but you expect everybody to calculate the valuable information by himself. Where is the problem plotting resistance over temperature, putting some fit on it and show how you calculate your values? Which ramp did you use for your T.C. calculation, ramp up or ramp down?It's pretty easy to miscalculate such paramter using the time domain data as high dynamic range of it hides true behavior.-branadic-

author: TiN
author_rank: Super Contributor
timestamp: January 28, 2018, 12:51:36 pm
content: Quote from: branadic on January 28, 2018, 12:15:04 pmI'm lost with term "lack in temperature". Temperature need to be higher? By looking at time-domain graph it's easy to understand why there was a difference in resistance at +60C point.Quote from: branadic on January 28, 2018, 12:15:04 pmVery true and correct, it's easy to miscalculate complex parameter, such as tempco (which is always combined figure of resistance tempco, setup quality, measurement noise, hysteresis and so on).Representation format or axis definition do not make a difference. We can shuffle numbers and pretty plots, all we want, but end result is going to be same - complex equation with many unknowns (which effect we try to reduce when measuring interested parameter, with different level of success).I don't expect anyone to calculate, I just present the data I got, that's it. I was interested to see if such DIY resistance device can be stable over time and small temperature span, not so much the absolute tempco itself. I got my answers, that it is not the case in this particular example, confirming my own experiments years ago, since which I decided it's not worth the time to try own DIY resistors. If you expected me to present traceable tempco calibration data with uncertainties and all, well, sorry, that's not happening.P.S. for all tempco numbers I ever post box method is used.

author: branadic
author_rank: Super Contributor
timestamp: January 28, 2018, 01:34:41 pm
content: QuoteA conclusion based on one measurement with only one ramp up and down.-branadic-

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 28, 2018, 07:32:23 pm
content: Ramon:That spool you acquired appears to be an older spool from Kanthal (possibly from the 1970s), I checked the spools I have in stock and even the older ones from the 1970s indicate clearly whether they are bare or enameled, the reason being that the spools used to be supplied in paper boxes with a clear window in the end for the spool label. One did not want to have to open the boxes to see whether or not the wire was bare or enameled. The boxes on my newer spools are plastic so you can see through but the labels still indicate bare or enameled as the boxes are almost always stacked so you still need the information on the label. The spool's TCR is accurate, I've only had one incident in many years where the spool's TCR was wrong and verified as wrong. I have one spool of wire with a similar TCR (not exactly the same) and it does indeed produce resistors with TCRs within the specified range if, and this is a very big if, it is made correctly. All of my resistors are very close to the TCR printed on the spools unless something has gone wrong in production, which it can, there are a lot of variables to deal with and the resistance wire terminations are just one of a long list that affect the end product.Cupron wire does indeed withstand high temperatures, well above the +140°C you indicated, in large gauge sizes, Cupron is used for heating coils, while Evanohm can operate at high temperatures, its TCR increases above approximately 150°C and if operated at much higher temperatures, the TCR can be changed. That is why PWW resistors generally have +150°C as their maximum operating temperature (and that is at zero power mind you). You are quite correct that Manganin (and derivatives) has a somewhat low operating maximum temperature of about 60°C, but you are also assuming that the 15 PPM/°C spec (maximum by the way) is what you get, it can actually be anywhere within the 15 PPM spec, even close to zero. Yes it is very sensitive to processing, the act of soldering will change the local characteristics and keeping it within 15°C to 35°C makes it happiest for temperature.Cupron is generally the alloy of choice for power resistors since it tolerates the +350°C maximum temperature without big changes in characteristics, you can't do that with Evanohm and keep its low TCR intact.I can state with complete confidence that over the years a great many variations have been tried for terminating resistance wire to various types of lead assemblies, in my published paper, The Last Half Century: Wire Wound Resistors (in EDN, also posted here in another thread), we detail how the industry struggled with the new Evanohm alloy. You are merely repeating history here and while I encourage it as a learning experience, you are not going to produce high grade precision resistors with any DIY methods unless you've got the money to setup the correct equipment and materials and that isn't going to be cheap. I agree with TiN's assessment (and not just because I make PWW resistors), it has been tried many times with the same results, frustration and pretty much wasted time and money trying to reinvent the 'wheel'.

author: branadic
author_rank: Super Contributor
timestamp: January 28, 2018, 08:31:37 pm
content: QuoteYou force people to repeat history and reinvent the wheel, cause you don't want to share your "secret knowledge". I do understand that you live from selling wirewound resistors and thus don't want to share your solution. But by the looks of it you try to stop people from investigations on that topic, which seems like you are scared that someone can come up with a proper solution. And if this solution is shared in the web those guys won't buy wirewound resistors anymore. Just my personal impression also based on statements like this:"...95% of what they're doing is nothing short of a waste of time and pointing it out to them will get you nowhere. They don't know the minutia of the components nor how to properly do tests and when to do the correct tests, they end up chasing their tails around in circles and can't understand why..."I can remember a time, when people couldn't afford soldering irons nor soldering stations for home lab, when they couldn't afford a scopes analog or digital. I'm sure there will be a time, when you can produce precision components at home. Digital process chain such as Inkjet printing is getting better and better, first Inkjet printers enter homelabs. It's just a question of time and noone will stop this development.-branadic-

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 28, 2018, 09:06:28 pm
content: Quote from: branadic on January 28, 2018, 08:31:37 pmhello,i don't think Mr. Edwin is forcing anybody to do anything. i think he is only pointing out the difficultyof the process, and graciously so, if he is not willing to share his "secret knowledge", he is perfectlywithin his rights to do so, since he has spent decades acquiring the necessary skills, and is not willing togive it away for free (to un-grateful buggers like me!), which is his perfect right and sweet will.whosoever wants to acquire the necessary skills should either work for it, or buy it, period.as far as giving away your knowledge for free, it is anybody's sweet will, and i for one amcompletely and utterly grateful for the knowledge i acquire, courtesy of all people on thisforum.let us have a positive attitude and be thankful for the *free-bees* we get frombeing members of this enlightened community.as far as new developments are concerned, yes, time moves on, and low-tech countries becomedeveloped, developed countries become poppers, these are historical cycles, and only the greatones become timeless, and great ones are pretty rare.regards.-zia

author: JoeN
author_rank: Frequent Contributor
timestamp: January 28, 2018, 09:30:12 pm
content: Quote from: Edwin G. Pettis on January 27, 2018, 11:41:15 pmI have not indicated any technique yet because I haven't tried to build my own resistors yet. I am just gathering information to see how feasible it would be to do. But no attempt has been made at this point. Thank you all for your replies to this post so far. I am reading the other indicated thread and some of Edwin's writings I have found on the internet to improve my overall understanding of this subject and why it is so difficult to make high-precision and low-tempco resistors. Quite a bit more involved than it might be expected and it sounds like millions and millions of man hours has already been dedicated to this art.

author: Echo88
author_rank: Frequent Contributor
timestamp: January 29, 2018, 11:33:34 am
content: I agree with zhtoor: Lets be grateful that experienced people like Edwin, TiN and Mister Diodes spend their time here and give hints how to do it. You can not expect them to write all their knowledge down and give it away for free.If you expect such a behavior i can point to lymex who did an absolutely remarkable job in sharing his knowledge onhttp://bbs.38hot.net/thread-485-1-1.htmlabout a wide range of thinks including rare beasts like DCCs, Warshawsky Bridges and other things. A pleasure to read, just a little bit tiresome due to the sometimes crude translation with google translate.

author: ramon
author_rank: Regular Contributor
timestamp: January 29, 2018, 04:21:48 pm
content: Quote from: Edwin G. Pettis on January 28, 2018, 07:32:23 pmI really thought it was quite new ! I compared several pictures of many different spools and the one I bought didn't seem like 70s style. What do you think is the date of this other spools? (this is what is currently available at auction sites)Even the 2012 catalog from Kanthal shows the picture of my spool (please, check the smallest spool on page #3).https://www.kanthal.com/globalassets/kanthal-global/downloads/materials-in-wire-and-strip-form/resistance-heating-wire-and-strip/s-ka026-b-eng-2012-01.pdfOh my good, those sweeden guys really love retro style. (The more I look at the catalog picture, the more I think they made the picture with some wood camera and kodachrome film).I don't pretend to reinvent the wheel, this is just like any other hobby. Some people collect stamps, or coins, or even insects. I just enjoy winding wire on weird substrates and check resistance value with a meter (with needle or leds). It takes little time, almost no money, don't need that much space, and is relatively safe for childrens ... and It's funny !!Please, could you help me to buy some bobin of cupron (or anything else)? I havent found any on the action site, and no manufacturer is willing to sell me.

author: branadic
author_rank: Super Contributor
timestamp: January 29, 2018, 05:38:53 pm
content: Noone wants Edwin to offer his knowledge here, all I'm asking for is to not stop people from investigating on their own.And never underestimate the power of curiosity and analysis capabilities. It took a minimum amount of time using EDX to get a material composition of the intermediate material used in his resistors.-branadic-

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 29, 2018, 06:10:16 pm
content: Quote from: branadic on January 29, 2018, 05:38:53 pmhello,just because you know the composition of the intermediate wire using x-ray spectroscopy,would you be able to supply PWW resistors at a cost lower than Edwin?, i think not.i am pretty sure a *lot* of work goes into making ppm level stability PWW's.regards.-zia

author: branadic
author_rank: Super Contributor
timestamp: January 29, 2018, 06:28:41 pm
content: QuoteWho would have thought that people are able to have voltage references at home? Who would have thought that people do have the possibilities to weld at home? Who would have thought that people do have inert gas and nitrogen at home? Who would have thought that people do all the metrology stuff you guys are presenting here?You think it is air that you are breathing? Okay, you better take the blue pill-branadic-

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 29, 2018, 06:44:04 pm
content: Quote from: branadic on January 29, 2018, 05:38:53 pmAre you going to share this info?

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 29, 2018, 06:59:31 pm
content: Quote from: branadic on January 29, 2018, 06:28:41 pmAgreed. But also, since Edwin is making resistors for profit, he has to keep the material and labor costs low so he can sell his resistors at a reasonable price. There can be no doubt that there were compromises made in his resistor designs to hit a price target. Conversely, a hobbyist can take all day long (if we want to) to make just one resistor, and since (we) are only making one (or a few), more expensive (even exotic) materials can be used, where that would be totally impractical for a resistor production line. This means that (we) can make resistors even better than Edwin's (and even better than VPG foil resistors), if enough time and effort is put into it-- and this process will go much faster if we share information. PWW resistors are an old technology-- so there is a lot of information available (for free) from the various patent filings and also from the many national metrology laboratories. If you have access, there are also many peer-reviewed scientific papers available on this subject.

author: branadic
author_rank: Super Contributor
timestamp: January 29, 2018, 08:42:02 pm
content: QuoteNot in detail, all I say is: It's a NiCr alloy.-branadic-

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: January 29, 2018, 10:19:31 pm
content: Back to the basics--Why could not the wire be terminated by some mechanical means? Some sort of screw terminal?(No one would want this in production, but should be ok for hobbyist use, surely).

author: amspire
author_rank: Super Contributor
timestamp: January 29, 2018, 10:46:41 pm
content: Quote from: RandallMcRee on January 29, 2018, 10:19:31 pmDefinitely try it. It is just it doesn't take much to ruin the stability. If you use 10 meters of wire to make a 1ppm/C stable resistor, then it only takes a problem in 10s of micrometers of the wire to affect stability. You would want a screw terminal with an accurate edge of contact and minimal stress on the wire.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 29, 2018, 10:46:50 pm
content: Quote from: RandallMcRee on January 29, 2018, 10:19:31 pmWell, the position of the wire can move, changing the value of the resistor and/or the joint can corrode, changing the value of the resistor-- and also, there might be more thermal EMF than a properly welded joint. Just my $0.02 worth...

author: Gyro
author_rank: Super Contributor
timestamp: January 29, 2018, 10:48:54 pm
content: Quote from: RandallMcRee on January 29, 2018, 10:19:31 pmI was thinking the same thing - with the right shaped clamps / washers / whatever, it ought to be possible to make a gas tight contact with the wire at a very defined point (soft knife edges?). They could also be arranged to be isothermal.

author: amspire
author_rank: Super Contributor
timestamp: January 29, 2018, 11:17:32 pm
content: Quote from: Gyro on January 29, 2018, 10:48:54 pmEven with the soft knife edges, at the sub mm level, the contact point will have a width and the leading edge of that contact point may only have 1% of the pressure further down the wire. Initially, that area with the 1% pressure will be making contact, but with time and corrosion, it may stop making contact. That is all it needs to change the resistance by perhaps 10-100ppm.I imagine that the welds Edwin uses have a very precise and mechanically stable contact boundary.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 29, 2018, 11:28:46 pm
content: If you can't afford the thousands of US$'s for a Sunstone Pulsed-TIG welder, and you don't want to DIY, then here is a less expensive alternative:http://www.aliexpress.com/item/220V-Stainless-Steel-Spot-Laser-Welding-Machine-Automatic-Numerical-Control-Touch-Pulse-Argon-Arc-Welder-for/32818492226.htmlYou would still need the industrial Argon tank for 99.99% Argon gas, and a regulator for the tank, plus some tubing to hook up the gas from the regulator to the back of the welder. I use an aluminum 60-L tank I bought from eBay-- you might find a good used one near where you live (or rent one from the gas supplier).Bonus: You can do a hell of a lot more with this tool than just make resistors.This should be a "must have" item in your shop.If you want to DIY, just a suggestion that you can buy the stylus and a matching connector from one of the sellers on aliexpress; then just do all of the rest of the electronics yourself. You might end up spending more than just buying one (above) though.Don't worry about "ruining" the TCR of the wire; the pulse is so fast that it does not heat the wire except very near where you are actually welding.

author: JoeN
author_rank: Frequent Contributor
timestamp: January 30, 2018, 12:22:17 am
content: Quote from: Magnificent Bastard on January 29, 2018, 11:28:46 pmHow long have you owned and how much have you used this? I always have a problem buying stuff on Aliexpress more than a hundred dollars or so because Chinese stuff can work for a while and crap out. Seems this happens quite often.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 30, 2018, 12:36:27 am
content: Quote from: JoeN on January 30, 2018, 12:22:17 amI never bought one from AliExpress; I have a Sunstone welder. You could buy one of these, and gut the electronics and then build your own; and make it as reliable as you can.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: January 30, 2018, 12:43:11 am
content: Quote from: Magnificent Bastard on January 29, 2018, 11:28:46 pmActually that's about opposite of what happens. The weld joint is absolutely critical to keep low TC, and you're not even at the tip of the iceberg. How you weld is even more critical.You take a spool of wire labeled 3ppm TC, and that's what it was when it was placed on the spool. The exact moment you unwind it, or tug or bend or crimp or weld or look at it the wrong way: The TC is going up. That TC spec on the spool is about as good as it gets from the day it was alloyed and drawn at the wire mill.Again: Crimp, fold, bend or kink the wire and TC (and noise) goes up. So much for crimped or screwed connections. That's why. Take the time to research the long history of PWW resistors to find out all the mistakes made by trying to using mechanical clamp/crimp methods. In other words: Read and Learn.Look: I'm not being negative here, but the fact is: I'm absolutely positive you probably never make a precision resistor for less money than thousands of good purchased parts that will work better, and are built on better equipment by people who actually know the process (and many decades more experience). This is not really a hobbyist project - if you're in it to save money on your precision resistor needs. Can you DIY a mediocre, UN-stable, noisy resistor? Probably. You'll still be out many times the cost of a good part though.Bran: Edwin has been very patient here, and he's giving you a lot more info than any other manufacturer - for free. He's offered to help you replace your broken parts for free. He's giving you tons of information - and why on earth should he divulge the exact process he uses?? That's called Intellectual Property, and that has a lot of value - and you never expect that knowledge at no cost.Buy some resistors if you want support of the manufacturers. Expect to pay a fair price if you want those quality parts again next year, because you -want- the suppliers to stay in business if YOU are going to stay in business. That's how it works.Always true: If you don't like the price of the parts, then you're not buying enough parts. Nobody is running a charity for parts or exact manufacturing tech info - please don't expect that to happen.

author: amspire
author_rank: Super Contributor
timestamp: January 30, 2018, 02:59:03 am
content: Quote from: MisterDiodes on January 30, 2018, 12:43:11 amGreat post. And I am still actually going to have a go at making some resistors myself. I just want to see what happens.Probably start out with a divider that just needs temp coefficient tracking, but it is just interesting to see how good or bad the results are. I am particularly interested in short term stability for transfers - and particularly trying to get a very low resistance versus voltage coefficient.If I was making a 10V reference standard for home, I would be buying resistors for sure. I could never trust a home made resistor in a primary reference.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 30, 2018, 04:01:00 am
content: I am encouraging experimentation, you could possibly understand better how resistors work even if you can't throw together a <5 PPM°C TCR. Knowledge is always better when you learn from doing even if you fail.It wasn't that long ago that making resistors under 10PPM/°C was difficult, then here and there somebody managed to get closer to 5 PPM/°C. Mind you those 'improvements' came along with significant losses in yields. Even today resistors with very low TCRs, such as those very expensive Vishay hermetic with <2PPM/°C come with yield losses, they don't have 100% yields. While I consistently have greater than 90% yields at ±3 PPM/°C, there are still small losses for various reasons, sometimes the TCR is off a tad and I know why in almost every case. This all comes from experience and interestingly enough, most failures are from mechanical sources (in my case). In some cases, I know of yields that are only 60% to 70% for a given set of specs at some other competitors. Some of those resistor manufacturers have been at it for decades and have yet to even get past 80% yields for tight parameters, that is just the beast that PWW resistors are. And it isn't just wire wounds, film/foil have their own set of alligators in the swamp to contend with, different construction, different problems.Depending on who is making the resistors, their yields are going to be most likely different to some degree than someone else's, TCR and stability are just two parameters that are hard to hit.So have at it and try to have some fun while learning.

author: TiN
author_rank: Super Contributor
timestamp: January 30, 2018, 04:06:59 am
content: And failure is the best teacher, the more you fail the better you learn. Especially when each attempt tied to significant cost.That will optimize your efforts a great deal.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: January 30, 2018, 05:07:05 am
content: Exactly...You learn about a ten million ways to do it wrong, and then you begin to appreciate how hard it is to get right.The really difficult part is testing a resistor on week one and it looks pretty good, maybe you got 8 or 10ppm TC if you're lucky - and then come back 6 months or 6 years later to find it absolute crap for stability and high drift rate. You might be able to claw your way to lower PPM but after a while just purchasing quality resistors is a much more practical solution if you want to get your circuit idea completed sometime this century.In the end what you might learn is: you're grateful to be able to buy a lot of custom precision and stability for $5~$10 each in a quality PWW (and spend your time designing areas of a circuit that you CAN solve)...but you had fun learning why they are very hard to build along the way.Have fun!Again: If you haven't done so and think making a PWW resistor means wrapping up some wire and making a couple crimped connections - Make sure you read Edwin's excellent article that appeared in EDN and learn what's been done long ago, and why various techniques that seem obvious solutions won't work.:https://www.edn.com/design/analog/4427151/The-last-half-century--Wirewound-resistors-Part-onehttps://www.edn.com/design/analog/4427940/The-last-half-century--Wirewound-resistors-Part-two

author: ramon
author_rank: Regular Contributor
timestamp: January 30, 2018, 03:46:01 pm
content: Quote from: Magnificent Bastard on January 29, 2018, 10:46:50 pmWhat if we apply a silver coating at the ends? Won't this solve all this three issues at once?Too easy, right? I guess someone tried that before and failed.

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 30, 2018, 04:20:35 pm
content: Quote from: ramon on January 30, 2018, 03:46:01 pminteresting... high conductivity and solderability, but coatings could result in semiconductor action resultingin maybe a diode in series with a resistor.regards.-zia

author: David Hess
author_rank: Super Contributor
timestamp: January 30, 2018, 05:52:50 pm
content: Quote from: zhtoor on January 30, 2018, 04:20:35 pmI have seen this done with copper but not silver. One problem with silver is that the tin in the solder would very quickly dissolve it unless silver bearing solder was used.

author: branadic
author_rank: Super Contributor
timestamp: January 30, 2018, 05:59:43 pm
content: In therory there is a possibility to use homebrew galvanic kits to coat some nickel + gold or silver on the end of the wire, to make it solderable. You don't want to mess around with chemical deposition. But don't forget to remove the enamel.-branadic-

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 30, 2018, 06:02:13 pm
content: Ramon: Besides Goodfellow, this source has various wire wlloys available:https://www.pelicanwire.com/product-category/resistanceheating/insulated-resistance-wire/?filter_alloy=advance-294&filter_product-availability=in-stockAs I've mentioned before, resistance wire is not cheap, particularly good wire and there is a lot of questionable wire floating around on the internet. Sometimes you will find some 'surplus' wire available on eBay for a good price but I haven't seen much showing up there.All alloys have a standard ±xxxPPM/°C range, a given spool will be somewhere within that range. The only way around getting a random TCR on a spool is to order wire with a specific TCR such as I do, and yes that does cost a pretty penny. For the finest wire gauges, I can expect to pay upwards of $20,000/lb., that isn't a typo. Of course nobody orders a full pound at those prices. The more common wire sizes that I use for LTZ resistors are still mostly above $1,500/lb. That is one of the reasons why PWW resistors aren't bargain basement cheap, every manufacturer pays roughly the same high costs for wire, mine is a bit higher because of my specifications. You simply can't make a low TCR precision resistor with cheap wire. Wire costs have ballooned since about 2000, they have stabilized in the last few years but are much more expensive than they used to be, I've got wire quotes from some years ago and current pricing is just about 300% higher. If you're wondering, film/foil uses the same/very similar alloys as I do, just in a different form.

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 30, 2018, 06:14:58 pm
content: Hello,question for Mr. Edwin:your comments on silver plated evanohm wire soldered to terminals, what would you expect in terms of TCR?regards.-zia

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 30, 2018, 06:18:45 pm
content: Many years ago I walked into H.P. Reid in Palm Coast Florida and asked if they had any partial spools I could buy. They fixed me up with a nice quantity of manganin and Rediohm 800 for under a couple hundred bucks. Times have changed and I doubt they'd (now Kanthal) do it today, at least not for what I paid back then.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 30, 2018, 06:39:06 pm
content: Yes, H. P. Reid was the best resistance wire supplier in the country, sadly (in a manner of speaking), they were bought by Kanthal about ten years ago, while the quality of wire is still top notch, they are expensive. I don't know if they still would sell to 'walk-ins' these days or not. I do know that they have maintained a very small inventory of unsold wire from customer runs that didn't require the minimum line item purchase the last time I checked in with them. I have no doubt that you won't get wire for anywhere near what you paid years ago. The commodity metals market went crazy after 2000 and while some metals have come down in cost since those crazy days, there has been no such drop in wire pricing that I can see.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 30, 2018, 06:58:12 pm
content: Silver has a TCR of +3800PPM/°C, close to copper and while silver is 'friendly' with nickle, it will add to the overall TCR, how much depends on what percentage its resistance is to the Evanohm resistance, this will include the 'interface' resistance between the silver plating and the Evanohm. In other words, you are adding two small silver resistors in series with the Evanohm and just like the copper leads will add a small resistance in series. EMF wise, these three materials, Evanohm, Silver and Copper all have close compatibility so thermal generation isn't going to be a big issue. Of course, low resistance values will show a larger effect than on higher values.

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 30, 2018, 07:03:17 pm
content: Quote from: Edwin G. Pettis on January 30, 2018, 06:58:12 pmthanks.it seems that this is a good solution for DIY PWW's at least for medium ohmage 100 Ohm -> 100K Ohm,assuming EVANOHM is available with a reasonable (sub 5ppm) TCR.regards.-zia

author: Echo88
author_rank: Frequent Contributor
timestamp: January 30, 2018, 08:31:26 pm
content: So we now need someone who can galvanize stuff, ramon ships him some of his wire, branadic prints the needed bobbins and now we can get in competition with Vishay and Edwin.

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 30, 2018, 08:49:10 pm
content: Quote from: Echo88 on January 30, 2018, 08:31:26 pmAndreas/TiN : TCR TestingRamon: Wire SourceBranadic: Bobbin (Maybe collapsable for stress relief)Xxxxxxx: Silver Plating (preferrably electroless)if silver plating can be found in Germany, most of the work can be done in Germanyregards.-zia

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 30, 2018, 09:20:51 pm
content: One of the old precision resistor companies was Julie research. They made KVDs and various other things. I have some of their old literature and they were quite proud of their resistors. Loebe Julie was a bit of a character. By modern standards, does anybody know how good their resistors were? I know my KVD (serial #1) has remained within a couple ppm.

author: IconicPCB
author_rank: Super Contributor
timestamp: January 30, 2018, 10:25:05 pm
content: I do immersion silver plating.

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 30, 2018, 10:31:51 pm
content: Quote from: Conrad Hoffman on January 30, 2018, 09:20:51 pmone hell of a chap.definitely a man on par with (or exceeding) the jim williams and pease, its a pity he was not as "visible"as others, and as many other greats, was not treated in the best of ways in the US.such a man should have been revered by the electronics engineering community.regards.-zia

author: zhtoor
author_rank: Frequent Contributor
timestamp: January 30, 2018, 10:33:57 pm
content: Quote from: IconicPCB on January 30, 2018, 10:25:05 pmmaybe you could post the method to immersion silver plate the wire by ramon?without changing the TCR of the wire.regards.-zia

author: ramon
author_rank: Regular Contributor
timestamp: January 31, 2018, 04:22:16 pm
content: Edwin:Thank you so much. Actually I already saw that pelicanwire had evanohm and several months ago asked them about their spools. What they sell on the website is wire by length (50, 100 or 250 ft) without any other information. I didn't wanted that. As you know, the manufacturer sell spools by weight, and they specify the TCR in the spool label, and put some serial number or code to track any quality issue. However pelicanwire doesn't have any single picture of the actual spool. So far I have no response from them (don't know if that was because I clearly said in my first contact that I was in Taiwan, so maybe they don't sell overseas).The only manufacturer that didn't completely ignored my quote request was Jelliff. They asked me what I wanted, how much quantity ... and that's it. So I am very happy with them that the took the time to write a few emails.So, as last resource, I bought the wire on a famous auction site. According to the seller it was from military surplus. It has at least a serial number, the actual weight (gross/tare/net), the measured resistance per length, and the TCR specified. I don't expect less than that.I completely understand that some business don't want to deal with one-off customers, or low quantity orders. At work, sometimes a customer wants to buy some product we don't have stock, and production is costly, and our providers ask for a minimum order quantity, and we ask the customer too for a minimum order quantity or we offer them an alternative product in stock. But not a single alloy distributor or manufacturer have ever replied to any quote I asked, they haven't either told me the required MOQ.And this reminds me that we are really lucky to have your presence here in this forum. It is amazing that we have one of the most respected manufacturers willing to sell a kit of 5 precison resistors to individuals all over the world (and even discussing how to make them !). With all the troubles, and loss of time, that that will carry. Exactly the reason that no single manufacturer or distributor has ever replied to me. So, I just want you express grattitude for what you are doing.I wonder If you ever had that problem too. Don't know If you have ever needed some spool gauge at hand for some customer request but you couldn't meet the MOQ from your suppliers, then you was forced to look for alternatives or even deny the job.Now I ask you (because you already offered to sell resistors) if you could sell a high quality spool of wire too?I suppose that it will not be that much different to sell the bare spool than the finished resistor. It can be even less work for you, and you maybe can have the added benefit of any volume discount (well, maybe not, as not sure how many here will be interesed too). For me (or us), the benefits are that we can get a good spool from a reliable source. Also some people want to do some special resistors (like a PWW resistor network) that no manufacturer is willing to make just only because it doesnt fit the production line, then they will have a way to make it. Obviously, we are not going to get the performance of your resistors, so this will not decrease any of your sales. And those who want a high quality resistor will buy it directly from you anyway, as they will not want to loose their time and mess with winding wire.What do you think?

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 31, 2018, 07:20:11 pm
content: Hello Ramon,First, thank you for the compliments. You are quite right about sellers like Pelican and Goodfellow, they buy wire and resell it in small quantities for a significant markup, they fill a niche for buyers who don’t need a lot of wire or can’t meet minimum purchase. They really should supply the wire data for the wire they sell since all wire manufacturers provide that information on the spools, most resistor manufacturers would not accept such unlabeled spools. MWS is another source which does not provide wire to specific TCR as I have talked with them and they won’t provide anything beyond the stated ‘normal’ range of TCR, such as ±20 PPM/°C for Evanohm. They may have changed that policy since I last talked to them but I kind of doubt it. When I asked them for a given TCR, all the salesmen will say it’s within ±20PPM/°C. There aren’t that many ‘original’ wire sources around and they know it so they can be selective in their customers and have no interest in small orders. It may be that is it simply too expensive for them to handle small orders but that doesn’t mean they shouldn’t at least reply to inquiries. More than likely, if you’re using an individual’s e-mail, they won’t reply, a business e-mail might at least get a burp out of them.In general, a PWW resistor house can make any value of resistor if they have the proper wire/bobbin size available, some will charge a premium for nonstandard values but that is more marketing policy than real cost involved. I will make any resistor value if I have the wire and bobbin size available in inventory and there is no premium for oddball values. The main limitation I have is limited inventory of wire and bobbin sizes available in stock, replacement costs are high so it is difficult to increase the number of wire sizes I have. Yes I have had to no bid some resistors due to inventory limitations. Unless it is a rather large order that can support the purchase of inventory to do it, I have to turn it down. Another limitation is very high values, since the huge metal price increases since 2000, wire cost has nearly tripled and that makes resistors up around >500K more expensive. I’ve generally only made high values to repair ESI decades or instruments that need PWWs because factory replacements are even more expensive.On the one hand, I don’t have any problems purchasing wire, that said, I’m still subject to their minimum order and I’ll also note that because they make the wire to my specifications, it raises the price from the standard stuff. The price of the wire depends on the wire size, larger sizes are lower cost, finer wire more expensive. Each purchase is quoted at the time and sometimes, they’ll change the quote more than once before all is said and done. How much wire is on a given spool also varies by the size (you’ll normally see the weight given on the spool), most of the time a given spool is in the hundreds of dollars each or higher.I know this is going to sound self-serving but….while I encourage experimentation, if you really need <5PPM/°C TCRs for a project, you’re much better off in the long run to just buy reputable resistors. As proven in actual industry, it took decades to get TCRs down below 10PPM/°C with reasonable yields. It isn’t a case of somebody just tripping over a solution, it requires considerable knowledge of all of the factors that goes into a low TCR and stable resistor and most resistor houses are still struggling with many of those factors, the customer just never is privy to all that goes on behind closed doors and manufacturers don’t like their faults being put on display very much for some unknown reason (tongue in cheek).One of things I see here is that a given parameter such as TCR is latched onto and is pursued, unfortunately unless all of the given parameters of construction are correct, the wire’s TCR is the least of your worries. I can guarantee that even if you have wire with 0PPM TCR, the end result is not going to be even close to that. There is a laundry list of parameters that will affect the finished TCR of a resistor and stability.Experimenting will help you learn how things work and what causes this effect or that effect or even more than one effect but you don’t need very low TCR wire to do that.Best regards,Edwin

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 31, 2018, 08:17:20 pm
content: I notice that when I search for Rediohm 800, it's used for biological work (cell recording?), and not resistors. It's also about $2 per foot! That's just crazy.Does anybody know how Julie terminated the wire in their resistors, especially the NBS resistors?Because the thermal conductivity of resistance wire is low, thermal effects should be local to the joint. How much of an effect might be expected if a very short portion of the wire was overheated by brazing it into a small copper socket or similar? It seems like all forms of termination involve some degree of compromise.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 31, 2018, 08:52:41 pm
content: Rediohm800 is what H. P. Reid called their recipe for Evanohm, there are a number of slight (and I do mean slight) variations on the metallurgy of the original Evanohm developed back in the 1940s but basically there is little difference. The mixture is tweaked to get a specific characteristic of the end result. Every batch (ingot) of an alloy is slightly different, there are limits as to how precisely the metals mix can be controlled. Each batch has to be processed slightly different in order to get a specific characteristic, such as TCR, depending on how good things go, they might hit the target on the first round, if not, they have to start over and redo the heat treatment process from the start. All of this has a bearing on final cost.If you're thinking that Loeb had some special termination that was better than others, not so much, he had yield problems like everybody else and some of his claims were disproved in the end. Loeb was a very good engineer for that matter, but when it came to resistors he had manufacturing problems like everybody else. Like most manufacturers, they don't like having their flaws put out in the public and it often leads to a silly argument merely to 'protect' inside knowledge. In the resistor industry, there are very few real secrets if truth be told, most industry knowledge has been spread around over the years so the any given manufacturer is using whatever techniques they think works for them and if you read my white paper on the history of resistors, you'll find a whole lot of 'borrowing' (to be kind) that has gone on for years.The problem here when shooting for very low TCR is that even localized effects can have significant effect on the overall end product. You are quite right, all forms of termination involve some form of compromise, a good match of materials and processes can produce a very low TCR consistently but that is not easy to do even with the right materials and processes. Years ago when we did a long term study of resistor terminations with the help of Northrup Metallurgy Labs, brazing did not produce a particularly good termination, it did produce a better termination than many commonly used methods in the industry for that matter but that isn't saying much, a little better isn't really good enough for very low TCRs and long term stability. Resistor termination has been a thorn in the industry for many years.I don't recall specific numbers for brazed terminations but they were still significant in the 0-10 PPM/°C range. Short of a correctly welded joint, a proper brazed joint was about the middle of the range of termination effects as measured by Northrup.

author: branadic
author_rank: Super Contributor
timestamp: January 31, 2018, 10:04:38 pm
content: I found an interesting paper by NIST, that could be of interest for you and give you some inspiration of what is possible with the resistor wire:Calculable Coaxial Resistor for Precision Measurment-branadic-

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 01, 2018, 12:48:39 am
content: Edwin, that's great info- thanks! I may still try a braze. I've got a Smith "Little Torch" that's capable of very tiny work and I should be able to keep the area exposed to high heat reasonably short. Ultra low TC is nice, but I can live with a bit higher, so long as the resistance is stable over the years. I note you worked at Shallcross. Though I have many GR resistance boxes, the Shallcross is the one I use the most. Not terribly sexy, but a great product.

author: cellularmitosis
author_rank: Unknown
timestamp: February 01, 2018, 01:20:09 am
content: Quote from: ramon on January 31, 2018, 04:22:16 pmMe too!

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: February 01, 2018, 01:24:48 am
content: David:You're welcome, no reason why you can't try it and see what happens. Actually it was my colleague Michael Chesselet that was at Shellcross, he was the front man so to speak and I was consulting from here. He was able to move around more than I could. We had a presence at quite a few of the resistor houses around the country over the years. At one point, Vishay tried to hire me as an applications engineer and Mike as consultant but that's another story.

author: ramon
author_rank: Regular Contributor
timestamp: February 15, 2018, 01:36:27 pm
content: Edwin,When I asked you for help to find wire I didn't mean that I want the same wire provider and specifications that you use on your resistors. Of course, I don't want you to disclose (to any competitor) any information about your wire manufacturer and specifications.I currently have a 0.03mm (.0012") spool. It has a high resistivity, 1920 ohms/m, and that's a good thing. But when working with such a thin wire diameter I really need to be careful handling it, and I also need to slow down everything. I cannot see the wire if it's not over a some white background. It bends so easily ... and I always lose the ends.So I think I need a thicker wire: between 0.0024" and 0.0040" (0.06mm or 0.1mm). I also want it to be enamelled: to speed up winding and for compactness. And I also want it to be of Carpenter's Evanohm S alloy. I've just found in their website/datasheet that Evanohm S has much better TCR coefficient than Evanohm R: 1ppm instead of 3ppm (Due to its different composition: Ni72, Cr20, Al3, Si1, and Mn4).This wire is almost impossible to find for anyone that wants to experiment on his own.I have been lucky lately. I have just found (and ordered) a enamelled spool of Evanohm 0.03mm with 0 ppm TCR between 25C and 125C. Yes, I don't need zero ppm wire to find out which parameter (substrate, winding, epoxy, leads ...) is changing the TRC, but it doesn't hurts either.I haven't received the spool yet, so I don't know if this enamelled wire (extra thickness and stiff?) will be easier and faster to work with than my current bare 0.03mm wire. So still wondering if you could help us to order enamelled Evanohm S spools with .0024" - .0040" diameter and low TCR (1ppm or less).Best regards,Ramon

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: February 15, 2018, 07:54:03 pm
content: Ramon,I know you weren't asking explicitly about my wire specs or source, that information would not directly help a competitor anyway since those specs are specific to my processes.Welcome to the world of fine wire handling, what you have is at the bottom range of the mid-size diameters, Evanohm can be drawn down to a mere ,0004" diameter and if you think .0012" is tough to see and handle, try some of that <grinning>. Some of the Evanohm variants are quite stiff and do not like to be wound around a small form, they will spring like a clock's main spring if let loose and it is a mess to straighten out. Handling is difficult to say the least with the finer wire sizes.The standard specification of TCR for Evanohm 'S' is 0±5PPM/°C, meaning that a given spool of wire can be anywhere within ±5 PPM and considered in spec. Indeed a given batch of wire can be any TCR within that range, if it happens to be 0 TCR, they just happened to hit the mark during heat treating, it is not a real precise process and depends a great deal on the knowledge of the heat treat process and the alloy. You can certainly 'order' a spool of 0 TCR but it isn't going to be cheap, base pricing is calculated upon the standard ±5 PPM spec, when you ask for a specific TCR, the marketing cash register starts dinging big time, the price is near exponential as the TCR approaches zero TCR.Carpenter is quite possibly the most difficult alloy source to deal with, they absolutely do not like small potato buyers, I've had quotes from them and you will likely end up with something on the order of 10 lbs minimum unless they've changed lately, that is a lot of wire. They will provide your spec willingly if you buy enough wire. I generally do not see very much Carpenter alloy on the 'surplus' markets, why I don't know for sure, they are one of the largest alloy suppliers around along with Kanthal. You were very lucky to find a spool with a zero TCR on it, those are relatively rare as the original owners usually won't part with that TCR wire. Be advised, like most alloys, cold working the alloy will change the apparent TCR, it is a mechanical stress and adds to all of the other mechanical stresses that accumulate during resistor manufacturing. As I've noted before, the finished wire TCR is inherent to the wire and cannot be changed by normal resistor manufacturing, it is the mechanical stresses introduced during manufacture that make the overall apparent TCR different and depending on the resistor manufacturer, they will have come up with various ways of trying to reduce that stress with varying success.The wire enamel (and there are several classes of enamel) won't have too much effect on the 'ease' of handling it.The short answer to your wire buy request is no, as I noted above, Carpenter's minimum is large and 0±1 PPM TCR would definitely be a special order from any wire source, including mine. What TCR you end up with on a 'finished' resistor will be different than the ±1 PPM/°C you started with. I am not trying to dissuade you from trying, I'm just trying to let everyone know how difficult it is to achieve very low TCR, there are so many variables involved, you just might get a resistor here and there with low TCR but it will likely be by accident. The wire is only the beginning.If I could persuade my supplier to produce 0±1 PPM wire, their minimum purchase will run pretty close to $600 my cost and depending on the wire size, how many feet that will get us won't be known until the quote is made. If you can persuade enough people to purchase enough of the spool, I can absorb some of the wire myself if it is a size I need.

author: ramon
author_rank: Regular Contributor
timestamp: February 18, 2018, 08:28:17 am
content: Yes, it's like a clock spring. If I touch or cut them wrong they will permanently curl. For me it's impossible to straighten out. Also If I put two or more wires together they will suddenly make a sailor knot. Impossible to untangle.The first spool I bought was 1 oz. (29g) and around 3 miles length (5 Km). I think that is already a life-time supply of wire. The second and third spools I've recently ordered weights 4 oz (112 gr) and 5.5 oz (155 gr). That is 11 and 15 miles (18 and 24 Km). That is a lot of wire, I certainly don't know what I will do with 10 lbs.The second spool is actually 0/3 ppm. 0 TCR between 25-150C, and 3 ppm between -55-25C. And the third one is -1/+1 ppm (-1 ppm between 25-150C; and 1ppm between -55-25C).Coating says 'TE 40' and I have not found any information on Kanthal website. Also I don't know how I will remove it without damaging the wire.As no one has expressed their interest I think no one other than me has interest in buying wire. I actually don't need to buy more spools (47 Km are already enough), It is just only for the reason of having another diameter much more 'easy' ... well, less difficult to handle. But 10 lbs is huge, If such an order is made I guess it will contain at least 20 spools. I don't think we will meet that number. (I gave some wire for free, and only three persons expressed interest.)Don't know how often do you order wire. If yearly, quarterly, ... or if you already has a shelf full of spools and you do not need to order more wire in years to come. The idea was that it first benefits you. If for some reason you need some wire diameter (between .0024" - .0040") for some customer and you know that you will not use all the wire required in the MOQ. I thought it can benefit you (and us too) if we can share an order that otherwise will not be made (as you can choose to made the resistors with wire of different diameters from your stock). Ideally, everyone that joins the shared order will have it's own spool, so you don't need to re-spool it and everyone will have is own spool with low TCR labeled on it.

author: TiN
author_rank: Super Contributor
timestamp: February 18, 2018, 08:44:42 am
content: QuoteSell to next enthusiastic person, who get to thinking of making high-stability resistor is easy-peasy wiring some wire on bobin and job done.On serious note, in production type designs and component production, I'd think most manufacturers prefer to overstock. Especially for high precision, this may result buying life-time supply for parts, as you never know when manufacturer will EOL that very specific component you need so much to ensure your design performance. Same reason why I try to buy things in as much multiple (resistors, connectors, LTZ chips, 8.5d DMMs....) as economically possible. Leadtime of such orders also not helping to "buy it when you need it" direction either.So I think Edwin likely to have many extra spools, proven spools and verified spools, that he will never sell at any cost, just to avoid the trouble of doing inbound QC and testing of each new batch of wire. It just makes no sense from business perspective. Once you figured magic formula that works, you'd do everything to keep it going. In the end, all that time spent on validating and testing worth way more than even 10lbs spool.Same reason why high-performance resistors, ADC/DACs and opamps cost so much. Usually it's not because use of voodoo materials or expensive process, but because of expensive testing time. Making ATE production test system to test uA-offset and mV-offset opamp (and it's test time) is very different to making same principle but high-performance ATE system to test fA-level offsets and nV-level offsets.

author: ramon
author_rank: Regular Contributor
timestamp: February 18, 2018, 01:22:16 pm
content: I certainlyneversaid it will be easy, and I cannot find that anyone else in this thread said that before.Also, if you have read carefully I am not asking Edwin to provide one of his 'proven spools'.DIYis exactly the opposite of production. Magnificient Bastard has already said before the advantages of that better than I can write.Quote from: Magnificent Bastard on January 29, 2018, 06:59:31 pmBTW, you don't need to wind wire on a bobin to get a high-stability resistor, you can just lay the wirestraight. Have you read the NIST paper that describes the coaxial straight-wire evanohm resistor?"used as primary transfer standards for characterization of frequency dependence".You will not find any manufacturer that does that for you even if you pay. 'Production issues'. The same reason you will never get a wirewound divider resistor from Edwin or any other manufacturer.

author: ramon
author_rank: Regular Contributor
timestamp: February 18, 2018, 01:31:39 pm
content: Well, maybe I am wrong on that! (So I will ask first)Edwin, have you read the NIST paper about the straight coaxial resistor?Is it possible that you can do that kind of primary coaxial resistor for us (the same way you provide LTZ1000 resistor kits) to allow us to have some kind of primary resistance for 8.5 dmm check?

author: amspire
author_rank: Super Contributor
timestamp: February 18, 2018, 01:44:19 pm
content: Quote from: ramon on February 18, 2018, 01:22:16 pmIt the paper you are talking about this one?Calculable Coaxial Resistors for Precision Measurementshttp://ws680.nist.gov/publication/get_pdf.cfm?pub_id=29665

author: ramon
author_rank: Regular Contributor
timestamp: February 18, 2018, 04:20:57 pm
content: Yes, that same (four pages) document by Randolph E. Elmquist. Don't you think it's brilliant?If you are going to get any trouble by winding the wire into a bobin, then use straight wire.If you are going to get any trouble by having a some coating on evanohm wire, then use bare wire.If you are going to get any trouble by applying any kind of epoxy to the resistor, then don't use any.If you are going to get any trouble by welding evanohm to any dissimilar material, then weld it to evanohm itself.If you are going to get any trouble by ...I propose to extend that way of thinking further: and for proper attachment to 8.5 digits DMM we will also need Kelvin test leads made with evanohm wire welded into a 100% evanohm-S mounting post (or banana plug). Just kidding (... or not?)Seriously. Going even further, If we take it to the extreme:If we are going to get any trouble by doing a weld, then do not weld it.If we are going to get any trouble with leads attachment, then don't use them.

author: TiN
author_rank: Super Contributor
timestamp: February 18, 2018, 05:48:51 pm
content: I'm just lost on what is the end goal of all this trouble?Resistors and their specs are built to purpose (usually). What is it in here for DIY?

author: zhtoor
author_rank: Frequent Contributor
timestamp: February 18, 2018, 07:16:44 pm
content: Quote from: Magnificent Bastard on February 18, 2018, 06:13:43 pmhello,i am not so sure about "very high" (100K +) and "very low" (100R -) to start with,middle ground should be the more reasonable target.and to do DIY PWW's a starter's approach may be use a manganin / zeranin (solderable)enamelled wire selected for a -ve tempco (say around -5ppm / degC) and use a copper-wirecompensator to "tune" the total TCR.as far as DIY hermetic packages go,how about taking the top off a 2N3055, disconnecting / removing the transistor dieand mounting the resultant 2 1/2 terminal posts into a milled aluminum box andmaking a metal-metal seal using some kind of loctite or "head gasket" sealing shellac.regards, and btw i agree with you 100%-zia

author: TiN
author_rank: Super Contributor
timestamp: February 18, 2018, 07:18:15 pm
content: Magnificent Bastard,I didn't say it is useless, quite the opposite actually. I spent hundreds of hours and $$$$$ on gear, that I don't need for day-day work,but if I go and post on forums, I usually try to specify what the goal is. This way other people can suggest some ideas towards the goal.Right now it's a lot of guesses here, but not clear direction where we going. And boundaries in resistor technology formetrologytoday advanced far beyond ppm level.There is not much learning in speculating about this and that, learning means the need in experimenting and documenting of observed results.

author: GerryBags
author_rank: Frequent Contributor
timestamp: February 18, 2018, 07:48:33 pm
content: Forgive me butting in with what may be an ignorant suggestion (especially if it has already been brought up and I missed it), but apparently silver solder is used to join evanohm wire to terminal hooks made from nickel plated oxygen-free copper.I know a few people who have been involved in the Hatton Garden jewellery trade for generations, who may know more reliable sources for SS than The Bay, I must remember to ask.

author: branadic
author_rank: Super Contributor
timestamp: March 12, 2018, 07:55:26 pm
content: Ordered some hermetic feedthroughs from they bay:Glass Hermetic 1 Pin Feedthrough Size 7.1mm x 4.5 mmGlass Hermetic 1 Pin Feedthrough Size 10.4mm x 5 mmthat received today. Together with some brass or copper tube it's possible to create hermetic resistor cases. Here is a frist put together.-branadic-

author: babysitter
author_rank: Frequent Contributor
timestamp: March 12, 2018, 10:34:39 pm
content: Quote from: branadic on March 12, 2018, 07:55:26 pmI imagine a smaller brass tube sticking out, soldered shut to keep some dry gas inside. Also, a Pt100 glued to the housing. And resistance wire wrapped around everything.

author: Gyro
author_rank: Super Contributor
timestamp: March 12, 2018, 10:55:55 pm
content: I have some of those feedthroughs myself and have similar ideas (possibly for a complete reference circuit).Just looking at your arrangement, it strikes me that thermal expansion of the tube could put significant axial stresses on the resistor. I would suggest including strain relief loops in the leads... or maybe bring them out radially from a soldered copper foil box rather than a rigid tube.

author: branadic
author_rank: Super Contributor
timestamp: March 13, 2018, 07:59:21 am
content: QuoteWell, looking at the CTE of the different materials there is only a slight mismatch between the copper leads and the brass tube. However, I will use a tinned copper tube instead, but for a first put together I used what I had laying around.CTE brass: 18.4 ppm/KCTE copper: 16.5 ppm/K-branadic-

author: Gyro
author_rank: Super Contributor
timestamp: March 13, 2018, 09:48:12 am
content: I was thinking more of the length of resistor package thatisn'tcopper wire - the bobbin etc. I'd still go for a slightly longer tube to leave space for strain loops.

author: iMo
author_rank: Super Contributor
timestamp: December 05, 2022, 10:02:27 am
content: I want to make diy wire-wound resistor - around 110ohm, for my ADR1001 vref (Izener). I found an old 20k ww resistor in my junkbox, took 20cm lengths of wire off it and it reads 136ohm. The wire is pretty thin, like 0.05mm, with cotton isolation. I can solder it after some effort.Let me ask you - after winding the wire on a small former, is there something special to do with the wire? Annealing with higher temp or something like that? Or just a drop of SG and that is it? This resistor is not about extreme precision or stability, but rather about fine tuning the resistance during my experiments..

author: Kleinstein
author_rank: Super Contributor
timestamp: December 05, 2022, 10:35:39 am
content: Soldering with quite some effort is a bit unusual. NiCr and similar alloys are very hard / nearly impossible so solder with soft solder. Manganin and similar alloys are usually relatively easy to solder, unless coated with some insulation.It can make sense to use some kind of burn in: heat the resistor to the maximum expected temperatue or a little higher and let is cool for a few cycles. This would be mainly for the thermal stess from different thermal expansion and maybe a few stress concentrations to relax.The more important part of cycling the resitor for some time would be to see how stable the resistor is. So more about testing than actual annealing. There are fine details and surprized that can make the difference between a good and poor construction. Even with experiance it is better to check the actual performance. It can range from surprisingly good to terrible.Reusing wire from an old WW resistor can be a bit tricky, as the wire is already bend somewhat and thus may show additional drift.Manganin wire is reasonable available and relatively easy to solder.For fine tuning / experiaments with different values I would more use parallel soldered SMD resistors, not a longer / shorter wire of a WW resistor.Onece the right value is found one may make the correct value as a WW resistor.

author: iMo
author_rank: Super Contributor
timestamp: December 05, 2022, 10:51:04 am
content: Yep, the wire is springy and curly, the original ww resistor is about 7mm outer dia and 10mm length. The wire itself is hardly to see with a naked eye..PS: ok, I resign for today.. After 11 attempts I made 4 working resistors, all couple of ohms off my target (wound on a 56k 125mW resistor). The wire has a good cotton insulation (perhaps impregnated with something) and it is extremely thin, perhaps like a hair. Soldering the wire goes ok, but to get rid of the insulation is difficult and with those 0.6ohm per millimeter it is rather delicate exercise..

author: iMo
author_rank: Super Contributor
timestamp: December 06, 2022, 09:29:32 am
content: FYI - below the comparison - 0.2mm enameled wire vs. the wire I cope with. The cotton coating has been removed by a gas torch, you may see the ball made of the melting material at the end of the wire. A little bit more temp and the wire evaporates.

author: Andreas
author_rank: Super Contributor
timestamp: December 06, 2022, 12:11:04 pm
content: Quote from: imo on December 05, 2022, 10:02:27 amhow much power was it rated?if it was a 2W resistor: now it has ~20mW.If it was a power resistor: the nominal value is for a wire temperture between 200 - 350 degC dependant on make.so I would not load this with more than ~2 mW.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: December 06, 2022, 01:37:47 pm
content: Andreas, power rating - no idea.. It is a set of ww resistors mounted on a multi-position rotary switch, with 4k and 20k values, which did something 50+ years back. Those resistors are wound on special bobbins with two pads/leads at its bottom side. The 4k wire looks a little bit thicker, the amount of wire is similar as the 20k one. So most probably a low power divider.During my experiments I wound ~107 ohm resistor 4x successfully (aprox 189mm long wire) and then I annealed it with 60-70mA current and it heated up, with little smell, it sustained even 100mA but the cotton insulation started to smell a lot and it changed the color.I think the 3-4mA of the zener current cannot hurt it, no way, the big issue is the construction - the de-cottoning of the wire is difficult (unless you use the torch, but most of the time the wire evaporates), often the tinned wire breaks during soldering it, when shortening the wire by a couple of mm you are immediately off by couple of ohms.I do not know how they soldered it 50+ years back..

author: dietert1
author_rank: Super Contributor
timestamp: December 06, 2022, 11:40:02 pm
content: Mouser still have those Dale (now Vishay) 1% wirewound power resistors, like RS-02B. They are durable and long term stable when used at low temperatures. At room temperature their TC is within +/- 5 ppm/K and with a little selection one can make a divider with +/- 1 ppm, no problem. They are cheap and available in many different resistance values.Regards, Dieter