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.
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