zhtoor:
If you think you're going to build a DIY precision resistor, I think you want to consider this: If you need a new light bulb, do you build one or just buy one ready-made? Think of all the technology that goes into a cheap incandescent bulb, or even an LED. You'll never build a light bulb on your own that's as good or as cheap as a factory made unit. You need the economy of scale to get the cost per unit down to something reasonable.
It's the same with resistors: You -could- buy the wire from a custom metals mill...which will cost at least several hundred or 10's of thousands of dollars for a minimum quantity of high-grade, low TCR resistance wire. It takes quite a bit of knowledge to even know what to ask for at the mill. Once you start asking for PPM tolerances, the price goes up more than exponentially.
I'm not sure what you're trying to DIY here, but you need to know that they don't give away good (or bad) quality resistance alloy. It's not cheap to make.
And then comes handling the wire, and what to wind it on. Sure, you could probably make some sort of tongs to hold on to stiff, very springy / ornery wire and try to make a spot weld. It might work once or twice by accident but with an incredible amount of frustration. There are man-centuries worth of research time just studying how to reliably and repeatably attach resistance wire & foil to just about -anything- else without screwing up the apparent TCR. The state of the art technology in this area still far from perfect, regardless of what Vishay datasheets tell you. This is very much a "Materials Science & Black Arts" area.
You want to flat-wind the wire on a glass slide? Just remember that every bend that goes into the wire can be a stress-riser to increase TCR. Every time you bend that alloy, you are destroying the carefully dispersed and annealed metal alloy structure within - and depending on what you're building you might need to anneal the wire after handling, etc.. There's a reason why round bobbins have been a very successful style for PWW resistors.
Do you want your home-made resistor to work only at DC, or do you want to handle low-freq AC? What noise properties, Does it pick up mechanical vibration? Magnetic interference? Magnetic interaction with other resistors? Voltage rating? Inter-winding capacitance? Inductance?
The list of those pesky details goes on and on and on and on.
Good resistors are incredibly complex beasts once you get to know them - this is not a trivial project. Cerebus seemed to be teasing Edwin in his message above, but the truth is resistors are very, very hard to get right: "The devil is in the details".
Before you build anything DIY: I suggest you order some resistors from Edwin if you haven't already - he's made a few million over his lifetime and knows a thing or two. Order from General Resistance, Vishay, Bourns and whomever else you can - and study and compare those first on your exploration journey - and learn -why- each type is designed the way it is. There is a resistor design style for every application, and you want to learn about that. The precision resistor thread here on this forum is also a good place to pick up some details.
Another suggestion: Look up the articles Edwin wrote on the history of resistors for EDN magazine...That's packed with good information also, and will give you a hint of the troubles in store for any resistor designer.
thanks misterdiodes for your extremely valuable insights into the subject.
i live in pakistan, a third-world under-developed country where almost nothing of precision is made. you have the luxury to live in a
society where precision components are available sometimes off-the-shelf and sometimes for a small lead time. and moreover, when
we translate the prices in USD to PKR, even a couple of bucks is quite a bit. (for example, Edwin charges USD6 per resistor, which is
actually the lowest price I have ever seen of this type of resistors, is equal in price to 40Kg's of wheat in pakistan), so, the combined
effect would be naturally to think about "doing it yourself".
now regarding the question you raise:-
as for considering a glass microscope slide, the material is readily available, mica is available easily too and one of the nearby towns
specializes in ceramics production so i though i could get ceramic plate type formers done at a very low costs, all these materials
share a couple of characteristics, relatively low thermal expansion and high insulation resistance. moreover i have seen a number
of very fancy pww's wound on mica (genrad decade boxes for example).
your point about introducing mechanical stress in the form of a sharp bend, i concede, i think round bobbins would be a lot better in
this department (you might get away with no-annealing after winding).
initially i want the resistors to work on DC only, i dont want to do any fancy non-inductive winding styles or start winding from the center-fold
of the wire to cancel inductance effect, neither do i really care about any kind of magnetics / microphonics pickup, interwinding capacitance
is not an issue for me at the moment.
i certainly agree with you that the devil is in the details, that is, if you get into the details. and yes i do expect a train of devils, anyone should
who does any kind of development.
and yes, i did read Mr. Edwin's articles a couple of months ago.
sorry for my naivete, but i just want to find out myself, what does handling a .02mm precision alloy wire really mean.
regards and thanks again.