Accurately Measuring Precision Resistors

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Hey, c'mon, I got a (relatively) accurate Leeds & Northrup Wheatstone bridge; it's fairly recent, due for recal in late 1968. ;-))

I got the Fluke 8600 decades ago, and it has earned its keep. I used a Fluke differential voltmeter in the army, and that was a real tedious PITA. So going to a DMM is like the best thing since sliced bread.

I think the best investment I've made in the last few yrs was the AADE LC Meter IIb. It's great for those odd coils and caps.

But I think I should get a 'scope that's better than the ol' 15MHz one that I now have. I can get a 100MHz one for a hundred or two on Ebay, but a pair of decent stinkin' probes for it cost more than that!

Maybe I should spring for a scope that has the DMM pod on it, or has all that buiilt-in and shows on the screen. I don't think I wanna get a hend-held one because they're too easy to steal or drop. Besides, the Fluke cable meter that I use at work had to have Ni-Cd batteries replaced and they were $158 apiece.

On Sun, 17 Apr 2005 08:48:05 -0700, Watson A.Name - "Watt Sun, the Dark

Ironically, a watch that's not even running shows the exact time twice a day!

Cheers! Rich

So, you're going to stand there and count 602,214,150,000,000,000,000,000 individual electrons?

Good Luck! ;-) Rich

On Sun, 17 Apr 2005 10:16:25 -0700, Watson A.Name - "Watt Sun, the Dark

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Before winding it, bend the wire into a long, narrow "U" shape. Start winding at the bend, in the middle of the form, and wind both ways. I was told that this makes the inductances "cancel out". ;-)

Cheers! Rich

Not digital watches. They just lay there and do nothing.

I doubt anyone made a digital watch with the flip panels. ;-) But I have noticed someone is making clocks like that again!

Best regards, Spehro Pefhany

Here's one:

It does when wound on a toroid. You wind 1/2 the turns in one direction, form a U in the wire (as if you were making a centertap) and wind the remaining turns in the opposite direction. I've never done it with an air wound, but it is a technique I've used on toroids.

Ed

How did you know that the wire was *exactly* 32 AWG? How did you terminate the resistor? How did you know the purity of the copper? Note that basic 'tough pitch' copper does not have the same resistivity as the ubiquitous OFHC wire, which does not have the same resistivity as pure copper. Which did you have, and which was used to build the wire tables you used?

With the same problems of accurate measurement of all parameters.

Yes, it's known as bifilar winding, and would drop the above resistor to less than a nanoHenry with reasonable care.

Not simply bifilar. The reversal scheme is called an Aryton-Perry winding, e.g., as in Ohmite's WN series of wirewound non-inductive resistors,

An 'Ayrton Perry' winding *is* just a standard bifilar winding, most easily made by cutting the length of wire you need, folding it in the middle, and winding carefully from that midpoint to the ends, keeping the twinned wire flat to ensure that adjacent turns always carry opposing current.

Unless it's wound as I described above, which most machines seem to do. The bifilar idea may be good for hand-made resistors, but perhaps not for machines.

Also, a bifilar winding isn't necessarily a low-inductance winding, that was my point. It may well be, if the end is connected, but it wouldn't have to be. For example, for very low resistances multiple parallel wires are helpful, wound all at once and shorted.

I'm coming in late here, but I always sort of liked the band Chicago's assessment of this. :-)

Tom

Bifilar means to me that the wires stay parallel throughout: Loop ^ \\ \\ \\ \\ \\_ \_ Leads

But the one I had in mind was more like this: Loop ^ Lead _/ / / / \ \ \ \_ Lead

I don't know what it's called.

Cheers! Rich

That's it, exactly. Someone called it "contra-wound" - but I don't know if that is the proper term. On the toroids I wind that way, the first half occupies about 40% of the diameter and the last half occupies another 40%. Ed

I'm just curious as to why you'd want to use a toroid. The whole idea of a toroid is to increase the magnetic field containment, so you get better inductance. Then you go winding it bifilar, to _get_rid_ of the inductance! This doesn't make sense.

Oh, one other point. This "contra-wound" method can only be used for a single layer. Anf it looks like the winding has to start at the hairpin bend of hte halfway point and go outward.

Indeed, but my point was that the word "bifilar" does not by itself imply low inductance, as the bifilar construction below illustrates.

. ---, . ^ . \\ \\ \\ \\ \\ . V . '---

On Wed, 20 Apr 2005 04:23:32 -0700, Watson A.Name - "Watt Sun, the Dark

Yes, that's exactly it. I saw it in a book once. :-)

Cheers! Rich