0.1 ohm Resistors from Scratch Again

With resistance below at 20-25 degrees C ONLY ROUGHLY, and close to varying roportionately with absolute temperature (degrees K):

18 AWG is close enough to 1 mm , 1 ohm per 160 feet 19 AWG is close enough to .9 mm , 1 ohm per 125-128 feet 20 AWG is close enough to .8 mm , 1 ohm per 100 feet 21 AWG is close enough to .7-71 mm , 1 ohm per 80 feet 22 AWG is close enough to .625-.64 mm , 1 ohm per 62.5-64 feet 23 AWG is close enough to .56 mm , 1 ohm per 50 feet 24 AWG is close enough to .5 mm , 1 ohm per 40 feet 25 AWG is close enough to .44-.45 mm , 1 ohm per 31.4-32 feet 26 AWG is close enough to .4 mm , 1 ohm per 25-25.6 feet 27 AWG is close enough to .35-.36 mm , 1 ohm per 20 feet 28 AWG is close enough to .314-.32 mm , 1 ohm per 16 feet 29 AWG is close enough to .28 mm , 1 ohm per 12.5-12.8 feet 30 AWG is close enough to .25 mm , 1 ohm per 10 feet 31 AWG is close enough to .22-.23 mm , 1 ohm per 8 feet 32 AWG is close enough to .2 mm , 1 ohm per 6.25-6.4 feet 33 AWG is close enough to .175-.18 mm , 1 ohm per 5 feet 34 AWG is close enough to .16 mm , 1 ohm per 4 feet 35 AWG is close enough to .14-.142 mm , 1 ohm per 3.14-3.2 feet 36 AWG is close enough to .124-.128 mm , 1 ohm per 2.5-2.56 feet 37 AWG is close enough to .11-.111 mm , 1 ohm per 2 feet 38 AWG is close enough to .1 mm , 1 ohm per 1.6 feet 39 AWG is close enough to .09 mm , 1 ohm per 1.25-1.28 feet 40 AWG is close enough to .08 mm , 1 ohm per foot 41 AWG is close enough to .07-.072 mm , 1 ohm per .8 foot 42 AWG is close enough to .0625-.064 mm , 1 ohm per .625-.64 foot 43 mm is close enough to .055-.058 mm , 1 ohm per .5 foot 44 AWG is close enough to .05 mm , 1 ohm per .4 foot

Now, take these values as well as those from any wire table with half a grain of salt. The reason: Tolerance, and some of the wire manufacturing processes. The wire gets drawn through dies, and the dies experience wear as they get used. The wire will get a little thicker as the last die experiences more wear. Who knows when the die gets thrown into the trash or maybe even the wire gets declared to be suitable for marketing by a different wire gauge or (GASP!) the next larger size?

Also, consider that most metals, including copper, have resistance varying roughly proportionately with temperature - as in having positive temperature coefficient of around .33% of 25 degree c resistance per degree C. (Nichrome has a significantly lower positive temperature coefficient.)

Now for the above chart - I did not have a chart in front of me, but I do remember that the AWG wire tables in the CRC handbook have figures close enough to and I consider almost certainly within expectable tolerances of what I call "1/3 octave numbers" - the powers of the 10th root of 10. (Diameter of an odd gauge was a "1/6 octave number"). Resistance in ohms per foot at 20-25 C was close enough to a 1/3 octave number, and resistance at 100 C was close enough to one "1/3 octave" higher. I like to use the 100 C figures when designing magnetic components. As for how I got the term "1/3 octave"? Ever see what they call a 1/3 octave equalizer? This is a somewhat common tool used by touring bands that drag along their own sound reinforcement ("PA") equipment and drag along enough equipment to drag along an "amp rack" and an "effects rack".

1/3 octave equalizers tend to be in the "effects rack". If you see one, look at the frequencies in Hz: 31.5-32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315-320, 400, 500, 630, 800, 1K, 1.25K, 1.6K, 2K, 2.5K, 3.15-3.2K, 4K, 5K, 6.3K, 8K, 10K, 12.5K, and 16K. Sometimes as high as 20K and as low as 25 and 20 Hz.

What's convenient about "1/3 octave numbers" is that any product or any ratio of any two of these is another one of these. Any square of one of these is another one of these. Half of these numbers have a square root being another one of these, and the other half have square roots being "1/6 octave numbers". Pi is one of the "1/3 octave numbers", and the number of feet in a meter is not that far away - although cubic feet in a cubic meter is close to a "1/6 octave number" that is not a 1/3 octave one. This enables easier rough engineering calculations in one's head. One who has practiced multiplying in the head via usage of "1/3 octave numbers" or "1/6 octave numbers" can get a final answer 5-20% off faster than a more exact answer can be obtained with a calculator, and tolerance in wire size (as affecting resistance per unit length) can get about that bad. It's a shame that most resistor and capacitor values come in 6, 12 or 24 per decade as opposed to 10 "1/3 octaves" per decade.

- Don Klipstein ( snipped-for-privacy@misty.com)

It seems that you have reasonably characterized the wire; so many ohms i so many feet; isn't that good enough? Does it really matter if it is #42, #43 or #44? Call the result a WWW resistor, for "Win's Wire Wound" resistor and do not ask What, Where, or When.

I read in alt.binaries.schematics.electronic that "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote (in ) about '0.1 ohm Resistors from Scratch Again', on Sun, 18 Sep 2005:

Why do you need to know the 'wire size'? The wire you are using will have been stretched slightly when it was wound on to the bobbin, so it won't be an exact size anyway. It seems to me that if you are trying to make a resistor, all you need to know is the resistance, and you can measure that. If you are concerned about current-carrying capacity, any figure you get from a table probably won't apply to wire wound on a resistor. So what you do is to pass current through your resistor and see how hot it gets. When you can't hold it for 10 s without pain, it's carrying just a bit more current than is prudent.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

kilometer to

Whoops...

has a calculating thing at the bottom

DNA

It looks to me like Watsun doesn't trust his ohm meter, and is trying his hand at making some standard resistors... unfortunately, copper wire manufacture isn't all that standard... especially in the hair fine sizes.

Manufacturers only spend money on tightening up specifications that truly matter to their customers. Cu wire size, though important, is only about 5% important. Additional parameters that affect resistance are oxygen content, copper purity, hardness, smoothness, imbedded stress and strain (if you don't believe that stress and strain affect the characteristics of wire, lookup "Wiegand effect" (for ferromagnetic wire).). When you buy spools of wire that was meant for making precision WW resistors, you will notice that the spools are hand stamped with ohms-per-foot values that have been precisely measured.

Large users of copper wire can order 'tween sizes that exactly match their physical requirements. You will find motors, and transformers, wound with odd sizes that you could never buy in an electronics store (17.5 AWG...). Wire that you salvage is especially likely to be a 'tween size.

When NBS made their standard resistors, they used pure metal rods finely machined, and polished, to standard diameters, and lengths. It was nothing like the brutally inaccurate process used to draw wire through dies. (put some drawn wire under a good microscope, and look at the grooves and trenches in the surface.)

NBS's higher value resistors were made by using bridges to adjust the higher value cut-and-try resistors against the low resistance physical "standards".

-Chuck

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[snip]

Thanks, Don. But this is the exact chart I found online as I described above. As I said above, what I'm looking for is a metric wire table, not AWG.

Thank you.

see

to

any

it's

Am I 'chasing rainbows' by asking for a metric wire table? I was just unpleasantly surprised that I couldn't find one online.

Thanks.

selection.

wrote

copper

Thanks. I'm a bit confused as to what the columns mean, especially the IEC stuff. The calculator seems to give an answer, but if I plug my predicted .07 mm value into it, does it give me an answer that is an increment of the metric wire table? IOW, can I find a .07mm metric wire?

I'm just kind of unwilling to believe that there isn't some kind of metric wire table online.

Thanks.

I read in alt.binaries.schematics.electronic that "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote (in ) about '0.1 ohm Resistors from Scratch Again', on Mon, 19 Sep 2005:

What did you Google for? "Giraffe"? "Metric copper wire sizes" produces enough hits to appease anyone.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

spools

It's trivial to measure resistance of a 0.1 ohm resistor fairly accurately using a bench power supply, an ammeter, a voltmeter and Ohm's law.

Wire taken from the same spool will have very similar diameter, since it has all passed through the same drawing die.

A resistor as described also makes a reasonable* temperature sensor... pure copper has a tempco in the +3800-3900ppm/K range, similar to that of platinum or other more stable materials that are often used. As such, you should consider the temperature when measuring the resistance if you care much about the resistance value.

*not so reasonable if you use very fine wire

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

So trivial, that I didn't think it required mentioning.

Watsun, I believe, wants to be able to go to a wire table, and read out that XX mm wire is YY ohms per 1000m, so he needs *exactly*

0.1 ohm x 1000m / YY ohms,

meters of wire to make his resistance. This will work, but not to within the inherent accuracy of precision ohm meters, such as on a HP3478A DMM.

I am just guessing as to why he really wants to make a precision 0.1 ohm resistor from scratch. My guesses are based on some of the themes of past postings by Watsun on the subject. I am probably totally off base.

True, but the die will increase in size somewhat by the time it reaches the end of a large spool. Copper is a very abrasive material, and eats dies for lunch. When the #38 die becomes too big, it becomes a #37 die.

-Chuck

Then the best way to find the resistance per unit length of a given spool is to take a known length, measure the resistance (using the ammeter/voltmeter/bench supply if you don't have a Kelvin connection milliohmmeter), and divide by the known length. ;-)

He'll *never* make what I would call a 'precision resistor' from copper wire. A few degrees C temperature change and the resistance will have changed by a whopping 1%.

If you're happy with the 10% tolerance range + tempco, maybe you have a hope. I was able to get away with it in that general region, although I hedged by having more like 20% adjustment range.

Thus you should avoid making one resistor from the beginning of the spool and the next from, say, the other end of the spool. ;-)

Magnet wire is drawn through diamond dies which don't wear very rapidly. I suspect that surface issues have a lot more to do with variations at smaller diameters. Personally, I wouldn't go much below AWG30 for this sort of thing. Another issue is that the finer wire stretches pretty easily. If you stay below the elastic limit that might even be a useful feature.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

[snipped lots of words asking about where to find standard metric wire size data]

Use the IEC60317 chart. The following chart covers the equivalent of

24 to 50+ AWG. Bold values are common sizes.

--
Mark

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kilometer to

I suppose I could mark the bobbin "1510 milliOhms/Ft" or something like that. But then when I come back in six months and want to use the wire for something, and I really have to know the wire gauge, then I'm still nowhere. Why? Because I still haven't directly correlated the one single measurement with the wire gauge. I suppose I could make a half dozen or so resistors each with a foot of wire as close as I can get, and then measure them and take an averagee so that I can be reasonably certain that the one and only measurement I'm now relying on wasn't distorted by stretched wire or shorted turns or a bad solder job, etc.

Yes. "Size Does Matter!"

BTW, it has a couple microhenrys inductance. :o)

[snip]

their

with

AWG...).

This is exactly wnat I'm noticing. The wire seems to be something like

41.5 gauge.

Or as someone else mentioned, the dies were so worn that the wire is considerably larger diameter than it's supposed to be.

[snip]

ONe thing I found out is a pound of copper makes enough 42 AWG wire to stretch across the city! More than ten miles.

Thanks for the info.

It doesn't have to have all that much, bend the wire into a hairpin loop before you wind, and most of the inductance will cancel out.

-Chuck

I read in alt.binaries.schematics.electronic that Rich Grise wrote (in ) about '0.1 ohm Resistors from Scratch Again', on Mon, 19 Sep 2005:

Well, in one way, but consider how many miles (and even more km) there are in a big reel of 40 gauge. 'An awful lot' can pass through in just a few hours.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in alt.binaries.schematics.electronic that "Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote (in ) about '0.1 ohm Resistors from Scratch Again', on Mon, 19 Sep 2005:

I'd tell you how to do an Ayrton-Perry winding, but it would upset the Moebius Coil people. (;-)

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

"John Woodgate" a écrit dans le message de news: snipped-for-privacy@jmwa.demon.co.uk...

Oh, you're too fast for me.

--
Thanks,
Fred.