Resistance variation with thickness

AFAIK the resistance of wire is proportional to its Cross Sectional Area. Period. If this remains unchanged, so does the resistance.

Gareth.

That is correct, but the length also has to remain unchanged The formula for the resistance of a conductor is R=r*L/A where R= Resistance r=Resistivity of the conductor (1.7x10^-8 for copper) L=Length A=cross section area

As you can see, the resistance remains constant as long as L and A remain the same, or change in a manner that produces the same ratio.

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So that begs the question, how much can a piece of copper wire be compressed? If you do squash it into a different shape, does or can its volume change significantly?

Gareth.

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So it may be an effect of work hardening , relative increase in the effect of imperfections/micro fractures or some other metallurgical effect. Mackie speaker voice coil failures due to this flattening/ribboning process to make the tails to the outside world. Previous failure at the juncture of round to flat (0.07mm round to about

0.02 x 0.2mm) so at the peak stress point. This one along the length of the ribbon section, but the whole 50mm or so run was brittleised and disintegrated on touch, not the slightest sign of overheating on the remaining 25 turns of round wire. broken end marked B on this pic Cannot expore the metallurgy as that curve of "wire" as totally disintegrated to dust.

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The shape of the cross section can change to virtually any dimension so long as the length remains the same. IOW, if you squeeze a bar of 10mmx10mm down to

2mmx50mm, its cross sectional area stayed constant (only the shape of the area changed). Its length will remain the same, since the volume didn't change; hence, its resistance will remain the same. So long as material is not added or removed, the volume will remain the same. The formula says that the ratio of length to cross-sectional area must remain the same in order for resistance to remain unchanged. If cross sectional area is changed, the length must change to maintain the ratio. The volume must remain constant.

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What I believe Norm is questioning/proposing is that the wire may have been made more "dense" by being compressed without lengthening, and that would probably decrease its resistance.

Bob Hofmann

IMHO, if the wire is squashed it will defin8itely get work hardened and this will mean an increase in resistivity

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Thanks for your time

Archer

I think a little experiment is in order. A length of fine wire, measure R and diameter , then squash between shim spaced flats and re-measure R and dimensions. Whether work-hardening (xtal structure change ?) or micro-fractures or whatever does not really matter , just some data on resistance change.

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Needs repeating with a proper milli-ohm set up rather than DVM 0.1 ohm resolution or preferably actively with some higher current through it. Probably really only becomes manifest close to the current carying limit of the original round wire, replicating the problem in speaker voice coil production and use.

Tried about 1m of 0.09mm (including varnish) wire around a 15mm a side flat and squashed between 2 flats in a protected vice and made no difference to

4.0 ohm, despite 32 theoretical, not obvious, flats. Repeated with just a single 15mm length of that 1m long wire, reducing 0.09 mm to about 0.03 mm , and repeated further along, so 2 squashed bits of 15mm . May have increased to about 4.05 ohm overall but not as obvious a change as I was expecting.

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Again 1m length , ps set for constant voltage and unlimited current. Set V for 0.5 amps , 2 flats and no change, set for 1A and 4 flats added, no change ie less than 0.01 amp change, if any. Set to give 1.7 amp and varnish burnt off. So no further forward.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

All your testing seems to have verified that the formula for resistance vs. cross-sectional area does work. No matter how many 'flats' you make on the wire, you haven't changed its resistance. The small amount that it changed can easily be contributed to variations in meter connections and/or minute changes in length due to the squeezing of the wire to make the 'flats'. Barring any crystalline structure changes in the metal itself, so long as the cross-sectional area and length doesn't change, the resistance doesn't change.

I suggest that the speaker winding that you're trying to diagnose failed because of metal fatigue, possibly due to loose mounting, broken adhesive or just an imperfection in the coil at manufacture.

Time to move on?

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Life is like a roll of toilet paper; the closer it gets to the end, the faster 
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If it was just a one off , then fair enough. But 2 separate Mackie amps with the same problem in the same 2 squashed percent of the wire of the speaker voice-coils ?

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Poor quality control, or a defective manufacturing process.

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Its more than that. In both Mackie cases the main active coils showed absolutely no overheating let alone burning. In all other odd o/c speakers I've poked my nose in, have had large sections of charring or completely burnt and broken coil formers. Not failure in the tail sections because they are usually different / larger conductors soldered or welded to the main coil. These Mackie tails are acting as fuses, protecting the voice coil, which is ridiculous.

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No, it isn't. Who would manufacture speakers that way for Mackie? If they did, all of them would fail. They would have to pay extra, and get no warranty. It sounds like the tinsel wire was the wrong type for the power level and was likely got through due to poor QC. Poor quality tinsel wire also suffers from work hardening, and broken strands. When enough break, the rest burn. Also, if they are a few percent too short, they get more mechanical abuse, which destroys them. Since you didn't see them when they were brand new, you have absolutely no idea what really happened.

I have worked in failure analysis in electronics manufacturing, and I can tell you that production people can be some of the biggest idiots in the world. Monday mornings they have hangovers, and Fridays they don't give a damn what they do, as long as they can leave on time to drink their paychecks.

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The problem is not at the tinsel wire, ie the flexible bridge. But before that, a flattening of the voice coil wire into ribbon. Production seems to be a precise length of say 2.5m with the ends squashed giving some precise run of round voice coil wire. One end has the ribbon fixed to the tinsel in the ideal spot but the other end arrives at maybe half a turn from ideal and has to make a half turn to join the tinsel for the other termination. Tinsel ribbon is presumably higher current carying than the crresponding voice coil wire, so no problems there. This Mackie problem is something to do with a run of same gauge but squashed wire. Remember the whole half-turn of ribbon disintegrated so not due to a nick or rubbing on metalwork.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Perhaps it is chemistry . Bear in mind that the wire is silver and not copper and the surface area of a squashed wire is more than when it was round and any corrosion on that surface will have proportionally more effect on the thin flats than the bulky round.

The voice coil wire looked like copper because it was under a coppery brown lacquer. But, before it disintegrated, the ribbon section was darker brown than the round section. Now if air could get under the lacquer and tarnish the silver to black copper sulphide, or whatever that blackening is, then that would explain it. Unfortunately none of that section remains as it literally turned to dust after photographing it, you could not pick it up even with fingers, it was little more than a wraithe.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Are you sure that voice coil isn't aluminium? Some aluminuium v/cs are copper coated btw.

Ron(UK)

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The voice coil remains unaffected, do you know of a simple test for silver v aluminium ? I would say it looked brighter, more silvery indeed, than aluminium but I am not familiar with seeing .07 to 0.09 mm diameter Al wire or silver wire for that matter.

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