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I've just bought some 6mm^2 copper wire for my project where I will have large pulse currents - ~500A peak (driving very large Xenon flash bulbs, 1000J pulse in 2ms).

But I know that skin effect make it useless to use thick wires on high frequency, but does it apply to sharp fronts? Is that correct that having multiple non-isolated wires make it even worse? (mine cable have about 20 tiny 'wires')

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Somewhat a nitpick, but the skin effect does not mean that thick wires are useless at high frequencies, rather it means that the interior of the wire is not useful. There's still more conductor in the skin deep circumferential ring of a large wire than that of a small one. Tubing is sometimes used in high power transmitters. –  Chris Stratton Feb 8 '11 at 5:19

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+100

For 6mm2 copper (~1.4mm radius), the skin effect impacts the wire at ~2.5kHz.

Your 2ms pulse is equivalent to a 0.25kHz square wave. Mathematically, that square wave is a sum of all the odd-multiple sine waves (0.25kHz, 0.75khz, 1.25khz, etc). Since the skin effect drastically reduces frequencies above 2.5khz (more as the frequency goes higher), you're creating an imperfect square wave (aka slowing down the ramp-up and ramp-down times of your pulse).

So, the rise-time on an "ideal" 2ms pulse will get slowed to maybe 80uS. This assumes you're able to generate 500A pulses with very high rise times in the first place.

And yes, having stranded cable (20 tiny wires), does make this a small bit worse, but not much. As one other poster suggested, litz wire (http://en.wikipedia.org/wiki/Litz_wire) helps for 100kHz+ applications by providing many insulated tiny wires.

Ref: http://www.marcspages.co.uk/pq/3250.htm (Skin Depth Tables) http://www.nessengr.com/techdata/skin/skindepth.html

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Wouldn't stranded be a bit better? –  tyblu Feb 7 '11 at 6:31
    
If it's Litz wire (which has each strand insulated), yes. But for regular stranded wire it's worse for a given (overall) wire diameter because the stranded wire still has some gaps between the wires, meaning you have less overall copper. The strands all short to each other, so its effectively a big wire with some holes in it as far as skin effect is concerned. –  vandee Feb 8 '11 at 7:33

A sharp pulse is a sum of many different frequencies, you can model each frequency as hitting a different impedance. Then, take how much loss each receives, and recombine to see the new wave front.

Chances are you will get a pulse with a slower rising edge.

Stranded vs. Solid

The difference will exist, but will be minor to you. Inductance will play a larger part I would suspect. The stranded wire should behave a bit better.

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More to OP's question, there isn't really going to be a tremendous difference between solid and stranded unless you're using Litz wire. –  Nick T Jan 24 '11 at 22:59
    
@NickT, I missed that part somewhat, rectified. –  Kortuk Jan 25 '11 at 0:29

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