RE: surface area of wire

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

 >Hi all,
 >
 >I'm considering winding a small test coil with litz wire. I would like
to
 >know if theres a chart which states the skin depth of wire at high
frequency
 >in mm. Ive seen a chart in inches floating around though I can only get
to
 >grips with mm. From what ive found 200khz is around 0.15mm skin depth.
It
 >would be useful to have exact figures.

Bear in mind that the skin depth is not a black and white interface.
There is a (exponential) continuum between 0 & 100% of the current
distribution between the surface and the core, not a specific depth
below which the current suddenly ceases.  The generally stated skin
depth refers to the point where the current is reduced to 1/e (~37%) of
its DC value.

The strand diameter in Litz wire must be chosen for the operating
frequency, and a good cook-book reference for this may be found at
http://www.coonerwire-dot-com/Products/Litz/DesignD_2.html.

 >Also, Is there any comparrisons to surface area in relation to solid
 >core,multistrand and litz wire ? For example if I have a wire of 1mm
dia,
 >surface area would say be 3mm, now if I had a 1mm Dia wire with
multistrand
 >how would that be worked out ? All the wires are shorintg out in all
 >combinations so in comparrison to the solid core wire I wouldn't
imagine
 >multistrand to give much more surface area than solid core ?

See my data at http://www.laushaus-dot-com/tesla/primary_resistance.htm

 >Litz wire would be the best. I plan to build a high Q secondary with
this
 >wire to see what happens. I have compaired solid core to stranded wire
and
 >was amazed to find the multistrand coil gave 2" arcs with little input
 >power, though a solid wire which should have been better , wasn't and
gave
 >no output at all. Despite the stranded wire had spaced turns ( due to
 >insualtion ) there should be a lot less output since the solid core
verson
 >was a normal wound secondary. Length of wire and wire dia was as close
as I
 >could get.

Maybe.  If two secondaries were wound, one with Litz, and one with solid
wire, both with the same inductance, DC resistance, number of turns,
geometry, I would also expect the Litz coil to have the performance
edge.  Whether the improvement is measurable or significant remains to
be seen.  I have heard anecdotal reports of folks being disappointed
with Litz secondaries, but I think they were simply no better than their
solid counterparts.  I have no specific information on these reports, so
take that with a grain of salt.

If your solid coil has zero output, something is wrong, and I don't
think any useful conclusions can be drawn.  Were the inductances
identical, or did you attempt to retune to compensate for differing
inductance?

 >I think it would make a interesting test since multistrand worked
better
 >than solid wire. which is why im interested in testing out Litz wire
which
 >would give me 10times the surface area and should give a much higher Q
coil.
 >
 >Many thanks,
 >Chris

Don't forget that what makes Litz wire "special" is that each strand is
insulated from the others.  Normal stranded wire will behave very
differently, and in RF applications, will have a much higher AC
resistance than a solid equivalent.  It's not just an issue of surface
area.  Spacing between adjacent turns (proximity effect) also figures
prominently.

Regards, Gary Lau
MA, USA