RE: [TCML] Stranded/silver plated wire (was Scientific Method)

["Lau, Gary" <Gary.Lau@xxxxxx>]

Hi Jim,

I understand that the situation would be far worse with a braided conductor rather than more common parallel-stranded wire.  But I base my opinion on the AC resistance measurements that I performed, documented at http://www.laushaus.com/tesla/primary_resistance.htm

The two most pertinent cases are #14 solid vs. #10 stranded.  The stranded wire I used is the parallel conductor variety, shown close-up on the web page.  It would have been nice if I could have compared the same wire gauge, but I didn't have it handy (and I no longer have access to the instrument).  The two coils had identical geometries and inductances.  Below are measured AC resistances, Ohms, vs. test frequency:

Freq		#14 solid	#10 stranded	#14 stranded (calculated)
DC		128.5		53.2		128.5
40KHz		257		209		504.8
80KHz		362		303		732
100KHz		388		320		773
200KHz		552		520		1256
400KHz		760		847		2045
800KHz		1060		1620		3913

The last column, #14 stranded, is an estimate of what a #14 stranded conductor might measure, from scaling the measured #10 stranded value by the DC resistance ratio of the #14solid/#10stranded.

It's clear that the resistance of stranded wire rises MUCH faster with increasing frequency than for a solid conductor.  It's likely that the effect would be less-so if the wire was silver plated, but I don't believe that the stranded conductor would ever result in a lower resistance than the equivalent solid conductor, silver plating or not.  Your comment concerning skin depth confirms that silver plating would not lower the resistance from a skin-effect perspective.

Regards, Gary Lau
MA, USA


> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of jimlux
> Sent: Tuesday, July 28, 2009 11:45 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Stranded/silver plated wire (was Scientific Method)
> 
> Lau, Gary wrote:
> > Are you saying that you use silver-plated STRANDED wire for
> > secondaries?  Granted, AC resistance and Q are far less important on
> > the secondary side, but the notion of this wire being superior to
> > common magnet wire is incorrect.  Stranded wire is inherently more
> > lossy at RF frequencies.  This is because the skin effect causes
> > current to travel on the outer surface of the bundle.  If a strand on
> > the outside of the bundle weaves to the interior of the bundle, the
> > current in that strand will try to find its way back to the surface,
> > and this means traveling to adjacent conductors, through any
> > resistive oxide layers between them.  This strand-hopping results in
> > a much higher AC resistance than if a single conductor were used.
> > This is the reason that Litz wire insulates the strands from one
> > another.
> 
> Most stranded wire is NOT braided. It's something like 7 strands, so the
>   outside strands stay outside. THe other thing is that 100kHz-ish
> frequencies have a fairly thick skin depth (0.2 mm in copper). It's not
> like at 15 MHz where skin depth is a few tens of microns.

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