Re: Why do primaries get hot?

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Goinbonkers-at-aol-dot-com>

Hi all, 
  I have never noticed my primary getting warm.   Must be because I use 3/8 or
1/2 inch tubing.   Added benefit to that is it is harder to accidently bend
wrong when building the primary. 
Mike 


>
> Hi Greg, 
>
> Let's suppose we have 10 amps RMS current in the primary which is pretty 
> close for a 1kW coil.  (The duty cycle is a few percent and the pulse width 
> is 100's of uS not just a few.) 
>
> Now we grab 100 feet of 1/4 inch copper tubing.  We need to know what its 
> resistance is at say 250kHz.  The "skin depth" of copper at 250kHz is 
> around 0.0052 inch.  For 1/4 inch tubing we have a conductive area of about 
> 0.004 square inches or about the area of 13 gauge wire.  The resistance of 
> 100 feet is about 0.2 ohm.  I^2R gives 10 x 10 x 0.2 = 20 watts.  So we 
> have as much heat as maybe a 25 watt light bulb in the tubing which makes 
> it warm to the touch after a while of running.   
>
> The real key is the "skin depth" which is a measure of the fact the RF 
> currents run on the outside of conductors and not evenly through them as 
> with DC.  For copper, the depth in inches is: 
>
> Din = 2.602 / SQRT(F) 
>
> So a very thin copper tube is just as good as a solid tube since there is 
> no RF current flowing inside it.  All the current is just in a thin layer 
> in the surface of the tube. 
>
> There are some small Eddy current effects and such too but resistive 
> heating is the big contributor. 
>
> Cheers, 
>
>    Terry 
>