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Re: Help needed for primary...

Tesla List wrote:
> 
> Original Poster: "Sam Barros" <sambarros-at-hotmail-dot-com>
> 
> >Date: Sat, 10 Oct 1998 11:23:13 -0600
> >To: tesla-at-pupman-dot-com
> >Subject: Re: Help needed for primary...
> >From: Tesla List <tesla-at-pupman-dot-com>
> >
<SNIP>
> 
> Ok. So I guess I'll just stick to my original design... Are you sure
> there is enough voltage difference from turn to turn to cause sparks to
> form between the turns? Since the primary is basically a short circuit,
> I would expect it to have the same voltage all around...

The primary is far from being a short circuit - it is an inductor. At
the instant the main gap fires, the full voltage of the tank cap appears
across the ends of the primary. The voltage across the primary will
oscillate at the resonant frequency of the coil, and the turn-turn
voltage between adjacent primary turns is typically many thousands of
volts. 

<SNIP>
>  But is the primary voltage large enough to pose a problem? Or would my
> dry, varnished, Plexiglas covered coil form be good enough? I need this
> coil to be very efficient!

The term coilform is usually used in the context of the secondary - I
now see you are actually referring to the primary. Your best bet will be
to use plastic (Plexglass is fine) at all points directly in contact
with the primary winding. 

<SNIP>
>  Some time ago (not very long) I saw this spark length/power input table
> based on actual Tesla Coil Results on this list. Could someone re-post
> it for me???

John Freau posted this table recently: 

Power input  (revised)   spark length      toroid dia (inches)
 680 W         620 W      actual  42"           20
2100 W        1570 W      actual  64"           30
8400 W        6280 W             128"           60
33.6 kW        25k W              21'          120
  67 kW                           31'          240
 134 kW        100 kW             42'          480
 538 kW        400 kW             84'          960
 1.6 MW                          168'         1920 
 5.1 MW                          300'                

<SNIP>
> >Since the Tesla Coil output is RF, you'd need to find diodes that can
> >turn off quite quickly. Most recitifiers are made for low frequency
> >power and won't do the trick. TV rectifiers are very low current, and
> >may still not be fast enough for your higher-frequency coil. Long
> >strings of HV avalancehe diodes could do the trick if immersed in oil.
> >However, this is far from a trivial problem to solve, and it won't be
> >cheap. Your best (most cost-efficient) alternative for reaching 100's
> of
> >kV DC would be to use a HV electrostatic generator, like a Van de Graaf
> >or Wimshurst machine. Try "Homemade Lightning" by R. A. Ford (Tab
> Books)
> >for construction plans.
> 
>  I know how to build a Van de Graff, but what I really need is a high
> power, continuous DC source... What kind of diodes would be ideal for
> this purpose. Anyone knows what the output would be like???
> 
>  Thanks a lot for answering all these questions!!!
> 
>  Sam Barros.

No real suggestion here other than tose already provided. The answer
really depends on how much power and how much you're willing to spend...
However, I'd be very surprised that you'd need very much power if all
you're planning to do are some electrostatic repulsion experiments... 

> 
>  BTW, I have a 8inches diameter, 25 inches winding height, 40inches
> toroid/strike rail clearance primary, wound with 1000 turns of 22
> (0.60mm) thick magnet wire secondary and I will build an 18 turn, 1 inch
> primary-secondary clearance primary, wound with 0.32inches diameter
> copper tubing and 0.32 inches turn to turn spacing. I also have a 20uF
> toroid. My secondary has over 1mm thick insulation, with epoxy, enamel
> and polyurethane. Any idea how much power such a system could handle?
> What would the voltage/amperage output and streamer length be?

This is a nice sized medium sized coil. The limiting factor for maximum
sparklength will be the secondary winding length. With an efficient tank
capacitor, good RF ground, and a decent sparkgap (what are you going to
use for these?), you should be able to get over 4 foot streamers, and
maybe even more with careful e-field management. With the large topload,
it will probably take 1-2 KVA to properly drive it, and perhaps more if
the toroid has a large radius of curvature. Ouput voltage will be a
function of your tank capacitor and input voltage. 

BTW, to get a better feel for Tesla Coil theory of operation, you may
want to check out the Tesla Coil section on Bart Anderson's site:
http://www.geocities-dot-com/CapeCanaveral/Hangar/3108/bhvl.html

Safe coilin' to you!

-- Bert --