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Re: Help needed for primary...

Sam and all,

My responses are interspersed below...

Tesla List wrote:
> 
> Original Poster: "Sam Barros" <sambarros-at-hotmail-dot-com>
> 
> ... Ok... It's me again bothering you people about my coil, in my
> non-stop quest for maximum efficiency...
>  Now, this time I have two relatively simple questions: One is: How
> close can I make the turn to turn spacing on the primary coil? I
> currently have one-inch primary/secondary clearance, and the primary is
> wound 18 turns of 8mm 0.32inches?) Thick copper tubing with an
> 8mm(0.32in?)  Turn to turn spacing. Now, since the first and the 15/16th
> turn will be connected to the primary circuit there shouldn't really be
> a large turn to turn potential. So, in theory, if I epoxy the tubing, I
> could make the turn to turn spacing 1mm(0.04in?) or less. What effects
> would it have on efficiency? Would it make the primary more likely to
> overcouple, what exactly would it do?

Making a smaller primary will have virtually no effect on overall coil
efficiency. Substantiall reducing the turn-turn spacing will
significantly increase the odds of a primary turn-turn flashover, will
require more primary turns to achieve the same primary tune-point, and
will increase the coupling coefficient somewhat. With the smaller
diameter primary, you may need more than 16 turns to achieve the same
tuning point, all other things being the same. HV RF will jump a longer
distance in air than low frequency AC or DC, particularly if you've got
any surface imperfections that promote corona - it would be quite
difficult to prevent turn-turn flashovers at a 1mm turn-turn spacing.   

You should be able to reduce the turn-turn spacing to 1/4" if you're
running below 5 KW, and smaller spacings at lower power levels, and
there's only marginal space advantage in trying to reducing this below
1/8". You will need to think about how you can easily change the "tap"
on the primary without reducing clearances or shorting to adjacent
turns. 

<SNIP>

>  Now, onto the second question:
>  I see most people make their secondary coil forms out of plywood. I was
> even suggested to do so. But from what I have sing trough
> experimentation, Plywood will conduct electricity fairly well. Even when
> DRY!!!
>  I can see huge amounts of corona going right trough the plywood with as
> little as 25KV.
>  Will drying with a heatgun and varnishing solve this problem, or shall
> I glue a 2mm thick piece of Plexiglas to the top and to both sides of
> the supports of my coilform?

Wood is a very poor insulator for HV and RF. Because of this, most
people actually make their coilforms out of some type of plastic or
dried and well-sealed cardboard tube (Sonotube). Typical plastic
coilform materials used include PVC, ABS, HDPE, Polycarbonate, phenolic,
or acrylic (PMMA). While it IS possible to make a coilform from wood if
the material is thoroughly dried and well sealed throughout with
multiple coats of polyurethane, this should be done only as a last
resort. There are MUCH better materials available now - Tesla would have
immedialtely switched over to PVC and LDPE had they been available!  :^)

>  Now, the next one is a bit more technical:
>  How do you calculate the voltage of a coil trough the sparklength. For
> example: If my coil makes 1 and a half meter (5foot?) long sparks.
> What's the voltage output? And, How many watts do I need per meter of
> spark?

Throw away any sparklength versus voltage tables you may find - they
simply don't apply to Tesla Coils - there's no simple relationship!
Sparklength is a complex and not fully understood function of voltage,
frequency, toroid size and voltage standoff characteristics, breakrate,
etc. 

However, you can estimate the maximum voltage your coil can output if
you know a few things about your system. 

   Vout (max) = Vgap*SQRT(Cp/Cs)

where Vgap is the gap breakdown voltage, Cp = primary capacitance, Cs =
combined secondary and toroid capacitance. Alternatively, if you know
the primary and secondary inductance you can also estimate Vout as

   Vout (max) = Vgap*SQRT(Ls/Lp)

As a refernce point, coils generating 5-7 foot streamers typically
operate at voltages of only 300-400 kV. Only very large, high power 
systems can be expected to break 1 MV. A reasonably efficient system
should be capable of achieving 1 meter long sparks with 500-700 watts.
Your mileage may vary... :^)

>  Oh yeah, so, if a coil puts out 1million volts, what would the amperage
> be? I suppose just a few uA, but than there is also the toroid charge,
> so I don’t really know how one can calculate it…

A coil capable of achieving 1 MV can deliver substantial amounts of
current! The topload "looks" exactly like a capacitor, and like any
capacitor, can store energy. A topload that can support a buildup to 1
MV will be quite large, and will have significant capacitance. The
current that's available will depend upon how rapidly you discharge this
capacitance - a streamer may discharge it over a period of several
hundred microseconds, while an arc to ground may do so 100 times more
rapidly. Maximum steamer currents may be in the 1-10A range, while a
power arc can easily be 10-20X greater (but last for a much shorter
time). Because these discharges only exist for a relatively short time,
the average system power is much lower - but still 10's of KW for a 1MV+
system.

>  And now, the most imaginative one: I want to use my coil for
> electrostatic repulsion. So I’ll have to rectify (make DC) the output.
> Will simply stacking several TV 60KV rectifiers in series do the job?
> Anyone ever tried this? You think it would work? Would it be nice, would
> the voltage be reduced substantially, what would happen?
>  Sorry for all this questions…
> And
>  Thanks a lot guys!!!
> 
>  Sam Barros.
<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.

Safe coilin' (or whateverin') to you, 

-- Bert --