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Re: Should I use a 4" or a 6" secondary? and a few more questions.
In a message dated 4/25/00 1:08:19 AM Pacific Daylight Time, tesla-at-pupman-dot-com
writes:
> Original Poster: "Bob Berg" <berg_bob-at-hotmail-dot-com>
> 4" route is cheaper and a little easier to obtain. But will the 4" handle
> the power of 12K -at- 180 ma? Or will I get better output with a 6" form.
> Second, if I build the coil properly, what kind of spark length output can
I
>
Bob,
You'll be using about 2200 watts. Using my formula for input watts vs.
spark length:
spark length (inches) = 1.7*sqrt input power (watts)
we get about 80" expected spark length. However, you probably
won't achieve this spark length without the use of a sync rotary gap.
But if you use a sync rotary, and even if you don't, it is possible that
with certain sized tank caps (reso or LTR), you may cause the NST's
to provide more power than they're rated at, esp if you use a 0 -140V
output variac. My small system uses a 12kV, 30ma NST (360 watt
rated), but actually draws 620 watts, and gives 42" sparks.
Assuming that you use the proper cap size, along with a sync gap,
your system could draw about 4500 watts or so. Using the formula
again, we now obtain 114" sparks. This is beginning to become a
large coil, so your secondary would need to be correspondingly large.
I'm also assuming above that you use 120 bps. At 120 bps, the
bang size will be large.... and this too will demand a taller secondary
to prevent breakdown. If you run at a higher breakrate, the bang size
will be smaller, so the secondary can be shorter.... but the coil will
probably not be as "efficient".
The sparks should not be more than 2.5 times the secondary length,
so for a 114" spark, figure a 46" secondary coil. Now there's no reason
that a secondary needs to be wide, and in fact I prefer narrower ones.
Figuring a 5.5:1 ratio, the secondary would be about 8.5" wide. Or
10" wide would be fine too.
There's no need to use thick wire. In fact the use of thick wire may
reduce your spark output because thick wire results in the use of
fewer turns, which results in fewer primary turns, which results in
higher gap losses. Since most of the losses in a TC are in the gap,
this is where efforts at loss-reduction must be directed.
You could certainly use # 20 on a coil of this type. If your capacitor
is large, this will permit you to use fewer turns on the primary. If
the cap is too small, you may need a great number of turns on the
primary. There's nothing wrong with this, but it makes it physically
more difficult to constuct especially if you use copper tubing. (The
primary gets too wide.) If the primary is too wide, the coupling may
be to low, esp with a narrow tall secondary. So you can see how
these issues all interact, and lead to specific size and shape
guidelines for TC secondary construction.
But now let's assume you don't bother with the sync gap, or you
use smaller capacitors, and obtain only 60" to 80" sparks. Let's
assume too that you run at a higher bps, with a smaller bang size.
Under these conditions (high bps, small bang size), you can let the
spark go more than 2.5 times the secondary length. You can
actually to to almost 4 times the secondary length this way. This
would suggest a secondary length of only 20" which is too short
of course....but serves to illustrate the point. Certainly a 30"
secondary can easily give 80" sparks at a higher break rate. Still,
I would probably select at least a 35" tall secondary. Figuring a
5.5:1 ratio gives a 6.4" diameter secondary. So I would recommend
at least a 6" diameter secondary for your TC, not the 4" secondary.
Using this smaller secondary, you can also make the wire thinner
which may help the spark length. You could use #22 wire, or even
#24 wire. Or you could use # 20 wire, but you'll get barely 1000 turns.
> The other concern I have is smoking my transformers. They are so mint I
> afraid to smoke 'em. But I know its part of the game, and I have been
> reading so much about protection circuits and filters for NST's I'm
confused
>
> beyond belief! The only thing I feel sure of is a saftey gap.
> I'm afraid to ask the next question!:) Should I be using an L-C filter or
> R-C filter? gulp!
Terry Fritz has come up with a nice protection scheme which is shown
at his website. I just use safety gaps for my coils.
Have fun, and be safe !!
John Freau
> Thanks
> Bob
>