Re: What Size Secondary (BIG or BIGGER) Should I Go For?

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


>  I should have enough AWG 18 to wind both secondaries.
>  Since I'm using AWG 18 for either one, they'd both be
>  the same height, assuming the same number of turns.
>  I've done some what-ifs using several programs, but
>  not being an EE, am probably missing some important
>  relationships which are more obvious to an EE.
>  
>  Well guys, what are the pros and cons of using either
>  one? Obviously, the 16" will weigh a TON, and will
>  require more Cp and Lp. Is there anything else I'm
>  missing?
>  
>  thanks
>  Adam

Adam,

The 16" coil will not necessarily need more Cp, just more Lp.

I'll discuss how I would deal with the overall issue.
It seems to me that your power will be limited by your 
capacitors as far as bang size is concerned.  If you put
the caps in parallel, you wouldn't want to use resonant
charging... the voltage will go too high.  Because of this,
it's better to use a higher BPS rather than 120 bps, in this
case.  Higher BPS is useful for getting longer sparks from 
relatively small capacitors.  Once a decision is made to
use high BPS, this helps to determine the secondary
size, since the secondary can be smaller at high BPS,
since the bang size is smaller.  Since the bang size is
smaller, the wire size can be smaller also at high BPS.

If you put some of the caps in series for a higher voltage
handling capabilty, then it will be even more important to
run at a high BPS due to the even small bang size.

I just noticed that you have a SRSG, I was unable to open
the webpage, but if it's 240 bps, that may work well for
your system.

Since the wire you have is #18 enameled wire, it's helpful
to consider how many turns you'll get with a certain
winding length.  For instance a 45" tall winding will give about
1000 turns if close wound.  Richard Hull used a secondary
of those parameters, and it was 14" diameter.  His sparks
were 15 feet long and he used a high bps also.  Now if you
plan to apply more than 10kVA to the system, your sparks
may be longer, and will tend to hit the ground using a short
secondary.  You could use up to 1600 turns on the secondary
which would give you a height of 76" for the winding which just
happens to be the length of the 11" pipe.  If the secondary is
too tall and narrow, it's harder to get a tight coupling.  Although
I think you can get a close enough coupling with the 11" pipe.
You could use just 1000 turns if you want to keep the coil
shorter.  More turns should reduce the spark gap losses 
though and give a little better output.  Another option would be
to space wind the coil, and make it 76" tall or taller, using 
fewer than 1600 turns maybe.  Space winding is theoretically
more efficient.  I would probably close wind on the 11" pipe
to keep the inductance high.  At a higher bps, the spark lengths
can reach 4 time the length of the secondary or greater.  For
a 76" tall coil, this would permits sparks up to 25+ feet.

It is possible that if the 16" diameter pipe is used with a tall
coil of many turns, the wire losses may be getting kind of 
high.  If you were using thick insulated wire, I would definitely
recommend using the 16" pipe, to get enough inductance.
Using the #18 wire, you'll get a lot of inductance even using
the 11" pipe, if you use 1600 turns.  An option would be to
use the 16" pipe, but space wind it using 1000 turns, or 1200
turns or so, and make it 76" tall or so.

If you have the room, you probably want to make the coil at
least 76" tall, which will tend to prevent the sparks from arcing
down to the ground and to the primary.  By stacking larger
and larger toroids on top of the coil, the sparks can be made
to emit from a higher vantage point will less likelihood of a
primary strike even using a short secondary however.

I would imagine that the results would be very similar using
either secondary.  If you want the coil to be smaller, lighter
and more compact, you can use the 11" pipe.  If you want the
coil to be massive, heavy, and perhaps taller, you can use the
16" pipe.  Many coilers believe that a large coil needs a very 
large diameter secondary.  They believe that the secondary
diameter needs to scale up with the spark length.  I think this
is completely wrong thinking.  I think it is the height that needs
to scale with the spark length.  I think that a 11" diameter
secondary can give 24 foot sparks just as readily as a 16"
diameter secondary, if they are both the same height.  The
11" diameter secondary would of course use a thinner wire
diameter such as the #18awg wire you have.  If I were building
the project, I would go with the 11" by 76" secondary, and wind
it fully with 1600 turns of the 18awg wire.  Despite what I just
said, I did seem to get longer sparks using my 6" by 24"
secondary than with my 4" by 23" secondary both wound
with the same size wire.  In another test though, the results
seemed about the same, so it wasn't clear cut. You may need 
to use an inverted cone type primary to get a tight enough
coupling using the tall narrow secondary.  You could limit the
secondary length to 67" or so to help tighten the coupling also.
I recommend calc'ing the coupling using a coupling program
such as Mark's program to verify that you can get a tight
enough coupling.  I certainly like the idea of getting just as
long sparks from a narrow, more lightweight secondary.  
I certainly believe that the same spark length will be obtained
from a narrow secondary having many turns compared to a
wide secondary having fewer turns, for the same resulting
overall inductance.  The inductance is more important than
the secondary diameter.  The inductance is also more important
than the wire diameter, since the gap losses tend to be more
important than the wire losses.  In a certain sense, a smaller,
narrower secondary is a benefit because less energy is trapped
in the Cself of the coil, and is available for the Ctop, permitting
a larger Ctop to be used.

A coil of this size can benefit from a rather large toroid.
Richard Hull used a 15" by 60" corregated toroid, and obtained
2 or 3 simultaneous streamers.  Keven Eldredge uses an 
even larger toroid, but he's running at 120 bps which demands
larger toroids anyway.  His system gives 24 foot sparks at
20kVA or so.  

Even though your variacs are rated at a combined 10kVA, you
can push them to 20kVA for coiling use.

Cheers,
John Freau