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Re: Hello from a new coiler.


From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Tuesday, August 05, 1997 8:58 AM
To: 	Tesla List
Subject: 	Re: Hello from a new coiler.

Tesla List wrote:
> 
> From:   Sulaiman Abdullah[SMTP:sulabd-at-hotmail-dot-com]
> Sent:   Monday, August 04, 1997 8:25 PM
> To:     tesla-at-pupman-dot-com
> Subject:        Hello from a new coiler.
> 
> Hello Coilers !
> I'm a virgin coiler just about to join you all.
> I started winding my main coil before I was aware of the expertise
> available here, so it's less than optimum and I'd like some help.
> 
> I've loads of questions so I'd really appreciate some help.
> 
> 1  My coil is wound on a nominal 4" uPVC pipe
>    0.7mm magnet wire, 33 turns per inch
>    108mm actual winding diameter (4.25")
>    576mm actual winding length  (22.67")
>    753 turns total
>     11mH measured
>    14ohm measured
>    Heavily coated with polyurethane varnish
>    Warmed to 60 C (140 F) for one week to reduce moisture
> 
>    Is this workable or do I REALLY need to start again with a larger
> diameter former ?
> 
> 2  I intend to use a Neon Transformer rated as follows:
>       Primary   240 v  1.9 A a.c.  (Malaysian a.c. like U.K.)
>      Secondary  15 kV  30 mA a.c.  centre-tapped.
>    The secondary measured at 21 k ohm.
>    I intend to make two 10 nF capacitors to use in series (0.005 uF).
> 
>    What is the largest value capacitance I can use with the above
> transformer WITHOUT cooking it ?
> 
> 3  Could someone explain spark-gap quenching to me ?
>    I'd REALLY appreciate a photo, .gif, .jpg etc. or even a
> description of waveforms and voltages.
> 
> 4  I'd like to operate at low-frequency / high top-capacitance,
>    is it necessary to operate at the quarter-wavelength frequency ?
> 
> 5  I just saw a photo in July's National Geographic of Christian
> Ristow's 1.8 MV TC launching great bolts from what looks like the
>    last few turns spaced-out and strengthened, which brings me to my
> last question,
> 
>    Don't toroids and guard-rails form an effective shorted-turn ?
> 
> Thanks for your endurance reading this long e-mail !
> 
> I will really appreciate all your responses.
> 
> Thanks.
> 
> ______________________________________________________
> Get Your Private, Free Email at http://www.hotmail-dot-com

Sulaiman,

Welcome aboard!

Here's some answers to you questions:

1. Your coil form is fine. BTW, did you coat the inside of the form? PVC
will absorb moisture from the atmosphere until it reaches equillibrium
unless it's completely sealed. If not, you may see slight changes in
performance when it gets humid. Smaller diameter coils typically tend to
have a somewhat higher length:diameter ratio, and your's is right in the
ballpark. It's also just about right for the power source you're
planning on using. The self-resonant frequency of your resonator will be
around 550 kHz with no top load (toroid).

2. An easy to remember "rule of thumb" for 15 KV neons is 0.01 uF for
every 60 MA of faceplate rating for 60 Hz power. For 50 Hz the value is
a little larger (0.013 uF). The size tank cap you're planning on using
is just about right. Larger than optimum values will tend not to charge
as rapidly, leading to fewer "bangs" per second. If you are running off
a 60 Hz power source, you will be resonating your transformer inductance
with your tank cap - don't open the spark gaps too widely (generally not
a good idea, but especially important in this situation) otherwise you
may short out your neon or tank cap. 30 MA neons are fairly fragile, so
make sure you have a spare one or two - the chances are quite high that
sooner or later you'll lose one in Tesla Coil use. 

3. When the charged tank cap is discharged through the primary, energy
that was initially stored in the tank cap is electromagentically coupled
to the secondary. If the primary and secondary are "tuned" to the same
frequency, this energy can easily and relatively efficiently be
transferred from the primary tank (LC) to the secondary tank (made up of
the secondary inductance combined with it's self-capacitance plus any
additional top load capacitiance - like a toroid). As llong as the gap
is firing, energy is being transferred. However, once all of the energy
has transferred to the secondary, the process reverses, and starts
transferring the secondary LC energy back into the primary! Quenching
means that, at the right time, we'll "break" the primary arc, and
prevent this backflow of energy from ocurring. The actual process and
calculations are fairly involved, but some idealized simulation pictures
of the process for both the quenched and unquenched cases can be seen on
the funet site. Go to:

   http://www.funet.fi/pub/sci/electrical/tesla/pictures/misc

and download file QUENCH.ZIP. Inside this ZIP file are a number of
simulation runs for a quenched (QNCH.JPG), and an unquenched
(UNQNCH.JPG) coil. In the quenched case, the primary current can be seen
to ring down to 0 at the "first notch" - if the spark gap is quenched at
this instant, we'll have transferred maximum energy to the secondary. 

4. Whenever you add capacitance to the top of the coil, you'll reduce
it's overall resonant frequency. For example, if you were to add a 4" x
14" toroid to the top of your coil, it would add around 12 pF of
capacitance to the top of your coil. When vertically mounted, with the
base grounded, your secondary has about 8.75 pF of sel-capacitance. The
effect of adding the toroid will lower your coil's resonant frequency to
about 340 kHz. The resonant frequency will be the 1/4 wave frequency,
where the base is the maximum current node, and the top is the maximum
voltage point. This is the frequency for best output.

5. Unfortunately, the National Geographic did not give proper credit to
the person who actually made that coil - Greg Leyh. BTW, Greg is on this
list. Being at the top of the resonator, the toroid is too far removed
from the primary to have much effect as a shorted turn. The bottm-most
strike rail, however, is immersed in the very strong alternating
magnetic field from the primary. The strike rail must have an insulated
gap so that it does NOT form a shorted turn. 

Hope this helped, and safe coilin' to you!

-- Bert H --