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Re: My first coil



Paul !, welcome aboard!

Your transformer won`t work, because the voltage is so low, that it will
not be able to make a spark gap fire.
If you are unable to find any NST`s, I can recommend that you dig up a
couple of MOT`s, that`s Micro Wawe Transformers. Get 3 while you are at
it.

Connect the primary`s in parallell, and the secondaries in series. The
secondaries are connected to the core at one end, so by connecting the
cores together, you series them. If you get no output this way, reverse
the leads to one of the primary`s.

This will give you a nice Tesla transformer.

If you connect this transformer to a short, like a capacitor or/and a
sparkgap, the current draw will be excessive. You need ballasting. The
ballast is an inductor in series with the primary windings, and it works
this way:

When the transformer starts to draw current, there will build a voltage
drop across the inductor, because it opposes any change in current
trough it. Thus the voltage available to the transformers is lower, and
a lower current will flow. However, when the capacitor is charged up,
and the current decreases, the voltage agross the inductor decreases,
and the full voltage becomes available to the transformer, so that it
can charge the capacitor to full voltage. (this can get more
complicated, but this is the basic mecanism)

One way to make a suitable inductor, is to use the 3rd. transformer as
the ballast. Connect the secondary in series with the transformers
primary. Most MOT`s are designed with a high idle current in mind,
around 4 amps is not uncommon. Still this is too litle, you would like
the coil to draw around 8 amps. Unwind some turns at a time, untill it
does, or if you have a variac, connect the inductor across it, and
unwind primary turns untill it passes 8 amps at 240 V. Just leave the
secondary floating, but don`t touch it, of course.

A variac would make a great current limiter also, but it has to be able
to deal with the current, though.

With a power supply like this, you should make a 4" secondary, 20" long
and with a topload of 4" by 16" out of dryers duct, at least 30" of
spark is within reach.

A good gap for a coil like this is made off tungsten welding rod, 8mm
thick is fine. Just 2 pieces, with the ends rounded slightly off, and
set ar a couple of millimeters distance. Clamp them in holes drilled in
heatsink profiles, or brass or copper details. The better coolint, the
longer the coil will run, but for short 10-15 cec. bursts, anything will
do.

Capacitor? 112 nF is good, and 18 caps will doo, 1600VDC/56nF, 6 strings
of 3. In fact with 8 caps, 4 strings of 2, the coil will run for around
15 minutes before the first cap blows. Phillips 376 series pulsecaps has
worked very well for me.

I will not try to take all the fun from you by going into more detail
here, play with Wintesla, get the numbers sorted out, and have fun
sparking!

Cheers, Finn Hammer

Tesla list wrote:
> 
> Original poster: "Paul Benham" <Paulb-at-woodanddouglas.co.uk>
> 
> I am looking towards building my first coil.  I have looked at various
> web sites and the archives on pupman, as well as ordering a couple of
> books and some software.  Some questions:-
> 
> I have a 900Va.c. 0.5A transformer.  This requires too much capacitance,
> but should I contemplate building with this as a cheap start, with less
> capacitance, or am I wasting my time.
> 
> I have thought about a 1" diameter secondary as a start, or should I
> think about a 2" Secondary(I only have a small garage).  My worry is I
> will build something which will not spark at low enough power inputs,
> and I believe that both sizes will take about the same amount of time to
> build.
> 
> Does anyone know of anywhere in the U.K where I can get an NST on the
> cheap, or a good supplier of Polycarb caps that I can use to make a
> MMC?  I have looked at RS/Farnell, but they do not seem to have anything
> suitable.
> 
> Are there many people in the U.K. coiling?
> 
> Thanks in advance,
> 
> Paul.