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First coil- need help

Original poster: "Jolyon Vater Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>

First TC -need help.

Last year I built my first tesla coil -a mini-coil- the performance of which
has not been totally satisfactory. I am writing from England as a newcomer to
this activity and while I have gleaned quite a bit of information from various
sources I do not presume to have all the answers; it would however be
interested to know as to what improvements could be made to the coil design.

The statistics are as follows:-

The primary consists of a flat spiral 11.75 turns of 24 SWG Tinned CopperWire
(wire diameter 0.022") which is supported by four 15-way terminal blocks
mounted at 90degrees around the centre of a 12" square plywood base (which is
not treated in any way). When the primary was tapped for max. spark length (at
7.75 turns) the Inductance was calculated to be 9.1936275uH.


The secondary consists of 130 turns of PVC insulated hookup wire (1 amp?) wound
on a plastic tube (industrial cotton bobbin) measuring 2.8946305" diameter, the
winding extending to 7.25" along the tube.

Inductance of the secondary was calculated at 413.91928uH.

The primary cap was measured at 350 pF; it is composed of sixteen capacitors
divided into eight strings of 2 caps -each cap is a plastic film can filled
with saline wallpaper paste gel on the inside and covered with foil on the

The Tesla coil is powered by an auto ignition coil via transistor ignition
driver (Velleman KD2543) driven by a 555 oscillator - spark length form the
ignition coil alone is about 1 inch long. 

Unfortunately the maximum spark length from the toroid to an ungrounded object
is little over 1" with about 0.25"of primary spark; it it appears to be EVEN
LESS to a grounded object, although in subdued light I have observed contiguous
thin filamentary discharges of up to 2" from the toroid to a sharp point
-although it is doubtful as to whether these could be accurately described as
either sparks or arcs. Performance is worse without the toroid but worsens when
the topload capacitance is further increased; form which I presume the toroid
size is near optimal. 

I have also noticed that the tone of the primary spark gap changes whenever
discharges from the toroid are drawn- is this normal or does it suggest a

The Medhurst capacitance of the secondary was calculated at 3.7838601pF.
Primary resonant frequency was calculated at 2.846.6706 Mhz which means that
the balance of the capacitance needed to match the secondary to the primary (a
total of 8.437785 pF) would have to be provided by a toroid with capacitance of

pF. I have not yet calculated the capacitance of the toroid yet -I have seen
formulae which claim to do this however I believe the distance above the ground
plane also affects the capacitance and I am not sure how this is calculated -am
I incorrect on this latter point?

Why is performance so poor and why is spark length increased to ungrounded
objects as opposed to grounded objects. Is the problem likely to be found with
the primary cap or or primary wire diameter, or frequency-splitting, or what