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Re: First coil- need help

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Tesla list wrote:
> 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
> 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.

Good job so far..at least you've built something...

Make one change at a time!

> 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.

24 is really, really small for a primary... Try something like 1/4"
diameter tubing, or bare AWG 6-10 (no idea what that is in SWG.. probably
close though..AWG10 = 0.10 inch diameter).. even bare #14 (stripped from
scraps of house wiring) will work much better than your #24..

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

This is a very small secondary (turns wise..)... Typical coils have more
than 500 turns (say, 1000 turns as a round figure)..and the secondary wire
is fairly small (#24 enamelled would be about right.)

You didn't mention the diameter of the primary.. Is it comparable to the
height/length of the secondary?  

> 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
> outside.

A very small primary cap, but, given the small size and high resonant
frequency of your secondary, this might be ok...  You'll have to run 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.

how much power (watts) is going to the coil?  That will give you a starting
point to assess performance.. 1.7*sqrt(power) gives roughly max spark
length in inches (from empirical data...) Don't expect this, though...
(Typical power through an ignition coil in an automotive application is
around 50-100 watts, by the way)  Ignition coils may not be the best way to
power a TC though...The current/voltage characteristics aren't very well
suited to capacitor charging.

> Unfortunately the maximum spark length from the toroid to an ungrounded
> 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
> 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
> problem?


> 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
> 4.650918361

At 3 MHz, you are definitely in the RF range... skin effect is going to be
a BIG factor, and small capacticances could add up to significant losses.
What sort of ground plane are you using under the coil?  At this frequency,
the wire from base of secondary to ground plane should be pretty short,
fat, etc.   If you're not using a ground plane, try putting a 2 foot square
piece of metal down (ground to earth for safety) and run your coil over
that, connecting the bottom of the secondary to the ground plane.  You
might notice a substantial improvement in performance.

> 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
> 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
> the primary cap or or primary wire diameter, or frequency-splitting, or what