Re: First coil- need help

["Tesla list" <tesla-at-pupman-dot-com>]

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

I have rechecked- the primary OD is 9" and the ID is 3"
The height of the secondary is 8.125" (almost as big as the primary
diameter) so I
presume they are comparable.

Ammeter says 3 A on the primary of the ignition coil -I am using a Radio
Shack
13.8VDC 3A regulated supply so max power would be 41.4 Watts.

I am using a 7 tube zigzag gap with 1mm between the copper tubes -which are
15mm
in diameter, so if breakdown voltage is 3.3MV/ metre between rounded
electrodes the
max voltage (perhaps) is 19.8kV.

The ignition coil-based HV supply has been improved with the addition of a
diode
string (60 x 1N4007 so total PIV = 60kV) to prevent the capacitor
discharging back
into the HV winding - this has resulted in more reliable firing of the gap
especially
with larger capacitors; sadly these do not seem to give good performance -is
because they lower the primary resonant frequency so far it becomes
impossible to get a good "match" with the secondary?

Why is spark length to grounded object less than spark length to ungrounded?
Is this anything to do with the secondary being more in tune with the
toroid-to-ungrounded-to-ground capacitance than it is with the ordinary
toroid-to-ground capacitance perhaps?

Jolyon

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, May 08, 2002 1:15 AM
Subject: 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
> 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.
>
> 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?)
> wound
> > 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
> numbers..
>
> >
> > 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
> 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
> > problem?
>
> Normal..
>
> >
> > 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
> 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
>
>
>
>