Re: First coil I built (More tests!)

To: tesla-at-pupman-dot-com
Subject: Re: First coil I built (More tests!)
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Wed, 21 Oct 1998 23:30:59 -0600
Approved: terryf-at-verinet-dot-com
Hi Richie,

> Original Poster: "R.E.Burnett" <R.E.Burnett-at-newcastle.ac.uk> 
> 
> First coil I built (More tests!)
> 
> I posted a message about a month ago about my first Tesla Coil, which
> I was impressed by the performance per watt of power input.
> 
> Firstly thank-you to Bart, Bob Golding and John Couture who took
> interest in this message,  and even fed some values into their
> computers.  Sorry I didn't provide more information.
> 
> Since I am now currently out of work,  my Telsa Coiling has taken
> a back seat due to lack of money and job hunting.  However I have
> operated my coil some more in order to do some testing with an
> oscilloscope.
> 
> My new (more comprehensive) coil specification is as follows:-
> 
> Power supply:       240 VAC, 50 Hz, roughly 6 Amps RMS.
> Supply filter:      250 VAC 16 Amp RFI filter.
> Transformer:        2 x 6Kv 100mA Neon Sign Transformers in parallel
>             (With centre-tapped secondary,  made by
>             Tunewell transformers of London, England.)
>             Open circuit voltage 6.4Kv RMS,
>             Short circuit current 130mA RMS,
>             Secondary Inductance 150H.
> Primary capacitance:    33 nF Total,  (made of 6 x 22 nF Polyethylene
>             caps in oil, series/parallel connected.)
>             Each tested to 18KVDC from TV-Flyback power supply.
> Spark gap:      Static RQ-gap,  5 gaps of roughly 0.5mm each,
>             Fires at roughly 10Kv.
> Primary Coil:       9.5 Turn Saucer shape spiral inclined at 20
>             degrees.  Tapped at 6 Turns for biggest sparks.
>             Overall diameter 450mm.
> Secondary Coil:     Roughly 1000 Turns of 0.6mm enamelled wire,
>             100mm diameter x 700mm length varnished PVC tube.
>             Measured 16.5mH
>             Resonates at 430KHz with no toroid,
>              indicating 8.2pF of interwinding capacitance,
>             Resonates at 304HKz with 300mm toroid,
>              indicating 16.4pF total capacitance,
>             Resonates at 272KHz with 2x 300mm toroids,
>              indicating 20.5pF total capacitance,
> Discharge Terminal: 2x 300mm Toroid from 100mm Aluminium vent ducting.
> Safety gaps:        None yet,  (woops)
> 
> 
> Secondary resonant frequencies were found by feeding the primary from
> a powerful signal generator,  and watching the brightness of a nearby
> 8 inch flourescent strip-light.  I found that if I stood too close
> to the TC during the test,  the resonant frequency would be lowered.
> 
> I also took some measurements with my oscilloscope using a standard
> probe fixed in the air about 15 feet away from the OPERATING Tesla Coil.
> With Y-gain set to 1V/div the damped oscillations were easily visible.
> 
> In my last message I speculated about whether my spark-gap may be
> firing more than once during each half cycle of the supply.
> Using the oscilloscope it could clearly be seen that there were roughly
> 7 or 8 damped RF oscillations fired off during each half cycle of the
> mains supply.  This corresponds to a break rate of around 800 BPS.
> I assume this is due to the relatively small tank capacitance being
> re-charged many times in each mains cycle by the large current
> available from the neons.
> 
> All this means my calculated power figures were out by a factor
> of about eight!   Not 70 Watts but about 600W.  This agrees with the
> supply current.
> 
> I have recently read much about small tank caps versus big tank caps,
> Maybe small caps fire more times per second giving similar overall
> power too big capacitors.  Any thoughts ?

True. The difference is that the a larger primary cap will give a 
higher primary energy at fewer BPS and the higher energy will give a 
higher output voltage and more energy per spark. The limiting factor 
is going to be your secondary height. Ep is currently 1.6J or 
thereabouts giving you a theoretical maximum Vout of 400kV with your 
highest secondary capacitance. It would be less in practice with
losses. You could try it with more primary capacitance but I expect 
that much more and secondary flashovers will become a problem. I'd 
try it briefly anyway.
     Alternatively, tightening the coupling incremetally might be 
worth a try.
  
> I found all this very interesting,  but had to cut short my testing
> due to the noise.  I would liked to have tried tests with one neon,
> two neons,  various combinations of capacitors,  and tried altering
> the coupling.  Maybe another time.
> 
> I have already fitted two 4inch fans to the RQ-gap to increase run-time
> from about 10 seconds to several minutes.
> Can anyone suggest any improvements to my design as I seem to be
> having trouble beating the 3 foot barrier,  for streamers into the air.

I'd say primary energy is your major limitation. I'd be tempted to 
try a bigger coil with that power supply. Be interested to hear 
other's views.

Malcolm

> PS. I don't have a reliable source for neons,  I got mine second hand
> from a neon sign maker in Gateshead.  He only had two of this type which
> had been taken off "burnt-out" signs.  He let me have them for 15 pounds
> each as he said that 100mA was too powerful !  I just pray that these
> units are robust and last long,  as they are over 100 pound each new !
> 
> If anyone has any feelings about my test results or suggestions for
> future tests please let me know,
> 
>                     Cheers,
> 
>                         Richie,
> 
>                 (Making some noise in sunny Newcastle.)