Re: Thinner wire and more turns
Hi John and all,
For a while now I have thought that the efficiency of my 4" coil may
have been limited slightly by the low coil diameter. My spark lengths
have always come in slightly under John's results for the same input
power. I was puzzled as to why this occured, but always put it
down to John's greater coiling expertise :-)
John's most recent post about the increased inductance of using more
than 1000 turns was the final straw, and yesterday I decided to wind a
coil of identical dimensions (w=4", h=22") to my original but with
1500 turns instead of 1000. ie. Thinner wire.
Original coil: 4x22" 1017T -at- 24swg (23awg 0.56mm) Ls=16.8mH
New coil: 4x22" 1503T -at- 28swg (27awg 0.37mm) Ls=43.1mH
The original coil resonated at 218kHz whereas the new one resonated at
164kHz. In order to get the primary to tune to the new frequency I
had to remove one toroid and move the primary tap out considerably.
I left the power supply, rotary phase, tank cap, ballasting etc. all
the same, to keep power the same for both tests.
With my original coil I got mostly one streamer, and occasionally two,
with frequent 36" strikes to a ground rod with 1020 Watts of measured
input power (200BPS sync rotary.)
I videod both tests for examination later in the house (in the warm !)
When I first powered up the new coil, the difference was immediately
1. Firstly there were 5 or 6 simultaneous streamers from all around
the top toroid.
2. I got frequent hits to ground at 36" and got a few rare hits to
other objects and the garage walls slightly further away.
Sole streamers were generally longer than one of many.
3. With the new coil breakout started at around 30% on the varaic
compared to around 50% with the old coil. (Higher toroid voltage ?)
4. The rotary gap seemed quieter and not as bright. Usually it lights
up the whole garage. Even the initial spitting sound was not as
sharp on the ears.
5. I noticed a rapid build up of ozone with the higher L coil also !
Maybe because of more simultaneous streamers, or higher voltage ?
6. Spark action was quite frantic. With rapid darting and surging
of sparks on either side of the toroid. With the original coil I
usually got one comparatively lazy evolving streamer. (This frantic,
chaotic spark action reminded me of my old static gap coil with a
very high break rate.)
Unfortunately, I had to remove one toroid from the coil to bring the
higher L coil into tune, so this is not exactly a "controlled
experiment". I don't know how much this affects performance. The maths
says that increasing Ls and decreasing Cs should produce a higher peak
I dont know if the change in performance is due to the lower operating
frequency, higher theoretical output voltage, higher pri surge imp.,
better match to the streamers, higher coupling, or maybe just due to
having one less toroid, but the improvement was quite dramatic.
I plan to carry out further investigations with this new coil tomorrow.
In particular I plan to add back the third toroid, and increase Cp
to regain tune. Hopefully I can get one sole streamer of great length !
Despite adding to Cp I will keep the input power the same to do a fair
comparison of the two coils. (Then I will crank it up higher to take
best advantage of the higher Cp !)
I know little about matching the output impedance of the sec to the
streamer load imp., so I cannot add much to that discussion.
However, from an engineering viewpoint, impedance matching comes up
everywhere like RF amplifiers, audio amplifiers etc. I would think it
would be just as important for Tesla Coils in order to acheive maximum
power transfer to the "load".
Also, since small diameter coils effectively have their inductance
crippled because of the geometry, I see no reason not to add a few
extra turns in order to increase inductance and ultimately let them
fair better against larger diameter coils.
Comments, suggestions, flames always welcome.
- Freezing in a cold garage