[TCML] Tuning a new coil
b alex pettit jr
a_pettit_jr at yahoo.com
Sat May 17 08:51:25 MDT 2008
Without Attachments ...
Thanks for the details.
I agree, the ResFreq should be closer to 295. The coil dia is 4.5",
and wire resistance is a total of 29.7 ohms. I did a quick turn count
and it is right at 44 TPI.
I will check this HP 5315B counter agains the scope today. It would
be ideal to have the JAVATC model match the hardware.
As I said, I built many TCs over the decades- most not
too impressive in their performance. My last venture was
in the 1980s in duplicating one from a book of the 1915 era.
With stained and varnished wooden cases for the transformer,
glass plate cap, rotary spark gap and coil base, it looks
quite authentic for that time.
It is not an optimal design, but I'd rather leave it for its
antique appearance and build another.
The referenced coil has an 8" dia tube, 16" long and wound with
string spaced enameled wire. It gives ~ 12" sparks.
SO, I am excited about this coil performing a bit more impressively !
Bart, from your JAVATC 'demo' it appears you overdrive your
NST to 140 volts. I was considering this - is that ok ?
Will it gain me anything if I do not increase the pri cap
from 0.0188 to 0.0199 uF as specified ? A wider spark gap
would be easy.
Thanks again for the guidance,
I just retired from industry and began teaching a senior
college level mechanical eng course and lab in mechanical
dynamics, vibration and modal analysis.
The parallel between that and these electrical systems is AMAZING !
I'm showing your coil should be nearer to 295 kHz. Your 323 kHz seems a little high to me. I know it's a small amount, but in this configuration, Javatc is always much closer. Usually in situations like this, the inputs to Javatc are not exact. For example, secondary is slightly smaller than stated (like 4.25") or things of that nature. Turns affect this also, but your 920 turn count seems about right and the secondary diameter would affect this frequency quite a bit (20 kHz just for a 1/4" difference in diameter).
On the primary side, cap tolerance will affect how much primary is needed as well as the primary itself. Good to measure the cap to be sure of it's total C. In any case, 1 full turn will affect about 44 kHz on the primary side for your coil. Thus, if the tuning is close to begin with, the ideal tap will be within inches. I suspect your coil will perform well at about turn 6.
When you tune the coil in at low power, you also get small sparks. As you increase power, spark lengths increase. Those sparks add a small amount of loading changing the frequency of the secondary slightly. So, a small amount of increased inductance is often added to match streamer loading. Say your coil tunes in at low power at 5.6 turns. When full power is applied (and if good streamers are forming), you may want to increase the inductance a few inches (but that is something you have to play with).
Take care, Bart
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