RE: Tesla coil grounding and other questions

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

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

Hi Joe:

MANY things have changed since that article was written.

1) It used to be that candlestick shaped secondaries were in vogue.  Now
a height-to-width (aspect) ratio of 5-1, or thereabouts, is recommended,
using roughly 1000-1500 turns.  What you have will work, I just wouldn't
recommend that length to someone starting from scratch.

2) If you have a center tapped NST secondary, connecting the TC
secondary base to either side of the primary could be lethal if it or a
streamer were touched.  The secondary base should only go to ground.  If
the coil is low powered, connecting to the AC ground is acceptable, but
yours is borderline, and a seperate RF ground may be worth considering.
The choice won't affect performance, but rather how much "bad" stuff
gets in your household wiring.

3) The gap/cap circuit placement won't affect performance, but the NST
will see less abuse id the gap is across the NST output.

4) Glass caps were state of the art in the 60's and at Tesla's time, but
cap arrays of store-bought polypropylene film caps offer far better
performance, longer life, and more compact size.  Search for "MMC".

5) A helical primary will typically overcouple to the secondary, causing
sparks to race over the surface of the secondary and damage it.  A flat
spiral roughly even with the lowermost secondary turn is best and
provides adequate coupling.

6) I assume the 15kV wire you propose for the primary is similar to what
is used on neon signs.  A couple problems here.  Stranded wire on a
primary is lossy at high frequencies as compared with a solid conductor
of the same gauge.  And the thick insulation will make it difficult to
easily tap to find the best tuning connection.  1/4" copper tubing is
very easy to bend, nothing special is needed, and is very low loss.
Wiring in a Tesla coil does not need any insulation at all if care is
taken that adjacent things don't touch.  There's no need to use special
HV wire.  If you don't want to spring for the tubing, common 14 or 12
gauge solid copper insulated household wire will work nearly as well.

7) The primary and secondary must be tuned to the same frequency, you
can't use a harmonic.  52 or even 26 turns is a lot.  Most flat
primaries use 8-15 turns, although maybe the smaller diameter of the
helical form needs more turns.

Regards, Gary Lau
MA, USA



Original poster: JOSEPH CACCIATORE <jocatch-at-us.ibm-dot-com>


Hello. I am trying to resurrect my old tesla coil I made in high-school
30
years ago with plans that came from the July 1964 Popular Electronics
magazine. I still have the secondary coil and 12kv, 30ma NST which I
want
to use. The secondary was built pretty close to the article, 4.75"
diameter, 34.625" long using #26 (or was it #28) wire. The NST is center

tap to the case.

First question is grounding. The 120 vac for the NST of course is not
grounded (only 2 leads to the outlet). The design I have shows the
bottom
of the secondary connected to one side of the primary tesla coil in an
auto-transformer arrangement. That is how the original design worked and
I
was able to get 2 or 3" sparks from it.

But reading on the net I see some places which say ground the bottom of
the
secondary to the case of the NST only and some show the bottom going to
earth ground only. Which is correct? If I want to run it in a house or
display like at school, there won't be an earth ground to begin with.

Also, if you have seen the PE issue, they had the spark gap in series
with
the primary but all circuits I see on the net today shows the capacitor
in
series. Is that better?

For the capacitors, I am using thick window glass and aluminum foil,
each
glass is about 18"x18". I am using 2 of them.

Lastly, since I have about 75' of 15kv wire I want to make the primary
using the wire in a helical coil arrangement (I don't have copper tubing

nor anyway to bent it nor a mount for it). I assume I will need all the
wire since my basic calculations show I would need 52 turns for the
primary. Since 52 would require more wire than I got, could I go with a
harmonic and use 26 turns for the primary?

Thanks for any input you guys may have. I got this beautiful secondary
coil
I made and NST and I really would like to make it work.

JC