Re: Tesla

CONTINUED FROM A PREVIOUS THREAD with mmcarty-at-dnaco-dot-net...

> That's a definite plus for neon transformers, I'm going to see if I 
> can scrounge one up in Dayton tomorrow.

The last time I checked, I could get a 15kV, 60mA neon sign transformer
for about $35.
> ...That's what I figured, I could figure the inductance, but wasn't sure if 
> there was any way of calculating the parasitic capacitance.

There are formulas that can be used to give you some indication of the
distributed capacitance, but I'm not sure how accurate the are.  I really
don't know if anyone actually uses them, especially given how easy it 
is to hook up a signal generator and some LED's or a VOM and get the 
resonant frequency "straight from the horse's mouth".  Harry Goldman 
from TCBA sent me a formula that was in turn given to him many moons 
ago -
          C = .55 + .3(l/d + d/l)
Where C = picofarads, d = coil diameter in inches, l = length of coil in
inches (accuracy is claimed to be 3%, but I've never really tried it out).

> I realized I could just hook something up and tune it, but in my mind 
> it's kind of cheating if I don't do the math right down to the last 
> detail (or at least some measuring in this case).

Building a coil by the numbers rarely seems to work, since there is a
whole lot more involved than what you normally account for.  
Coefficient of coupling, electrostatics, corona loading, etc... all come 
into play and are usually not considered in the simple theoretical, 
steady-state analysis of RLC circuits you would normally find in an EE 
textbook.  Running the numbers can tell you if there is something badly 
mismatched, but will generally just get you in the right ballpark.  The 
real "work" (fun?) comes after you have everything wired together and 
you fire it all up.  It is very rare that you will achieve maximum spark 
output without tweaking things (coupling, primary inductance, gap, 
etc...).  As an example, my first coil ran off a 15kV, 60mA neon sign 
transformer and put out 4" sparks and I was tickled pink.  After lots and 
reading, lots of questions, and lots of rebuilding and tweaking, my 
current coil can put out discharges 5 times what I was getting from my 
first coil, with essentially the same input power!  My latest find was that 
by raising my secondary coil up and reducing the coupling, I increased 
my discharges by about 2".  The two new coils that I'm working on now 
are built according to Richard Quick's and Richard Hull's 
recommendations, and should be even better performers.

> I was added to the mailing list a couple of days ago, but have only 
> seen some gifs come across there.  I figured I'd wait and get a feel for 
> what's going on there before jumping in.

Don't worry about asking what you think might be simple questions.  
There are a lot of tips and tricks about Tesla coils that aren't 
necessarily obvious, but will help get you going without spending a lot 
of money on expensive electronics equipment.  There are also some 
very experienced people on the list and it's a great opportunity to get 
some good tips.  There seems to be a wide range from Tesla coil 
"newbies" who aren't even sure what a neon sign transformer is all the 
way up to multi-kVA, pole transformer veterans like Richard Quick and 
Mark Graalman.  If you aren't sure, ask!  Everyone benefits from the 
questions and answers.

> These hobbies get to be expensive.

Just keep reminding yourself how much money you're saving by 
building Tesla coils instead of buying a yacht or something :-).

Another tip is to make friends with some amateur radio operators and 
find out when and where the hamfests are, and go to them.  You can 
pick up all sorts of equipment and supplies at hamfests for very 
reasonable prices.  Variacs, scopes, meters, high-voltage capacitors 
and transformers, coils, wire, RG8 solid poly core coax (used to 
connect the HV transformer to your primary tank circuit and also to 
connect your coil to your RF ground), signal generators, frequency 
counters, etc... can all be found in abundance and sometimes 
practically for nothing but the hernia you get by carrying the thing away. 
 Also, Dayton has an annual hamfest that people typically describe as 
being very close to a Religious xperience; I've heard that if it's 
electronic and you're looking for it, you can usually find a truckload of it 
somewhere at the Dayton hamfest!

> By the way, what's "thin wall" PVC?  The thinnest I've ever seen at 4" 
> is schedule 40, which we use for electrical duct, it doesn't seem too 
> thin to me.

There is a thinner walled PVC tubing that is used for drain fields, but I 
forget what it's called (DWV, DVW or something like that???).  Check 
your local Hechinger's or Ace Hardware store.  I saw some plastic 
tubing in (of all places) a Spot-A-Pot that was labeled HDPE.  If it really 
was high-density polyethylene and you can find it, that would be even 
better for your secondary.

Steven Roys (sroys-at-radiology.ab.umd.edu)