Re: Need help finding Inductive Reactances

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

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Eric,

At 03:48 PM 1/24/2001 -0500, you wrote:
>I am a first time telsa coil builder. I need someone to explain how I find 
>the total inductive reactance of the telsa coil I built so I can find a 
>capacitor that matches for resonance. So I need to know how to find 
>Inductive Reactance of the primary and secondery windings of the Telsa coil.
>
>Also Does the power supply (neon sign transformer) play a part in the total 
>inductive reactance?
>Thanks
>Eric B

I don't think you want the "inductive reactances" but rather a way to find
the resonant frequencies of the primary and secondary coils.  Then you can
work on matching them to the same frequency.  The NST you have will pretty
much determine the primary capacitor size.

First off, there are a ton of great equations at:
http://home.earthlink-dot-net/~electronxlc/

in the formulas section.  They will tell you how to find the inductance of
the primary and secondary coils and many other things.

There are charts at:
http://hot-streamer-dot-com/TeslaCoils/Misc/NSTStudy/NSTStudy.htm

that can help with finding the right cap size to go with your NST.  Those
charts are a little complex for a first time builder as Dan mentioned in
another post but I'll try and go through that.

So let me go through a example that I hope will help clear up this messy
business us or at least give you some ideas to ask further questions about.

1.  Suppose I find a 12000 volt 30 milliampere NST transformer.  This find
is a critical part of the coil since that is were all the power comes from
and much else will be based on it.

2.  I know I need a spark gap and, for a first time builder, the fancy
rotaries are too messy so I go with two bolts or a "static" (nothing is
moving) gap.

3.  There are zillions of equations (many forms of one) that match the cap
to the transformer but this new "LTR" stuff is changing that fast.  Since I
am a first time builder I like the LTR's greater safety margin in not being
as likely to blow things up, I want a cap matched to the LTR value.  So I
go to the NST study chart above and look up the data on 12kV/30mA NSTs.
Under the cap values the 6.631nF number stands out at the resonant value.
To find the LTR value I look for the cap size that gives 120 break per
second operation.  That value is 9nF.

So you now have a "12/30 NST, static gap, LTR system, with a 9nF primary
cap".  That my not mean much to a first time builder but it means
everything to coil designers who will help you :-))

4.  Next step is the secondary coil.  The only rule here is to build them
pretty much like everyone else does :-)  Not too tall, not too short...
About 4 to 6 diameters tall is good.  4 inch PVC is common so you have a 4
inch diameter by 20 inch tall coil (plus an extra 1/2 inch on each end).
Then you have to find the wire.  Electric motor repair shops are a great
place since they have lots of extra leftovers laying all over the place.
You want #24 to #28 gauge.  I really like #24 wire.  You can order it
on-line but the price is high or someone on the Tesla list probably has a
"deal" they will make you.  Let's suppose I use #24.

5.  Wind the wire on the coil.  Use epoxy for the start (test it to be sure
it holds really good) and hold the wire with masking tape as you go.  First
time builders almost always just do this by hand while watching football
:-)  The fancy power winders are often a project unto them selves.  Wind
the wire tight and snug so it does not unravel.  After awhile it will be
wound and epoxy the other end.  Best to coat the coil with clear
polyurethane as used in wood finishing from the hardware store.  This
protects the coil, holds the wire, and provides some electrical insulation.
 Don't use anything else since other material my do "bad" things.

6.  You will need to know the number of turns on the coil and you probably
didn't count them :-)  There is another chart bellow that has all the info
on it.  A big file but you can frame this old chart and hang it on the wall:
http://hot-streamer-dot-com/TeslaCoils/Misc/WireGaugeChart.jpg

7.  This chart says that enameled #24 wire has 46.3 turns per inch.  So the
20 inch coil has 926 turns.  If you were John Freau he would use #28 for
1454 turns since he likes higher numbers of turns (probably right) but this
is my example ;-)

8.  So we have a 926 turn, 20 inch long, and 4 inch diameter secondary
coil.  The inductance can be found at Matt's sight above.  L(uH) = (N^2 x
R^2) / (9 x R + 10 x H) = 15734uH or 15.73mH.  That is a pretty important
number especially when you coil is not working and you are asking for help
to determine why.

9.  Now you need to know the secondary capacitance.  This could go on for
days here...  There are many powerful new tools to do this.  I have a
program that will figure it out at:
http://hot-streamer-dot-com/TeslaCoils/Programs/E-Tesla6.zip

10.  But really I have another suggestion.  Just guess it at 30pF and you
will be pretty close :-)  If things get rough we can really figure it out
later, but for most coils in this size with a decent size top terminal 30pF
will be fine.

11.  Now we can figure out the all important resonant frequency.  This is a
really important number.  Turning back to Matt's great site the formula is:
F0 = 1 / (2 x pi x SQRT (L x C))  

12.  In our case L is 0.01573 and C is 30E-12.  That all works out to
231684 Hz.  Our goal now is to make a primary coil that can tune to that
frequency with plenty of room to spare.  Tuning the primary coil is really
the only good adjustment you have with the possibility of radically
changing the top terminal size.

13.  So the 9nF primary cap and the primary coil have to tune to 231684 Hz,
or... you will be writing to us about how your coil is not putting out
sparks and you can't tune it :-O  In that case we will first have to grill
you for all the above information...  However, you have a good head start
and are doing this all first ;-))

14.  Like everyone, you get 50 feet of 1/4 inch copper refrigerator tubing
from the hardware store.  Your secondary is 4 inches in diameter do the
center of your primary should be 6 or seven inches to give 1 to 1.5 inches
spacing between the primary and secondary.  "I" have always used 1 inch
spacing.  Using Matt's site again the formula is:
L(uH) = (N^2 x R^2) / (8 x R + 11 x W)

The value you want is:
1 / ((2 x pi x Fo)^2 x C) = 52.43 uH

15.  Now you can just try like 5 and 10 turns with the calculator to see
what you get.  Assuming 3/4 inch turn to turn spacing, I get:
5 turns = 7.40uH
10 turns = 33.38uH
15 turns = 86.83uH

If you work for awhile you will see you need about 12.4 turns.  But give
yourself a bunch extra like 15.  This is where a lot of first time builders
get into trouble, not having enough primary turns to tune their coil.  Not
easy to fix...  Far better to have 10 extra turns than 1 too few!


So you are sort of done!!  You know a ton about your coil and you know it
will work.  Just a matter of building it and if something does not work you
can answer all the questions we will ask to help you out.

There is another way to make a first time coil too.  Look through the coils
on the web ring:
http://nav.webring.yahoo-dot-com/hub?ring=TeslaRing&list

Find a coil you like and think you can build and COPY it!!  That takes most
of the design stuff out of the way so you just need to build it and start
playing with it.


You can also just start blindly putting a coil together like we all did
years ago and hope you can get it to work.  That's how we all did it :-))
However, we are trying to SAVE you from all the pain and frustration we
went through here!!  My first coil barely put out an inch before it burned
up...


Depending on how much you know and have learned so far, this will either
make sense or it won't.  Don't worry if it doesn't since it is pretty messy
but at least you will get a start on what the thinking is.  If you already
knew all this.  There is somebody out there who can use it I hope :-))
Once you build the first coil, this will all be much more clear!

This is just a post I put together while sitting here, so if the numbers
don't workout somewhere I probably goofed.  Not everyone may agree with me
since I go about these things pretty technically, but to each their own...

I would strongly recommend taking a good look at Matt's web site since he
has ton's of great info there in a real nice readable form.  He also has a
bunch of programs that can help with the calcs.  And, of course, this Tesla
list is in the business of answering questions...

Above all, be real careful with the high voltages!!  I have a nice safety
read at:
http://hot-streamer-dot-com/TeslaCoils/Safety/safety.htm

Almost all sites do now since it was last year's safety project to put them
on web sites.  If you have any questions about safety, be SURE to ask while
you still can...

Hope that helps...

Cheers,

	Terry