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Tuning a coil with out a function generator

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LAN_403

H~Tank~V

Tue Apr 15 2008, 12:42AM

Registered Member #1253 Joined: Tue Jan 22 2008, 02:06PM
Location:
Posts: 39

Does anyone have any links to a website to tune a coil, without using a function generator?

Im going to be using a variac, if that means anything.

J. Aaron Holmes

Tue Apr 15 2008, 02:08AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

EDIT: My comments here assume we are talking about spark gap coils (SGTCs).

Nobody tunes their coils with function generators. That's like suggesting that a piano tuner just holds a ruler up to your piano and calls it good

Tuning means applying Real Input and looking for Real Output. And just like tuning a musical instrument, tuning a coil is a trial and error process; you make an educated guess, apply Real Input, observe, adjust, repeat. A scope and function generator can tell you what the resonant frequency of your secondary coil is without streamers (which effectively alter its resonant frequency), but that just helps with a starting point, and JavaTC and similar programs are good for that too. Coil theorists like Terry Fritz and others enjoy coming up with numerical models that make computer tuning and simulation of coils more accurate, but that's just one facet of the hobby, and those programs and models will never beat a methodical, throrough job of trial-and-error tuning.

And on top of all that, the vast, VAST majority of coilers don't even have function generators, which means that the vast, VAST majority of "How to Build a Tesla Coil" books and web sites describe methods of trial-and-error tuning that don't involve use of function generators. Best do a bit of reading, methinks.

Cheers,
Aaron, N7OE

HV Enthusiast

Tue Apr 15 2008, 02:47AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

wrote ...

Nobody tunes their coils with function generators.

Thats not true at all. Although I agree that final tuning is always done experimentally, function generators (or whatever) are a valuable way to establish an initial tuning point or at least verify
your design calculations.

If you have a function generator (audio oscillator, etc...), it would be a lost not to establish initial tuning with a function generator. Its easy, and takes minutes. And the amount of understanding you would get in
would be invaluable - you can verify your initial calculations, have a very good idea of what your initial tap points are tuned to, etc...

Now, once the initial tuning point is established, you can start experimentally varying tuning points, coupling, etc.... to fine the optimal tuning point.

J. Aaron Holmes

Tue Apr 15 2008, 03:51AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Dr. Shock wrote ...

Thats not true at all. Although I agree that final tuning is always done experimentally, function generators (or whatever) are a valuable way to establish an initial tuning point or at least verify
your design calculations.

Right. As I said, you don't "tune" with a function generator

You "characterize" an element of the total system. The results may be independently interesting (just like numerical models are independently interesting) and may figure in your decision about where to start the tuning process, but there are a lot of different (and far more common) ways to get to that point. I won't downplay the interesting-ness of scoping your coils. The practical importance, IMO, is far far less.

Cheers,
Aaron, N7OE

H~Tank~V

Tue Apr 15 2008, 09:41AM

Registered Member #1253 Joined: Tue Jan 22 2008, 02:06PM
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Posts: 39

Do either of you have suggestions for something to read online? I cant seem to find anything that doesn't involve a function generator.

Thanks

Andy

J. Aaron Holmes

Tue Apr 15 2008, 01:53PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

First off, just to avoid confusion, "tuning" a Tesla coil generally refers to the process of effetively adjusting the number of turns on the primary coil for maximum streamer length while holding all other parameters constant. If you're not sure what your other parameters should be, I'd recommend spending some time searching the TCML archives on

for, e.g., "toroid size" and "secondary diameter" and so on. Beginners often assume that these things are critical and that there must be some way to compute every coil attribute exactly, and are often frustrated to discover that most coilers just use what's cheap or already in their parts bin, then get everything to work (sometimes well, sometimes not) through trial and error. Really, Tesla coils are all about trial and error, and never moreso that with tuning, IMO.

Tuning, however, assumes that you've already selected all of the critical components: Primary and secondary coil dimensions (length, width, number of turns, wire diameter, etc.), topload (toroid, sphere), primary capacitor, power transformer, etc. If you are satisfied that you've chosen these things well, then the simplest thing is to plug them into a program like JavaTC (as I alluded to above) and see how the primary and secondary resonant frequencies align.

For the most part, JavaTC is just applying well-known formulae for inductance of coils, resonance of LC circuits, etc. So I don't consider it "cheating", as long as you have an intuition for what it's doing. There is an "auto-tune" feature that will basically tell you where it thinks the primary tap point should be. This is just computing the expected resonance of the primary coil and tank cap, the resonance of the secondary coil and topload, and then adjusting the primary "L" so that the two resonances are the same. Like measurements with a function generator and scope, however, it doesn't account for many Real World effects such as "streamer loading", which only present themselves when Real Input is applied to the system. So use this tap point as your starting point. You'll then briefly run the coil many times, making small adjustments to the tap point (both in and out) in between runs until you believe you have observed the best/longest streamers. Many people use the so-called "Freau Equation" (spark length = 1.7 * sqrt(input power in watts)) to get a feel for where their coil should be when tuned, if nothing is terribly wrong with it. Like the numbers you'll get out of JavaTC, the number you get from the Freau Equation is just a good starting point and is not to be taken too seriously.

Anyway, hope that helps.

Cheers,
Aaron, N7OE

Arcstarter

Tue Apr 15 2008, 03:34PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Im just going to tell you same basic thing as everyone else. Manual tuning will be best but if you want you can download tesla map. Punch in all of your coil parameters and it will tell you a good starting point for tuning. But after that you will need to do some manual tuning. Here is a site with info on how to use an oscilloscope:

And here is tesla map which is a great coil calculator.:

Download the standard version because it has been much better with my experience with it.
Hope it helps!

HV Enthusiast

Tue Apr 15 2008, 05:45PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Just as a note, although J. Aaron Holmes comments may apply to spark gap coils, they do not necessarily apply to DRSSTCs.

Tuning on DRSSTCs is quite complex and typically requires the use of a function generator, current transformer, and oscilloscope. With a spark gap coil, you typically tune for maximum performance, but with DRSSTCs where you have sensitive switching devices, you must find an optimal tuning which is a compromise with maximum performance and maximum reliaiblity.

Fine tuning on all my DRSSTCs was always finding the optimal point between maximum arc length vs. voltage, and primary current.

Dan

J. Aaron Holmes

Tue Apr 15 2008, 06:45PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Dr. Shock wrote ...

Just as a note, although J. Aaron Holmes comments may apply to spark gap coils, they do not necessarily apply to DRSSTCs.

Yes, sorry Dan; I have indeed been assuming we are talking about SGTCs, based in large part on the I-just-want-sparks tone of the question. I would not presume to debate anybody on the subject of tuning solid-state coils, as I have no experience to guide me there.

Cheers,
Aaron, N7OE

HV Enthusiast

Tue Apr 15 2008, 08:15PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

J. Aaron Holmes wrote ...

Dr. Shock wrote ...

Just as a note, although J. Aaron Holmes comments may apply to spark gap coils, they do not necessarily apply to DRSSTCs.

And my apologies as well. Its not very common to see spark gap based threads on this newsgroup, so i was assuming we were discussing solid state coils, in particular DRSSTCs.

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