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secondary coil resonant frequency

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Dr. Dark Current

Sun Oct 07 2007, 02:05PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi, just a simple question, but I can't seem to find an answer anywhere. When I have just the secondary coil, without topload, how can I calculate it's resonant frequency?

Shaun

Sun Oct 07 2007, 03:16PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

There are a lot of programs out there that make it it easy; TeslaMap is a good one, and JavaTC is a bit more in-depth.

EDIT: BTW those two assume you are making an SGTC, but the calcs for secondary resonant frequency are still the same.

Hazmatt_(The Underdog)

Sun Oct 07 2007, 08:00PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I've been doing some work in this category (9 different coils to be exact, but all same form dia. with different wire gauges and H/D).

Either you can calculate the Medhurst self capacitence from the H/D (which is interesting because wire size doesn't matter... kinda neat). OR you can calculate resonance from transmission velocity. I'm doing both and the velocity I'm finding is about 92% c, so with your winding length you derive the quarter wave frequency because you have f = c/lambda, its all in meters. That give's you the approximate resonant frequency.

Of course now... that's one of the coil's resonances, and the only one that people pay any attention to. There are a couple more modes further up in frequency because its not as simple as you would think.

After you calculate that frequency and capacitence, calculate your toroid, then when you go to load your coil the Cself will drop by about 75% (I'm going to work on that too), so the result is more like Ctotal = .25Cself + Ctoroid. Then its just a mater of CpLp = Ctotal Ls

Sulaiman

Sun Oct 07 2007, 09:26PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Hazmat,

What do you mean by "then when you go to load your coil the Cself will drop by about 75%"
I haven't noticed that.
And how did you determine the transmission velocity factor is about 92%?

P.S. On your site under Tesla Coils you've got a hv differential probe;
Does it work well at TC frequencies for differential signals?
If so can you give some info/diagrams please?

Hazmatt_(The Underdog)

Sun Oct 07 2007, 11:36PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

HEH, I'm sorry for the information 'blackout' I screwed up my computer so I'm recovering files while trying to motivate myself to do some college reports right now.

1. First you calculate your Ctoroid. Then you sweep the coil for resonance, then determine your Cself and make a note of the frequency. When you load it with a toroid the expected frequency of Ctor + Cself will be lower then the observed resonant frequency because Ctor Z is much smaller then Cself Z. I dunno if I can find the plot of it now because of my chaotic filing system, but I'll try to send it when I can.

2. Transmission velocity. If the conductor were perfect, and the wave didn't bounce around inside the conductor, then f = c/lambda. BUT since it does, the propagation is impeded a bit. Knowing the length of the wire, you then have a 1/4 wavelength standing wave on resonance, but when you go to calculate this the expected frequency is higher then the observed, so that's your loss factor. It's well documented with coax too.
92% is my experimentally measured value which holds true pretty close within the 9 coils I'm playing with. There are some adjustment factors like 95% wire spacing, some form shrinkage, its not all exact but I've been tuning my parameters to fit the actual coils I have as my research set.
I think I lost the picture of my coil set... I'm in the process of recovery, so im sorry but it may take soem time.

3. I post as much of my work here as possible. The probe details are actually here. Nice huh.
It was designed to measure what was going on with the NST 60Hz stuff. the resistive ballast goes on either side of the terminals so you get a current and voltage output continuously. I ran into problems with the huge di/dt because it idles at 60mA then spikes at like 300A. Its sorta impossible to watch real time. I might need a log amp for it.

some details about the probe

I did some digging insted of my homework, and found it.
I'm having a lot of problems getting out of the vacation mindset, but meh.

Anyway most of that is all here.

Steve Conner

Mon Oct 08 2007, 08:52AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The best analysis I've ever seen of this is by Paul Nicholson. He uses finite element methods to model the resonator as a transmission line. Everything else is just a kludge.

If you really want to know what's going on, you owe it to yourself to read and understand his two papers:

Hazmatt_(The Underdog)

Thu Oct 11 2007, 02:33AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

On the other hand, I just got a copy of The Ultimate Tesla Coil Design And Construction Guide and I highly recommend this book. It has just about everything you need to get started on analysis of Tesla Coils, AC Resistance, Mutual inductance calculations, and a bunch of other stuff!

It's one of the few books I've found so far that's a real help, and will probably be more so once I get into it later.

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