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Several resonance point of secondary

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steven968

Mon Jan 09 2012, 10:20AM

Registered Member #3516 Joined: Wed Dec 15 2010, 10:40AM
Location: Australia
Posts: 32

Hi all,
I found two "resonance point" when I was measuring my secondary freq of my DRSSTC using o-scope and oscillator.
one at 202kHz and the other at 521kHz.
at both point the amplitude reached its maximum.(as the pics shows)
(the primary wasn't removed when measuring the secondary freq)
Is that normal? which point I should use to tune my coil?
Thanks.

Steve Conner

Mon Jan 09 2012, 11:21AM

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

Yes it is normal, the higher one is the 1/2 wave mode of the secondary. Use the lower one, unless you want sparks shooting from the middle of your secondary.

teravolt

Mon Jan 09 2012, 07:56PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

another way would to switch to a pulse squar wave and change the pulse width gradualy and you will see each frequency as the amplitude increases. do yo have a ordinary pulse generator?

zzz_julian_zzz

Tue Jan 10 2012, 03:35AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Use lower pole frequency, to avoid secondary arcing

steven968

Tue Jan 10 2012, 04:17AM

Registered Member #3516 Joined: Wed Dec 15 2010, 10:40AM
Location: Australia
Posts: 32

teravolt wrote ...

another way would to switch to a pulse squar wave and change the pulse width gradualy and you will see each frequency as the amplitude increases. do yo have a ordinary pulse generator?

Yes I have, the pulse generator is combined with my signal generator, and I can vary the pulse width (duty cycle?)

Coronafix

Tue Jan 10 2012, 11:03PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Hi Steven, I notice you have one of those DDS function generators. How do you find it? Would you recommend getting one? I've seen them on ebay pretty cheap.
Depending on the coupling, you should find two resonant frequencies on the lower one too.

steven968

Wed Jan 11 2012, 04:08AM

Registered Member #3516 Joined: Wed Dec 15 2010, 10:40AM
Location: Australia
Posts: 32

Coronafix wrote ...

Hi Steven, I notice you have one of those DDS function generators. How do you find it? Would you recommend getting one? I've seen them on ebay pretty cheap.
Depending on the coupling, you should find two resonant frequencies on the lower one too.

It worth trying. It is pretty cheap but well designed. The waveform looks very accurate,but when I generate square wave @ 5Mhz,it looks like sine wave

Anyway it's a good product.

Hazmatt_(The Underdog)

Fri Jan 13 2012, 01:17AM

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

If you take the time to characterize your coil you will see it has some very interesting things going on. I've spent a lot of time on it, but have not had the time to compile everything yet, I'm still working on some detials.

Pictures are a swept response of a real coil, and approximate circuit equavilent which in reality has "n" branches in parallel, "n" being frequency modes.
If you look carefully at the right end you can see the half wave response peaking up again, that's where I stopped with the sweep.

*Matt

Uspring

Fri Jan 13 2012, 11:20AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

I think the secondary can be seen much as a transmission line i.e. a long series of little inductances with little capacitances inbetween, which are connected to ground. Its behaviour can be best understood if you look at the voltages as you go up along your secondary, starting from the grounded base.

At the bottom the voltage swing is zero. It then increases in a sinusoidal way as you go up along your secondary. For the lowest resonance frequency the voltage profile is about a quarter period of a sine wave.

The currents in the secondary are proportional to the rate at which the voltages increase as you go up along the secondary. In the case of the lowest resonance frequency you will have the largest currents at the bottom. They will then decrease as you go up.

An interesting special case is a secondary without a top load. The current at the top then must be zero since it has nowhere to go. The voltage profile is then an exact quarter wave because the rate of increase of a sine is 0 after a quarter wave.

At higher frequencies the voltage profile as you go up along the secondary oscillates more rapidly. If you e.g. triple the frequency at which you get a quarter wave, the voltage will first go up, then go down through zero and then will reach again a max as you arrive at the top.

So in theory the second resonance of the coil without top load should be at 3 times the lowest resonance. There also should be a resonance at 5 times the lowest frequency and so on.

If you have a top load with some capacitance there will be currents going into it so the voltage profile will not end with a flat part at the top. The resonances therefore don't follow the simple 1,3,5,7.. ratios as in the case without top load. There are still many of them there, though.

I must confess, that I naver made any such experiments with my coil and I'd be interested to know, whether the theory coincides with your observations.

Steve Conner

Fri Jan 13 2012, 11:50AM

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

Paul Nicholson's TSSP code was capable of calculating the first 10 or so resonances to quite high accuracy. They don't follow integer ratios even without a topload. This is why, for instance, they aren't excited by the harmonics of the square wave drive from a SSTC's inverter.

The Seibt coil is a Tesla coil with a very tall, thin secondary excited in a higher mode. It produces several bands of corona.

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