DC pwr supply vs AC pwr supply Tesla Coil

[Jeg]

Hi to all.
This is my first post to this fascinating community. I am really very glad that there is a place, where many enthusiasts - researchers like you, are gathered sharing all this valuable info and experiences. 

I would like to build a Tesla coil with the use of a flyback tv transformer. I 'd like to be in a range of 5 to 20KHz. By the use of a simple 555 and a mosfet, i can drive the flyback transformer, and as far as i understand, this could create a rectified output (cause of the internal output diode of the transformer) which can be used for the secondary Tesla coil excitement. Even if this is a rectified dc input signal, what is gonna be the output of the secondary Tesla coil? Will it be an AC output on the specified frequency or not? My logic says that even if this is a dc fluctuation, the output coil will "see" just the fluctuation even if it is a dc voltage, and the output is going to be an AC HV. Is that correct?

My next question is how can i produce resonance on the output Tesla coil? I feel that frequencies on that range are very low for building a Tesla coil resonating at so low freqs. Is there any way for making this resonance without the need of building a huge output coil? Do u think that a Flyback transformer can handle frequencies as high as 300KHz?

Tnks
Jeg

[TinselKoala]

Hello Jeg. You might like to take a look at my recent YT videos showing my MOT DC blown spark gap coil.

A slide show of some psarks:
http://www.youtube.com/watch?v=tTLFlRhsa5U

There are some other vids giving the construction details, check my channel.

1. A flyback transformer won't produce the current output you need for good performance at "CW" or continuous operation. You can get single discharges from your coil several times a second perhaps,  that can be pretty spectacular with a low-current DC input to charge your primary tank capacitor, but the flyback can't provide the current necessary to keep a big cap bank charging and discharging fast enough to make a decent coil work continuously. A MOT (microwave oven transformer) has the reverse problem: it is not current limited and provides too much current for good spark gap performance, unless you use some tricks.

2. For driving a flyback transformer efficiently a ZVS oscillator using two mosfets is a much better choice than the 555 timer based driver. Easier to build, more robust, wider input voltage range, better power thruput and just a bit more expensive (the IRFP260 mosfets I use are about 3 or 4 dollars each, but I haven't blown one yet, in very rough service.) My ZVS flyback driver will actually drive a flyback at only 5.5 volts input, but prefers at least 12 volts and is really cooking at 24 volts input.

Again, check my channel for demonstrations of the ZVS driver. Over the past week or so I've been using it to drive a flyback connected to my "TKTerella" vaccum triode apparatus. This is pretty spectacular but not really relevant to your quest.


3. The lower the frequency of a Tesla coil the bigger it has to be. My MOT DC coil secondary is 750 turns on a 4 1/4 inch diameter form -- just over 12 inches of winding -- and resonates at around 250-300 kHz, depending on topload. This is already high for a spark-gap coil. A 20kHz Tesla coil aircore secondary will be correspondingly larger.

4. Yes, you can use rectified DC as input to your primary tank circuit, and there are some advantages to this. Yes, the output of the secondary will always be AC at high voltage, and if your coil is working right it will be a clean sine wave with not much hash on it. The way it works is this: The DC input charges the primary tank capacitor until the voltage is such that the spark gap breaks down and shorts out the coil-cap circuit. This allows the primary coil-cap tank to "ring" at its resonant frequency. This ringing is AC, and "pumps" the resonant secondary, which is electrically a quarter-wavelength of the primary's ringing frequency. This causes the voltage at the free end (usually the top) of the secondary to rise very high, a phenomenon called "VRSWR" voltage rise by standing wave resonance ... the metaphor often used is the cracking of a whip. The resonant frequencies of the primary tank and the secondary are matched by adjusting the cap value, the turn count of the primary, the wire length of the secondary, the top capacity of the secondary, and the coupling between the two coils.

5. Using a high-voltage primary source like a flyback transformer means that you will also need high-voltage capacitors in your primary tank circuit. Even though the flyback charges them slowly, they still have to withstand the full 15 or 20 kV plus a safety factor, just like the caps of a NST-powered TC would.

6. The spark gap is very important. In fact it is the key to a successful SG Tesla coil. Spend some time researching and experimenting here. A proper gap can make the difference between no or very little secondary output, and spectacular HV displays.

7. The advantage of using air-core coils is that they do not saturate magnetically. A flyback is designed to operate at 20-40 kHz; I doubt if you'd get much action at 300 kHz driving. The strategy for resonating in a DC supplied tank circuit is a little different from that in an AC supplied coil. So you'd go ahead and use the flyback's normal pulsed DC output as driven by the automatically-resonating ZVS oscillator at 38 kHz or so, pumping up the tank capacitor, even though the tank might resonate at 300 kHz when the spark gap actually fires.

I'm happy to help out in any way I can. If you are stuck on using a 555 and a flyback for your tesla coil you can still get some great displays, psarks and corona, just smaller and at a lower rate than you might with a higher-current power supply.

[Farmhand]

Hi Jeg, Tinsel, I tried using an AC circuit with series caps and parallel spark gaps which kinda worked with an inverter type supply but
not so well, it did also work from the grid but caused problems.

So I decided that for a plug in the wall setup I needed the HV rectified and a DC charging circuit with lots of inductance to limit the current,
I made the HV supply like in the picture and use a rotary gap of a shorting bar design at about 500 BPS is good, but higher rates can put more power through.
I used two anti parallel MOT's and the charging inductance is a MOT secondary ( over 20 Henry's) but it does work with 4.7 Henry's as well (with the primary shorted).

The spark gap doesn't fire to ground it fires to the negative floating supply which i think is important for keeping noise off the lines.
I have filters and such on the input side, the HV supply transformers are grounded to the Tesla coil ground stake not the house ground.

I've blown one series set of caps in the cap bank, there are over thirty caps in the HV cap bank. I think that was caused by a mistake of my own.

http://i227.photobucket.com/albums/dd168/Toey1/DualMOTPrimaryCircuit12.jpg

Also Tinsel I was wondering what your opinion is of the three coil Tesla coil designs, as in with the fairly close coupled primary-secondary and the loose coupled secondary - extra coil ?

Like the transformer in the attached picture. In the second picture I put a long aluminium tube on top of the secondary so it hung out over the front to get the sparks to ground there.

Maybe it might be possible to load the secondary of such a transformer or pair of them while still maintaining some resonance either by directly loading the powerd transformer or by loading the secondary of an induced transformer, like in the third picture.

Here's a short clip showing the light from the output coil of an induced transformer, do you notice the very slow rhythmic sound almost like the frequency is building slowly then
suddenly changing back. Any idea's what might cause that ?

http://www.youtube.com/watch?v=ZVx1FzLFFXc

I figured this was as good a thread as any to talk about Tesla coils without, imposing on other threads.

Cheers

P.S. I did a rough turn count the secondary has 38 turns and the extra coil has about 165 turns but i keep loosing count due to going cross eyed. 
Not many turns wire is 1 mm, I'm considering winding a coil 115 mm diameter from 0.7 mm wire so that it's frequency matches the setup as it is now then use it for an
extra coil sometimes for more voltage, I'll need to use more primary capacitance then and maybe go to 2 primary turns to keep it low enough. I can go to about 40 nF of primary caps rated to 20 kV without too much trouble.

The black wire behind the sparking coil in the third picture is hanging in free air at some distance behind, it's just a wire through the purple coil to ground, for safety, the coil is to keep the vibrations
of the discharges to ground fairly low frequency, also to reduce extra HF ground noise. Trying to damp any discharges to ground is all.

..

[Jeg]

Hi to all
Thank you very much Tinselkoala, you were very descriptive and informative. I am at work now but when i'll be back at home i will check your channel and i'll come back on this.

Nice job farmhand! I really admire you guys. I wish i 'll built my first coil soon!

Greetings Jeg

[Farmhand]

Hi Jeg, Tesla coils are really wonderful things to experiment with, I started with 12 volt powered stuff not knowing anything much, but because
the Tesla coil itself can produce such weird effects it's kinda addictive. Try to remember safety first always when you use medium to HV and capacitors.
Try also to resist using yourself as a conductor, I think it can cause issues under certain circumstances.

My setup is fairly high frequency at about 750 kHz, when the frequency is high and the voltage and oscillating energy is high some really curious effects can be seen.
The number of different experiments that can be done with different arrangements of Tesla coils is huge.

So a warning of possible "Tesla coil fever" or "experiment addiction" is prudent. 

Cheers