"Direct Drive" Tesla coil (without primary winding)

[Zbig]

This is my first post on this forum, and i'd like to apologize for my English (I'm still learing) and lack of all these usefull three-Letter-shortcuts about Tesla coils and all this stuff. I will learn and use it later (I promise...).
I've discovered Tesla coils some time before, but after some experiments I've give up.  Lately I give them another chance 
After some burned MOSFETs and IGBTs  I can produce several centimeters sparks, but nothing more. Reading several articles about Tesla coils spreads some light on my experiments, and gives answer to some riddles. But answers was like that: build quite complicated electronics, with hundreds of volts supply and hundred of amperes current, use expensive MOSFETs/IGBTs, and  spend hours tuning primary and secondary windings

I try to analyse secondary Tesla coil as RLC circuit, but nothing really works as expected. And then some idea comes to my head:  Tesla coil is NOT and RLC resonance circuit, but simply 1/4 lamba dipole antenna  twisted in a shape of coil , working on its own resonance frequency! And many pieces of puzzle started to fit together.
And another idea comes to my head: If Tesla coil is a kind of antenna, it may be supplied like antenna, without primary coil at all!  I tried to find something about this idea, but I can't - everybody uses two coils (If i'm wrong let me know).

After several hours everything was built and tadam! DDTC - Direct Drive Tesla Coil is ready and working! It produces 20cm/8inch sparks:

Power generator (2x cheap IGBTs in TO220 case + SG3525 as a VCO and driver + NE555 for duty cycle) is supplied from 48V and consumes less than 3 Amperes. With 36V and 2A spark is 15cm/6inch.
Burst lasts 200 microseconds and repeat time is 2 miliseconds (duty cycle 1:10), coil works on 200kHz. 
As you can see coil is driven by single wire (3 metres wire in my case - may be even longer).
It seems that this kind of supplying Tesla Coil is very easy and efficient.
What do you say?

[Uspring]

The coil seems to work well :-) but for me it is surprising. A base fed coil, what yours seems to be, requires large driving voltages, usually several kV. Your driver supplies maybe 48V of AC. I'm puzzled. Can you supply a bit more information about the driver and how it is connected to your coil?

[Mads Barnkob]

That sounds like some very interesting experiments you have had going on for quite a while, good to see that you get some sparks afterall

What is the bulged out part at the bottom of the secondary coil where the wire goes into?

Regarding the RLC or 1/4-wave theory, did you remember to take arc loading into account?

Uspring once wrote: "I'm aware of 2 streamer models, one implemented in Terry Fritzs scantesla and one by Steve Conner, the hungry streamer model. AFAIK scantesla assumes a 220k resistance in series with a 1.5pF capacitance per foot streamer length. Conners model assumes a streamer capacitance of 25pF per m streamer length and a resistance, which is equal to the capacitive reactance. The results I got seem to match the hungry streamer model much better. I'd be interested in the logic behind that model." http://4hv.org/e107_plugins/forum/forum_viewtopic.php?137883

[Zbig]

The coil seems to work well :-) but for me it is surprising. A base fed coil, what yours seems to be, requires large driving voltages, usually several kV. Your driver supplies maybe 48V of AC. I'm puzzled. Can you supply a bit more information about the driver and how it is connected to your coil?

Yest, there is ETD44 ferrite transformer  with 1:60  Winding, so as you guess some KV are supplied.

Here is my generator (sorry for the mess on my desk)
 
2xIGBT on small radiator (with fan, but not powered yet), SG3525 board (from Aliexpress) as VCO/Driver, ETD44 transformer and  200us pulse generator in aluminium case to suppress EMI noises from coil/sparks. That's all. The only tuning (VCO frequency equal secondary tesla coil resonance) is on SG's board.  It may be done without oscilloscope, just to produce largest sparks.

Of course it should be auto-tuning feature because sparks changes resonance frequency, but let me work on it.


"A base fed coil" - thanks for the hint!  I can't find this idea previously, but it seems that "direct drive" idea is nothing new

What is the bulged out part at the bottom of the secondary coil where the wire goes into?

It is just PVC  tube  and dedicated PVC cap,  nothing inside.  I have short PVC tube, so I need to wind coil on this bulged part. looks  veired, but do not impact coil performace.

Regarding the RLC or 1/4-wave theory, did you remember to take arc loading into account?

Uspring once wrote: "I'm aware of 2 streamer models, one implemented in Terry Fritzs scantesla and one by Steve Conner, the hungry streamer model. AFAIK scantesla assumes a 220k resistance in series with a 1.5pF capacitance per foot streamer length. Conners model assumes a streamer capacitance of 25pF per m streamer length and a resistance, which is equal to the capacitive reactance. The results I got seem to match the hungry streamer model much better. I'd be interested in the logic behind that model." http://4hv.org/e107_plugins/forum/forum_viewtopic.php?137883

I need to read this article.

[oneKone]

this is simply amazing! when i first read it i thought surely its not base fed, as i've only ever seen base fed vttc.  i'm looking forward to seeing more of your future projects. including this one!

[Uspring]

You can find more about this kind of driving TCs when you search for "Tesla magnifiers". Actually an old technique.

[Zbig]

You can find more about this kind of driving TCs when you search for "Tesla magnifiers". Actually an old technique.

Thanks for pointing it. It is exactly what I've done, but with ferrite transformer rather than coreless (open-air) transformer. It makes design more compact, but requires some care when winding transformer, due to high voltage on secondary, and risk of insulation break.  Also number of turns (wire total length) required in coreless transformer is far higher, unless you have RC resonance circuit on its primary  winding (but it seems to be very similar to classic double resonance configuration with all its drawbacks and tunings).

I think ETD44 (of course with air gap) working on 200kHz can handle 2-3 kW, but it may be difficult to get much higher secondary voltage, due to limitted space for windings and insulation between winding layers.  I need to try bigger core, maybe UU shape for better  high voltage winding insulation.
 



   
 

[Hydron]

Would any of the other high frequency transformers commonly used for high voltage be suitable as a step-up for a base-fed coil? I'm thinking of AC flyback transformers, or even those oil-filled X-ray ones.

[Zbig]

Would any of the other high frequency transformers commonly used for high voltage be suitable as a step-up for a base-fed coil? I'm thinking of AC flyback transformers, or even those oil-filled X-ray ones.

Of course any (true) high frequency transformer can  be used, but flyback transformers (e.g. from old TV sets) are not good. Original secondary winding cannot operate at high frequency, and its ferrite core is too small (saturates and becomes hot in my experiments). 
I do not know X-rays transformers and can't tell if they will be suitable.

In my opinion to get good results we need ferrite transformer working in forward mode with symmetrical supply (half/full bridge  or  symmetric primary windings with 2 transistors) to get maximum power from core, and good (close to sinusoidal) symmtrical output voltage shape. 
 

[T3sl4co1l]

Yeah, it's a 1/4 wave transformer, but the impedance at the feedpoint (bottom) is pretty high already, and the impedance at the top, even higher.  So it takes a lot to get it going in the first place.  Upside: VTTCs are usually close (optimal plate load ~kohms), hence why you see a lot of those.

And, impedance being what it is, you can get a lower impedance, and higher transformation ratio, with a lower characteristic impedance (less inductance, more capacitance), and more Q, respectively.  Of course, a Q over 100 is a rather challenging prospect at low frequencies, and a Q over 2000 is nearly impossible (Litz won't take you more than that -- you might need heroic efforts to achieve more, like using type 1 superconductors).

So, the [conventional] transformer helps a lot.

It follows that any transmission line structure that exhibits voltage gain and impedance step-up, can be used here, and therefore counts as a Tesla coil.  You might construct a transmission line with a relatively low impedance (say 50 ohms) to match a low impedance switcher (say 10 ohms) to a higher impedance (250 ohms), and so on.  Downside being, doing this at even 10MHz requires quite a lot of transmission line, so the losses are large, even at modest Q's like this (Q >> 5).

There wouldn't seem to be a good solution -- aside from the humble [wideband] transformer -- that is practical at low frequencies.  For example, "pulling" a 1/4 wave resonator (whether a straight line or a helical resonator), by loading it with capacitors, increases losses disproportionately.

Tim

[Zbig]

I agree with you.   My coil impedance is approx 1.5 kiloOhm, and vacuum tube will drive it with ease.
Transformer may be a problem with high power.  There may be another problem - parasitic serial resonanace between TC and transformer (68kHz in my case).   
I've made some tests with primary winding directly on secondary (2+2 turns  it is SSTC configuration), but I can't get close to "direct drive" power (too much windings for such a small supply voltage).

Much better when putting capacitor across winding (2+2 turns and 100nF - it makes dual resonant configuration) - power and efficiency becomes  close to "Direct drive", but still can't beat  DD, no mention that whole electronics must be as close to coil as possible.
Interesting thing is that I get  better results when removing diodes parallel to IGBT (my IGBT has no internal diode) - reisonance amplitude goes up (and now LC tank do not "pump" energy back to supply through these diodes).

I also build driver to get rid od fixed frequency generator.  To avoid potential problem I've added dead-time for IGBT switching, and pulse length limit for each IGBT (there is no need for current limiting). Feedback is from TC "ground" current. Works fine.

[Acid Byte]

My 10.87 mhz coil is also base fed
and also on 48vdc!
never seen a base fed lower frequency though
Nice job

[Teravolt]

nice work Zbig, A circuit with a microwave transformer, a tube and a resonant tank could feed base feed your coil. So essentially a class C amplifier would have low enough impedance and have the voltage and frequency to drive the base. it may become a double resonant system but a parallel tank circuit would be less affected buy the secondary because of its low impedance. like Uspring said magnifiers are old tech. in the 80"s and 90's there were builders who would do this with SGTC's. one of the biggest that I know of is the Golca coil and the work of Ed Windgate.

http://tb3.com/tesla/golka/

[Mads Barnkob]

Thanks for sharing that Golka archive, I actually never heard or read about that system

[Teravolt]

magnifiers have been around since the beginning. the trick is feeding the secondary with a low impedance RF to the base if the secondary to get a sympathetic vibration. hear is a vid of Ed Windgate's magnifier     

it is a spark gap TC but there is no reason this could not could be done with solid state. I think that Steve Ward explored this once but clamed that there was not any benefit. However what Zbig has done bears some exploring and I have some ideas. a parallel tank circuit has a low impedance and high circulating current or a PFN

[Scott Fusare]

Nicely done Zbig!

Years ago, circa ~1990, many of us built out first SSTCs following Duane Bylund's base fed design. I see he has a web page up now reprising that work;

http://www.duanesradios.info/html/tesla_coil_photos.html