speakerguy79

I think I might have figured out why he's calling it FM.

When I was working with HV at my last job, I pulled apart an electronic NST. I ripped out the output transformer. It had a primary wound with magnet wire, a continuous ceramic core, and a potted secondary. It was a resonant transformer, which I thought was odd. When driven at 30kHz it would massively step up the voltage on the secondary to 10kVpk-pk. If you shifted down to 25kHz or up to 35kHz, it would drop to <1kVpk-pk.

If you were to take an audio input and use it to modulate the frequency of a sine wave so that it fluctuated between 25 and 30kHz, you could achieve an amplitude modulation of the spark voltage on the secondary. The linearity would depend on the linearity of the transfer function of output voltage as a function of frequency, but there might be ways to avoid or compensate for that.

Does that sound like a possibility AG? I'm tempted to go whip one up. I've been meaning to do a plasma speaker for a while, and this I could definitely whip something like this up. I've got a Tektronix rep swinging by next week to show me their new DPO3000 scope and I could ask for a quote on one of their HV probes.

audioguru

A super-regen "radio" has an AM detector. It uses "slope-detection" like you did with the resonating high voltage coil to detect FM when it is tuned to one side of an FM station.

Another guy on another site and I don't think a car's ignition coil will work at an ultrasonic frequency.
A V8 engine running at 6000RPM produces a frequency in the coil of only 800Hz.
Some cars use an ignition coil on each sparkplug so it can fire at a lower frequency.

The flyback transformer from a TV might barely reach 20kHz.

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speakerguy79

I've worked with ignition coils before. I could get a specialized drag racing coil to do 10kHz but the primary wasn't being fully charged. It was $250 for that single ignition coil, so I doubt there's much better out there. The more common ignition coil I had on hand was limited to a few kHz.

audioguru

One of the videos you linked to has an 8kHz carrier frequency that is audible.

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Hero999

Yes, the same technique is used in quasi resonany converters, except zero voltage switching is used to reduce the losses.

Could it have been a quasi resonant converter?

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speakerguy79

Maybe, I dunno. I'll be buying another one and ripping it apart.

It was a dimming electronic NST, and operated at 20kHz for the bright and 40kHz for the dim. Splitting the difference, I guess. I forget the drive circuitry but IIRC it had two FETs and I think the low voltage secondary wires were for a feedback type thing. I'll but another and rip it open. I just ordered some 320W 24V and 12V switching supplies from Meanwell for this project, time to get some flybacks and ignition coils (though I think I will have much more luck with the flyback types).

FWIW I observed the behavior of the transformer as described above just driving it with the frequency generator directly. Its output voltage sagged a lot when I hit resonance even though the secondary voltage went way high. I don't know how it would behave with a low Z high current capable source.

speakerguy79

Q: how could the transformer alone exhibit resonance like that? Could the inter-winding capacitance have been big enough to make it behave like that?

Hero999

I would have thought that they added a capacitor but it might be tacked on to the transformer in this case.

Also a transformer will have two resonances: the primary and secondary.

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Someone Electro

Yeah ignition coils do resonate at somehere from 5 to 10 Khz.And they have a pretty small output when not in resonance.They should work well FM modulated but there is going to be a very loud sqealing sound from it(The carier)

So i would recomend useing a flyback they normaly resonate at about 20 to 30 Khz.I guess even better would be a air core transformer since that would resonate at 100s of Khz.Thats basicly a tesla coil.You can also thurn a ignition coil in to a air core transformer by takeing it apart and removeing the iron core.Note that this is a very messy job since ignition coils are filled with oil for insulation.

Oh and recently i built a IGBT high freqency driver.And tested it out on a ignition coil.It made white hot and long arcs....for 5 seconds and then the electrical tape(my temoprary fix for arcovers) arced over and made a pretty big fire.I should have not thrusted electrical tape in the first place.Now im solving the problem with a garden hose and lacquer.Videos of it on youtube if anyone wants to see il give a link

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darkfury18

Wish I saw your post before I built mine. I got a nice (noisy) arc using an ignition coil, but the arc always died when I pushed the frequency over 1.5kHz.

As for frequency modulation, I used a 555 timer in astable mode and then fed the audio signal into pin 5 to modulate the frequency. It looks fairly clean on the oscilloscope, but I haven't been able to test it since I never got a usable arc.

Any recommendations for a cheap flyback transformer that I can get on ebay? (link?) Thanks in advance. Otherwise I might try taking apart that ignition coil and removing the core.

Someone Electro

You can just get the cheapest flyback.

There is also a nother way, you can wind your own high freqency transformer.Since they dont realy need a lot of thurns. like 5 thurn primary and a 500 thurn secundary will work.The trick is to wind i on a plastic tube but you have to wind it from one side slowly to the other, NOT going from one to the other side because it will arc over.Then you can paint it with varnish or like some clear laqour for wood to further keep from arcovers.Stick a ferite core inside with a small thick primary on it and your done.

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darkfury18

Winding my own coil doesn't sound too fun. I'll try to grab one from ebay first and see how it goes. Thanks for the help.

Mifffo

actually i found a site with a working plasma speaker and complete schematic with instructions to wind the proper coil for it aswell.

http://www.hvlabs.com/plasmasonic.html

i just built it using a TV flyback as coil, and it does work and produces clear audio without distortion. only problem is the TV flyback coil must be of the wrong windings to work with the secondary windings he suggests at his site, so my plasma speaker would love to use moar then 11 amps at 24 volts but the powersupply i use will only output 11 amps at 30 volts on 2 terminals so i suppose if i parallell couple them id get 30 volts and 22 amps. but i have too small heatsinks on the mosfets so they will burn for sure

so i think i will increase the number of windings on the secondary of the flyback coil from 8 windings to 16, which should lower the current used and increase the voltage? and put some larger heatsinks on the mosfets

the proper HV coil for this project is a total of 6400 windings on a ferrite core custom built by 2 separate ferrite cores from tv flybacks :O

but anyway this speaker design works

audioguru

Boy oh boy it downloads slowly. I have waited for about 15 minutes for it to download. Ho hum.

Guess what? It sounds horrible!
The plazma tweeter produces 5kHz to who knows how high, then the sub-woofer produces from about 200Hz down to who knows what. All the frequencies between 200Hz and 5kHz are missing.

Boom boom and tweet tweet. It ain't musical.

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darkfury18

Thanks for the link.. guess I'll give that one a shot