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Can air cored inductors be influenced by proximity to other conductive things such as heatsinks even if nothing actually enters the inside of the windings? I'm talking about simple ones such as this:

Yes, but much less so than similar ferrite-core inductors.  With air-core, most of the magnetic reluctance is where field is concentrated through core center.  With lots of area for field to return outside of the coil, blocking some of that return path has only a tiny effect.  Unless you block much of it, such as if you milled a slot in a heatsink and placed it over the coil blocking most field return paths.

Thanks Dave, here's some photos of it. Excuse the messy amateur DIY construction.

It would be very dependent on the aspect ratio of the coil - long skinny vs wide flat.  This is how basic metal detectors work, after all, by using a wide flat coil.

It's a single layer coil measuring 32.5mm x 15mm and about 26 turns of litz wire (was aiming for a couple uH). It's 12mm at its closest point from the grounded heatsink. Flying blue wire is circuit ground.

Turns out I've been running it in forward mode by having the primary coil connections reversed, are the calculations still the same for forward vs flyback?

Output voltage in forward mode is lower but the arc is thicker and current draw is about 500mA higher from the 24v source with all other parameters being the same.

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Thanks, how would I convert that number into minimum turns?

60%/40kHz = 15us maximum on-time.  15us * 24V = 360uVs.  360uVs / 38.4uVs/turn = 9.4 turns.  Or, in other words, your 8 turn coil is generating peak flux a bit higher than 0.3T.  Specifically:  360uVs / 8turns = 45uVs/turn.  45uVs/turn / 128mm^2 = 0.35T.  Probably fine for your core.  If the core is designed for flyback use, it likely has high saturation flux, so perhaps 0.5T at room temperature dropping to 0.4T by 100C.

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Drain voltage waveform from a simulation Dave uploaded in 2020.

That simulation has some different part values.  Most important, it has no secondary winding with its coupling factor and parasitic capacitance and arc load.

Thanks! That's much easier to understand after seeing a working out example. BTW the 60% is just the max the PWM can swing to, it's default "center" is set to 30% and the audio signal PWMs the duty cycle between close to zero and 60% to vary the arc power and make sound.

Doesn't sound too bad and can get louder than my phone without distorting/clipping with the right kind of music (80's stuff seems to fit that criteria best). Actually I recently found out the background hissing sound comes from the bluetooth to aux receiver I'm using, the arc is silent on its own or if I use an old phone with a direct 3.5mm to 3.5mm aux cable.

I wonder if some kind of feedback from the output could help cancel out the non linearities of the TV flyback and make the sound even better.

Sounds like you are running a non-resonant flyback (no capacitor across primary).  Will be more efficient if using existing primary winding under secondary since leakage inductance will be lower.  However existing primary likely has more turns, designed for ~200Vdc supply and ~1500V switch transistor, with correspondingly lower current.  For ZVS-driven flybacks, some leakage inductance is helpful.  That's an ideal case for winding a new primary on other ferrite leg.

No simple rule-of-thumb I'm aware of.  For a lower limit on turn count, calculate maximum volt-seconds/turn to stay under maximum flux density.  0.3T is probably OK for most ferrite materials.  Some can go to 0.4T (or even 0.5T if kept close to room temperature).  Core cross-sectional area in meters^2 * flux (0.3T) gives volt-seconds/turn.  Max_on_time * 24V / turn_count needs to be under that limit.  This calculation avoids saturation, but doesn't guarantee reasonable ferrite heating due to AC losses.

Thanks Dave. Core measurements are as follows plus the small 0.3mm core gap if that counts, the legs to the left which run down through the center of the flyback are rounded rather than square however.

I'm having trouble calculating the volt-seconds/turn from these numbers. The core and flyback do warm up, after 15 minutes of audio modulation it was like a little hand warmer.

It is indeed running in single ended flyback mode, output stage is the same as this except with a more modern fet and without the current sensing part and a smaller 33nF drain capacitor since the frequency is higher at 40khz (peak drain voltage is well under the fets maximum rating).

Hi, is there a rule of thumb for selecting the minimum number of primary turns when driving these things using external primary windings on the exposed part of the core? For example in flyback topology to produce arcs what would be a good volt/turn rule of thumb to use for extended run times to reduce losses and saturation etc.

Also does switching frequency and max on time/off time ratio count with selecting turns? Right now I'm using 40khz and a max on time of 60% (audio modulated PWM limit) with 24v and 8 turns, arcs get bigger and power consumption increases if I reduce turns but obviously I assume there would be problems going down to say 3 turns. The driver currently doesn't have any peak current control so I'm being careful about keeping the arc continuous and steady rather than pulling them until they extinguish or letting the flyback run unloaded.

Hi, been away for while but back messing with electronics again 

Has anyone tried those bluetooth to aux adaptors to get an audio signal into their high voltage projects? These are the things I'm talking about.

I'm finding they have a lot of background hiss which makes the plasma speaker sound worse than it actually is (I've isolated it to the bluetooth adaptor).

Problem is these days phones all have touch screens which makes them behave weird when directly connected to such RF heavy high voltage projects so isolation is needed, old dumb phones with buttons behave when connected directly using an aux cable but then you can't use youtube videos for audio content on the fly with such old devices.

Only thing that helped was to heavily passive RC low pass the input at the plasma speakers aux jack (hiss from the bluetooth is still there but nowhere near as noticable in the arc), but the corner cutoff frequency is right down around 30khz so it cuts into the audio range too.

Is there some elaborate filter circuit I could use or maybe even some chokes to filter out this bluetooth induced hiss? Or maybe someone has found a bluetooth to aux adaptor which has clear hiss free audio.

Please share thoughts and ideas here.

Weird, looks like core saturation in one direction. Are you really using 22v TVS diodes or zeners? Can you add a pull down resistor of about 10k between input of the gate drive IC's and ground, maybe it's not being shut off correctly.

Show pic of your circuit.

Quite a punch there! 

Since I'm a fanatic for minimizing parasitic inductance, here's an over-kill low inductance H-Bridge using two CM600DY-13T bricks.  This would be more practical using an ECB rather than copper foil.  What I built makes any future repair difficult.

This is the H-Bridge before VBus planes.  VBus+ leads are all towards the center.


VBus+ plane added, soldered to capacitor + terminals and temporarily bolted to brick + terminals:


VBus+ plane covered with polyimide insulating layer:


VBus- plane added on top and soldered to capacitor - terminals:


Cables added, including current transformer on one H-Bridge output.  Note that parasitic inductance on the H-Bridge outputs is not critical.  No need for planes on the output.  Cables are fine.  Cables are fine for power input too as long as there is enough local capacitance on the parallel planes to handle high-frequency currents.


Used this bridge for my low-frequency QCW experiment:
https://highvoltageforum.net/index.php?topic=1268.msg11784#msg11784

Wow that's really cool Dave! Do you need special tools to make it using copper foil or will scissors work?

I suspected his scope might be mistakenly adding some offset compared to the center of the screen, it's the asymmetrical ringing amplitude what's got me puzzled.

I second what Dave said about where to put the probes too, put it right at the pins of the MOSFET gate to source and use the short ground spring rather than the long ground wire.