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Old 17th Jan 2021, 7:26 pm   #1
Al (astral highway)
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Default Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

After overcoming obstacles of many kinds over a three-year period, I’m delighted to be sharing these photos of my latest build!

Drum-roll please for this small but powerful DRSSTC (Dual Resonant Solid-State Tesla Coil). Main inductor is 50 cm high.
Plasma discharge after initial tuning exceeds this height.

The mechanical build isn’t finished, but the electronics are doing what they’re supposed to.

There's still room for more tuning, too.

Output. First light. 220mm of plasma into free air at 200uS on-time and 200Hz PRF (not a grounded target).


After initial tuning, 340mm of plasma into free air


Max test so far, 530 mm of plasma at 210V on the DC bus. This was intentionally a destructive test of 3mS duration and took out two of the IGBT’s in the H-bridge. They're pulse rated-at only 180A. Such blunt-instruments of evaluation are par-for-the-course for this kind of build. I will post an analysis of this later on in the thread.

It has helped me to evaluate the peak current capacity of the devices, compared to the manufacturer’s ratings, and so to set current protection circuitry with enough headroom for a resilient, stable, final build.

Background

This is a dual-resonant solid-state Tesla coil - DRSSTC. Mine classes as ‘small’ on a range from mini to very large (three phase mains powered). My design operates with a small heatsink and no fan.

The topology is well-established with a lot of helpful research online.

This doesn’t make it any easier to build. It’s by far the most demanding project I have completed to date.

It puts together hundreds of passive components, a couple of handfuls of IC’s, a home-designed gate-transformer, a tricky-to-design H-bridge and a DC bus that won't instantly destroy the switches connected to it.


To make things spicier, I once had access to machine-tools and test equipment in a local HackSpace but it gave up its lease with no notice to me. Lost inside were about 9 months’ work, including the original secondary coil and quite a lot of finished modules.

For a period, I hit a brick wall with inadequate test-equipment. I was getting side-tracked by completely spurious readings on my oscilloscope, which took me on wild detours until I realised and downed tools.

Things changed during lockdown when I acquired a decent 100MHz lab-grade instrument and 10:1 probes. The great thing is that by then I knew how to drive the thing properly.

My initial vision for this thing was totally unrealistic. I built giant current transformers and started building a full-bridge of 600A IGBTs, until I realised I’d never be able to move the thing around.

So I rescaled and the resulting inverter now weighs less than a single IGBT module and heatsink.

The whole rig will be able to fit into a small wheely-case.


It's worth pointing out (to those not familiar) that the timeline to destruction of the power-electronics can take place on a very compressed timeline compared to say, a vintage radio or amplifier.


There's no time for smoking components or fizzing transformers. In just a few RF cycles, it can all go bang.




Basic (non-technical) details in brief
Self-oscillates at 300KHz.
Pulse-frequency modulator outputs a logic-level square-wave output (approx. 30uS to 300uS.) Repetition-rate variable,10-200Hz.

(Among the photos are ‘scope capture of a stingray-shaped modulation envelope. This was the moment when I knew everything had come together.)

I built this interrupter into an old US Army Signals voltmeter case. Pix of the whole build will follow soon.
It is fibre-optically coupled to logic including waveform-shaping and conversion. This board samples feedback from the primary tank circuit in order to self-oscillate. It also provides vital over-current protection and finally beefy gate-drive for an inverter. All of this is contained in a screened case inside the inverter assembly. [/LIST] The inverter is coupled to this module via a gate-drive transformer. It contains an H-bridge of TO-3 IGBTs, powered by a DC bus that has a low-impedance connection directly to the mains via a DC link capacitor. (Safety sidebar - see below)

Output coupled to a primary tank circuit, tuned to resonance with the secondary coil. The tank capacitor is home-built and good for several hundred A at 8kV.

RF current of several hundred A flows in the tank circuit. [/LIST]
  • Coupling (k) is critical. I have designed a simple mech to easily adjust this.
  • A combination of theory, maths, curiosity, patience and test-test-test are vital to get results and to avoid destruction of sensitive and sometimes expensive components.


Why?

In recent years I decided to produce a table-top source of plasma – one of the four fundamental states of matter.

The deeper reason was to be able to photograph and analyse the properties of the plasma it at my leisure, including making Lichtenburg figures for possible exhibition.

I hope it can also delight and entertain others once adequately finished, and in a suitable environment.

A sidebar – safety

*Inverters for DRSSTC's are intentionally connected to low-impedance mains. The DC link capacitor contains a lethal charge.

*During operation, copious ozone is produced.

* The sound produced is like a small helicopter.

All precautions should be taken. Only operate when galvanically isolated via fibre-optic cable, in a sufficiently ventilated room and with ear protection.

It is impossible to operate my design without following a specific start-up protocol. I have written this down as a tick-list after research into operating-theatre and aircraft pilot best-practice.[/FONT]

The inverter also contains an interlock. If the lid is removed, the mains is automatically disconnected. The DC link capacitor also discharges quickly and the power-down protocol speeds this up.
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Old 17th Jan 2021, 7:37 pm   #2
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Default Re: Table-top but powerful DRSSTC - first run and plasma photos

Brilliant ! ( No pun intended )

Please post a video.

I remember a few years ago visiting a side-show at a fair. Professor Voltini he was called, and the tesla thing he had in a cage. It looked like a writhing sci-fi monster when it was running, and sounded like a chainsaw.

Fantastic stuff !
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Old 17th Jan 2021, 7:51 pm   #3
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Default Re: Table-top but powerful DRSSTC - first run and plasma photos

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Brilliant ! ( No pun intended )

Please post a video.

Fantastic stuff !

Thank you!


I'll post a video when I've finished all the mechanical build and resilience tests! Could be a few weeks yet, but I wanted to share this landmark stage as it happened.

I've posted hi-res images in an album in my user profile if you want to get a better sense of the scale of things.

Meantime, here's the inverter not showing interlock or auto-discharge of link capacitor. The H-bridge is under the busbar. I designed it so the top can be flipped back to replace IGBT's in the event of a blow-out.
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Old 17th Jan 2021, 9:19 pm   #4
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Default Re: Table-top but powerful DRSSTC - first run and plasma photos

An epic build sir, I salute your knowledge & sheer determination. I've dabbled in these creations myself many years ago but, at the time, there just wasn't the solid state kit around that could deal with it. If it's of any interest to you I think I've got a couple of 1200V 100A IGBT modules kicking around, yours if you want them & if I can find them.
Well done chap.
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Old 17th Jan 2021, 10:27 pm   #5
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Default Re: Table-top but powerful DRSSTC - first run and plasma photos

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An epic build sir, I salute your knowledge & sheer determination.
Thank you for stopping by and for those generous comments. Glad you like it!
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Old 18th Jan 2021, 12:08 pm   #6
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

Marvellous pictures Al! Congratulations on your success.

I've been following your progress for a couple of years now and it's great to see "first light" even though there's still more work to be done, I've got lots of pictures of me and a friend using partially completed telescopes just to provide a proof of concept!

Looking forward to seeing the video, if you could be wearing a "mad scientist" white labcoat that would be perfect.

"It's Alive! Mwah ha ha..."
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Old 18th Jan 2021, 12:40 pm   #7
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

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Marvellous pictures Al! Congratulations on your success.


Hi Ken, thanks for following the project over a couple of years and for your supportive comments.
It's great to celebrate it here with others on the forum!

My calcs show that I've probably reached a ceiling for spark-length at the existing 500mm or so (20 in. appx) with the TO-3 IGBT's that I'm using in my full-bridge. This meets one of the criteria I selected before I began, so I'm happy with that.

This is to free-air, not a grounded target.
Resilience and reliability are higher priorities, and I'll be operating the rig within its proven limits from now on.

P.S I share your interest in all things astronomical.



Cheers!
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Old 18th Jan 2021, 1:24 pm   #8
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

Quite impressive!

Have you got a cover story prepared for when the neighbourhood, on seeing flickering blue light in your window, come round to accuse you of 'experimenting with 5G'?

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Old 18th Jan 2021, 2:24 pm   #9
Al (astral highway)
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

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Quite impressive!
Thanks David!


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Have you got a cover story prepared for when the neighbourhood...accuse you of 'experimenting with 5G'?
Haha! Or even 6G!?
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Old 18th Jan 2021, 3:57 pm   #10
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

This is way out of my league. Quite astonishing skill, perseverance and not only knowledge, but the ability to apply theory to get to knowledge you haven't got yet. Really impressive and very well done.

Of course, we know Bill Gates is a reincarnation of Tesla and the coil is now coming into its own as a way of capturing the souls stolen in the vaccination programme by the lizard people...etc.
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Old 18th Jan 2021, 4:16 pm   #11
Al (astral highway)
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

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Quite astonishing skill, perseverance and not only knowledge, but the ability to apply theory to get to knowledge you haven't got yet. Really impressive and very well done.
Thank you UncleB - that's generous praise, and I appreciate it greatly

On your second para: ahah, that explains why a large, Chinook-sized drone is suddenly flying overhead and filming the rig! Ducks for cover...
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Old 18th Jan 2021, 4:20 pm   #12
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

We'd better not give the conspiracy theorists any clues towards new material and stick to the tesla coil itself. Just so long as Al has his defences primed in case of crowds of peasants with flaming torches.

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Old 18th Jan 2021, 4:40 pm   #13
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

Fantastic! A very complex project. The last YT video i saw of something similar used an angle grinder with a bespoke 'chopper' attachment as part of the build- that was the easy bit..

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Old 18th Jan 2021, 6:33 pm   #14
Al (astral highway)
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

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Fantastic! A very complex project.

Thanks Dave, I'm glad you like it!

IMO this DRSSTC mode is by far the best, although it is also by far the most complex...

I did start off with a static spark-gap type years ago. The rotary ones are extremely noisy, unless quenched imaginatively and effectively. I've seen giant rigs using them in far-off places with cheap land but the state of the art is definitely DRSSTC these-days.
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Old 19th Jan 2021, 12:35 am   #15
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

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Quite impressive!

Have you got a cover story prepared for when the neighbourhood, on seeing flickering blue light in your window, come round to accuse you of 'experimenting with 5G'?

David
Just tell them that your pet alien must have forgotten to close the blackout curtains before experimenting.
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Old 19th Jan 2021, 12:39 am   #16
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Default Re: Wild sparks from this small but powerful homebrew Tesla coil (DRSSTC).

The mods have been discussing this, and have decided that it's off topic for the forum. While Al may have the appropriate skills for these sorts of experiments, HV electricity is potentially very dangerous, and we have all sorts of people visiting the forum as a result of casual Google searches. Consequently, we have decided to close this thread.
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