Small-ish 3D printed SGTC via cheap ZVS flyback build, humbly asking a couple ?s

[Michelle_]

Looks great.

I'd wonder about making contact to the primary ring for tuning. From the top you have to get
down into that narrow canyon. From the bottom cutting holes...

Not a criticism just curious...

Cheers.

Yeah that's a good question, what I intend to do is basically make a metal P-clamp into a 90 degree connector that mates to an insulated wire using a 3d printed housing (if that makes any sense). I'll post it after I design it. I was going to drill through the bottom once tuning is established then cover the top. Also the grooves are big enough for 1/4" tubing but I was thinking about trying the 1/8" tubing first which will give me more room to make the insulated connector. It didn't seem to make a big difference in javaTC whether it was 1/8" or 1/4" and considering the relatively small amount of power I'm going to use and the skin effect, I am not convinced 1/8" will be too small but I don't know. I'm trying not to screw this up but this is the first tesla coil I've ever built unless you count the slayer exciter.

[alan sailer]

Great. I'm not saying 1/4 inch is wrong here but a few weeks back I buttoned up
a small coil with primary current of ~200amps and 3/32" copper tubing. Worked great.

One disadvantage of having the connection and wire come out from the top makes
secondary strikes to the primary more likely. Not only does it reduce the primary to
topload distance it also introduces sharp edges rhat encourage flashover.

In my limited experience tesla coils are all about screwing up so be prepared...

Cheers.

[Michelle_]

Great. I'm not saying 1/4 inch is wrong here but a few weeks back I buttoned up
a small coil with primary current of ~200amps and 3/32" copper tubing. Worked great.

One disadvantage of having the connection and wire come out from the top makes
secondary strikes to the primary more likely. Not only does it reduce the primary to
topload distance it also introduces sharp edges rhat encourage flashover.

In my limited experience tesla coils are all about screwing up so be prepared...

Cheers.

I wonder if I can make the adjustable connection an insulated wire that runs on top of the primary and stays inside the groove but then keeps following the spiral to the outside then goes down through an exit hole at the end similar to how the beginning of the primary is run through a hole at the start of the spiral. Then I can always have the top covered but still open it to adjust it. The connector will be trickier to make but it should be possible with the 1/8 tube.

[Benjamin Lockhart]

With small spark gap coils the primary losses are so much lower due to the higher voltage compared with DRSSTCs that even 14 awg wire would be fine. I used just 10 AWG wire on my dual mot sgtc making 5 foot arcs and the primary didn't even get warm.

Also you don't want the primary tap wire coming in from the top or the sparks will likely hit it. A smaller outer diameter primary is good for the same reason. I would think it's a lot easier to just make 6 or 8 individual upright primary supports with slots and tap from below with an alligator clip. For a low powered SGTC (like under 1kW) that's just fine.

[Michelle_]

With small spark gap coils the primary losses are so much lower due to the higher voltage compared with DRSSTCs that even 14 awg wire would be fine. I used just 10 AWG wire on my dual mot sgtc making 5 foot arcs and the primary didn't even get warm.

Also you don't want the primary tap wire coming in from the top or the sparks will likely hit it. A smaller outer diameter primary is good for the same reason. I would think it's a lot easier to just make 6 or 8 individual upright primary supports with slots and tap from below with an alligator clip. For a low powered SGTC (like under 1kW) that's just fine.

Thanks for the feedback. I'll go with the 1/8" for now. The reason I made the primary holder this way is because it reflects the dimensions of what I modeled in javaTC, which probably could have been different. I agree that it seems too big lol, I guess we'll see what happens because I already printed it. I was worried about arcs between the primary windings if they were spaced too close, is this even an issue?

Instead of having a conductive tap coming in from the top I'm going to run the tap inside the channel then outwards along the spiral and down with an insulated wire, and cover the top with a PETG cover that mates into the spirals, as well as possibly add a strike ring, in addition to a vertical breakout point in the middle of the toroid, so hopefully I won't get strikes to the primary.

At this point I'm more worried that I picked bad capacitors but if they light on fire I'll try to find cornell dubilier capacitors.

[Benjamin Lockhart]

Arcing between primary turns would only be an issue at the tap point where there's less clearance.  You can think of the full supply voltage being across the primary coil and it's divided between the number of turns.

[Michelle_]

Arcing between primary turns would only be an issue at the tap point where there's less clearance.  You can think of the full supply voltage being across the primary coil and it's divided between the number of turns.

I see that would have been good to know! I thought the 24kv could have arced between any point. Oops. Should have spaced the coil closer together I guess.

I also wonder would tapping the primary then running the tapped wire, even if insulated, along the rest of the spiral, just make a longer primary and defeat the purpose?

[Benjamin Lockhart]

If the tap wire runs flat right across the primary coil from the outside to inside, the insulation might not survive the 24kV. The best insulation for high voltage is physical distance.

[MRMILSTAR]

Instead of holes I cut eight slots under the primary coil. This allows a wide latitude for moving the tap point around under the primary coil. I have attached a picture.

[Michelle_]

Instead of holes I cut eight slots under the primary coil. This allows a wide latitude for moving the tap point around under the primary coil. I have attached a picture.

That seems like a good idea, nice construction.

[Michelle_]

What are everyone's opinions on RF ground for this thing? I'm on an upper floor apartment now (I really miss living in a house)

My options seem to be either connecting it to mains ground, or, I have a big ol oil drip pan (3x5 foot sheet metal) I can use for a counterpoise. I'm just wondering about electrical potential on the counterpoise, I was thinking of setting it on a rubber mat so it's at least not directly on the carpet, or I could set it on something to raise it up higher.

This is probably a stupid question but what if I use a counterpoise under the coil but also connect the counterpoise to mains ground?

It seems like a lot of "small" tesla coils I see people build online are either hooked to mains ground or seemingly nothing? At least some of the more popular youtubers are doing that. It kinda seems like some of them may or may not REALLY know what they're doing though?

[MRMILSTAR]

For a small coil inside an apartment I would use a counterpoise. I would avoid connecting it to the house ground. The quality of that ground is unknown. You could end up destroying electronic equipment that is plugged in.

[davekni]

This is probably a stupid question but what if I use a counterpoise under the coil but also connect the counterpoise to mains ground?

Not at all a stupid question!  Opinions vary widely.  I always use a counterpoise under my coils and always connect them to mains ground too.

[Michelle_]

This is probably a stupid question but what if I use a counterpoise under the coil but also connect the counterpoise to mains ground?

Not at all a stupid question!  Opinions vary widely.  I always use a counterpoise under my coils and always connect them to mains ground too.

Thanks for answering all my questions (and thanks MRMILSTAR too).

Would that mean the coil mainly uses the counterpoise and the mains ground is just extra security, or will it transmit interference to the mains no matter what even if the capacitance of the counterpoise is significantly higher than the toroid?

Side update: I would the "perfect" secondary I was talking about and it's almost perfect, and has a much thicker coat of epoxy and more even windings. I need to check the resonant frequency but it should be close. I also uh... put glitter in the epoxy (the glitter isn't metallic so I think it should be fine) - I'm sure it's not to everyone's taste but I wanted to see what would happen. I also finished the MMC and am currently printing the spark gap/fan holder assembly.

Who knew making a tesla coil was so much work!

[davekni]

Would that mean the coil mainly uses the counterpoise and the mains ground is just extra security, or will it transmit interference to the mains no matter what even if the capacitance of the counterpoise is significantly higher than the toroid?

The larger the counterpoise is the less anything else to do with grounding matters.  Modeling the electric field around a Tesla coil including all the building wiring and plumbing and construction materials is too complex to be tractable.  And the location of wiring and plumbing is rarely known either.  That's why there are varied simplified views and opinions.
My goal for inside use is to think of the building more as a low-quality Faraday cage with random wiring and plumbing all around, all "grounded" for the purposes of TC frequencies.  Of course, wire inductance makes the cage far from perfect.  But most of that inductance is common-mode between hot and neutral and safety-ground.  There is very little differential noise introduced.
With any option, your small coil is extremely unlikely to damage any electronics, at least any devices more than a meter or two from coil.  At worst it interferes with radio communication.

[Michelle_]

Would that mean the coil mainly uses the counterpoise and the mains ground is just extra security, or will it transmit interference to the mains no matter what even if the capacitance of the counterpoise is significantly higher than the toroid?

The larger the counterpoise is the less anything else to do with grounding matters.  Modeling the electric field around a Tesla coil including all the building wiring and plumbing and construction materials is too complex to be tractable.  And the location of wiring and plumbing is rarely known either.  That's why there are varied simplified views and opinions.
My goal for inside use is to think of the building more as a low-quality Faraday cage with random wiring and plumbing all around, all "grounded" for the purposes of TC frequencies.  Of course, wire inductance makes the cage far from perfect.  But most of that inductance is common-mode between hot and neutral and safety-ground.  There is very little differential noise introduced.
With any option, your small coil is extremely unlikely to damage any electronics, at least any devices more than a meter or two from coil.  At worst it interferes with radio communication.

Thank you for the explanation, that makes sense. I'll go with the counterpoise and probably also ground it to mains just in case. That makes me feel better that it should be OK to use this inside, that was my hope.

Also I just checked the new secondary and it resonates at 438KHZ versus 472KHZ on the other one, which is still within the initial javaTC estimate and should only be 1/2 turn difference in the primary tapping point interestingly. The new one is 120 ohms vs 107. They both use the same gauge wire and almost the same size form, but I think the natural wire has a thinner layer of coating.

[alan sailer]

Glitter sounds fun. It will be interesting to see what happens.

In general, as far as I understand, any dielectric discontinuity ie air to epoxy (definitely) epoxy to glitter (possibly)
will cause changes in the electric field. Also in general, with high voltage fields you want to avoid discontinuities.

The glitter would also tend to trap tiny air bubbles which will definitely cause electric field issues. Whether this issues will
cause a problem remains to be seen.

Cheers.

[Michelle_]

Glitter sounds fun. It will be interesting to see what happens.

In general, as far as I understand, any dielectric discontinuity ie air to epoxy (definitely) epoxy to glitter (possibly)
will cause changes in the electric field. Also in general, with high voltage fields you want to avoid discontinuities.

The glitter would also tend to trap tiny air bubbles which will definitely cause electric field issues. Whether this issues will
cause a problem remains to be seen.

Cheers.

We'll definitely find out. If there are air bubbles I can't see them, as far as I can tell they're [the glitter] small and encased in the epoxy. At any rate now you've seen something nobody has done before so there's that.

Here's an update on my spark gap. I am aware of how ridiculous it is but I had these fans sitting around. I also screwed up the spacing to mount them. They are held in with nylon screws and the spark gaps are about twice as close to each other as any metal in the fans so I'm hoping for the best. If I have a catastrophic fan meltdown I will take responsibility. Also the PETG melts at 260c (compared to PVC closer to 200c) so the mounting brackets might melt, I might have to redo some or all of the spark gap setup. My title  of "high voltage experimenter" is apt. I will run these in short bursts then monitor the temperature with the thermal camera by the way.

With any luck I might be able to try powering this on tomorrow.

Do people use fuses or circuit protection in these things?

[Michelle_]

I should have done more research because now I know my MMC has inadequate dV/dT. Pretty disappointed that I overlooked this and added capacitors to my last digikey order without doing all my homework. Not sure if I should even try to use them, certainly not at 24V. The combination of 24KV input voltage and 400+KHZ makes the dV/dT requirement crazy high.  :'( (50KV/uS!!!!). For my secondary at 12V it's still 25KV/uS... The capacitors I have are 750v/uS*10 so only 7500v/uS total.

I guess I can just run it at a way lower voltage for now. I guess I could add my spares to my series and tap the main coil farther out since I gave it a few extra windings for tuning?

Like what would it even take to make an MMC for this? ($1000 worth of Cornell Dubilier capacitors?)

How the hell does this one even work for example: It should require 150KV/uS unless I'm mistaken and I only see 4 capacitors, those can't possibly be rated at 37KV/uS!

https://highvoltageforum.net/index.php?topic=2442.0

[MRMILSTAR]

The most expensive component of any SGTC is the HV capacitor bank. There really is no cheap way around it unless you resort to beer bottle capacitors but those are very lossy and frequently fail.