Author Topic: BrOdin coil - High power Big sparks!  (Read 17118 times)

Offline Mads Barnkob

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Re: Large coil experimenting with long on-times.
« Reply #20 on: February 21, 2021, 12:24:41 PM »
Such an awesome coil!, that is quite amazing that you can run it at such long on-times. I am wondering if it really is needed to grow long sparks, your longer on-time just makes them more fat?

High impedance coils usually also like long on-time, so if you experiment with 1uF+ MMC, you are getting into the territory of Kizmox / Dalus like coils, which operate with very low impedance, high peak currents and short on-times.

Regarding the power meter and its measurements. The PF does seem high. It "should" be 0.5-0.6 without any kind of PFC. With a variac in front it could climb to 0.7'ish from the added inductance. Maybe I am just used to being too close to a transformer station... The mains destribution network in Denmark relies on much bigger transformers, we do not have polepigs. Could it be that the power meter is not suitable for high frequency current peaks?

When I built my PM700 power meter box, I made sure to find a meter and current transformers that was good up to 10 kHz harmonics.
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Offline fh89

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Re: Large coil experimenting with long on-times.
« Reply #21 on: February 21, 2021, 07:18:52 PM »
Quote
Such an awesome coil!, that is quite amazing that you can run it at such long on-times. I am wondering if it really is needed to grow long sparks, your longer on-time just makes them more fat?
Thanks, I appreciate it! I'm not sure how much longer the sparks are with the longer on-times because they seem to be going straight down for the most part. As I mentioned in an earlier post, I have a few things to try to fix this.

I also have another interrupter in the works that will be able to run much longer on-times, and I'll see if I can get some extra spark length out of longer pulses at lower PRFs.

Quote
High impedance coils usually also like long on-time, so if you experiment with 1uF+ MMC, you are getting into the territory of Kizmox / Dalus like coils, which operate with very low impedance, high peak currents and short on-times.

Wait, I'm confused here- so if I want to design an MMC optimized for long on-times, I want a larger capacitance value?

Quote
Regarding the power meter and its measurements. The PF does seem high. It "should" be 0.5-0.6 without any kind of PFC. With a variac in front it could climb to 0.7'ish from the added inductance. Maybe I am just used to being too close to a transformer station... The mains destribution network in Denmark relies on much bigger transformers, we do not have polepigs.

Hmm. There is only maybe 150' of wire total between my coil and the distribution xfrmr, you can see it right behind my yard in this pic:
 [ You are not allowed to view attachments ]

I have no idea how to calculate how much inductance this adds to the circuit.

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Could it be that the power meter is not suitable for high frequency current peaks?

This very well could be true, it was a vey inexpensive ebay meter- like $25 USD. I was unaware that the PF reading could be inaccurate with HF current peaks.

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When I built my PM700 power meter box, I made sure to find a meter and current transformers that was good up to 10 kHz harmonics.
That is a much more sophisticated device, I have no doubt it gives more accurate PF reading.


**side note** [ You are not allowed to view attachments ]  Why does this keep happening? It seems to work sometimes and not others.
« Last Edit: February 21, 2021, 07:22:21 PM by fh89 »

Offline davekni

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Re: Large coil experimenting with long on-times.
« Reply #22 on: February 21, 2021, 10:19:33 PM »
Agreed that 75% PF sounds high, but I think it is likely real.  Line resistance is the key contributor here.  Notice that RMS voltage drops from 241V at low load to 220V at high load.  That fits my spice simulation with 0.17ohms.  With just that resistance, power factor is 72.7%.  With 100uH added, power factor is 75% as measured.  (I've updated simulation parameters a bit based on the most recent video with more meter data.)  Both 0.17ohms and 100uH seem plausible for the pictured power connection.

At light load, power factor is lower as would be expected.  Power factor matters only at high load, since that is what limits total power obtainable from a given circuit.  Series resistance can increase total available power even though it is wasting power.  That is because (up to a point) resistance reduces RMS current more than it reduces RMS voltage.
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Offline fh89

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Re: Large coil experimenting with long on-times.
« Reply #23 on: February 22, 2021, 06:32:34 AM »
**Update** Longer sparks and explosions?

Raised toroid, used shorter breakout point, and adjusted tuning resulting in improved performance and dramatic explosions!

This ground strike is 14' linear from breakout point to ladder.



Explosion!



This was one of the steel fasteners becoming superheated and melting/exploding from the high current. No damage to the coil/electronics.



There was one other fastener with some melting:



I can't wrap my head around why this one fastener in particular took the brunt of the heating and all the rest were fine. All of these fasteners were very tight, and the current path was not directly through this area. The arc hit the strike rail on the left side of the below photo. The melted fastener is the near corner. The strike ring is grounded to the frame on the right side primary stand-off, behind the near corner. The frame is connected to RF ground in the rear of this photo.



I would expect the current path to be like this:



Here's video of the exploding fastener:


It's really interesting that after the initial strike rail hit and subsequent shutdown of the coil, the fastener remained hot enough to instantly melt/explode again through inductive heating alone
upon firing the coil back up.


Here's some video of the improved spark length. The shorter breakout point does seem to help push the sparks out instead of down. Still think the coil could perform better higher off the ground. The interesting thing is that I had to change the tuning to get maximum spark length at this higher pulse width. Does this make sense? Is it expected to require a different tap point with longer on-times? Maximum spark length at 360us was 6.25 turns; tap point for 1.12ms performed best at 7.5 turns (the full primary on this coil, maybe it would be even better with more turns?)


In this video, I turn down the PRF and the output becomes very sputtery for lack of a better word, almost like it's out of tune. I then turn down the PW and the output almost disappears even at like 300us, like it's way out of tune
« Last Edit: September 11, 2021, 09:09:18 AM by fh89 »

Offline Hydron

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Re: Large coil experimenting with long on-times.
« Reply #24 on: February 22, 2021, 11:19:03 AM »
Wow, great videos and the fireworks from the bolt are pretty spectacular given that nothing important went pop.

In the second video I see what looks to be an arc from something internal (not the bolt that sparked earlier in the clip) - do you know what that was?

Finally tuning the primary lower for longer on-time makes perfect sense - as the arc grows with on-time it de-tunes the secondary, pulling it's resonant frequency lower. When the primary is tuned lower this improves the match and energy transfer between the two.
With lower PRF you're putting less power into the arc, therefore it isn't as long and the secondary resonance frequency remains higher. I have especially noticed this back when I had my bigger coil tuned for low-Z bus operation with high primary detuning - streamer length was pretty poor until you got the power to a certain level (via increasing bus voltage/pulsewidth/PRF), and then it really took off as the detuning came into play. Gave impressive and chaotic streamers, but wasn't so good for MIDI mode, where you want the streamer growth to be more linear with increasing pulsewidth/PRF.

Offline Mads Barnkob

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Re: Large coil experimenting with long on-times.
« Reply #25 on: February 22, 2021, 12:21:15 PM »
What a great capture of that failure! When people ask for advise, the one I give the first is to always have a camera rolling!

I think I know what happens before and during the continues "explosion".

The aluminium profiles are most likely anodized and that is actually an electrical insulation. So your high voltage arc knocks through the anodized layers and forms a nice current path for the upper part of the frame, it is now seen by the primary coil as a single turn winding underneath it and it starts to pull some serious power and thus is why you see steel bolts melting instantly.

The flashover between bolts and plate would be in the place with the worst connection, just like a bad connection on a mains feeder will heat up from the increased resistance, its just the point of failure and it has really nothing to do with the sparks anymore, it was just the initiator of the underlying problem (single turn metal construction too near the primary coil)
« Last Edit: February 22, 2021, 12:23:46 PM by Mads Barnkob »
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Offline Hydron

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Re: Large coil experimenting with long on-times.
« Reply #26 on: February 22, 2021, 02:56:02 PM »
Makes me thing about what could be done deliberately to add some hot metal & sparks to the show of a high power DRSSTC... maybe a single turn coil with a high-resistance part designed to emit the fireworks. Would need to be careful about where said hot metal flies though.

Offline fh89

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Re: Large coil experimenting with long on-times.
« Reply #27 on: February 22, 2021, 05:58:01 PM »
Quote
In the second video I see what looks to be an arc from something internal (not the bolt that sparked earlier in the clip) - do you know what that was?
It was from somewhere in this area. There is some carbon tracking on the primary lead coming from the MMC. I think it was probably from there to the current transformer that was left floating.



Quote
Finally tuning the primary lower for longer on-time makes perfect sense - as the arc grows with on-time it de-tunes the secondary, pulling it's resonant frequency lower. When the primary is tuned lower this improves the match and energy transfer between the two.
With lower PRF you're putting less power into the arc, therefore it isn't as long and the secondary resonance frequency remains higher.

This is extremely interesting. I wouldn't have expected this effect to be so dramatic. I may have to rebuild my primary with more turns to run longer on-times!


Quote
What a great capture of that failure! When people ask for advise, the one I give the first is to always have a camera rolling!
Thanks, good advice!

Quote
I think I know what happens before and during the continues "explosion".

The aluminium profiles are most likely anodized and that is actually an electrical insulation. So your high voltage arc knocks through the anodized layers and forms a nice current path for the upper part of the frame, it is now seen by the primary coil as a single turn winding underneath it and it starts to pull some serious power and thus is why you see steel bolts melting instantly.

The flashover between bolts and plate would be in the place with the worst connection, just like a bad connection on a mains feeder will heat up from the increased resistance, its just the point of failure and it has really nothing to do with the sparks anymore, it was just the initiator of the underlying problem (single turn metal construction too near the primary coil)

This makes sense and is a good explanation. Still surprising to me- I thought I would be ok because of aluminum being non-ferrous, the entire frame is located outside of the outer turn of the primary, and the distance the primary is from the frame.

You can see in this photo the corner with the melted bolt is well outside the primary, and you can also see the primary standoffs are all outside the outer edge of the primary.


The primary is also 8" above the frame:


Obviously none of this matters as much as I thought it would! I guess I need to try to figure out how to put a gap in the upper frame rail without compromising the structural integrity of the frame because this thing is really heavy! Any idea how much of a gap would be sufficient to remedy the shorted turn effect? Would 1 inch work?


« Last Edit: February 22, 2021, 06:06:48 PM by fh89 »

Offline davekni

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Re: Large coil experimenting with long on-times.
« Reply #28 on: February 22, 2021, 06:38:04 PM »
I'd say that Mads and Hydron "hit the nail on the head", except that you are using screws :)

Could you make some of the triangular corner-connector pieces out of polycarbonate or fiberglass?

Besides avoiding the shorted turn at the top, you still want a ground path for all metal frame parts.  My thought is to make each top edge extrusion connect at one end to the vertical post going down, but not connect at the other end (via plastic connector pieces).

Yes, you have large spacing, but your coil is large and high-power.  Relative spacing is what matters.

For detuning, consider the arc length compared to top-load dimensions.  The arc is much longer, making total capacitance much larger.  The advantage of skip-pulse is allowing long runs at max current - long enough until an arc manages to pull the secondary frequency down to match primary.  My arc length is limited because if I detune any further I hit OCD before enough arc forms to bring it back in tune.

Concerning intentional metal sparks for Tesla coil shows, I suggest using sparking metal as the breakout.  I do this on my SSTC, both with fine metal springs and with aluminum window screen.  (It is only 3kW, so barely enough to spark the breakout.  Your coil should be impressive with such breakout points.)

It looks like you have enough yard space to move the breakout point even farther onto the top of the coil.  That may help even more to direct arcs away from ground.  (I couldn't go farther onto the top without arcs going back to my roof.)
David Knierim

Offline Mads Barnkob

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Re: Large coil experimenting with long on-times.
« Reply #29 on: February 22, 2021, 07:47:07 PM »
Quote
In the second video I see what looks to be an arc from something internal (not the bolt that sparked earlier in the clip) - do you know what that was?
It was from somewhere in this area. There is some carbon tracking on the primary lead coming from the MMC. I think it was probably from there to the current transformer that was left floating.


You should move your CT. The primary circuit voltage is at its highest just at that point between the L and C of the primary resonant circuit. Please the CT at either output leg of the inverter, but never between the primary coil and MMC.

Your CT has properly only survived because of its internal resistor for 50 Ohm output impedance. A open loop CT secondary side can produce very high voltages and fail from it. https://voltage-disturbance.com/power-engineering/open-circuit-current-transformer-characteristics/


Quote
I think I know what happens before and during the continues "explosion".

The aluminium profiles are most likely anodized and that is actually an electrical insulation. So your high voltage arc knocks through the anodized layers and forms a nice current path for the upper part of the frame, it is now seen by the primary coil as a single turn winding underneath it and it starts to pull some serious power and thus is why you see steel bolts melting instantly.

The flashover between bolts and plate would be in the place with the worst connection, just like a bad connection on a mains feeder will heat up from the increased resistance, its just the point of failure and it has really nothing to do with the sparks anymore, it was just the initiator of the underlying problem (single turn metal construction too near the primary coil)

This makes sense and is a good explanation. Still surprising to me- I thought I would be ok because of aluminum being non-ferrous, the entire frame is located outside of the outer turn of the primary, and the distance the primary is from the frame.

You can see in this photo the corner with the melted bolt is well outside the primary, and you can also see the primary standoffs are all outside the outer edge of the primary.


The primary is also 8" above the frame:


Obviously none of this matters as much as I thought it would! I guess I need to try to figure out how to put a gap in the upper frame rail without compromising the structural integrity of the frame because this thing is really heavy! Any idea how much of a gap would be sufficient to remedy the shorted turn effect? Would 1 inch work?
[/quote]
1 inch would be fine, it is only the induced magnetic field you are fighting here, not a high voltage. Even less could properly do it too.

Just for the fun of it, I simulated your primary coil + MMC with a 1 turn secondary coil underneath it, 8" like your frame and the coupling is actually higher than I expected.



**side note** [ You are not allowed to view attachments ]  Why does this keep happening? It seems to work sometimes and not others.

I think there is some issues between attachment plugin and preview function, did you use preview?
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Offline davekni

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Re: Large coil experimenting with long on-times.
« Reply #30 on: February 22, 2021, 08:42:57 PM »
Using Mads JavaTC numbers, I'm calculating 600V peak around your rail at 2000A primary.  Rail inductance is reported to only one digit (2uH), so my calculation is correspondingly imprecise.  Still, a 2mm gap should be plenty.

Of the primary magnetic flux passing through the that top square rail, 1/4th of it passes through each side, for 150V peak.  That is why I suggested breaking the top rail connections at each corner, not just once in the loop.  A path to ground is necessary to handle strikes from the secondary.  So each edge of the top square can connect to one vertical post (but not the other) to provide that path.  It would be best for the intended connection paths to be robust.  I'd suggest sanding/grinding through the thin anodization layer where electrical connections are intentional, and using plastic where connections are to be avoided.
David Knierim

Offline fh89

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Re: Large coil experimenting with long on-times.
« Reply #31 on: February 22, 2021, 09:08:44 PM »
Quote
Could you make some of the triangular corner-connector pieces out of polycarbonate or fiberglass?

Besides avoiding the shorted turn at the top, you still want a ground path for all metal frame parts.  My thought is to make each top edge extrusion connect at one end to the vertical post going down, but not connect at the other end (via plastic connector pieces).

I have some leftover 1/4" G10 that I can make a sturdy brackets out of. Right now, the vertical pieces and edge pieces all butt up against each other. I could make a top bracket out of the G10 and cut one of the edge pieces 1/4" short to make a gap. This will unshort the turn and hopefully still maintain a very rigid and strong frame.

Quote
Concerning intentional metal sparks for Tesla coil shows, I suggest using sparking metal as the breakout.  I do this on my SSTC, both with fine metal springs and with aluminum window screen.  (It is only 3kW, so barely enough to spark the breakout.  Your coil should be impressive with such breakout points.)

Would be cool to vaporize some metal (intentionally this time.) Pretty sure I have some screen laying around somewhere.

Quote
It looks like you have enough yard space to move the breakout point even farther onto the top of the coil.  That may help even more to direct arcs away from ground.  (I couldn't go farther onto the top without arcs going back to my roof.)
I'll give this a shot.

Quote
You should move your CT. The primary circuit voltage is at its highest just at that point between the L and C of the primary resonant circuit. Please the CT at either output leg of the inverter, but never between the primary coil and MMC.
I think Dave mentioned this also in a previous post. I'll get it moved before the next run.

Quote
1 inch would be fine, it is only the induced magnetic field you are fighting here, not a high voltage. Even less could properly do it too.
I'll try Dave's idea of replacing one of the corner brackets and I'll introduce a small gap. Hopefully it will be enough.

Quote
Just for the fun of it, I simulated your primary coil + MMC with a 1 turn secondary coil underneath it, 8" like your frame and the coupling is actually higher than I expected.
That's interesting! Clearly quite a bit of power is being transferred to the frame. Hopefully the corner mods will help!

Quote
I think there is some issues between attachment plugin and preview function, did you use preview?
I've tried it both ways- Previewing first, and posting without previewing first. Doesn't seem to matter.


Quote
Of the primary magnetic flux passing through the that top square rail, 1/4th of it passes through each side, for 150V peak.  That is why I suggested breaking the top rail connections at each corner, not just once in the loop.  A path to ground is necessary to handle strikes from the secondary.  So each edge of the top square can connect to one vertical post (but not the other) to provide that path.  It would be best for the intended connection paths to be robust.  I'd suggest sanding/grinding through the thin anodization layer where electrical connections are intentional, and using plastic where connections are to be avoided.

That would be ideal, but it is much more work- I would need to replace all 4 top corner brackets and cut each of the 4 edge pieces. I'm hoping replacing a single bracket and using a larger gap will be good enough. If I continue to have issues, this will be the way I go.

Offline davekni

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Re: Large coil experimenting with long on-times.
« Reply #32 on: February 22, 2021, 10:19:48 PM »
If you were to make 4 gaps, it's only 150V per gap.  A plastic shim would be sufficient insulation - something like 4 or 5-mil mylar transparency film.  That way perhaps you could avoid cutting any of the extrusions.  For that matter, 4-mil mylar would be fine at 600V.  Success of a single break will depend on which other joints happen to conduct or not (ie. on what bits of anodization have been scraped through or not during construction). 

All joints solidly conducting is an option too.  A solidly-conducting shorted loop would reduce primary inductance, so require a bit of retuning.  If conducting, that upper frame ring would conduct about 1.1kA peak.  A solid connection for that current would require non-anodized parts well clamped together.  Breaking connections is probably easier than making connections good for 1.1kA peak.
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Offline fh89

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Re: Large coil experimenting with long on-times.
« Reply #33 on: February 22, 2021, 10:43:49 PM »
If you were to make 4 gaps, it's only 150V per gap.  A plastic shim would be sufficient insulation - something like 4 or 5-mil mylar transparency film.  That way perhaps you could avoid cutting any of the extrusions.  For that matter, 4-mil mylar would be fine at 600V.  Success of a single break will depend on which other joints happen to conduct or not (ie. on what bits of anodization have been scraped through or not during construction). 

All joints solidly conducting is an option too.  A solidly-conducting shorted loop would reduce primary inductance, so require a bit of retuning.  If conducting, that upper frame ring would conduct about 1.1kA peak.  A solid connection for that current would require non-anodized parts well clamped together.  Breaking connections is probably easier than making connections good for 1.1kA peak.

Insulating the edge pieces from the verticals would be easy enough with this method, but the top corner brackets would still be shorting everything together. They would be much harder to insulate. I guess I could drill out the bolt holes and press in nylon sleeves, then use nylon washers on the bolts.

Offline davekni

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Re: Large coil experimenting with long on-times.
« Reply #34 on: February 22, 2021, 11:40:29 PM »
Yes, I was thinking of your G10 for corner pieces.  Isolated connections using metal pieces is complex (ie. nylon sleeves and washers as you suggest).  Not that I know what I'm talking about when it comes to mechanical construction.  My DRSSTC base is a stack of plastic milk crates.  Hardest part was cutting out the steel reinforcement rib at the top without damaging the plastic.  Had to remove the steel rib for the same reason - it formed a shorted turn.
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Offline fh89

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Re: Large coil experimenting with long on-times.
« Reply #35 on: February 23, 2021, 04:42:02 AM »
Yes, I was thinking of your G10 for corner pieces.  Isolated connections using metal pieces is complex (ie. nylon sleeves and washers as you suggest).  Not that I know what I'm talking about when it comes to mechanical construction.  My DRSSTC base is a stack of plastic milk crates.  Hardest part was cutting out the steel reinforcement rib at the top without damaging the plastic.  Had to remove the steel rib for the same reason - it formed a shorted turn.

Well I was all ready to start cutting G10, ordered some mylar sheets off Amazon, popped off a corner to use as a stencil, and instantly had a DOH! moment.


Of course the side corner pieces sort the rails together also! No way I'm going to make 12 G10 corner pieces! I think my best bet at this point is to cut a small gap in the rear rail, then reinforce on 3 sides with strips of the G10.

Offline davekni

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Re: Large coil experimenting with long on-times.
« Reply #36 on: February 23, 2021, 06:06:27 AM »
It would be only 8 corner pieces, not all 12.  You want a ground path for each of the four horizontal extrusion pieces.  I had no idea how hard or easy it would be to make eight G10 corner pieces.

I'd still consider making two G10 corner pieces and isolating an existing corner rather than cutting in the middle of an extrusion.  That way if you avoid a patch in the middle of your clean build.  Also, if you find that more breaks are needed, you are already 1/4th of the way there.

Is there enough clearance in the existing corner piece holes that you could add a small section of heat-shrink around the head-end of the screws and still fit?  That and nylon washers and mylar (or kapton tape) around one end of each horizontal extrusion would work.  That's a total of 16 nylon washers, as only the four screws at one end of each extrusion need insulating.

BTW, I found a scrap of what looks like the same extrusion as you are using.  The scrap I have is definitely anodized.

Of course, this is your build, so modify as you see fit!  Good luck with the upgrade!
« Last Edit: February 23, 2021, 06:15:30 AM by davekni »
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Offline fh89

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Re: Large coil experimenting with long on-times.
« Reply #37 on: February 23, 2021, 07:27:25 AM »
Quote
Is there enough clearance in the existing corner piece holes that you could add a small section of heat-shrink around the head-end of the screws and still fit?  That and nylon washers and mylar (or kapton tape) around one end of each horizontal extrusion would work.  That's a total of 16 nylon washers, as only the four screws at one end of each extrusion need insulating.

I think I'll go with this idea. McMaster Carr has nylon sleeve washers https://www.mcmaster.com/91145A165/ that would be perfect for this. Washer and insulating sleeve in one. Drilling out the holes in the brackets to press-fit the sleeve washers will be way easier than making the G10 corner brackets. The Kapton tape is a good idea too. I appreciate the feedback! Maybe you are better at structural engineering than you give yourself credit for!

Quote
BTW, I found a scrap of what looks like the same extrusion as you are using.  The scrap I have is definitely anodized.
It's 80/20 brand T-slot aluminum. Definitely anodized, sorry for not acknowledging that before, I took it as an observation.

 
Quote
Of course, this is your build, so modify as you see fit!  Good luck with the upgrade!
Thanks!
« Last Edit: February 23, 2021, 08:11:41 AM by fh89 »

Offline Mads Barnkob

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Re: Large coil experimenting with long on-times.
« Reply #38 on: February 23, 2021, 11:20:12 AM »
How about replacing one of the horizontal extrusions with something made out of non-conductive material, then you do not need to redo any corners.

You could also just cut out a 1" piece of a horizontal extrusion, as you suggested earlier and replace the missing piece with a machined counterpart of G10 knocked into each extrusion to give it some sturdiness or even mount some more pieces on the outside using just regular screws/nuts in the extrusion slots.

You should also use a dedicated conductor from strike rail to RF ground, do not lead that through the anodized frame.
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Offline Hydron

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Re: Large coil experimenting with long on-times.
« Reply #39 on: February 23, 2021, 04:39:06 PM »
Isolating both top rails at 2 opposing corners would be best (should eliminate the whole shorted turn path), though 1 should be a lot better than nothing (would leave half of the path there next to the primary and the other half via the bottom rails). Looks to me like some thin shims and isolating the screws from the triangular aluminium bits would do the job. If you're feeling lazy you could order some 0.8mm PCBs (without copper) with the correct shape and hole spacing (0.8mm was the same price as 1.6mm last time I ordered a thin PCB, going below this got pricey).

As for the CT on the primary, looks like the rating is 700A RMS (and it is internally terminated) so you're probably safe there to leave it connected unless you start pushing a few 10s of % duty cycle, but as Mads said, put it between the bridge and one of the primary tank components (capacitor or primary coil), not between the cap and primary. It's likely possible to insulate it from the primary voltage seen in the current location, but is unnecessary effort vs just putting it somewhere safer.

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Re: Large coil experimenting with long on-times.
« Reply #39 on: February 23, 2021, 04:39:06 PM »

 


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