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Marx generator powered tesla coil

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Dr. Slack

Tue Nov 29 2016, 07:16AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Hi Benjamin,

I think you have your m confused with your micro, m means milli to most of the planet. Use u for micro, when the unicode 0B3C character "Î¼" isn't handy.

Johnf was being facetious when he suggested you build a 500mH air core inductor, you could fit a small person inside it (50" x 15" x 2300 turns, inductance goes as volume, other things being equal).

A 5" x 1.5" 500uH (500Î¼H) built as you suggest has an SRF of over 4MHz, so will be usable on any practical SGTC. The effect on primary tuning will be small, and will be tuned out / corrected for during the normal TC tuning process.

Benjamin

Tue Nov 29 2016, 04:42PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

Ok sorry for the confusion, I meant micro not milli. How would one go about calculating the smallest usable value of inductance that would work in the marx tank circuit? I'm assuming that a larger inductance is better but what is the practical limit for say a 250kHz 1kW tesla coil and a 4 stage marx.

Dr. Slack

Wed Nov 30 2016, 06:34PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I'll give you a methodology, though not necessarily solid numbers.

Think of how much energy is stored in the primary coil itself, compared to how much is stored in other inductors. For instance the loop through the MMC and spark gap will store energy, as will current flowing through these charging chokes. The trick is to have *most* of your energy stored in the primary, where it's doing something useful.

It would probably be infeasible to get the energy stored in those non-primary inductors down to 1% of total. But if you had 50% stored elsewhere than the primary, that would make for a pretty poorly designed primary circuit. So it's going to be somewhere between the two extremes. 10%? 20% 5%. Whatever you can achieve and feel comfortable with.

Benjamin

Wed Nov 30 2016, 08:02PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

So if I was designing for 5% of the total energy stored in the inductors, would I want the combined inductance of all the charging chokes to be 20 times the inductance of the primary coil? How do I calculate the stored energy in the inductors? I know that a larger inductance will store more energy with the same current flowing through it, however a larger inductance also has a greater resistance to change in current.

jpsmith123

Thu Dec 01 2016, 05:03AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Here's a paper describing a 500 KV, 500 J Marx generator that uses charging inductors. The coils are about 1 mh with 33 ohms resistance and they don't look like anything special.

You might be able to use some cheap audio crossover coils e.g. like this as charging inductors:

BTW sometimes the Marx capacitors are themselves the primary capacitance of a Tesla coil.

Edit: Imagine that the device described in this patent is your complete primary circuit:

Benjamin

Thu Dec 01 2016, 05:47AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

I am planning on using the marx caps as my tank caps. I would rather wind my own charging inductors because I already have a bunch of 24 gauge magnet wire. I just don't want to wind 8 coils before I know roughly the best inductance for them. I've attached a revised schematic of what I'm planning to build.
]marx_powered_tesla_coil.pdf[/file]

edit: This schematic is different from the other one even though it has the same name.

Dr. Slack

Thu Dec 01 2016, 09:36AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The reason I was encouraging you to think in the ratio of stored energy was that the energy in an inductor is 0.5LI2, directly proportional to L, for series connections when they're running at the same current. As you might imagine, even without doing the maths, for a parallel connection the energy goes as 1/L for the same voltage.

I guessed at some nominal figures for a biggish primary for javaTC, 10 turns with 10cm internal and 17cm external radii, and it's around 40uH. So with several inductors in shunt with that, you would not want to go much smaller than 500uH, and a tad bigger might be nice. Depending on the configuration, so all parallel, or inductors cascaded up the ladder, you will have different numbers of charging inductors being 'seen' by the primary. You will obviously have to draw it out in detail. A cascade along your marx (the conventional configuration) will act like a transformer so each of your inductors sees just part of the primary resonant voltage, that is, just the voltage on that part of the MMC.

I would be inclined to connect a Terry Filter between the MOTs and the Marx. MOTs do not have the HV insulation that NSTs do, and the thought of a whole bunch of high Q inductors connected to spark gaps and stuff has me feeling breakovers.

Benjamin

Thu Dec 01 2016, 04:16PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

Thanks, that makes sense. I have another question, I was planning on using a single parallel copper pipe spark gap for each stage. Will this quench effectively enough or should I build a multiple air blast gap? The total effective capacitance of the cap bank is only 0.0105uF and the firing voltage should be greater than 16 kV.

Dr. Slack

Thu Dec 01 2016, 09:04PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

one gap per stage *is* a multi-gap for the whole MMC. Given the low voltage per stage, you might have difficulty setting up a single gap, let alone 2 in series. Don't forget to let them have a good sight-line to each other and keep them as close as electrically practical for UV to help triggering of all gaps at the same time.

Benjamin

Thu Dec 01 2016, 10:00PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

By multiple air blast gap I meant one gap per stage, four gaps total.

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