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Graham Armitage

Tue May 27 2014, 12:33PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

I think many of us started with ultra low budgets on our first coils. My first caps used thin glass sheets and aluminum foil. Not great but we got them to work. What I later found to be a good homemade cap is using polyethylene sheet (painter's drop cloth). It has a very high dielectric constant and good breakdown strength. Calculate how many sheets you need to sustain the primary voltage and double that. Then place alternating layers of aluminum foil and poly and roll it up with a terminal on each end. If you can, place the whole capacitor roll in a plastic pipe or tube and seal the ends after filling it with cheap vegetable oil from the supermarket. This will prevent corona discharge that will destroy your poly sheet very quickly. I have used these for years with minimal failures.

This should be a cheap and very efficient capacitor. Good luck.

Here is a large coil running off the caps I just described -

You can see the white pvc tubes in the bottom right.

skyler

Thu Aug 07 2014, 10:55PM

Registered Member #42712 Joined: Fri Jan 10 2014, 01:21AM
Location:
Posts: 21

How would 6 or 8 of these in series hold up?

Mads Barnkob

Fri Aug 08 2014, 05:53AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

skyler wrote ...

How would 6 or 8 of these in series hold up?

Big snubber capacitors work great in DRSSTC, just be sure to check its numbers to your coil and current setting, there is a MMC calculator on my site for this:http://kaizerpowerelectronics.dk/calculators/ mmc-calculator/

loneoceans

Fri Aug 08 2014, 06:41PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

Hey Skyler,

It looks like your coil is really big. Did you start off with a 4" pipe and are now using a 12" pipe? A 4 x 1 foot secondary is pretty big! For reference, today's electronic DRSSTC coils at that size take in about 5 to 10kW input power to make ~10+ to 13ft of spark. Are you also now using a NST or your 4-MOT stack? As a reference, I built a 4" secondary rotary spark gap coil about ten years ago and it's still going strong. It used a 4 MOT stack in series for 8600VAC, which was I think what you originally planned to do? Would be good to know the current status of the project in more detail.

12221404836 7f58379b44 C

Above's my coil running at about 4kW input power. Not too overkill if you ask me

. Check out my build page here which you might find useful:

As for the caps you linked, I'm sure they'll work fine, but you need to size them correctly. What sort of power supply are you using now? 8 in series would give a 14.4kV 225nF bank which looks like a bit too much capacitance for a RSG coil. But despite the 1.8kVDC capacitor rating, they're probably closer to 500VAC. If I was building one for my 4-MOT power supply, I'd use more like 14-20 in series, and then it gets pretty expensive. If you're using a NST, I'll go for the smaller individual polypropylene film capacitors and have more of them in series.

skyler

Sun Aug 10 2014, 12:43PM

Registered Member #42712 Joined: Fri Jan 10 2014, 01:21AM
Location:
Posts: 21

I realize that the size of our coil may be way overkill.

Theres not much we are going to do about it though, because we spent a long time winding that secondary.

We do know the resonant frequency, which is about 83 Khz, and I now have 100 feet of copper tubing for the primary instead of the 20 feet before.

Reason we are going back from NST to MOT:

The rotary spark gap keeps failing when we try it because the NST keeps GFI interrupting, and they promised that the NST was non GFI but sure enough the tripper in it breaks. We opened the top off and there was all of this solid insulator stuff covering it, so we did not want to mess with that. The NST is the old type, not the new semiconductor. So we are going back to MOTS.

I can't spend lots of money on primary caps. Only about $100, so if anybody has suggestions, please say.

The bucket cap seems like it won't work because when I tested it with the NST, a beer bottle just broke.

Thanks for the help

skyler

Thu Sept 11 2014, 10:16PM

Registered Member #42712 Joined: Fri Jan 10 2014, 01:21AM
Location:
Posts: 21

â€¢I finally got the MOTS (microwave oven transformers) to work, by correctly wiring the secondary coils (using the core of two mots as the center, and removing the connection to the core on the outer MOTS, as well as submersing the entire power supply in oil) and have a HIGH current 8KV power supply.

â€¢I have experimented with several ballasts, including a MOT with the secondary shorted, a big hunk of aluminum etc, and am close to finding a good setup with the amount of current I have available (20amps-30amps)

â€¢I used a 3450 RPM pool pump motor for the rotary spark gap. (Yes, I know it isn't Synchronous, and blah blah but this youtube video has an awesome Quad mot coil using 3450.

The motor is very similar to this setup, using tungsten rods. --> Srsg240 200

â€¢I have a primary coil of 100 feet copper wire wound, but it might be a bit uneven (like goes up and down a bit).

â€¢I still have my 12-15nf home-brew bucket capacitor as well as the large secondary with 12ft diameter and 4 feet height.

THE PROBLEM

I got the spark gap to fire Perfectly, so next step, I hook up the Series string of primary coil and bucket cap in parallel with the spark gap, which was firing perfectly

But.... Nothing, the primary coil fails to fire until I disconnect the capacitor primary string, and then it fires again.

What is going on? Does it have to do with it not resonating and the cap having a wayy off value (which i know is true, I thought I could compensate by buying 100 feet of copper tubing)

Thanks for any help, we are out of ideas and would love any input!!! HD Pictures of setup will come as soon I can get to my friends house again (as he has the coil)

loneoceans

Thu Sept 11 2014, 10:40PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

Photos and more specific details of the setup will help.

Do you mean that your spark gap isn't firing, but you have made it close enough such that without the capacitor/primary in series (parallel with the gap), the spark gap no longer fires? As for your ballast, I found that using a secondary-shorted MOT as a ballast does help in current limiting, but at least for my transformer, I was getting more like 1 to 1.5kVA maximum. This will limit the output to about 2-3 feet or so. I now simply run my quad MOT coil on a 30A circuit with no ballast and it draws about 17A at 240V with great performance.

Since your gap is not firing, do you have a short somewhere in your capacitor bank? Is your MOT stack drawing lots of current without the spark? If it is, there might be some arcing-over or a short somewhere in which case the gap would not fire when the caps are attached.

Once you get the gap to fire, you'll need to work on your tuning, which isn't going to be solved by adding a 100ft primary without matching the frequencies. The primary circuit should be slightly lower frequency (5 to 10% is a good place to begin) than the secondary. Fine tune for maximum performance - no shortcuts here.

skyler

Fri Sept 12 2014, 10:06PM

Registered Member #42712 Joined: Fri Jan 10 2014, 01:21AM
Location:
Posts: 21

My MOTS are definitely not shorted, high currents are not drawn when the mots aren't connected to anything, and we built a fully functional Jacobs ladder out of it to test it.

To get this straight forward,
A. the spark gap fires when nothing is connected (exept the mots of course)
B. the spark gap fails to fire when the primary capacitor and 100ft primary coil is connected

Capacitor have a short?

Last time I measured (a few months ago), it did not have a short cirucit but I don't know now, something could have happened. I emailed my friend to test for shorts and will get back as soon as possible.

skyler

Mon Sept 15 2014, 09:42PM

Registered Member #42712 Joined: Fri Jan 10 2014, 01:21AM
Location:
Posts: 21

Yup the capacitor had a short!!! Dead short.

skyler

Mon Jan 26 2015, 07:58PM

Registered Member #42712 Joined: Fri Jan 10 2014, 01:21AM
Location:
Posts: 21

I am getting frustrated, my coil project has been going on for over a year, and it still barely works, 4-6 inch sparks with help with a grounded material stuck right next to it is all that I get.

I have gotten the primary and secondary to resonate at the same frequency, first the primary was a bit high in frequency, but we added some extra coil, and they are both somewhere around 100khz.

Any suggestions on what I can try next? I will upload all of the videos and pictures on YouTube when I get home today and maybe the visual will help.

Primary capacitance: 24 nF (two beer bottle bucket capacitors like above)
Primary inductance .23 millihenries (and varied by position on coil)

Rotary Spark gap 3450 RPM, yes I know it should be 3600, but I have seen plenty of videos where 3450 works great with arcs order of magnitudes bigger than mine.

Power supply, 4 MOTs
Ballast , 1 MOT

What am I doing wrong, can anyone give me steps that I can troubleshoot? The coil is built to everything it should be, and I have no idea why it is not behaving even close to the power input.

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