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My First Coil...

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iceowl

Fri May 28 2010, 03:11AM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

Unfortunately, after weeks of building, I tried for first light and got breakouts and arcing in the secondary. At the moment I am not thinking it's overcoupled...but rather, I performed some surgery on the secondary which I bought on eBay. I didn't like the fact the builder suggested connections internal to the core, so undid what he did, and mounted the connections to the toroid and ground as Dan McC suggests in his book(s). I think that in doing that, I must have used the wrong glue (cyanoacrylate) or perhaps the wrong epoxy to put the pvc plugs into the ends (Loctite 5 min setting epoxy). As I raised the voltage on the variac I started seeing corona around the base of the secondary, suspiciously near where I had tried to plug the holes in the secondary (made by the guy I bought the secondary from).

I should probably break down and wind my own secondary. I just can't get my head around building a jig. But given everything else I've built in the past month, it shouldn't stop me. Meanwhile, I've ordered a new secondary from Eastern (Dan M.). Hopefully I won't wreck it.

Cheers,
Joe

[

iceowl

Fri May 28 2010, 03:20AM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

Ah ha...I could have a bad ground. The breakouts and arcing appear at the lower points of the coil. Hmm... As this is RF, and I've measured the resonant freq at ~223kHz, the ground length of a couple feet shouldn't be all that awful. In ham radio, when we're operating at some mHz, a ground connection of a couple feet could be as good as no ground at all. I'm wondering if my ground wire is too long... Of course, by now I've got all sorts of marks on the secondary base. Hopefully it's not totally gone.

Joe

Herr Zapp

Fri May 28 2010, 05:39AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

iceowl -

I suspect that the bottom of your secondary is not grounded, or you have some other ground-related problem. How is the last turn at the bottom of secondary mechanically terminated, and how is this termination tied to your earth ground wire?

What, exactly, is your earth ground, and how far from the secondary is it? Have you measured the DC resistance between your earth ground and the AC line ground at a nearby outlet?

Also, what is the minimum distance from the innermost turn of your primary to any part of the secondary windings?

I don't think the type of adhesive you used should have any affect on arcing at the base of the secondary, unless you used a metal-powder-filled epoxy. Post some well-focused, close-up photos of the base of the secondary where the arcing is occurring, including the nearest parts of the primary.

You might want to consider adding bleeder resistors (typically ~ 10 megohm) resistors across each cap in your MMC to bleed off any stored charge. They can give a really nasty shock if you are not cautious, especially with all the adjustments of the primary tap you will
be making as you "tune" your coil.

Finally, what is the value of the individual caps in your MMC, and the voltage and current ratings of your NST?

Regards,
Herr Zapp

coillah

Fri May 28 2010, 07:28PM

Registered Member #1517 Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304

Coupling looks very tight to me. I had coupling issues, by moving out about 4cm I took care of most of the issue with no noticeable decrease in power. My ground has always been janky, but the coil runs just fine. Definitely look into your coupling. Other than that, the craftsmanship looks great man!

Herr Zapp

Fri May 28 2010, 08:02PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Herr Zapp

Fri May 28 2010, 08:10PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

iceowl -

I built a spark-gap coil a decade ago that is amazingly similar in appearance to yours ........ I was finally able to find a small photo of it.

Regards,
Herr Zapp

1275077439 480 FT89990 Nor Cal Teslathon Sh 6in Tc Sm

iceowl

Fri May 28 2010, 10:57PM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

Hi All,
Thanks so much for all the suggestions. I have first light as evident from the attached photo. But it is not without wailing and gnashing of teeth.

Last eve, at the suggestion of a local coiler, I coated the secondary with Minwax Polyurethane (also suggested in Dan M's book). I let it dry overnight, but in the morning, I had the same arcing between turns of the secondary.

All along I had suspected the grounding system. I am a ham radio operator (AL3A) and I happened to have a 8' ground rod lying around in my backyard from the last time I had grounding issues (with my HF rig). I brought the rod to the front of the house and pounded it into my wife's garden at 6AM. After about 45 minutes I got it through various layers of rocks, etc. Then I took some automobile jumper cables, cut off the calipers, and used the wire as connection to the ground rod. That wire is stranded. There's about 25-30 strands of 16ga copper. I figured if the combination of grounding rods and jumper cables didn't give me a good enough ground, nothing would.

And alas, still didn't work. I began to seriously hate my secondary at that point, which had developed brown burn spots where the arcing liked to occur with regularity.

But then I got an e-mail from someone who read one of my posts and said: you are just seriously - heinously out of tune, my friend. Use JavaTC and figure out what your problem is.

Well, at first I didn't want to think about that. After all, I had lined up the resonant frequencies with my scope, function generator, and LED power indicators. But I began to despair for lack of time.

As it is, I've put nearly 100 hours into the coil. I can't bear to see it fail. And I don't know when again I will get 100 contiguous free hours to do nothing but coil.

And so, I went to JavaTC, plugged in all my measurements, and here's what it said: you need 15.1 turns on your primary to get good power transfer.

Woah. I only had 13 turns on my primary - and not only that, but my LED+oscilloscope+functiongenerator method had said I should tap the primary around the 5th turn. I was beginning to despair of having made a 13 turn primary (through lucite with closed holes...another story all together). As it turned out I had drilled my primary stanchions to accept 15 turns and I had some 1/4" copper tubing left. So I threaded it through the unused holes. I spliced it to the existing primary coil by cutting of a small bit of tubing and cutting just one wall of the tube lengthwise with a hacksaw. Then I folded/rolled it in upon itself and used that as a plug/splice.

When I energized the system I could hardly believe my eyes.

Wake up and smell the ozone.

iceowl

Fri May 28 2010, 11:01PM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

Oops, maybe this video file will upload

iceowl

Fri May 28 2010, 11:04PM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

Here's the video....

iceowl

Sat May 29 2010, 04:24AM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

If anyone can tell me how to take good night pics...I'm looking for suggestions.

Thanks!

Joe

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