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Problem with Secondary to Primary Strikes

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MaxD

Wed Dec 31 2008, 08:11PM

Registered Member #1753 Joined: Mon Oct 13 2008, 03:03PM
Location:
Posts: 12

Hey everyone, I'm just tinkering with my first spark-gap type tesla coil.

I keep getting strikes going between the secondary and the primary coils. I've tried raising the secondary coil up in small incriments and this cures the problem but then I have no streamers coming from the breakout point. :(

I honestly don't know what I'm doing wrong here. I've experimented with the size of the MMC, tapping the primary at different points, fiddling about with my sparkgap, tinkering with the coupling.

I am actually at a total loss, so if anyone could shed some light on the situation I'd be rather thankful :)

Myke

Wed Dec 31 2008, 08:21PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Try adding a strike rail that is placed around the primary. Make sure that it isn't a closed loop otherwise it will sap power from the resonant circuit. Ground the Strike rail to the RF ground. The sparks should hit the rail instead of the primary.

MOT_man

Wed Dec 31 2008, 08:56PM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139

start off by tapping all the way out on your primary and use no top load - then tap towards the center until to achieve break out with top load. Sounds like a tuning problem. What is the distance between secondary and primary? How many volts ~ watts are you using for your input?

MaxD

Wed Dec 31 2008, 09:37PM

Registered Member #1753 Joined: Mon Oct 13 2008, 03:03PM
Location:
Posts: 12

Here is a video of the problem.

Video

I'm not sure if a strike rail would help since its arcing to the first turn on the Primary.

I'll try tapping the primary in different points, I have only stuck with the point that seemed to get the longest arcs so far.

MOT_man

Wed Dec 31 2008, 11:01PM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139

:)
This is a classic tuning problem with inverse conical primary! But saying that it could also be the ground.

Herr Zapp

Wed Dec 31 2008, 11:39PM

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

Max -

Actually, the primary-to-secondary arcing shown in your video clip looks much more like poor secondary grounding than anything else.

Two questions:

1. How is the bottom of your secondary winding connected to "ground"?

2. What (specifically) are you using as an "earth ground"?

The first thing I'd do is to move your coil to a location where you can connect to a known good-quality earth ground, like a metal rod driven 2 feet into moist earth, or to a copper water supply pipe that's known to be buried underground.

As confirmation that your secondary is actually grounded, you should be able to measure continuity from your topload to an independent earth ground, using a multimeter.

Aside from very poor output, there's no evidence in your video of any tuning or coupling problems, although these problems may show up as soon as you eliminate the pri-sec arcing. I think your next problem may be overcoupling from the inverse conical primary, although I've seen coils with IC primaries run very well, but with intense corona. Have you calculated your pri-sec coupling using JAVATC or any of the other TC design tools? At this point, you should try to achieve coupling within the recommended values for spark-gap coils (K of approximately .1 to .15).

Regards,
Herr Zapp

MaxD

Wed Dec 31 2008, 11:58PM

Registered Member #1753 Joined: Mon Oct 13 2008, 03:03PM
Location:
Posts: 12

Ahh, thanks man. I'll do it next year. :)

I changed from a flat primary to a conical because I thought that I'd get better results.
I got better results but had bigger problems too.

This coil is probably one of the most ghetto you will ever see.
Calculations done - Nill.

I started building before I knew much at all, so I can't fault the build quality.
It's just been undergoing several facelifts for a few weeks and is changing as I learn more and more.

The secondary - is not grounded.
I had it grounded before but I then had no air discharge from the breakout point.
Maybe I should run the end of the wire through an RF choke and then into ground? I'm not sure.

Frosty90

Thu Jan 01 2009, 12:56AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

You need to ground the secondary. this would be the main cause of your problem. If a proper ground isnt an option, connect the bottom of your secondary to the inner turn of your primary. I know a number of other members here are going to say 'no dont do that' i would disagree. it is not a problem if you dont touch the secondary while its running (you shouldnt do this in any case), and ive done it on my 4" MOT coil.

Cheers,
Jesse

Myke

Thu Jan 01 2009, 01:25AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I would think that the HF from the bottom of the sec would travel back down the connections and destroy the insulation in your transformer. (I would think that an NST is more sensitive to HF than some other transformers)
What sort of transformer are you using to power it? (MOT, NST, OBIT, etc.)
I think you need a top load for your TC output.

Herr Zapp

Thu Jan 01 2009, 01:25AM

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

Max -

As is obvious from your video, you didn't have your secondary grounded. It's absolutely essential for a Tesla coil to have the bottom of the secondary grounded to either an earth ground, or to a counterpoise ground. Please disregard Frosty's bad advice and DO NOT connect the bottom of the secondary to any part of your primary circuit; this creates a potentially fatal shock hazard from the streamers.

If you cannot locate the coil where it can be connected to a solid earth ground, you can construct a counterpoise ground out of thin sheet metal, chicken wire, metal mesh, or even aluminum foil. The counterpoise is a "floating" (not connected to earth ground)horizontal ground plane with the Tesla coil positioned in the center. It should be at least four feet in diameter for a coil of your size, and if it's made from several pieces of material, they all need to be electrically connected together, and then connected to the bottom turn of your secondary. A single piece of chicken wire or metal screen works well, as it's easy to connect to and can be rolled up for storage. Any "strike targets" that you want the streamers to arc to also need to be connected to the counterpoise.

As soon as you ground your secondary, the type of pri-sec arcing shown in your video should be eliminated. If you did no design work, no design calculations, have no idea of the resonant frequency of your primary and secondary circuits, no idea of where the initial primary tap should be, etc, etc. you may be unable to bring the coil into resonance.

I'd suggest that you go back and do your design homework, and you'll have a much better chance of getting the coil operational. JAVATC is one of the best software tools to use to design and analyze a coil. If you input all your known parameters, JAVATC will help you find where to start the tuning process, or will indicate if your design is so far off-target that it cannot be tuned.

Regards,
Herr Zapp

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