Re: Advice on bad coil performance.
Sulaiman, Sat Feb 24 2007, 02:22PM

Nice Job !

Welcome.

Try blowing air through the spark gap,
that should increase performance.

Some small improvement if you don't use steel/iron as conductors.

Re: Advice on bad coil performance.
ragnar, Sat Feb 24 2007, 02:31PM

Hi and welcome, Annie... just a very quick question for you... have you grounded the coil?

Re: Advice on bad coil performance.
Marko, Sat Feb 24 2007, 02:43PM

Yes, bad gap quenching is one of most common begginer (as well as mine) errors.

Use some kind of fan to blow air across the gap. Multi-gap also greatly helps with the quenching thing. It's usually made of copper pipes with air stream across them, and they can be made in many configurations: and like so


Once your gap quenches steadily, you can move onto tuning the coil. With that NST, I guess you can expect some 35 inch or so arcs at most, surely unless you add more power to it

Re: Advice on bad coil performance.
Hazmatt_(The Underdog), Sat Feb 24 2007, 09:05PM

Your spark pictures look really good. Looks like you have some nice output there.

If you want higher efficiencies then what bottle caps can provide, your project starts getting really expensive. I'd be happy with what you have for now. ^^

If you want to get into higher power systems I would probably have to say go to solid state and not bother with the spark gap type. It would be cheaper.
I know because in purchasing Door Knob caps becomes very expensive when you need to buy 20 to get a $20 each price.

Re: Advice on bad coil performance.
sparky, Sat Feb 24 2007, 11:05PM

Green glass - or for that matter any colored glass causes RF losses due to salt additives found within.

Re: Advice on bad coil performance.
Nik, Sun Feb 25 2007, 12:23AM

Congrats on your coil. A better spark gap will help for sure. You are getting the same output I got on my first coil but I had a 4kv 450mA mot stack.

Re: Advice on bad coil performance.
Annie, Sun Feb 25 2007, 01:01AM

Thank you so much for all these replies!!! I just got home from work and found my mailbox stuffed, LOL! Yes, I have grounded the coil, with a 3 ft. long steel tube, that reaches ground water level, which is very high here in the Netherlands. I will construct a different multi-gap spark gap with a 3 inch fan underneath, and replace the bottle caps with (maybe selfmade) polythene capacitors, I agree factory caps are quite expensive... Didn't know about the coloured glass being far less efficient, one more reason to switch to polythene caps. Well, lots of things to do! I am glad we could get the coil to work at the first try, so my son's project is in working order! In the forthcoming weeks, we can improve the coil, which is a nice hobby. I really would like to thank you all for these clues, and I will post my results in this forum as well.

Re: Advice on bad coil performance.
sparky, Sun Feb 25 2007, 07:38AM

Well its all about experimentation... the colored glass is a real problem and from experience I can say that these capacitors run very warm. I lost 20-25% of my spark length due to glass capacitors RF losses. CDE makes 1600 VDC 0.68uF MMC type capacitors - and they are super for a low cost Tesla Coil. I have 22 in series running an 12kV 120mA NST
and the capacitors were sent to me as a sample - FREE!

Re: Advice on bad coil performance.
Annie, Sun Feb 25 2007, 12:11PM

Hi Sparky! I will try to get these capacitors somewhere. Thanks for this suggestion

Re: Advice on bad coil performance.
Part Scavenger, Sun Feb 25 2007, 05:58PM

Hey! You have really worked hard on you coil and it shows!

Here is my 2 cent's worth...

Definitely pay attention to the spark gap, and whether it is quenching or not.

About the ionizing, and losses in general, just remember that anything you see (heat, light, movement, sound) that isn't a streamer is robbing energy from your streamers and anything you can do to decrease that will improve performance. Generally, you tend to lose a lot of power in the primary and your connections within the primary circuit. So pay attention, is anything getting hot. If so, make it thicker or of more conductive metal. Using steel wastes alot of power.

Your bottle caps... They're right about the colored glass, but I don't know how big a deal that is, I've never tested it. Make sure you've got a saturated salt solution in the bottles, that's important. The biggest problem I can see with your bottle caps is that the aluminum foil isn't really tight. I know how hard it is to actually accomplish that, but you're probably losing power to movement and corona. I would suggest taping them or better yet gluing the foil on like (shameless plug). A good tip when connecting the bottle caps together with aluminum foil is to fold over the edges to prevent corona. You only need it on the bottom anyway.

In the tutorial, I used really big bottles because they had more capacitance per footprint, but I have since learned that using smaller bottles and more of them would have worked better because it would decrease the resistance. The very last picture depicts the bottle caps glowing. That wasted energy and could have been fixed with tape, but I thought it was cool and didn't fix it. The steel strapping was handy, but got hot so don't use it.

With bottle caps, and your setup, you should be able to get about 20-22" A nicer spark gap and an MMC should get you over 30" If you decide to make your own rolled caps, look me up. I just did a ton of research on the topic.

Not to be to critical, just to help. It's very refreshing to see someone work so hard on their first coil. You might want to download Terry Fritz's ScanTesla too, it's helped me alot. But right now, you ought to get your spark gap fixed, that should really help.

Hope that helps.
-Part Scavenger

Re: Advice on bad coil performance.
Annie, Mon Feb 26 2007, 10:41AM

Hi Part Scavenger!
Thank you for this reply (and your compliments). I really appreciate it. Your bottle caps look super compared to ours, LOL. We are surely going to upgrade the spark gap, and construct one with a CPU fan underneath, they are high in RPM, compared to other fans. I saw a nice multiple gap one, made with 4 pieces of copper tube. Also, we are planning to replace the bottle caps with polypropylene caps. The only thing is that bottle caps give such a nice period look... The corona rings look wonderful! When we tested the coil for the first time, I noticed the spark gap fired for a few seconds, and then stopped. Thanks to this forum, I know now that is is a matter of quenching, and that performance can really increase when using a vented multi-gap. Being the coil a project for school, we can say it works, so mission accomplished. But when we saw our fist sparks, there was this we-want-more feeling

Re: Advice on bad coil performance.
Mike, Mon Feb 26 2007, 11:58PM

Hi, if you are having trouble with the caps, i might suggest one of my creations
It is basically a cap using plastic plates as the dielectric, they seem to be a much easier replacement to the bottle caps. Technically though they have not been tested in tesla coil use. So if you want to try them out, the thread has the details. Hopefully they should help and weigh quite a bit less then bottle caps.

Btw: nice looking coil there.

Mike
Edit: also, if your caps seem to arc over, it may be because of the NST's higher voltage. A simple fix would be to submerge the plates in a tub of oil or alike.

Re: Advice on bad coil performance.
Sulaiman, Tue Feb 27 2007, 12:14AM

I have tried home made capacitors, rolled and flat plate.
My advice is - don't go there.
Lots and lots of 'challenges' - Always ending in eventual failure.

- Removing air bubbles
- Corona
- flashover
- leakage
- spillage
- ........

If you don't use the 'traditional' bottle caps go straight to an MMC
Multi-Mini-Capacitors in a series/parallel array with voltage-balancing resistors.
Endless threads on this topic so I shall not elaborate any further.

Re: Advice on bad coil performance.
Mike, Tue Feb 27 2007, 01:38AM

hmm, with my testing with my plastic plate caps I so far haven't really had any troubles with them. the ridges on the plates stop flash overs and I used glue to hold down the al foil to prevent air bubbles. But yes, a mmc is really superior, though expensive...

Re: Advice on bad coil performance.
Annie, Tue Feb 27 2007, 08:24AM

Those mega-disc-capacitors look really super, Mike!

Re: Advice on bad coil performance.
sparky, Tue Feb 27 2007, 03:44PM

I'll give it a try!

Re: Advice on bad coil performance.
Annie, Sun Mar 04 2007, 03:55PM

Hi! Today we tested a vented gap, with 8 gaps of about 2.5mm / 0.1inch. http://vdhaaknl.dyndns.org/gallery/albums/album07/IMG_4047.jpg A Sunbeam case fan was mounted underneath. We tested it without the secondary coil, because we were testing indoors. We noticed, that we had to short 6 of the gaps to start the firing. What can this be? Is it the lack of load due to the missing secondary, or is it the bad quality of the bottle capacitors? I certainly hope it is the bad caps, since we ordered 25 0.47uF Cornell-Dubilier caps... We hope you guys can give us any clues....

Re: Advice on bad coil performance.
J. Aaron Holmes, Sun Mar 04 2007, 06:23PM

Did you have the cap and primary connected? Cap, primary, and secondary should always be present together, never separately. Just taking the secondary away is one of the worst things you can do to your poor primary circuit, since things will ring out of control with nothing to suck the energy away [quickly]. It's a good way to ruin your cap and NST. Best give those things a look by themselves to make sure they're still ok. If you have a Terry filter, it's possible it may have given its life (or half of if, anyway) to save your NST. If half your Terry filter failed short, or half your NST secondary failed open, you'd end up with only half the voltage out, which would certainly explain the need for less gap. Is your 8kV NST mid-point grounded?

Your gap looks real nice, BTW. Air flow will reduce the required gap spacing somewhat. Sometimes a lot, if you're using a real stong blast, but probably not too much in your case.

Regards,
Aaron, N7OE

Re: Advice on bad coil performance.
Annie, Sun Mar 04 2007, 06:44PM

Hi Aaron; Thanks for your message! We had the primary and caps connected.... I sure will not run the coil without the secondary anymore! I think the NST and bottle caps are still OK, I have a center-grounded NST, but the center (NST housing) is not connected with anything (floating). I think, our CDE caps will arrive this week, hopefully this improves results. Maybe, our spark gap and bottle caps work OK, but with the missing load of the sec. coil, failed to work properly? Can you confirm this?

Re: Advice on bad coil performance.
J. Aaron Holmes, Sun Mar 04 2007, 08:40PM

That is a little mysterious...can you draw an arc on your NST from either output to the case? Are you using a Terry filter to protect your NST? (highly recommended) I'm not too familiar with the ins and outs of bottle caps, so maybe there's something to look for there. Certainly having too *much* capacitance could cause shortness of voltage on the gap, but you said you'd already had it jumping .5" in the prior gap?

Hopefully somebody else jumps in with some thoughts.

I'd definitely recommend grounding the case of your NST to RF ground. You'll give the RF nasties someplace to go and equalize the stresses on your NST secondary. Don't use the mains ground, as a strike from secondary to primary will probably find its way to your RF ground in a very undesirable way (through your house wiring to ground, and from that ground to your RF ground--probably passing by a lot of other expensive electronics on the way!).

Regards,
Aaron, N7OE

Re: Advice on bad coil performance.
Annie, Sun Mar 04 2007, 09:00PM

Hi Aaron; Thanks for your message! I do not use a filter, I think, since the spark gap operates in parallel with the NST and primary LC resonance circuit, that any voltage that is too high, will be shorted by the gap automatically. I think the NST is working correctly, although I did not test the secondary windings separately. I am a little afraid to connect the NST housing (middle of secondary) to the RF ground, because current from the NST to RF ground will flow when sparks go from toroid to RF ground. They will be carried by the streamers from toroid to earth (this is dangerous, because then the current will be very high, and when one is accidentally hit by a toroid streamer, it could be lethal). When the caps arrive, I will test both sides of the NST separately and tell you if they are ok. Tnx for your reply, 73's!

Re: Advice on bad coil performance.
J. Aaron Holmes, Sun Mar 04 2007, 09:40PM

It's true that having the gap across the transformer helps a lot with RF, however you still have the spike caused by the spark gap to contend with. The snubbing action of the cap in the Terry filter will virtually eliminate this. For this reason, Terry filters are generally regarded as indispensable for NST-based coils.

I'm not sure I completely follow the comment about getting hit by streamers. By itself, I don't see how grounding the NST secondary midpoint makes the streamers more of a hazard. To have mains-frequency current flowing in the streamers, I would think you'd really have to have a primary-to-secondary flashover happen at the same time. And if that happened with the secondary midpoint floating, then the chances of the NST insulation standing off 100's of kV between secondary and mains ground (which is, to some degree, connected to your RF ground) to save you ought not to be relied upon!

EDIT: Not sure where you're at, but in the U.S., "neutral" runs straight to the mains ground at the breaker box, so even if you don't connect the green wire to your NST case, mains ground will still effectively be very near your NST's core by way of the neutral lead. Not sure how this works in other places, but I'd still wager you're not going to be saved by the differences.

Regards,
Aaron, N7OE

Re: Advice on bad coil performance.
Annie, Sun Mar 04 2007, 10:47PM

Hi; If the middle of the NST is connected to RF ground, together with the lower end of the secondary coil, there wil be a potential of 4kv 50 mA (8kV center earthed NST) between primary and secondary circuits. This voltage can run through the body, carried by a spark coming from the toroid. That is what I read on other sites, I don't know is this is really going to happen, but that is why I decided to keep the NST free from any earth connection. I doubt if the floating center of the NST decreases performance. Maybe someone has more clues on this ?? P.S. I live in the Netherlands. The mains here consists of Neutral and Live. In addition, an Earth electrode, grounded in the soil under our house, is connected to all earth pins in the wall outlets. Neutral to Earth should be 0V, Live to Earth is 230V, 50Hz AC. Equipment without necessary Earth connection is, of course, connected between Live and Neutral, LOL!

Re: Advice on bad coil performance.
J. Aaron Holmes, Mon Mar 05 2007, 03:37AM

Maybe I need somebody else to help me understand the argument here

Just because you have your secondary midpoint grounded to RF ground, the streamers are no more harmful than with the secondary floating. Whether you ground or not, you still need a strike from secondary to primary or something like that. Otherwise there is no circuit for your "4.5kV." Please explain how this 4.5kV gets into the streamers in your scenario.

Without your midpoint grounded, high voltages and RF kicking back into your transformer (esp. in the case of a primary strike) are likely to go bad places. Now, if you really want to avoid primary strikes, then proper spacing of primary and secondary, use of a strike rail, and (in some cases) grounding of the inner turn of your primary to RF ground are often indicated. This latter approach is common on coils powered by large pole transformers but is generally not done on NSTs because the midpoint ground prevents it (you'd be shorting half the transformer).

...but even if you do all these things, streamers can go unexpected places, and so they should always be regarded as dangerous.

Take care,
Aaron, N7OE

Re: Advice on bad coil performance.
Hazmatt_(The Underdog), Mon Mar 05 2007, 07:58AM

One of the major reasons I use a supply grounded source circuit neutral and separate earth ground strike rail & secondary ground is that I don't want the high RF energies feeding back into the house. Now they still do because of reflected load through the transformer, but in my case that usually just drives the light dimmers crazy. If I were however feeding back my ~750W worth of streamer energy to the neutral, it could be pushing 10-20A worth of RF current on the neutral and high enough voltage transients to arc to the low voltage wiring, causing a huge fire in the conduit.

Separate grounds in this case I think are safe grounds.

I should also mention just as a silly tidbit. One of the new guys working at the electronics store I used to work at and still frequent, has a friend who was testing a coil in his apartment. Turns out he didn't know what he was really doing when he had all of his grounds tied together. Long story short, he blew out his apartment's wiring and 3 other unit's wiring, destroying his electronics and other tenant's electronics. He's being sued for damages now and has to find a new place to live.