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Advice on bad coil performance.

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Annie

Sat Feb 24 2007, 01:36PM

Registered Member #547 Joined: Sat Feb 24 2007, 01:27PM
Location:
Posts: 9

I hope I can ask you guys for advice on the Tesla coil my son had to build for a school project. We designed It using Tesla Coil CAD v2. I, being a dad, also put a reasonable amount of effort in this thing... Pictures of construction and testing are on my webpage: http://vdhaaknl.dyndns.org/gallery/view_album.php?set_albumName=album07
As we tested it for the first time, there was arcing between the common bottle caps terminal and the outside electrode of the caps themselves. Since I had connected the outside electrodes with wire to the common rail, we removed these wires and covered the caps compartment bottom and sides with aluminium foil. This foil was then secured under the common rail. After this, the arcing vanished, but the thin layer of air between foil and outside of the bottle caps was ionising. I don't know if this is bad or not... We put the cover plate with the primary back on top of the case, and continued testing. The maximum of spark length we were able to reach was about 4 inches. Also, the spark gap's maximum distance was about 0.5 inch with our 8 kV, 50 mA NST. When set larger, the spark would not fire constantly. Now I would like to ask, how to improve performance on our coil. Maybe there are losses due to the MDF woodboard on which the spark gap was mounted, or is the ionising at the outside of the caps not a good thing? I certainly hope to hear from someone,

regards, AndrÃ©

Sulaiman

Sat Feb 24 2007, 02:22PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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.

ragnar

Sat Feb 24 2007, 02:31PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

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

Marko

Sat Feb 24 2007, 02:43PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

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

Hazmatt_(The Underdog)

Sat Feb 24 2007, 09:05PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

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.

sparky

Sat Feb 24 2007, 11:05PM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

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

Nik

Sun Feb 25 2007, 12:23AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

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.

Annie

Sun Feb 25 2007, 01:01AM

Registered Member #547 Joined: Sat Feb 24 2007, 01:27PM
Location:
Posts: 9

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.

sparky

Sun Feb 25 2007, 07:38AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

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!

Annie

Sun Feb 25 2007, 12:11PM

Registered Member #547 Joined: Sat Feb 24 2007, 01:27PM
Location:
Posts: 9

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

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