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My first coil, What can I do with 15w? - help needed

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LAN_403

Storken

Sun May 10 2009, 09:49PM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48

Hello 4hv!

I've been a lurker for Ã¥ long time now so it's time for me to try some of the same things as you do :)

Me and another friend (both 18, soon done with high-school "videregÃ¥ende") are planning to build a small sparc gap tesla coil for the end of this semester. Our science lab has a powersupply that we can loan, it's putting out 6kV, but is rather weak as it's only putting ut a current of 2,5 mA.

Following the guide on deepfriedneon.com I've come up with the design (that follows under here) as a suitable coil. Since I have no experience, I'd like some critiqe on whats good/bad and possible improvements.

We haven't ordered anything yet and so everything can be changed. Here is my calculations so far:

Everything here is calcutated using the equation tool found at deepfriedneon.com.

Spark length: 15w input shoud give 15 cm spark length, so this will be our goal.

Transformer: 6kV 2,5mA output.

Secondary: I'm thinking a width of 3cm with 1000 windings of 0,2 mm coated copper wire. If I calculate 0,2mm wire width + 10% as insulation I get a total heigth of about 220mm. The heigth/width aspect ratio is at about 7,3 then. Inductance: 3,7mA Capasitans: 3pF

Toroid: Image says it all, I will probably be getting this done at my school in aluminium. Capasistans: 6,5 pF
Toroid

Resonant frequency secondary circuit: Total values: Inductance: 3,7mA Capasistance: 9,54 pF. Gives a RF at 836 kHz.

Primary tank capasitor: Using the calculator I find that the transformer needs a 1,3 nF cap. I'm thinking of using this teqnicue (

<-- found there to make a cap that is a lot larger than whats needed. Is this a problem?

Primary coil: Using the value from the amount of charge that's "ideal" (1,3 nF) I found that I needed a Inductance of about 0,0028 mA on the primary coil.
I think I will use 1mm solid core copper wire for the primary. I will take this wire from some normal wire found in hard wall cords. (fantastic spelling xD) ~15 turns with 7mm spacing, 50 mm inne diameter gives a inductance that's near the required one. Will have to be at like more 14,25 turns to have it properly tuned... But that will be fixed once the coil is assembled. Total length of wire will be about 8 meter.

Spark gap: i'm not quite sure what to build here - but this is a draft on how I'd like it to be.
Sparkgap

Some questions:

1. Should I have more windings on the secondary?

2. Spark gap, is a pc-fan enough to allow for enough "quenching"?

3. Toroid, is hi-gloss polishing it positive?

4. Secondary, should it be slimmer and taller?

Hit me with your constructive replies and ideas please! :D cant wait to order the parts and actually start building it :P

101111

Mon May 11 2009, 12:05AM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

1. I think that will pretty much do the trick.
2. Yes.
3. It will look neater, but in performance It won't matter to much as long as the toroid don't have too sharp edges.
4. That really depends on how you want it to look.

All this calculating is good, but can also make a few problems when something didn't become as expected. Therefore I advise you to get a tube roll a few meters with insulated copper wire on it. Hook up a frequency generator, measure the frequency then calculate a capacitor bank and spark gap that will fit to exactly that secondary coil.

BTW where in Norway (obviously) do you live?

Regards Christian
Med vennlig hilsen Christian.

Storken

Mon May 11 2009, 12:26AM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48

Just outside Trondheim, Malvik. Attending the local hi-school here ;)

I'm abit worried my 15w transformer is weak and will give a lousy result, can anyone confirm/reject my thoughts?

So, I got abit confused over that way of building and measuring my build so that I can finish my build (spelling, dunno how to say it). As I see it: this is the way to get things done right:
1. Make the tesla coil.
2. Tune the coil with the primary windings so that the inductance matches good. (RF matches maybe?)

A comment on that question 1:
More secondary turns = higher resistance = smaller sparks. More secondary turns = higher voltage = higher abillity to lead current = longer sparks. Am I right?

Also, the 6kV will have no problems breaking down the air in the spark gap - so this isn't a problem right? If the spark gap is adjustable - I do not need to calculate "opening" in advance - I can do it after, Correct or not?

I promise you alot of naive questions here, just bear with me and I'll do my best!

101111

Mon May 11 2009, 01:07AM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

More secondary turns equals higher resonance frequency.
Less secondary turns equals lower resonance frequency.

You dimensioning the copper wire diameter roughly after your resonance frequency and wire length, then you won't have a too big loose.

When it comes to spark length, isn't necessarily lower frequency the best or higher frequency the best but something in the middle. It depends on what "switching topology" you are using.

6 kV @ 15 Watt might work then again it might not... So personally i would aim for something a little higher. Try to find a few old microwave ovens. And for gods sake read the 4hv Wiki about safety when it comes to high voltage!

Tribologist

Mon May 11 2009, 01:26AM

Registered Member #1879 Joined: Mon Dec 22 2008, 04:11AM
Location:
Posts: 40

Oil ignition transformers are an other decent choice. They should be similar in current and voltage in Scandinavia as they are here. Check with some local plumbers if they can help.

Ulf

Diarrg

Mon May 11 2009, 02:25AM

Registered Member #1904 Joined: Sat Jan 03 2009, 02:40AM
Location: Pittsburgh
Posts: 14

If you need to lower capacitance of the cap bank, stringing the caps in series will cut capacitance but increase the voltage it can handle (voltage is additive, capacitance is the inverse of the sum of the inverses of the individual capacitors) So if you put two caps together, the voltage it can handle doubles (You should shoot for around 8kV just to be safe), while the capacitance is cut in half.

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