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First light on my little coil

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LAN_403

Coronafix

Mon Oct 16 2006, 10:37PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Hi All,
I finally finished my little coil over the weekend and got first light with it.
It uses a 1560 NST run through a variable autotransformer,
3 strings of 0.047uf 3kv capacitors, 8 in each string, giving 17.6nf total capacitance.
4.7Mohm bleeder resistors over each cap.
A 5 electrode static spark gap that is fully adjustable.
The secondary is wound from a microwave fan motor wire with approx. 1430 turns,
on a 55mm diameter pipe and 220mm high. Ratio 4:1
The primary is made of 3mm wire soldered together to get 7 turns, tapped at 6.5.
I also have a safety spark gap across the NST.

I adjusted the static gap to spark at 5kv, because that is all I thought the little coil could handle
without getting racing sparks or perhaps going up in flames

, but so far I have cranked it up
to about 12.5kv without an adverse reactions.
So far getting 8" sparks to grounding point.
I haven't got any good photos yet of it running at high power, will try again tonight.
Here is the setup and some crap action shots running at 5kv:

...

Mon Oct 16 2006, 10:53PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You have way too little coil for 900w of NST... That guy should give 3 foot sparks...

Your cap seems to match the transformer about right (if it isn't a little too big), but that coil just is too small. SGTC's can't do sparks 3x as long as the secondary like DRSSTC's can, you just get racing arks.

I suppose if it works it works ;)

Coronafix

Tue Oct 17 2006, 02:43AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

I know that it is too small for the NST, but I haven't sourced the wire yet for my larger coil.
This is why I adjusted the output for only 5kv, but was suprised to see that it has so far taken
12.5kv with no racing sparks.
The cap is built for my larger coil and made LTR for the static spark gap I'm using.

What is the advantage of increasing the spark gap spacing?

...

Tue Oct 17 2006, 04:08AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Well you want to adjust the gap so that it fires when the cap is at the max voltage, not when it is only half charged up (as then all of the power that would have gone into charging the cap is wasted)...

Coronafix

Tue Oct 17 2006, 02:21PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Here is a higher power photo after I opened up the gap to fire at 10kv.
Even though i had cranked the power up to full potential previously, with the gap set
at no load sparking 5kv, there was no racing sparks.
Here you see the racing arcs, and the 8" spark jump to ground.
And let me say it's a lot louder.
A friend measured the db at 101 the other night and that was only at 5kv input.
My ears are still ringing after tonights play.
This goes to show how crucial spark gap setting is!
I'm sure most of you know this, but does anyone have certain techniques for setting
the spark gap? I just use the output of NST and variac set to appropriate voltage.
By the way, I have a lot to learn about tc photography, it seems crap compared to other photos I've seen.
ie: TDU, EVR, etc.

EDY19

Tue Oct 17 2006, 05:05PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

When setting the spark gap, most people set it so it just arcs when there is no primary capacitor in place. This ensures it fires at the highest voltage that you designed everything else around (Specifically the caps) I think :-0

Jorrit

Tue Oct 17 2006, 08:58PM

Registered Member #140 Joined: Sat Feb 11 2006, 11:43AM
Location: The Netherlands
Posts: 14

I would rise the secondary a little bit more. This lowers your coupling but should stop the racing sparks. It also could be that the coil is detuned. When the coil is detuned you also will get racing sparks. You should be able to get a lot more out of that coil without racing sparks. Also try to put a vacuum cleaner or something that moves a lot of air on your sparkgap. If you do this the apark gap will quench better and you will get bigger sparks.

Coronafix

Wed Oct 18 2006, 01:16AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Didn't think to try some air across the gap, it doesn't get very warm, but could be good
for quenching as you said Jorrit.
It may also be slightly out of tune too, I haven't played a lot with the tuning.
I thought of adding another top load as well because it breaks out very easily.
I'd like to see the top get some more potential on it, but will need to recalculate the tapping point.
May need more primary turns for that.
All together, I'm fairly happy with my first coil.

Dr. Drone

Wed Oct 18 2006, 01:17AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Coronafix

Wed Oct 18 2006, 04:32AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

That is raised a fair height! Will also try raising the secondary.
The reason I designed the spark gap the way I did is for the adjustability,
I could raise each gap individually to get the distance I wanted.
My theory also was to follow the natural curve that an arc takes in the air.

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