How to optimize spark gap Tesla coils?

[Alex Karatzky]

My elementary school had its first annual science fair a few weeks ago, so me and my friend signed up and we decided to do it on wireless electricity. So naturally, we built a tesla coil. It took a couple of tries, but we eventually got it to barely illuminate a portion of a fluorescent light bulb. We somehow managed to make it to the regional competition, but we need to make it at least power the full lightbulb.

The lightbulb only blinks when the spark gap flashes, which is every 1-2 seconds.
We used an HV transformer from a bug zapper which had 2 HV input and 2 HV output wires. The 2 input wires were attached to a 9V battery and the output wires were attached to each end of a capacitor bank comprised of 2 150k capacitors.

We used 18 AWG speaker wire to connect the capacitors to the spark gap and primary coil.
The secondary coil was made of 24 AWG magnetized copper wire wrapped around a 20cm long piece of 1 1/4' PVC pipe, while the primary coil was made from 10 AWG wire wrapped ~6 times around the secondary.

The coil seems to only power fluorescent lights (we tried 2 incandescent and 2 LEDs).

We used these 2 videos as guides during the building process, they were fairly easy to understand.



Any suggestions on how we can improve the coil? The regional competition is on March 28th.
Picture ---> https://postimg.cc/sMHVzjG8

 

[tech99]

I am not sure how to choose the capacitors for the primary spark circuit. Are they in series with the primary winding? Do you have a circuit of what you did?
(As a matter of interest, when the spark transmitter for the first trans Atlantic radio communication was built, the capacitors and tuning of the primary circuit were an area of disagreement between Fleming (an old style electrical engineer) and Marconi (a hands-on RF artist).

 

[tech99]

Without seeing a circuit diagram, I realized this morning that your set up has only one spark gap and is not the double system used by the trans Atlantic transmitter.
In this case, the primary circuit must resonate at the same frequency as the secondary, and the objective will be to adjust the capacitor to resonate the primary at the same frequency as the secondary. I will think about this more.

 

[Alex Karatzky]

Thank you very much for the help so far!

I found the circuit diagram
https://postimg.cc/mPZMXLfn

I just have one more question - what can I do to make it powerful enough to light a different type of lightbulb?

 

[tech99]

Alex
Thank you for the circuit. This is how it works.
The capacitor is slowly charged during one cycle of the stepped-up AC supply. When it reaches a certain voltage the spark gap breaks down. This creates a very low resistance path and a resonant circuit then exists with C and the six turn winding, and several cycles of RF occur.
The current in the six turn winding induces a voltage in the large secondary. For best results, the secondary and the six turn winding need to be resonant at the same frequency.
One of the design variables is the size and coupling of the six turn winding. The two windings form a double tuned transformer, which needs to have critical coupling. This is described in many textbooks. For design purposes, the secondary load probably consists in the main of the loss in the winding.
The secondary will step up the voltage to a very high value, but can supply only a small current. Therefore any load you apply must have very high resistance. A fluorescent tube meets this requirement but an incandescent lamp or LED does not.
It would be possible to check the primary and secondary resonances using various RF techniques, such as the old style GDO or a network analyser.

 

[Alex Karatzky]

Wow, thank you so much! I did not expect this much detail on my first post, but I'll be working on it over the next few weeks, now that I know what I'm doing :)

 

[tech99]

Wow, thank you so much! I did not expect this much detail on my first post, but I'll be working on it over the next few weeks, now that I know what I'm doing :)

The secondary contains about 36m of wire, so we will expect a resonance in the region of 5 to 10 MHz. It might be worth trying just one primary turn instead of six.