Stuck on Tesla Coil

[skyler]

(go get coffee to wait for the images to load)
Hi everyone,

For the semester, I have been working on a friend on a tesla coil at a club we started at my high school. We seem to get closer and closer, yet more and more problems come up. We are almost totally out of funds, yet everything seems like it is breaking.  Here is a bunch of photos on our process, and we would appreciate any help or advice on what to do next.

First, the power supply. We have picked up a bunch of Microwave oven transformers. To be exact, we have 6 of them lying around, and another 2 with dead primaries. We were going to first put four in series to get the minimum voltage needed, 8k, but they arced internally, and we have figured out, two is the limit in series.



Here is the image of two MOTs together, and following is the rest.



With the two MOTS, we realized we would need a voltage doubler, though. We had 5 microwave capacitors, and 4 diodes, and we used 2 of the caps, 2 of the diodes, and another set of 4nf caps to get a 1nf cap across the diode. We used the circuit below:


We tested the circuit, and had great arcs, and they "looked" like high enough voltage for a power supply. Great! Until..... A capacitor popped. Testing it with all of the other capacitors left, the arcs are very small when we make the doubler, and we made sure the capacitors and diodes worked. Here is a few pics of our capacitors and diodes:

High Voltage diodes

1nf Cap bank across the diodes(the blue caps)

Here are the capacitors we have left (we actually might have 3 more microwave capacitors, and 3 diodes at my friends house Note they are all different sizes , but all close to 1 microfarad )


Oh yeah, we forgot to put a safety gap if that is important! We made one since:

As shown, we have an electrode for +V,-V,and Gnd from the output of the Doubler.


OK, so on the power supply, what do you suggest? We know we should probably get a NST, but we can't afford one, and for our size of tesla coil, we would atleast need two. (in Parallel)Does anyone know where to get one for a REAlly low price, or maybe someone who would sell one to us for cheap

Next is the Primary Capacitor:

Because we are cheap, we created a capacitor using a bucket, beer bottles, motor oil and salt water as shown in this youtube video:

•https://www.youtube.com/watch?v=hjqKcRQB3RI

Here is a photo of ours:


We measured the capacitor, and got somewhere around 15 NanoFarads. We are hoping this capacitor will work for the tesla coil, those Cornell Dubilier caps are pretty expensive!

Rotary Spark Gap
I do not have much pictures for this, we are working on this at my friends house, but I think that will be pretty straight forward, we got 5/32 Tungsten rods, and a 3400RPM pool pump motor, which we will attach using non-conductive plastic from instructions on tb3.com>tesla coil>spark gap We know to put a biggg box around this, because the force will be massive flying around on the motor, and the UV will be bad. I did find this picture, though of the tip of the motor, which we will be attaching the tungsten.


The coil!

The pancake coil on the bottom consists of copper tubing (I think it was quarter inch but it might be bigger). We got 20 feet, thinking that would be enough, but looking at the amount of turns, we may be short. ( I don't know though)

The diameter of the primary coil, which the secondary wraps around is one foot.




The secondary coil

For the secondary, we took a 4 ft by 1ft diameter Sona tubing, and coiled it with fishing line for spacing, and copper wire. We used almost the whole role of 100 ohms of 24AWG wire, which calculated to about 3500 Feet.

Does the primary and secondary coil "work" for resonance, and will this be adequate? We know the secondary is a bit weird just sitting on the base, and we will be able to raise and lower the coil, we have not glued it down



Top of Secondary coil

Toroid

We have a few things laying around for the toroid, will these work?



Thanks for any help, tips, or advice, We really wanted to get this coil done before the end of the school year, but without any help, we are just stuck.



Our work area!

[Halfdead]

If you keep an eye out on eBay you can usually grab a Franceformer 15KV 30mA NST with SCGFP for around 50 bucks. The SCGFP is easily bypassed, or can be made to run with it if you wire everything correctly.

[kc9qvl]

the primary and secondary coil/cap are not even close to resonant. Increase the primary coil. Better wind a smaller secondary. What you have can be made to work. but don't expect large sparks.

[FrankenPC]

I created a toroid using that aluminum dryer duct and a thick piece of cardboard cut into a circle.   The cardboard is the  form for the center.  Then, I used aluminum duct tape found at the hardware store and wrapped the whole thing with the tape while carefully smoothing (burnishing) the tape with a spoon as I put down each layer.  The end product was very solid. 

[FrankenPC]

For the best power output, use formulas. 
http://www.teslacoildesign.com/design.html#design_primarycap

Also, I chose to use an air-quenched spark gap.  It had dual tungstein gaps being quenched with a small box fan.  Cheap and decent performance vs mechanical rotary types.

[skyler]

the primary and secondary coil/cap are not even close to resonant. Increase the primary coil. Better wind a smaller secondary. What you have can be made to work. but don't expect large sparks.

Thanks, so get a lot more turns on the primary basically. How many turns do I need? That secondary took hour after hour of torturous winding, we went through many movies over weeks at my freinds house. We don't want to rewind that, can we get away with just the primary different?

[FrankenPC]

Thanks, so get a lot more turns on the primary basically. How many turns do I need? That secondary took hour after hour of torturous winding, we went through many movies over weeks at my freinds house. We don't want to rewind that, can we get away with just the primary different?

If you look at that formula link I sent previously, you can reverse engineer the primary from what you currently have with the secondary. 

I agree.  Winding the secondary is a nightmare without a jig.  I ended up using one of those geared 1/2" drills and a big variac to control the drill speed to spin the secondary while I guided the wire with leather gloves.  It's not too bad with a drill .

[IanB]

Thanks, so get a lot more turns on the primary basically. How many turns do I need? That secondary took hour after hour of torturous winding, we went through many movies over weeks at my freinds house. We don't want to rewind that, can we get away with just the primary different?

It sounds like you are  trying to design it as you go along. This is probably not a good idea for a first project. A Tesla coil is a finely balanced thing. If you get it slightly wrong it won't work.

For a first project, it is much better to build something to a known and working design, and follow the plans exactly. You can experiment a bit on the second and third one, after you have learned a little from the first one.

[skyler]

It sounds like you are  trying to design it as you go along. This is probably not a good idea for a first project. A Tesla coil is a finely balanced thing. If you get it slightly wrong it won't work.

For a first project, it is much better to build something to a known and working design, and follow the plans exactly. You can experiment a bit on the second and third one, after you have learned a little from the first one.

I guess our problem is the equations for all of the sizes and ratios etc. scared us away and we couldn't figure anything out like java tesla coil design.

[IanB]

I guess our problem is the equations for all of the sizes and ratios etc. scared us away and we couldn't figure anything out like java tesla coil design.

What I meant was to find some plans to follow where someone else has done the design. Such plans would say "build it to exactly these dimensions", "use exactly this many turns", "use exactly these parts", and so on. In other words, avoiding the need to do calculations.

[skyler]

For the best power output, use formulas. 
http://www.teslacoildesign.com/design.html#design_primarycap

Also, I chose to use an air-quenched spark gap.  It had dual tungstein gaps being quenched with a small box fan.  Cheap and decent performance vs mechanical rotary types.

Thanks for the link, just wondering, I think I found an error and i want to make sure, the primary resonant capacitance formula should be in farads instead of microfarads, is that correct, because it would not work with their example how they wrote it.
Primary Resonate Capacitance (uF) = 1 ? (2 × pi × NST Impedance × NST Fin)

Did they mean
Primary Resonate Capacitance (F) = 1 ? (2 × pi × NST Impedance × NST Fin)

I am also confused on what units to use:

"Secondary Coil Turns = (1 ? (Magnet Wire Diameter + 0.000001)) × Secondary Wire Winding Height × 0.97"

What Unit for the Magnet wire diameter?
What Unit for the Secondary Wire Winding Height?


And then the site does not even let me print the page or copy text because of "Copyright Infringement Issues" but clearly they are incomplete

[skyler]

If you keep an eye out on eBay you can usually grab a Franceformer 15KV 30mA NST with SCGFP for around 50 bucks. The SCGFP is easily bypassed, or can be made to run with it if you wire everything correctly.

Only thing up now is this Franceformer transformer 30ma 15kv

•http://www.ebay.com/itm/NEW-Franceformer-15kV-30Ma-Neon-Transformer-Gaseous-Tube-Power-Supply-w-Warran-/121030219757?pt=BI_Circuit_Breakers_Transformers&hash=item1c2df697ed

Wayyy Over priced for us, $170 Plus 30 shipping!! That is a nightmare 200 bucks for a stinking transformer, and we probably want two for 60ma. I hate being a poor highschooler with nobody to sponsor our club