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First tesla coil thread - Dual Mot

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Ash Small

Sun Nov 24 2013, 02:19PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'll make another shortly, but I add a bag of salt to a pan of water and boil it, then allow it to cool a bit, then pour into the bottles while still hot. Some further salt solidifies out, but it's still super-saturated. If it's cold, more salt will solidify out.

redruM69

Sun Nov 24 2013, 05:01PM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

Watched the video. I'd also say the cap is the weak point. I've had limited success with glass beer bottles with my first coil. You could try swapping those PET bottles with glass. Just make sure its clear glass, and not the brown stuff. (Corona or similar.)
They do occasionally explode with a 15kv NST, but should last longer with MOTs.
Also, what are the specs on the secondary? I'll figure out a rough guestimate on primary tap tuning.

MrFlatox

Sun Nov 24 2013, 08:07PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

I will not make glass bottle capacitor, because if I go with 25mL beer bottles, I would need 175 of those ... I will go with a MMC, in fact, the same as yours RedruM69.

As for my secondary, there is approx. 1100 turns of 0.45mm wire. The winding length is 50 cm, for a 125mm diameter. My toroid dimensions are : 60cm outside diameter, 150mm duct diameter. I measured the resonance frequency of the secondary with a signal generator and an oscilloscope, and it turned out to be about 165kHz.

redruM69

Sun Nov 24 2013, 09:33PM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

MrFlatox wrote ...

I will not make glass bottle capacitor, because if I go with 25mL beer bottles, I would need 175 of those ... I will go with a MMC, in fact, the same as yours RedruM69.

As for my secondary, there is approx. 1100 turns of 0.45mm wire. The winding length is 50 cm, for a 125mm diameter. My toroid dimensions are : 60cm outside diameter, 150mm duct diameter. I measured the resonance frequency of the secondary with a signal generator and an oscilloscope, and it turned out to be about 165kHz.

Tank capacitance value is not super critical in a async coil. You would just need to re-tune your primary coil

You could easily do it with 18 corona bottles. You'd have about 40-50nf.
That secondary has a calculated resonance of 147 kHz, so 165khz is in the ballpark.
With the ~50nf cap I describe, it should have a primary tap of around 7-8 turns.
It would work...

My first coil was a 4"x20", NST, static gap, and 12 beer bottles. I managed to achieve 24" streamers. Not optimal for sure, but at least it would give you something to tinker with until you built an MMC

MrFlatox

Sun Nov 24 2013, 10:37PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Unfortunately, my primary coil is no longer than 4.75 turns ...

redruM69

Mon Nov 25 2013, 03:23AM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

MrFlatox wrote ...

Unfortunately, my primary coil is no longer than 4.75 turns ...

Well, you would need to solder additional tubing to the end to make it larger then.

I was just trying to make the point that it didn't need to be ~130nf to be functional. I didn't even calculate mine to 131nf, it just ended up that way because its what I had, and it worked well. In fact the optimal size for mine is supposed to be ~293nf :)

MrFlatox

Mon Nov 25 2013, 11:06AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Yes, I know that 130nF is not an optimal value at all, and that it cans work with various primary capacitance as long as the LC constant (resonance frequency) is the same. In my case, my primay capacitor must be at least 130nF because of the primary max turns of 4.75. I don't think I will solder an extension of the primay because the structure was not made to accomodate a much longer primary. In that case, I would be better of doing another primary. However, there is already notches to have one more primary turn.

Kizmo

Mon Nov 25 2013, 12:43PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

And bigger capacitance means bigger bang energy which means more power and sparks :)

MrFlatox

Sat Dec 07 2013, 09:36AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Hi there !

Thanks to kizmo, i now have 942c caps to make a mmc, i will start soon, but i was wondering about the usefullness of balancing resistor in a ac operated tesla coil. Does it serve any purpose of balancing the voltage accross each cap ?

Thanks

MrFlatox

Sun Dec 15 2013, 02:11PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Hi !

At last, some good news ! Thanks to Kizmo for providing me some nice 942c caps, I built a mmc, and the tesla coil is now running fine. I went with a 7s7p configuration which gives me 14kV ans 150nF of primary tank capacitance.
I also redesign the spark gap to accomodate 8 moving electrodes, in order to achieve higher BPS. The results are quite good, and quite impressive for someone like me who had never built a tesla coil before. It's freaking loud, my neighbours are going to love me

MMC

New rotary spark gap

youtube video :

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