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My first tesla coil (SGTC)

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Dr. ISOTOP

Thu Aug 04 2011, 07:58AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

You use a synchronous motor, which rotates at a fixed multiple of the line frequency.
You need a fairly beefy motor for the sync gap; as I said, Richie's page explains it better than I ever could:

Herr Zapp

Thu Aug 04 2011, 08:23AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Teslacoil -

I think you may need to spend a little more time studying tesla coil theory of operation, construction techniques, and "tuning" before you start procuring materials or actually start construction. If you really have no idea what a safety gap is, or what it does, or how it's constructed, then you have some reading to do.

First, I believe the capacitor calculator at Deep Fried Neon may still be set up to give the "resonant" value for your tank capacitor. Unless you are VERY careful and conservative with your spark gap adjustment, you can easily destroy your NST and/or your MMC capacitor array due to "resonant rise". If you don't know what this is, then you need to spend some time at Richie Burnett's exceptionally in formatiave website learning about AC spark gap coils, and what resonant rise is. You will want a "larger than resonant" value for your MMC (LTR), around .008uF. Look in the 4HV archives for all the info you need on LTR capacitor value. Rather than trying to construct a giant MMC array of 90 capacitors with 90 bleeder resistors, I think you'll be better off using 18 or 19 Cornell-Dubilier 942C series capacitors in the .15uF/2kV model. These can be purcxhased for less than $4.00 each from C-D distributors or on eBay, and will probably be far more reliable than your 90-cap array.

Second, use the smallest gage wire you can wind, #27, #28, or #29. The lower the resonant frequency, the lower the losses in your spark gap, and the better the coil's performsnce.

It will take a large amount of tuning and optimization to get anywhere near 36" sparks out of a 15/30 NST. Be satisfied with 18-24" sparks initially, and then be prepared to spend time and energy to squeeze out each additional inch of performance. As noted, a synchronous rotary spark gap will likely be required to achieve 36" arcs, and building a reliable and safe SRSG is not a trivial exercise if you don't have access to a machine shop. Asking about the suitability of using a computer fan as the basis of an SRSG indicates that you may not understand what a SRSG is, or how one is constructed. Again, there is a tremendous amount of information on this topic in the 4HV archives, and on the web; you just have to do your homework.

Finally, using 1/2" copper tubing for your primary conductor is total overkill for a coil powered by a 15/30 NST. #10 AWG copper wire will work just fine, although 1/4" annealed copper refrigeration tubing is also commonly used and is very easy to form.

It's pretty much SOP for a first-time TC builder to be hyper-enthusiastic to start procuring parts and start construction;, however, you will save yourself a lot of frustration if you take the time up front to understand Tesla coil theory of operation, look at a few dozen different coils on the web to get a better understanding of construction techniques, and especially to understand TC tuning and exactly how you will need to "adjust" your TC to achieve resonance.

Here's a link to Richie Burnett's all-encompassing website. There's a huge amount of useful information here:

Herr Zapp

TeslaCoil

Wed Aug 24 2011, 05:17AM

Registered Member #4020 Joined: Thu Jul 21 2011, 06:40PM
Location:
Posts: 30

Can someone help me? I got the inductance to match but I really don't know where I'm going to find 764.537 inches of quarter inch copper tubing??

1314163005 4020 FT120573 Img 20110824 001435

Dr. ISOTOP

Wed Aug 24 2011, 06:03AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

764" is a bit over 60', which is neither terribly expensive nor very rare.

Herr Zapp

Wed Aug 24 2011, 06:59AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

TC -

Regarding your 18 turn, 64 foot primary:

1. 1/4" soft copper tubing is commonly available in 25' and 50' coils;

2. 18 turns is more turns than is commonly used for a TC of the size you are planning; 8-12 turns is a far more reasonable size. As you decrease the number of primary turns (decreasing the inductance), you'll need to increase your capacitor value to maintain the original resonant frequency.

3. Decrease the primary turns until you can make the primary from a single 50' coil of copper tubing.

4. Your calculated .006uF capacitor is the resonant value, which poses potential problems with resonant rise which can (under some conditions) can cause an uncontrolled voltage rise which can destroy your NST and/or your MMC. You want to use a "larger-than-resonant" capacitor, which in your case would be around .0075 - .008uF. Substitute this value in your calculations and see how many primary turns will be required (= how many feet of copper tubing you'll need).

5. Wind your secondary with #27 or #28 AWG magnet wire rather than #24. This will give more inductance and a lower operating frequency, which will reduce losses in the spark gap.

Herr Zapp

TeslaCoil

Wed Aug 24 2011, 11:22AM

Registered Member #4020 Joined: Thu Jul 21 2011, 06:40PM
Location:
Posts: 30

Right now I have it set up so that I'm using 1000 turns of 24 awg wire. If I choose a smaller wire wont that mean I will go over 1000? I heard 1000 turns is optimal?

Can I safely make my secondary smaller than 24 inches to keep it at 1000 turns without having to worry about arcs to the primary? Or do I just have to use more turns?

Herr Zapp

Wed Aug 24 2011, 07:33PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

TC -

There is absolutely nothing "magical" about 1000 turns on the secondary; anything between 800 and 1500 turns will work just fine, although more turns will provide more inductance, a lower frequency, and potentially less losses in the spark gap.

Many, many small TC's have been built with 15/30 NST power supplies, and the specifications for many of these can be found online. Check out the Tesla Coil Webring, if you haven't already done so.

The beauty of TC design is that there is no single "recipie" that's magic. There ARE general guidelines that will help extract the best performance and provide good reliability.

A 4" PVC secondary coilform is appropriate for a 15/30 powered coil, with a winding length-to-diameter ratio of 4:1 to 6:1.

For example, use a 23" long coilform with a 20" winding length, leaving 1-1/2" of exposed coilform at each end for mounting and wire termination. Wind this with #28 AWG wire, which will give ~1450 turns. Plug these specifications into your TC design program (I recommend JAVATC, from

), and determine the other coil parameters (primary dimensions, tank capacitor value, etc.). All these parameters are "interactive", will affect each other, and all can be "tweaked" to achieve final system resonance.

Herr Zapp

TeslaCoil

Thu Aug 25 2011, 03:38AM

Registered Member #4020 Joined: Thu Jul 21 2011, 06:40PM
Location:
Posts: 30

Which is better? A tungsten spark gap or a copper multi spark gap?

Herr Zapp

Thu Aug 25 2011, 04:49AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

TC -

For a 15/30 NST powered coil, a 5-6 segment copper-pipe gap will work fine, although the electrodes will occasionally need to be cleaned with steel wool, etc. to remove oxidation. Tungsten electrodes provide erosion resistance for high-power coils with very high primary current, but are totally unnecessary with a 30ma power supply.

You'll get better performance (via better quenching) if you build an enclosure for the copper pipe electrodes and add a an appropriately-sized fan to blow air through the gaps.

Herr Zapp

TeslaCoil

Fri Aug 26 2011, 12:50AM

Registered Member #4020 Joined: Thu Jul 21 2011, 06:40PM
Location:
Posts: 30

Can you look this over? The resonant frequency is still to high and I can't seem to desgin a primary coil that uses a reasonable amount of turns. Did I do something wrong?

I used DeepFriedNeons calculators.

Secondary Coil:

Diameter(D):4
Number of turns(N):1450
Wire Diameter (W):.0126
Turn spacing(S):.00126
-----------------------
Height(H):*20.079
Length of wire:18221.221in/1518.4350ft
Inductance(L): 38407.087uH
( Self Capacitance:7.778pF )

Â Top Load
Â D1 : 19in
Â D2 : 6in
---------
C : 21.089pF

Resonant Frequency of Secondary Circuit:

(C of topload)+(C of SecCoil)= (Total C of SecCircuit)
21.089pF+7.778pF=28.876pF OR .000028876uF
Inductance: 38407.087uH/38.407087mH
-------------------------- -
Resonant Frequency of SecCircuit:Â
Resonant Frequency(F): 151.128Â kHz

MMC

Calculator gave me .0053uF so I bumped it up to .008uF to compensate for resonant rise.

C: .008uF

Resonant Frequency of LC Circuit:

Inductance (L): .13863mH
Capacitance (C): .008uF
Resonant Frequency(F): 151.128kHz

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