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New at this, advice would be greatly appreciated

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Diarrg

Sat Jan 03 2009, 03:38AM

Registered Member #1904 Joined: Sat Jan 03 2009, 02:40AM
Location: Pittsburgh
Posts: 14

Building my first coil after several years of waiting and wishing I could. And by new, I mean that what I know is what I've learned from deepfriedneon et al.

Being somewhat limited in the budget for this project, I've decided to construct my own Hawg capacitors. Any advice on this or where to get cheap MMC components?

Right now, I have a 7500V @ 30mA NST, and the topload, a 4" diameter steel gazing ball. I'm planning on constructing a small coil, around 3"x18". The copper wire for the primary will be here shortly, and deepfriedneon tells me I'll need about 707 ft of #25 wire, which I'm relatively sure I have a source of. I'm going to make a spark gap on my own, which doesn't seem too terribly difficult.

I'm sure something in the preceding made it readily apparent that I am very new to this, and I welcome all the advice you'll lend me. Have I missed something completely? Have I thought something was far easier than it truly is? Have I gotten myself into something I cannot possibly accomplish without dropping large amounts of cash? Advice on the construction of Hawgs and spark gaps would be great, since I'm not 100% certain of the capacitance values for the Hawgs. Does anyone know a way of calculating it?

Finally, is there any way of testing the individual components out before hand, or is the best method to simply toss it all together, pray, and turn it on? No detail is too glaring or obvious for me to have had to notice it, so stuff on the level of "don't look at the spark gap" would be greatly appreciated.

Phillip Slawinski

Sat Jan 03 2009, 03:57AM

Registered Member #1732 Joined: Thu Oct 02 2008, 02:34PM
Location: Huntsville, AL
Posts: 112

- Bart Anderson's site and the home of JavaTC. JavaTC is great for designing your Tesla Coil and making sure you have the right number of primary turns, tank capacitance, etc... The size of secondary you're using seems about right for your transformer. You'd be better served using a larger topload than that gazing ball, I'd suggest building your own toroid if cost is an issue. You can buy two baking pans from a supermarket, some flexible aluminum tubing from the hardware store, and some aluminum tape. There are plenty of tutorials online on how to make your own toroid.

I would forget about the home-made caps unless you really can't spare the cash for MMC capacitors. D.C. Cox <**link**> has about 400 MMC caps in stock now, and his prices are quite fair.

Diarrg

Sat Jan 03 2009, 04:19AM

Registered Member #1904 Joined: Sat Jan 03 2009, 02:40AM
Location: Pittsburgh
Posts: 14

Thanks for the info. It only spawn more questions, though. What would the resultant size be for this toroid?
And what exactly is reasonable? And would plate capacitors be any better?

Herr Zapp

Sat Jan 03 2009, 06:49AM

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

Diarrq -

For help designing a first coil, you might also consider using WINTESLA, another software design tool that's not as comprehensive as JACATC, but a bit simpler to use and understand, at least until you get more familiar with Tesla coil design.

As Phillip suggested, your chances for quick success will be improved if you build an MMC (series/parallel array of small commercially-made high voltage pulse capacitors) rather than trying to fabricate a home-made capacitor.

A better topload for your 3" X 18" coil could be inexpensively made from a piece of semi-rigid convoluted-aluminum HVAC duct material. This is readily available from Home Depot or any hardware store carrying heating and air conditioning supplies, and costs less than $10. Form it into a 12" diameter donut shape, add a plywood or foam-core board center, and you have a very effective toroidal top load that will give better performance than your 4" gazing ball could.

The Tesla coil design software will allow you to input the parameters of one component (like your secondary: diameter, length, wire gage, etc) and then it will calculate the dimensions and values for all the other components. If you don't have electronic test equipment like an oscilloscope and RF frequency generator, the only way to get a coil to function is to carefully the dimensions and values that are required to obtain resonsnce between the primary and secondary circuits, and then carefull build these components to meet the dimensions and values.

Regards,
Herr Zapp

HV Enthusiast

Sat Jan 03 2009, 01:23PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

If you go to my website (listed below) and click on Amateur resource, i have a nice PDF paper which outlines how *I* design my coils. This should help out and show how i use different (free) programs to design my coils.

Phillip Slawinski

Sat Jan 03 2009, 03:34PM

Registered Member #1732 Joined: Thu Oct 02 2008, 02:34PM
Location: Huntsville, AL
Posts: 112

Diarrg wrote ...

Thanks for the info. It only spawn more questions, though. What would the resultant size be for this toroid?
And what exactly is reasonable? And would plate capacitors be any better?

Reasonable is (about $3.00) per cap. I know Bart Anderson has a MMC calculator on his site you can use to figure out how many MMC caps you'd need to construct your MMC. Of course you'll want to find out what size capacitor you are going to need first.

Plate Capacitors, Bottle Capacitors, and generally all Home-Made capacitors almost always end up leaving a big mess all over your floor, or performing very poorly. Since you're using a 7/30 NST there is not a lot of input power to begin with, you do not want a large percentage of this wasted in the capacitor.

Have a look at Gary Lau's Table Top Coil, as it is very similar to what you'll be building.

Diarrg

Sat Jan 03 2009, 11:23PM

Registered Member #1904 Joined: Sat Jan 03 2009, 02:40AM
Location: Pittsburgh
Posts: 14

I looked around on Digikey and found some 0.047 uF capacitors rated at 3kV. 4 of these should give me 0.0117 uF, and a rating of 12kV. Would this work? It's more efficient than 14 of DC Coxs .15 uF capacitors, I'd think, but farther away, considering Coxs give me .0107 uF. (The capacitance that deepfriedneon recommends for my NST is .0106 uF. How close should it be to this? Off by .01? .001?)

Herr Zapp

Sun Jan 04 2009, 01:24AM

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

Diarrq -

1. Your 7.5KV, 30ma NST actually needs a tank capacitor value of around .014 - .016uF. The value you mentioned (.0106uF) is a theoretical "resonant" value, and can interact with your NST to cause extremely high voltages that can destroy both your NST and tank capacitor. Unfortunately, the transformer/tank capacitor calculator at Deepfriedneon doesn't make it clear that it is calculating the resonant value, and not the "practical" value. What you need is a "LTR" value ("Larger Than Resonant") that will avoid creating these destructive high voltages. A good rule of thumb for an LTR MMC is to select a value that's around 1.4X the "resonant value", or in your case around .015uF.

2. To survive in a Tesla coil tank circuit, capacitors must have a dielectric material with low loss at high frequencies, must have an adequate voltage rating, and must be capable of handling very high peak current. Just selecting a randon HV capacitor from Digikey is unlikly to meet these requirements. Through years of experimentation in Tesla coil circuits, the Cornell-Dubilier 942 series of film-foil capacitors have been proven, time and again, to be the best capacitor to use in MMCs for small to medium sized Tesla coils. These can be bought directly from any of a number of Cornell-Dubilier distributors, or from several sellers on eBay, or from other sources. Buying direct from a distributor will yield the lowest cost per capacitor, but there may be a minimum order quantity. Buying from "middlemen" will add cost, but you will be able to buy only the quantity you actually need (in your case, 10 or 11 of the .15uF, 2KV caps, C-D model 942C20P15k).

3. Remember that to get a Tesla coil to "work", the primary and secondary circuits need to resonate at the same frequency. If they don't, your coil won't generate any output at all. To make sure that the resonant frequencies of the primary and secondary circuits will match, you need to completely design the coil "on paper" (using a TC design program) before you start constructing anything, or buying any parts. Then, you need to follow the design parameters EXACTLY as you build the coil.

4. The world's best source for information on how Tesla coils operate, with a description of the function of every part, and critical design information, is Richie Burnett's website at:

Spend some time reading the section on spark gap Tesla coils, and you'll have a much better chance of getting your coil running with minimal problems.

Regards,
Herr Zapp

Diarrg

Sun Jan 04 2009, 02:43AM

Registered Member #1904 Joined: Sat Jan 03 2009, 02:40AM
Location: Pittsburgh
Posts: 14

1. Thank you Herr Zapp, I wasn't aware of that LTR value.

2. I did in fact find those CDE caps, but the 940 series, not the 942 series, which I will look up.

3. Isn't the resonance of the primary coil changed by tapping it at different turns?

Herr Zapp

Sun Jan 04 2009, 03:55AM

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

Diarrq -

1. If you use JAVATC to design your coil, and are using a static spark gap, JAVATC will automatically recommend the appropriate LTC tank cap value to match your 7.5/30 NST.

2. The Cornell Dubilier 940 series capacitors utilize metallized polypropylene film plates, while the 942 series caps utilize actual metal foil plates. The 942 series caps have higher peak current ratings, and are better suited to Tesla coil use.

3. Yes, the resonant frequency of the primary coil can be adjusted via different "tap" points, but only within a very narrow frequency range. This is one of the most common problems encountered when you don't carefully calculate all the design parameters for your coil: it can't be brought into "tune". It can't be brought into tune because the resonant frequencies of the primary and secondary circuits are so far apart, there isn't enough range of adjustment in the primary to get it to match the secondary frequency.

In your first post, you commented: ".... or is the best method to simply toss it all together, pray, and turn it on?"".

If you just "toss" parts together, your chances of getting a coil to operate are about nil.

The design of a Tesla coil system is a well-understood science. If you carefully perform all the design calculations for your coil, and carefully build it to the dimensions and parameters provided by the design calculations, it WILL WORK, the first time power is applied. It may still need final tuning and tweaking to obtain best performance, but it will generate sparks the first time. There is no "praying" or black magic involved, just basic science.
That's why it is so important perform all the design calculations before you start buying parts, or building anything. You can either take many hours do all the calculations by hand, or do it in 20 minutes using Tesla coil design software like JAVATC; its your choice.

Regards,
Herr Zapp

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