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Hey guys! I just joined this forum to get some help with my first tesla coil. Im building it for a science fair and Im counting on it to make my team win :P. We're almost done with the construction, all we have to do is get the capacitors and put everything together. For now, I have two questions- 1. I have made a toroid by bending a RUBBER pipe to shape and wrapping it with aluminum tape. Will this work? 2. I need help with capacitors. We're planning to use an MMC arrangement. How do I calculate the capacitance of the MMC combination and how do I find out the best arrangement for 'em? I am using a 9kV, 30mA NST.
Oh, one more thing. Can you guys tell me some do's, dont's and tips I should keep in mind while constructing and operating the tesla coil? THANKS!!
PS: This is the third forum I signed up for to get some help with this project. I got few and unclear replies from the last two forums. So Im REALLY counting on you guys.
Alright I got that. So basically the equivalent capacitance of my MMC array should be 0.0106 uF right? But I dont get the 31kV part. Should the capacitors be able to withstand that voltage? And what about the toroid?
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
interloper -
Let's start with the basics.
If I am reading your first post correctly, it sounds like you have already built your primary and secondary coils. If so, what is the calculated resonant frequency of your system? What is the capacitance value of your primary capacitor that will achieve resonance with your secondary circuit?
MMC are typically constructed from series-parallel arrays of relatively low-voltage capacitors. A number of capacitors are connected in series to achieve the required voltage rating; for your 9kV NST, a target voltage rating might be 15-20kV. If you are using capacitors with a 2kV voltage rating, connecting 10 in series will provide a 20kV voltage rating (its actually more complicated then this, but this is the basic idea). Next, you need to determine how many of these 10-capacitor "strings" will need to be connected in parallel to achieve your target capacitance value (in your case, about .014uF).
The formula for determining the capacitance value of series-connected capacitors is shown at:
The formula for calculating the capacitance value of parallel-connected capacitors is shown at:
All this presupposes that you understand the concept of resonant circuits, and understand that the resonant frequency of your primary circuit (primary capacitor and primary coil) MUST precisely match the resonant frequency of your secondary circuit (secondary coil and topload).
An essential step in building a successful Tesla coil is to COMPLETELY DESIGN every component BEFORE starting construction. This way, you will know the exact dimensions of the primary and secondary coils, the exact dimensions of your toroid, and the exact value of your primary capacitor. All these values and dimensions must be correct for the system to achieve resonance; if the primary and secondary circuits do not resonate at bthe same frequency, your Tesla coil will not work (will not generate any sparks).
It might be best to plug the parameters of your primary and secondary coils, and your toroid dimensions, into JAVATC and see what the primary capacitor value needs to be. If it's very far from .014uF, you might need to wind a new secondary, change your primary coil, or make other changes so both circuits resonate at the same frequency.
JAVATC will help you see how changes in any one component will affect the values of the other components.
Alright, Im working on the MMC array now. What should be resistance of each bleeder capacitor which I will connect in parallel across each capacitor? and can I use the toroid I described in my first post? Also, how is the secondary coil connected to the toroid and RF ground?
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
interloper -
Your questions seem to indicate that you may have very little understanding of how a Tesla coil works, or how the various components are interconnected. I think you need to spend some time doing some basic research and gain an understanding of what a Tesla coil is, the theory of operation, and what each component part does. I've seen far too many cases where someone sees a Tesla coil on a website and blindly tries to "copy" it, not understanding anything about tuned circuits or resonance, and then gets discouraged or PO'd when the coil doesn't work. Asking the builder crucial questions like "what is the resonant frequency of your coil?" or even "how many turns of what gage wire did you use on your secondary coil?" elicit only a blank stare.
All the questions you are asking are very basic, and the answers are readily available on the web; you just need to do some research yourself. Check out the 4HV archives (links are on the 4HV home page), and the archives of the Tesla Coil Mailing List (links are also on the TCML homepage at ). Use the search box to enter specific questions, and be prepared to do some reading.
The website that does the best job of providing basic technical information on Tesla coil design, theory of operation, and the function of the various parts is Richie Burnett's site at: Spend as much time here as required to get a good idea of how a tesla coil works, what each part does, and especially what primary/secondary resonance is and how it is achieved.
At a minimum, study the sections on spark gap Tesla coils, including Tesla Coil Components, Tesla Coil Operation, and Quenching and Coupling. Without an understanding of how a Tesla coil works, you will never be able to "tune" your coil and get it to produce sparks.
Regarding previous attempts to help you, did you enter your primary and secondary coil data into JAVATC to determine what your resonant frequency should be, and what value primary capacitor and toroid size is needed to achieve resonance? Did you even download JAVATC? If not, please do it!
(JAVATC is a very useful Tesla coil design tool that will help you design every part of your coil so it will achieve resonance, and will allow you to evaluate various design parametsrs and optimize the coil based on components you may already have.)
While you're thinking about the above, here are a few answers to your latest questions.
1. You will want to connect bleeder RESISTORS in parallel with each of your MMC capacitors, not bleeder CAPACITORS. You do understand the difference between a resistor and a capacitor, right? A typical resistance value will be around 10 meg ohms, and use 1/2 watt resistors to help avoid flashover. Vishay type VR-37 resistors work very well because they have a much higher voltage rating than common carbon film or metal film resistors.
2. You state that you are "working on your MMC array now". What is the target capacitance value for the array you are building, and how did you determine the required capacitance value?
3. What type (manufacturer, model and value) of capacitors are you using in your MMC? The primary capacitor is the most highly stressed component in a Tesla coil, and usually causes the most problems if the correct capacitor type is not used, and if the MMC is not designed and constructed properly.
4. You didn't provide any dimensional data on your toroid, so it's impossible to state if you "can use it". The construction you described (aluminum foil over a "rubber pipe") can work, as long as all the foil segments make good electrical contact with each other, there are no sharp points or edges exposed, the toroid is of the correct dimensions to meet your topload capacitance requirements (from JAVATC), and the toroid is properly connected to the top turn of the secondary coil.
While you are doing your research, take a look at another current 4HV thread by a first-time coil builder ("Need some advise ....."). =P is also using a 9kV, 30ma NST, and has assembled a fairly clean setup using a MMC primary capacitor. His coil is running, but needs some additional tweaking to achieve best output. =P has clearly done his homework, and I'm sure that with a little help from 4HV members he will get his coil running properly. Take a look at his photos and construction techniques, and post some photos of what you've assembled so far.
I guess I came across as an unknowledgeable person lol. I do understand how a tesla coil works, if not COMPLETELY, then about 90% of it. I know what resonance is.. (I am a 12th grade physics maniac and basically all we will be doing in 12th grade physics is electrostatics and electrodynamics.) I have been to all the sites you have mentioned above. Almost all sites say that Aluminum ducting can be used as a toroid. It wasn't easily available here hence I thought it would be better if I went to more "responsive" sources for a good opinion. I have JAVATC and TeslaMap. I found TeslaMap better for designing the MMC arrangement. The total capacitance of my primary resonant circuit is 0.0106 uF (I calculated it using both JAVATC and TeslaMap, I posted it before). I have decided to use two strings each with 18 capacitors as im using a static spark gap. The peak voltage rating will be about 36kV (which is good, I require about 31kV peak rating) with a temperature rise of about 0.527 deg C.
PS: If I sound angry/offended.. trust me IM NOT.. Thanks
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
interloper wrote ...
Alright I got that. So basically the equivalent capacitance of my MMC array should be 0.0106 uF right? But I dont get the 31kV part. Should the capacitors be able to withstand that voltage? And what about the toroid?
With a 9KV RMS transformer that brings your peak voltage to near 13 KV, "9KV * 1.414=12.726" Peak, round up to 13KV now for safety sake and reliability double that number and that gives you 25.45KV, 31KV rating will be more than ample and also the thermals will not be a problem.
Yes the toroid you describe will work but becareful when taping it because any wrinkle in the tape will be a discharge point. The smoother you can make it the better off you will be.
I have a 4 inch SG that uses a flexible dryer duct as the toroid, seems to work ok.
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
interloper wrote ...
Hey guys! I just joined this forum to get some help with my first tesla coil. Im building it for a science fair and Im counting on it to make my team win :P. We're almost done with the construction, all we have to do is get the capacitors and put everything together. For now, I have two questions- 1. I have made a toroid by bending a RUBBER pipe to shape and wrapping it with aluminum tape. Will this work? 2. I need help with capacitors. We're planning to use an MMC arrangement. How do I calculate the capacitance of the MMC combination and how do I find out the best arrangement for 'em? I am using a 9kV, 30mA NST.
Oh, one more thing. Can you guys tell me some do's, dont's and tips I should keep in mind while constructing and operating the tesla coil? THANKS!!
PS: This is the third forum I signed up for to get some help with this project. I got few and unclear replies from the last two forums. So Im REALLY counting on you guys.
Here is a couple pictures of the 4 inch SG coil, the flexible ducting used for the toroid is not smooth and has some dents in it, the aluminum tape is not as smooth as it should be...etc you can see where the major breakout points are..."anywhere there is a bump" the second picture is where I made a breakout point for it...so keep the toroid as smooth as you can...
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
interloper -
The "resonant" value capacitor for your 9/30 NST is .0088 uF, but you definitely don't want to use a resonant value cap, at least not for your first coil (see Richie Burnett's website for the dangers of the incredibly high voltages that can result from a primary capacitor value that is resonant with the NST secondary.) What you want is a "larger-than-resonant" cap value, which is typically 1.5X the "resonant" value. This will prevent the destructively high voltages that can result from a "resonant" cap value. I think .0106 uF may be a little too close to the resonant value to provide protection from overvoltage, especially if you don't carefully adjust your safety gap, and don't use a "Terry filter" to help further snub voltage spikes at the NST secondary.
Your target primary cap value is therefore ~.0132 uF (1.5X resonant). Depending on the value of the indiuvidual caps useed in your MMC, the number of caps per string, and the number of strings, anything near this value will be fine.
Sorry if I initially came off sounding a bit harsh; I was really was just trying to encourage doing a little initial research on commonly asked questions.
Post the complete set of calculated specifications for your coil and some photos of the components you've already assembled, and the forum members will provide a "design review" that may help catch design or construction problems while they still can be easily corrected.
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Tesla Coil Help
