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Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Good day, everyone! I'm new here to 4HV, so bear with me... I'm working on building a 4-inch diameter Tesla coil that I plan to run off of my 9kV, 60mA neon sign transformer. I found a calculator online to match the transformer to a capacitor, and I know I need one rated 18kV, and about .02uF. Right now, I am stuck on finding a capacitor like this. I have been on ebay and I have found dozens of high-voltage capacitors with the right specs, but they are polarized. I know it is not possible to use polarized capacitors in a Tesla coil tank circuit. Anyway, my question is this: I found these that I am considering, but I need to know if they are polarized or not:
It says 20kV DC, but I believe that is just the dielectric breakdown voltage--It doesn't necessarily mean that the cap is polarized. Can somebody tell me if these would work? I appreciate your help, and I hope to hear from you soon! Best regards, Der Strom
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
bwang wrote ...
Use an MMC. It is the de facto standard for small to medium TC's now.
Thanks for the quick reply, bwang! I have avoided an MMC so far because of the cost. I generally have a very low budget for my projects, and buying a bunch of smaller caps can get rather expensive. However, if anyone comes across a bunch of caps that would work for a MMC for less than $35, please let me know! ;) But would it be possible to use these capacitors I mentioned in my first post? Thanks in advance! Der Strom
Registered Member #3599
Joined: Mon Jan 10 2011, 05:50PM
Location:
Posts: 15
it will work but it wont last long and cant take abuses. ive read some feedbacks that CJE caps fail. good for starting though. have you tried rolling alu foil and some transparencies? well im giving you just another option for sake of cheapness. i suggest u invest more for quality CDE 942c caps.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
haikara wrote ...
it will work but it wont last long and cant take abuses. ive read some feedbacks that CJE caps fail. good for starting though. have you tried rolling alu foil and some transparencies? well im giving you just another option for sake of cheapness. i suggest u invest more for quality CDE 942c caps.
Thank you very much. I'll look into it. I have not tried the foil and transparencies because I hear they need to be submersed in oil. I would like to avoid the "messiness" of oil, if possible. Sorry, I'm very picky about this sort of thing. You'll have to forgive me... I am also thinking of trying the classic beer bottle capacitors, because those might be a bit cleaner than the rolled caps. I would, however, rather use professional capacitors, hence my original question. Many thanks, again! Der Strom
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Der Strom -
Spend a little time in the 4HV archives, and you will find all the information you need on selecting appropriate capacitors for spark-gap Tesla coils.
The capacitors in your link are totally unsuitable for use in a Tesla coil (despite the seller's claims) because they utilize a very lossy polyester (Mylar) dielectric. Polyester is NOT usable at high frequencies because it quickly overheats and fails. Additionally, the capacitor's current rating is nowhere near adequate for even a small Tesla coil. Your proposed 18KV voltage rating does not have much voltage safety factor, as the peak voltage from a 9KV NST will be almost 13KV.
For a Tesla coil using a 9/60 NST and a static spark gap, the desired tank capacitor value is around .025uF. You can achieve the voltage and current ratings you need by using an MMC assembled with Cornell-Dubilier 942 series film/foil caps, which use a low-loss polypropylene dielectric and are specifically designed to handle large pulse currents. The .15uF, 2,000V model has been used for years in Tesla coil MMCs, and is well-proven.
For a fairly reliable MMC, you'd need two parallel strings of 12 of these capacitors, 24 total. These can be purchased directly from C-D distributors for less than $4 each (Richardson Electronics currently lists these at $3.30 each in a quantity of 50.)
If you're really short of funds, you can make a salt-water cap bank at neglible cost that will allow you to get your coil running, but it won't be pretty or convenient.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Herr Zapp wrote ...
Der Strom -
Spend a little time in the 4HV archives, and you will find all the information you need on selecting appropriate capacitors for spark-gap Tesla coils.
The capacitors in your link are totally unsuitable for use in a Tesla coil (despite the seller's claims) because they utilize a very lossy polyester (Mylar) dielectric. Polyester is NOT usable at high frequencies because it quickly overheats and fails. Additionally, the capacitor's current rating is nowhere near adequate for even a small Tesla coil. Your proposed 18KV voltage rating does not have much voltage safety factor, as the peak voltage from a 9KV NST will be almost 13KV.
For a Tesla coil using a 9/60 NST and a static spark gap, the desired tank capacitor value is around .025uF. You can achieve the voltage and current ratings you need by using an MMC assembled with Cornell-Dubilier 942 series film/foil caps, which use a low-loss polypropylene dielectric and are specifically designed to handle large pulse currents. The .15uF, 2,000V model has been used for years in Tesla coil MMCs, and is well-proven.
For a fairly reliable MMC, you'd need two parallel strings of 12 of these capacitors, 24 total. These can be purchased directly from C-D distributors for less than $4 each (Richardson Electronics currently lists these at $3.30 each in a quantity of 50.)
If you're really short of funds, you can make a salt-water cap bank at neglible cost that will allow you to get your coil running, but it won't be pretty or convenient.
Herr Zapp
Thanks for the reply, Herr Zapp. So you are suggesting that I find a capacitor with a voltage rating of at least 26KV? Also, as I think I mentioned already, I'm working on a salt-water capacitor bank (using beer bottles), so I'll see how that works.
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Yes, for reliable operation the 100% safety margin is necessary. At high powers, a low-loss dielectric is *very* important. Tesla coils place tremendous stress on the tank capacitor, which has to deal with high peak currents, high frequency operation, and enormous voltage reversal. Voltage rating is far from everything.
Your choices are, in order of increasing simplicity:
-Salt-water cap bank. Free, messy, inefficient. Good for getting first light, not good for optimizing your coil. -Old-school rolled poly+foil homemade cap. Messy, rather expensive, suffers from occasional reliability problems. Not recommended. -MMC. If your're not on a very low budget, this is the way to go. $100 will get you a reliable tank capacitor that performs well and lasts years. -Big pulse capacitor. Expensive. Nice and clean. Note that not all pulse capacitors are created equal; for example, the Maxwell 36xxx series performs much more reliably in TC service than the 31xxx series.
Random note: some TDK/Murata doorknobs have been known to work reliably in small TC's, despite the general guideline of "don't use ceramics."
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
bwang wrote ...
Yes, for reliable operation the 100% safety margin is necessary. At high powers, a low-loss dielectric is *very* important. Tesla coils place tremendous stress on the tank capacitor, which has to deal with high peak currents, high frequency operation, and enormous voltage reversal. Voltage rating is far from everything.
Your choices are, in order of increasing simplicity:
-Salt-water cap bank. Free, messy, inefficient. Good for getting first light, not good for optimizing your coil. -Old-school rolled poly+foil homemade cap. Messy, rather expensive, suffers from occasional reliability problems. Not recommended. -MMC. If your're not on a very low budget, this is the way to go. $100 will get you a reliable tank capacitor that performs well and lasts years. -Big pulse capacitor. Expensive. Nice and clean. Note that not all pulse capacitors are created equal; for example, the Maxwell 36xxx series performs much more reliably in TC service than the 31xxx series.
Random note: some TDK/Murata doorknobs have been known to work reliably in small TC's, despite the general guideline of "don't use ceramics."
Thanks, bwang. I have had enough experience with tesla coils to have found this list out, myself. My only real problem with TCs are the professional capacitors (MMC), and what type of dielectric, etc. works well and what doesn't. Otherwise, I understand the whole concept of operation, what each part does, many "dos" and "do nots," and basic tuning.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Der Strom -
A safety factor of at least 2X the peak supply voltage is recommended as kind of a minimum margin. Of course this requirement is affected by the voltage safety margin that the capacitor manufacturer has built into his capacitor design; something that is unknown to the end user.
To start, multiply the RMS output voltage of your HV transformer (9KV in your case) by 1.414 to find the peak AC voltage that the capacitor must withstand. Then, multiply by 2 to get a target voltage rating for the MMC string.
If you have access to military or industrial surplus, or ham radio swapmeets, you may find other types of capacitors that are suitable for Tesla coil use, but rarely will they be the exact value you are looking for. High-voltage pulse-rated capacitors are found in industrial and medical laser systems, military radar and sonar systems, indistrial induction-heating systems, and high-power military and commercial radio transmitters. Sometimes interesting capacitors show up on eBay, but you never know their history, and purchase is always a gamble.
Building a conservatively-rated MMC for your system using the C-D 942 capacitors should cost you less than $100, and will likely give the best performance and reliability in the long run.
If you're going to take the salt-water capacitor route, I'd strongly recommend getting an inexpensive multimeter with a capacitance scale to measure the actual capacitance of your bottles. Knowing the actual capacitance value can help eliminate a long and frustrating exercise in "blind tuning".
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Tesla coil capacitor?
