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Hello William, A salt water cap is easy to make and is great for a first-time coil. If you use beer bottles, the average capacitance of a beer bottle is about 800-900 pF, but you'll probably need to connect pairs of them in series to be able to withstand the voltage stresses from your 15 kV supply. Each pair of series connected bottles will be about 450 pF. If you need 6500 pF, you'll need about 14 pairs of bottles connected in parallel. An MMC cap is a series of low-loss polypropylene capacitors that are connected in series and parallel to create a tank capacitor with the right capacitance, current handling, and voltage rating for your Tesla Coil. They are normally constructed from "snubber capacitors" - these are capacitors that are used with high power switches (MOSFETS and IGBT's) in switching power supplies and inverters. An MMC is comparatively inexpensive, easy to repair, and provides better performance than salt water caps, and some types are even "self healing" - they can actually "fix" themselves if the dielectric breaks down during use. Bleeder resistors are simply high valued resistors that can withstand moderate voltage (1,000-2,000 volts). They "bleed" away any leftover charge on the capacitors once you remove HV power, and are are used as a safety precaution to prevent you from accidentally getting shocked from your MMC. Although bleeder resistors are not absolutely required, I would definately recommend using them. Here are some information sources for MMC's and bleeder resistors: MMC Information (Good and bad snubber caps for MMC's): Where you can order GOOD MMC caps: Good Bleeder Resistors: For a small coil, you can get away without using a special dedicated RF ground. Instead, you can make a counterpoise. This is simply a metal plate or screen that's about 4 - 5 feet in diameter that's centered underneath your primary and secondary. It "acts" like a local ground for your coil. It can be made from aluminum foil, chicken wire, window screen, etc. Connect the bottom of the secondary winding to the counterpoise, but do not let the counterpoise touch anything else since it will become electrified when you operate your coil. See the following for a discussion on counterpoises: You may also wish to peruse the archives or consider joining the Tesla Coil Builders Mailing List for lots of excellent Tesla Coil information. The web site also contains a searchable archive that is a treasure trove of technical information: Good luck and best regards, -- Bert --
: Hi Bert!
: I have almost completed my 1st TC except for the tank cap. & the RF
: groung. I was considering building a saltwater cap to get 0.0065 uF but one
: of my friends advised me to build an MMC. I have read that MMCs are simply
: dielectric caps joined in series & parallel. Is that right? Also, what
: are bleeder resistors? Are they required? Should I just ask for dielectric
: caps at elec. shops?
: I live non the 5th floor and thus the RF ground is a problem. Is it really
: nesessary? Is there an alternative? My system is 15kV@30mA NST powered.
: Thanks in advance!!
: --William--
:
http://hot-streamer.com/TeslaCoils/MMCInfo/good-bad.txt
The Geek Group: http://thegeekgroup.org/mmc/
(Good information on using MMC caps as well)
or Richardson Electronics:
http://catalog.rell.com/rellecom/scripts/SkuPage.asp?SKU=162589&PNO=&PNM
Jameco: http://www.jameco.com
Digikey: http://www.digikey.com
Digikey's part number is CFR-50JB-10M (carbon film, 1/2 W, 10 Meg, axial leads).
You can get a quantity of 200 at $0.0198/ea or a total of $ 3.96.
http://www.4hv.org/archive/topic.22660.html
http://www.pupman.com
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