Stoneridge Engineering Home |
MMC Help
Posted by William on 3/21/2004, 11:47 pm 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!!
|
Re: MMC Help
Posted by Bert Hickman on 3/23/2004, 11:33 pm, in reply to "MMC Help" 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 --
|
Re: MMC Help
Posted by Rockinriley14 on 3/22/2004, 5:06 pm, in reply to "MMC Help" --Previous Message-- I just thought I would write about the RF ground... there isn't hardly any info online about them from what I found. However, you can use some sort of large, flat piece of metal on the floor, this has worked for me at least. I used a scanner lid and covered the flat part with foil, and sat it on the floor. This worked almost as good as my earth ground. You are correct on the construction of an MMC, that it is a bunch of caps in strings (series) with the strings in paralell. Make sure and buy pulse-rated caps. I haven't built an MMC myself so I am probably not the most knowledgable in this field.... Bleeder resistors are just for your own safety definitely a good idea. =Rockinriley14=
|