Building a DRSSTC Pt. 11 - Building the MMC

Hi folks, welcome back!

I was originally waiting for some supplies for my primary coil to arrive, and then I was going to write a post about the primary. However, I made the mistake of ordering from the Home Depot online and they won't be shipping it out until next week. Therefore, I decided to write about a different part of the Tesla coil instead--the primary capacitor bank. It actually makes more sense to work on this part first, instead of the primary, because it's generally more difficult to change a capacitor bank. The primary is easy to change depending on the other parts.

As you may recall, I have gone through many different designs. I also went through several capacitor ideas. My first capacitors I bought on a whim, before I had really looked into Tesla coil MMC design basics. They were two 10nF 5kV Russian capacitors with a very low dissipation factor (which you really want for a Tesla coil). I was going to put them in parallel to get 20nF at 5kV. In general you'll want capacitors rated for at least 20x your bus voltage (30x-50x is even better), and since I was planning on running at 170VDC I figured these would work:

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However, there were several issues with these. First of all, 20nF is ridiculously low for a DRSSTC. While it may have worked, the primary would have to have many turns to make up for it. The voltage rating was also cutting it close, because a rating of 20x the bus voltage is the absolute lowest, and at that point you risk destroying components. I eventually changed my plans to use Cornell Dubillier 942C polypropylene capacitors which are some of the most commonly used ones for Tesla coils. By mistake I ended up buying some 940Cs instead, which were rated for 0.15uF 3kVDC (600VAC). I ordered 8 of them so that I could have two parallel strings of four capacitors in series, to give me a total capacitance of 75nF.

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However, I realized later that I had only paid attention to the DC voltage rating. When used in Tesla coils, you MUST go by the AC voltage rating. 4 600V capacitors in series gives me a voltage rating of a mere 2400V -- Nowhere near enough! 75nF was also going to require a fairly large primary coil, which I did not have space for. I decided to set myself a capacitance goal of around 250nF, and then search for capacitors based on that value. I finally decided on some Aerovox RBPS polypropylene film IGBT snubber capacitors from CTR Surplus. I bought 50 of them for $50 plus shipping.

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Before I go on, I want to give a few pointers on designing a capacitor bank:

1. Capacitors MUST have a very low dissipation factor (preferably <0.001). The best type of capacitor to use is polypropylene film, as mentioned earlier. MKP and MMKP (metalized film and double-metalized film) are excellent options.
   
2. Capacitors MUST be able to handle high frequencies in the hundreds of kilohertz range (Tesla coil operating frequencies). The PP film caps are great at these frequencies, which is why they are recommended.
   
3. Run different capacitor sizes through JavaTC based on your secondary coil and topload (which you should have designed by this point) and use the "auto-tune" option. This will set your primary coil to the best length, and will help you determine which primary capacitance gives you a reasonable primary coil length. Generally 75nF-350nF is good for a medium sized Tesla coil.
   
4. In my personal experience, I have found it best to choose capacitors with individual capacitances higher than that of your final MMC. I realize this may be difficult to follow, so let's start simple: Capacitances in series divide, capacitances in parallel multiply. Two 1uF caps in parallel gives you 2uF, two in series gives you 0.5uF. It generally requires fewer capacitors when you put them in series to obtain the final capacitance you want, rather than in parallel.