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:
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.
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.
Before I go on, I want to give a few pointers on designing a capacitor bank:
The Aerovox capacitors are rated for 2uF each at 1kVDC (around 530VAC). Recall that I am aiming for 250nF, and that it is easier to put caps in series to obtain your final value rather than in parallel. That is why I chose each capacitor to be 2uF. To get an acceptable voltage rating, I decided that 16 in series would give me approximately 8.5kVAC. That would mean that one string would have a capacitance of 125nF. Since I am going for 250nF, then I knew I would need two of these strings in parallel (125nF * 2 = 250nF). Therefore, I would need 16 x 2 = 32 capacitors total. This worked out pretty well since I purchased 50 of them--it's always a good idea to have some spares!
- 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.
- 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.
- 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.
- 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.
So, to review, I have two parallel strings of 16 2uF 530VAC capacitors which gives me a final MMC value of 250nF at around 8.5kV.
I assembled the capacitor bank on a sheet of plexiglass, each capacitor tab connected snugly to the next. The plexiglass does not offer any of the structural support to the bank, as it could melt or sag as it heats up, which could cause some issues. The main connections to the capacitors (where the primary coil and H-bridge connects, for example) are solid 3/4" wide 1/8" thick aluminum buses. Since a coil this size could see peak currents of 500A I wanted to make sure the connections could handle that kind of power. It is also important that you DO NOT use steel to make these connections. Steel is terrible at high frequencies and will heat up significantly. This would mean lots of losses and your coil will likely perform very poorly. Aluminum, brass, and copper are the ideal bus materials.
Here is my final MMC, as described above:
That's about all I can think to discuss at this point about the capacitor bank, so I'll end the entry here. I am sure I have left out a fair amount, so if you have any questions, feel free to post them as a reply or send me a PM.
Hope you've enjoyed this post, and I welcome any and all feedback and suggestions!
Building a DRSSTC Pt. 11 - Building the MMC
Blog entry posted in 'Building a Dual-Resonant Solid State Tesla Coil', Sep 16, 2014.