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Building a DRSSTC Pt. 2 - Secondary Coil Design

    Blog entry posted in 'Building a Dual-Resonant Solid State Tesla Coil', July 23, 2014.

    Hello all,

    Today I'm going to begin discussing the design process. DRSSTCs require a fair amount of math before one can even begin considering building their own. The math is fairly straghtforward, so once you have all of the formulas it should be reasonably easy. I recommend starting with the secondary coil and working your way backwards.

    For this project I decided to use the same secondary that I wound for my SGTC that I was working on a couple of years ago. It's approximately 1200 turns of #26 AWG wire around a 4" (ID) PVC pipe. The topload is a piece of 3" diameter aluminum dryer duct which I bought from a hardware store for a few dollars. The topload provides some capacitance with respect to ground, which is what makes the secondary circuit a resonant LC circuit--along with the primary coil (an inductor), the secondary oscillates at its resonant frequency. For the first step in designing a DRSSTC, I would recommend deciding on your secondary specs--height, diameter, and topload. From there you can calculate the resonant frequency, which you will need for the rest of your coil design.

    Normally in order to calculate resonant frequency, one would use the following formula:

    where f is the resonant frequency, L is the inductance, and C is the capacitance. However, at high frequencies like what we would expect to see on a Tesla coil, it's not quite this simple. The surrounding environment, the capacitance of the secondary, and lead length can all significantly change the resonant frequency. Therefore, I strongly recommend JavaTC to help you determine your coil's specs. Based on Paul Nicholson's GEOTC software, JavaTC was developed by Bart Anderson to help Tesla Coil designers determine, quite accurately, various information regarding their coils' operation. You can use or download JavaTC here.

    At this point all we are going to worry about is the secondary circuit information, namely the coil radius, height, number of turns, wire gauge, and topload data.

    Since I have all this data, I am going to go ahead and plug it in to JavaTC and see what it gives me.

    • Secondary radius 1: 4.5" (the PVC is 4" ID with 1/4" walls, so the total diameter is 4.5". This gives me a radius of 2.25".

    • Secondary radius 2: Same as radius 1
    • Height 1: This is how high off the ground the bottom of your secondary sits. I have mine set up on a stand that is 24" tall. The PVC pipe then sits on top of a 3/4" thick piece of wood which is attached to the top of the stand, so the total height off the ground is 24.75" off the ground.
    • Height 2: The coil is exactly 20" tall, so 24.75 + 20 = 44.75"

    • Since I didn't count the number of turns, I'll have to calculate it. I know I used #26 AWG wire, which has a diameter of 0.0159". Therefore, I can calculate the number of turns by dividing 20 inches by 0.0159 inches (one turn), which comes out to about 1257.86 turns. I'll just round down to 1257 for now.
    • As I mentioned, the wire gauge is #26, or 0.0159" diameter.
    • My topload is a toroid shape. The minor diameter is the diameter of the tubing. In my case it's 3 inches.
    • The major diameter is the overall outer diameter. Mine came out to be 12.75"
    • The toroid center height is right in the vertical center of the toroid. Since the toroid is 3" tall (determined by the minor diameter), then the center is at 1.5". Add this to the height of the coil (44.75 + 1.5) and you get 46.25 inches for the height. Make sure to click "add" to add the topload to the list. If you have more than one topload, then you can repeat this process and add your other ones. I don't recommend large or multiple toroids for DRSSTCs though, as it will likely cause too much loading and you will risk arcing between the secondary and primary, which could destroy your primary driver.

    These are the values that are specific to my coil. I plug these values in to JavaTC and here is the output:


    Right at the top, you can see the resonant frequency - 218.02 kHz. You will need this value to determine the specifications of your primary tank circuit.

    I was originally going to discuss the design of the primary in this entry as well, but I think I might save that for next time.

    I hope you all have enjoyed this so far. Once again, if you have any comments, questions, or feedback, feel free to leave a comment or send me a PM here on the site.

    Stan Smale M.D. (stan), July 25, 2014
    Hi Matt! Perhaps I misunderstood, but it seems that the coil diameter dimension was used where the coil radius input was needed.... I just 'tuned in', and maybe I missed something.... anyway, if your coil is 4.5" across (diameter), then radius would be 2.25". Not trying to 'flame' you, but just trying to follow along. I just finished winding a coil secondary is 6 inch diameter, 24 inches long, wound with 24 ga. 'magnet' wire. My primary coil is a flat spiral design, 18 turns of 1/4 inch copper tubing, outer diameter 46 inches. Neon-sign xformer/ fixed spark gap initial design (had to see if it 'would work'). Made my own primary capacitor from 0.050 glass/ aluminum foil. Plate size 12 x 14 inches, 11 glass plates, 12 aluminum plates (actually I blew up three home-made caps before this, this one worked.... for a while). Without any tuning, or adjustments, on first fire, I got 18 inch spark from top of coil. Then the glass cap went.... I guess I should be conventional, calculate it out and get commertial caps. I hope you have smoother sailing with your design, sounds like a really nice system. Best of coiling! Stan
    DerStrom8, July 25, 2014
    Hi there Stan! You are correct, the overall diameter is 4.5", which means the radius is, in fact, 2.25". I did enter 2.25" for the secondary radius in JavaTC, so the results are accurate. Thanks for pointing that out though, I can see how that could be confusing. I will edit the post to reflect the radius instead of the diameter. Sounds like you're on a roll with your coil! This TC originally began as a SGTC, and I decided that the dimensions were just about right for a DRSSTC, so I am reusing the secondary. The topload and primary will have to be tweaked though. Best of luck with your SGTC! I hope to see more about it in the future!

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