# Building a DRSSTC Pt. 4 - Pri. Tank Design (cont)

Blog entry posted in 'Building a Dual-Resonant Solid State Tesla Coil', July 26, 2014.

Hi all,

We left off last time by finding the formula to determine the number of turns we'll need for the primary coil in order to obtain the right inductance:

$n=\frac{\sqrt{2}\sqrt{L(200L(d+s)^2+9D^3)}+20dL+20Ls}{D^2}$

where n is the number of turns, L is the inductance, d is the wire diameter, s is the spacing between turns, and D is the diameter of the coil. I really didn't want to plug in the values manually every time I do the calculation, so I originally spoke with @croberts , who put together an Excel spreadsheet for me. Originally it was designed to assume a wire diameter of 0.0808" and a spacing of 0". While this worked in my case, I decided I would make my own that took more input variables, like the wire diameter and the spacing between turns. I wanted to make it more universal. I have attached the spreadsheet to this post in case you want to make a cylindrical primary coil and need to determine the number of turns required. My data is as follows:

• L (the inductance of the primary), according to the resonance formula, is 26.65uH
• D (diameter of the primary) is going to be 5.5 inches
• d (diameter of the wire) is going to be 0.0808 inches (#12 AWG wire)
• s (spacing between turns) is going to be 0 inches

When plugged into the Excel spreadsheet, I get a calculation of 10.87 turns. Great! Now I have enough information to put into JavaTC! We already have the information for the secondary and the topload, so I'll plug that in just like I did before. The primary coil values are as follows:

• Height 1: 24.75" (same as Height 1 of the secondary coil)
• Height 2: 25.63" (add "h" from the spreadsheet to Height 1 of the primary to give the overall height of the primary coil)
• Turns: 10.87 (as calculated by Excel)
• Wire diameter: 0.0808" (diameter of #12 AWG wire)

The input to JavaTC looks like this:

87542

Before clicking the "RUN JAVATC" button, I'm going to do a couple of things. First, I'm going to check the "Auto-Tune" box. This will take care of any error in the turns number of the primary. Second, I will tell it to automatically adjust the coupling to 0.13. Generally you do not want the coupling to be more than 0.2.

When I run the program, I get the following output:

87543

As you can see, the primary and secondary resonant frequencies are very closely tuned to each other. This was done by the auto-tune option. Also, the coupling coefficient is quite close to what it should be.

At this point I think it's fair to say we have completed the design for the primary and secondary tank circuits. We have the specifications of both circuits, such as the resonant frequencies, inductance, coupling coefficient, length of the wire, and so on. So now that we have designed the actual Tesla coil itself, it's time to start thinking about how we're going to power it. In the next section I will explain the H-bridge circuit used to drive the primary and how it works.

I hope you have enjoyed this post, and again, feel free to leave comments, questions, and/or feedback!