Building a DRSSTC Pt. 12 - Building the Primary

Hello everyone, welcome back to the build!

So far we have built the secondary coil and topload (the secondary resonant L-C circuit), and the primary tank capacitor. In order to complete the primary tank circuit we need an L (inductance). This will be the primary coil.

After building the secondary coil and the topload, we used JavaTC to determine the following information:

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According to that data, the secondary coil inductance and the topload capacitance will oscillate at a frequency of 122.48kHz. Our ultimate goal is to match that frequency precisely with the primary tank circuit. This is why the Tesla coil is said to be "Dual Resonant" -- It has two resonant circuits: The secondary coil / topload, and the primary coil / MMC. Right now, all we're missing is the primary L.

According to JavaTC, under "Primary Coil Output Data", we see that the required inductance in order to oscillate with the MMC at 122.48kHz is around 6.8uH. This is what we need to do today.

My primary coil material of choice for this small coil is 1/4" copper tubing. Notice that JavaTC states I'll need 10.69 feet of it. However, I bought 20 feet (cost me around $18 at my local family-owned hardware store) in order to provide plenty of turns for tuning. It is always a good practice to have plenty of turns like this, just in case you decide to make any changes later on that will require a higher primary coil inductance. Anyway, that's enough of the theory, so let's get started!

Let's begin with the stand. For this project I decided to re-use the stand that I built for my spark gap Tesla coil several years ago. It doesn't look pretty--it took me about a half hour to build--but it had a good shape for this coil, as well as a shelf where I can put all of the driver electronics. Here's what it looks like by itself:

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The first step was to set up some spacers to hold the primary coil in place. I decided to use a 1/4" wooden dowel cut into ~1" pieces. I drilled the holes on the base exactly 1/2" center-to-center. This would ensure that the 1/4" tubing would fit snugly between each dowel (1/8" on either side leaves 1/4" between the inserted pegs).

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After all the holes were drilled, I placed as many pins in as I could. I realized later I should have bought another long dowel, but this one was just long enough.

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Now that the supports were in, it was time to lay down the primary. When building a flat Tesla coil primary like this, buying the pre-coiled tubing is almost a necessity. It makes placing the secondary incredibly easy, and I would never dream of doing it any other way!

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If you look closely at that last photo, you'll see that I drilled two more holes straight through the base, and bent the primary copper to fit through these holes. This is the best way to do it for a couple of reasons: 1) It leaves the ends of the tubing pointing downwards to the "shelf", which allows for much easier connection to the drivers without much extra inductance, and 2) it leaves no sharp edges on the top. Sharp primary edges can sometimes lead to flashover from the secondary, which could cause severe, irreparable damage to the secondary coil. I have seen this happen, and it is not pretty.

That's it! The primary coil is done, simple as that! When designing your coil in JavaTC, it's important to keep an eye on how far away the first turn of the primary coil is from the secondary. I allowed about half an inch on mine, which in theory should be ok.

I almost forgot to mention, in the photo you'll notice that the "X" that the secondary fits over is set up on a wooden disc. I used this disc to adjust the coupling on my SGTC, but I have removed it since the photo was taken. The coupling was far too loose and the efficiency of the coil would have been fairly poor if I had left it there.

Now that we have the primary coil in place it's time to add a bit of protection. The last thing we want to happen is a streamer from the topload to reach down and strike the primary, sending very high voltage and high frequency surges back into our transistors and other sensitive driver electronics. Most Tesla coils use what's called a "strike ring", or "strike rail" that "catches" these strikes, thus protecting the primary coil and driver. The strike rail is connected to RF ground (the same ground connection as the bottom of the secondary coil). DO NOT connect the secondary base or the strike rail to mains ground, and most certainly not to the DC ground of your driver. The back-EMF can cause serious damage to your driver, and if you connect it to your mains ground you risks damaging any other electronic appliances connected to that ground. Instead, I highly recommend grounding your secondary and strike rail by driving a 3-4 foot metal spike deep into the ground, and using that as your RF ground connection. This goes for any Tesla coils, not just DRSSTCs.

I decided to use 1/2" copper tubing for my strike rail, but this time I only bought 10 feet of it. I only need enough to go around the outside of my primary coil one time, so 10 feet was certainly overkill. It was, however, one of the better deals I found. I also purchased a cutting tool designed to neatly cut copper tubing without crimping it.

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Once again, I bought the pre-coiled tubing which makes things much simpler.

Before I could place the strike rail on the stand, however, I needed some sort of support for it. Generally a strike rail should sit an inch or two (sometimes more, depending on your coil) above the primary to ensure it would be the preferred target for any errant streamers. I bought 4 3/4" PVC coupler, drilled a hole through one side, used a Dremel with a cutting wheel attachment to open up the hole, and finally a grinder attachment to widen the hole. This probably sounds fairly confusing, so here's a picture of the supports with the strike rail inserted:

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Notice that there is a gap between the two ends of the strike rail. This is very important. If the two ends were touching, it would act like a shorted 1-turn secondary coil. This could cause significant heating, and also would negatively affect the magnetic field surrounding the primary and secondary coil. In general the gap between the ends should be around 1/2" to 1". I miscalculated the length somewhat, so mine has a 2-inch gap instead. This shouldn't cause any problems though.

Finally, I decided to wrap the ends in electrical tape to cover up any sharp edges and to make it look a little neater. This really isn't all that important but I figured it couldn't hurt.

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And with that, both of our resonant circuits are complete! The only things missing are the connections (as well as the driver circuitry), which we'll get to in a later post. Here's what the final topload, secondary, primary, and strike rail look like:

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Really starting to look like a Tesla coil now, isn't it?

That's it for this post. In my next entry, we'll continue to work our way backwards and we'll discuss the H-bridge board(s) that we use to convert the DC power source to AC, which we send to the primary coil and capacitor bank. We're in the home stretch now!

I hope you've enjoyed this post. Please forgive any typos, run-ons, or other poor uses of the English language. I have been battling a nasty cold for several days, so my brain still is not working completely ;)

As always, I welcome any questions, feedback, and suggestions that you may have. Please feel free to post them as a comment here, as a thread on the forums, or send me a PM!