Building a DRSSTC [INPROGRESS]

Build log/tutorial on how we built our Medium sized DRSSTC including assumptions and a step-by-step guide.

[UPDATE] 30/12/2022 - One day I will come back and finish this. The current progress is it msotly works, just haven't had a place to run them up to full power. I had had some earlier issues where the bridge driver was backwards, causing it to work (somehow?...) but then blew the entire IGBT bridge set :(

Josh and I are planning to build 2 Medium-sized Dual Resonant Solid State Tesla Coils with a polyphonic music input. This guide/walkthrough hopefully can guide other people hoping to build a similar thing, and what decisions & why they were made (which I found a little lacking when following other guides!).

Step 1, How big do I want it?

For us, we wanted something not too small as we'd like to demo it around at things such as emfcamp, but not too big seeing as this is our first DRSSTC build. I had built small (<tabletop size) SSTC and also managed to hook in an interrupter circuit in order to play simple single note tunes so this seemed like a good progression.

Looking through other builds that people had done, we thought that something around Kaizer's DDRSTC I and Loneocean's DDRSTC 2 would be a good size to aim for.

From looking at all these designs and what is easily availble, decide on a secondary size (this can be approximate) of both diameter and length. We decided on a secondary diameter of 100mm and length around 500mm.

Step 2, Deciding on Other Parts

Secondary

Wire Size

Using Kaizer's DRSSTC Design Guide section on choosing secondary coils, we decided to use a wire diameter of 0.25mm.

Number of turns

From the wire size, and secondary diameter, we can calculate the number of turns using many handy calculators (we used http://www.classictesla.com/java/javatc/cst.html, linked off of JavaTC). Our secondary ended up at ~1850 turns.

Primary

Primary size for most medium coils from our research seemed to be pretty constant with 10mm copper tubing at around 9-10 turns (initial, this will be tuned to match the secondary later).

Flat rather than helical was chosen as the general consensus [1] seemed to be that helical coils on a DRSSTC were a bad idea and should generally be avoided.

Spacing between turns was assumed to be fine at 5mm (literally come to by looking at pictures of other people's coils).

Toroid

Chosen from again, looking at other coils and ensuring it wasn't so big as to look silly on top of the size of secondary we were looking at.

We planned to use 100mm ducting around a form in the middle giving a major diameter of around 600mm.

Primary Tank Capacitor (MMC)

This one is more tricky to assess what you want. This will partially determine the resonant frequency of the primary circuit and can be tweaked in the next step to ensure your primary circuit does not have ridiculous values (such as a 1-turn primary).

Comparing different capacitors that were available, their specs, and how much they cost, we decided that our MMC should add up to 0.2μF at 4000V. The actual amount of capacitors and their type was determined later. We had originally played around with this value in order to get a good multiple of capacitors available. Be aware though, that changing this value will mean re-calculation of most steps forward from this.

Initial Calculation

Using JavaTC (as everyone does) plug all the values in. We approximated the heights of the secondary and primary to be starting at 350mm above the ground (giving a good amount of clearance underneath for circuitry) and the topload slightly above the top of the secondary at 900mm.

For auto-tuning between the secondary and primary (assuming you have put in the primary capacitor size), click the Auto-Tune button. This will vary the number of turns on your primary and therefore it's major diameter.

For our tesla coil, we managed to achieve a resonant frequency of 115.13kHz. This is a little high compared to other designs of this size, but not overly high as to make switching hard. We tried to aim to keep this below 120kHz due to making IGBT selection harder in the next steps. If you're unsure, keep with the value you have (and/or compare with other coils of the same sort of size as what you're building) and continue through this guide/build log. As we're only doing calculations you can always come back and change it later!

(Coming back to this later, we found that the secondary impedance at resonant should be for a good spark-length output [2] around 50kΩ. It turned out ours was very close to this already!)

Important output values:

Secondary
Resonant Frequency = 115.13kHz
Wire Length = 628m
X at resonance = 53.989kΩ
Topload Capcitance = 22.138pF
Secondary Q = 185

Primary
Resonant Frequency = 115.13kHz
Length of wire = 4.3359m
Ldc = 9.609μH
Lmutual = 119.187μH
Coupling Coeff = 0.142
(recommended Coeff = 0.129)


Finding maximum values for components

Finding time before overvolting the MMC

The tank capacitors will experience a ringing current that increases in size as the system resonates until a spark breaks out. As $I = C\frac{dV}{dT}$, the voltage transient will also increase as the system resonates. For somewhat worst-case calculations, this ringing is taken to double in size every half cycle of the resonance. Therefore, the theoretical number of half-cycles before the MMC overvolts:
$$N_{\frac{1}{2}} = \frac{V_C^{MAX}Derating}{V_{DC}} = \frac{4000 * 0.9}{270\sqrt{2}} = 10.58$$
Therefore the theoretical maximum amount of time (assuming no losses, perfect resonance and that the current doubles each time) we can let the system oscillate for is determined by:
$$t_{burst}^{MAX} = N_{\frac{1}{2}} \frac{1}{2f} = 10.58\frac{1}{2*115.13\times10^{3}} = 45.98\mu S$$

From this, we can calculate the expected primary current

$$I^{MAX} = \frac{V_C^{MAX}Derating}{2\pi fL_{DC}} = \frac{4000 * 0.9}{2\pi * 115.13\times10^{3} * 9.609\times10^{-6}} = 517.91A$$

Capacitor Selection

So we need 0.2μF at 4000V. We searched around for a good day or so to find the best value capacitors we could get, and it appears the CDE capacitors from Farnell (onecall specifically - the student version) are the best value.
We had the following 3 to choose from (between stock/price/values) that we then used to calculate the cheapest cost:

python2.7 capcalc.py What voltage do you want? (V): 4000
What capacitance do you need (nF): 200

942C20P15K: You need 5.33333333333 (2.0 series, 2.66666666667 parallel) costing GBP 31.7333333333
940C20P1K: You need 8.0 (2.0 series, 4.0 parallel) costing GBP 22.24
940C20P22K-F: You need 3.63636363636 (2.0 series, 1.81818181818 parallel) costing GBP 13.4909090909


capcalc.py

Despite the 940C20P22K-F's being the cheapest, only 2 in series would not give us the required current. The current they can deliver is listed in their spec sheet up to a certain frequency at which point you can interpolate or guess if they can meet what we expect!

We decided instead to go with the 940C20P1Ks.