# Solid State Tesla Coil

## Step 11: Control Circuitry

I strongly recommend you to employ Steve Ward's design.  (Scroll to the bottom of his page for his final design).

...But, sometimes you will not be able to find some components. For example, I could not find locally the UCC37322 or UCC37321. So, I made my own variation of his circuit. See the schematic below.

In this instructable, I will not teach you to make a circuit out of a schematic; if you can't read them you should try simpler projects first. A disadvantage from my spinoff is that there is no isolation; bear this in mind when working. This is why I recommend Steve's Design if possible.

You may build this circuit in a strip board, veroboard, printed circuit board, point to point, etc; but do not use a breadboard (Protoboard); there is too much parasitic capacitance.

As always, place a fuse between the circuit and the mains plug. 5A or similar.

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Brief Technical Explanation

A solid state Tesla coil works by switching the primary at a resonant frequency. This frequency varies due to the height of the coil, the topload, and the environment. Thus, a fixed frequency oscillator is not ideal.

The following driver changes its frequency based on what the antenna gets from the Tesla coil. The antenna could be any straight piece of wire connected to the circuit. The other end is left unconnected. Thus, this SSTC uses antenna feedback.

To avoid overheating the mosfets, usually an interrupter is used. It's role is to turn the Tesla coil on and off at a certain frequency. This doubles as a power control if one varies the duty cycle of the circuit. A full explanation on how the Tesla coil works can be found on Steve Wards site. I also recommend to follow his schematic (the last one), and fall back to mine if some of the parts are not available  on your area.

The only difference between mine and his is that I did not use the gate drive transformer and the ucc chips. If you can find these elements, by all means do his design, as it has galvanic isolation between the half bridge and the rest of the circuit. (which is a good thing)

---The capacitors on the half bridge are must withstand at least 170V.
---This tesla coil is not audio modulated, but it could be made so by just changing the interrupter for an audio one.
---The 5v and 12v signals can be obtained from a transformer followed by a LM7812 and a LM7805 (plus capacitors).

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I have been trying to make an Audio Modulated SSTC for a while now, and I have found that your Ible has been the most helpful yet. So on step 11, where you say this SSTC could become audio modulated by changing the interrupter to an audio one, what exactly does that mean? Circuit diagrams are definitely not my thing, and I have not been able to find a sophomoric explanation of an Audio Interrupter or MIDI Microcontroller. I hope someone can help me!
Well, actually, this tesla coil is not audio modulated.There is nothing on the schematic that performs audio modulation.

However, if you were to replace the smaller schematic (which is the interrupter's schematic) with one designed for audio modulation... well, you would have audio. There are several diagrams over the net. However, I would recommend you to only go for audio modulation after you get the basic setup running.
The_Saint says: 1 year ago
Hi, its a nice work! Do you have any higher resolution circuit diagram? Thanks!
Michael Chen (author) in reply to The_Saint1 year ago
Just hover the mouse on the image and click the "i" that appears in the corner. Then click on original file.
florinandrei says: 1 year ago
Great instructable re: the mechanical part. The electronic part needs a bit more pampering.

What is the "interrupter"? Is it some kind of signal you need to apply? What's the frequency, amplitude, etc?

Is your schematic a "singing" coil? If so, where does the audio signal go?

The main IC (the one that feeds the MOSFETs) is not labeled. What is it? What does it do?

Various capacitors don't have the max voltage indicated. While this is not important for the low voltage parts of the circuit, I'd imagine it's kind of a big deal when it comes to the output stage. :) Same goes for resistors, if more than, say, 0.25 W is required. Please indicate the wattage where it's important.

It would help to add a few words about the main parts of the circuit. I.e., this is the start-up stage, it performs such-and-such function; this is the output stage, its function is to feed the primary; yadda-yadda.

I can't figure out what the antenna does, but perhaps I need to re-read the rest of the article.

Thanks!
Michael Chen (author) in reply to florinandrei1 year ago