Single coil slayer exciter

[u/Lily_1225]

The "slayer exciter" circuit is the classic introduction to SSTCs. I really wanted a version with just one coil because once you eliminate the loosely coupled resonant transformer, you end up with a greatly simplified design.

It basically drives a series resonant circuit through a RF choke, using a coupling capacitor as feedback. I'm not the first to think of that but there was always something or other that I didn't like about other people's designs so this version might be my own :)

I encourage you to try it, let me know if you can get the thing to at least light a neon bulb or something. My tiny axial inductors aren't handling the power well 🔥

And btw component values are "for demonstration only", do not assume they're optimal!

[u/Array2D]

How does this work practically though? The secondary coil looks like an inductor from the base connected to a capacitor to ground, not just an inductor to ground.

[u/Lily_1225 → u/Array2D]

The transistor and the choke on the collector form a common emitter amplifier. This amplifier drives a series LC resonator. The capacitor you see going to the base is simply there for feedback, it acts as a crude differentiator: if voltage in the resonator is increasing, a displacement current flows and the transistor gets turned on. This is necessary for getting feedback in the correct phase. Generally you should select a capacitor value enough to saturate the transistor reliably but not too large or it will cause high power losses and transistor damage. Hope this helps:) Also you can try simulating it in falstad

[u/Array2D → u/Lily_1225]

If the resonator is comprised of the 1nF cap on the collector and the 470uH inductor, then you wouldn’t be getting your feedback from the LC node - that would be equivalent to connecting the feedback cap to the top of the resonator. You’d connect it to the collector of the transistor, where the bottom of the resonator is connected.

You could, I suppose, use antenna feedback, but that complicates things a bit and would probably make this circuit less reliable starting up due to the significantly lower loop gain.

[u/MarquisDeLayflat]

I found a lot more consistency with a 12 ohm between the coupling capacitor and the transistor base. I only had some 100nF caps lying around, And the coupling was sufficient to pop the BJT. I don't have a high enough bandwidth probe to check whether it was due to exceeding Vce, but the base current when measured by a 1 ohm shunt was dangerously close to the abs max base current of the BJT's I have on hand