Solid State Tesla Coil Parts and Design Questions

[IanBerry]

Hello, I am working on a project for a simple solid state tesla coil but I am having some issues/questions. My schematic for the oscillator is attached.

1: When testing it on a small scale with a breadboard the circuit oscillated at 1.3 MHz. This is an issue because I was using a 10nF caps and 1.5 mH inductors on the same ferrite toroid and the resonant frequency of those two are around 40 kHz. The bizarre thing is that when I connected my alligator clips (I am connecting the inductors to the breadboard using alligator clips and a jumper wire) to the second coil on the ferrite toroid the inductance or the first coil dropped to 4.4 uH. What the heck is going on?

2: For the power source I would like to use rectified wall current, so about 170 volts, but for the life of me I can't find a matched pair of PNP NPN BJTs that can handle that voltage. I would also like them to be able to handle a beefy amount of amperage because I have no clue how much current this thing will draw. Does anyone have any suggestions?

3: That brings me to another question. Are there heat sinks for small value capacitors such as the 10 nF caps I was using on the breadboard or high current small value capacitors? I will probably have to use a similar value for the capacitors if not smaller and I imagine those itty bitty disk capacitors will explode or heat up tremendously when exposed to high current.

[Fungus]

1) Breadboards have all sorts of stray capacitances/inductances.

2) You might need to look at IBGTs

3) I don't think you can kill capacitors with "high current". Usually it takes "wrong voltage".

[Audioguru]

Your simple circuit has nothing to begin turning on a transistor when power is applied, both transistors will be off and remain off.
If the transistor ever begin conducting then there is nothing limiting their base current which will probably destroy them.

[IanBerry]

My apologies, I forgot to add the momentary switch that goes from positive battery terminal to the output of the transistors. It just starts the oscillation then the feedback from the coils turns the transistors on and off alternatingly. One is NPN and one is PNP so one will turn on in the positive and one in the negative portion of the sine wave.

About the capacitors, this might begin delving into beginner questions but since the capacitor is charging and discharging so rapidly and electrons have to flow onto or away from the plates of the capacitor for it to charge, wouldn't the electrons sometimes collide with atoms in the plates of the capacitor and cause heating? Especially because capacitors of that value are typically quite small and the sheets or plates of the capacitor are going to be very thin? So would high current through a capacitor cause heating?

[PChi]

You are correct capacitors designed for high power use have current ratings. It's just resistance of the internal connections and plates. No need to mention Tesla.

[Fungus]

So would high current through a capacitor cause heating?

Yes and no.

Capacitors only allow current through them for a very short time - until the plate charges up. At DC no current will pass, no heating.

At higher frequencies (eg 40kHz) the inductance of the wires will prevent much current from being available so again, you get very little heating.

Every capacitor will have a low point, a charge/discharge frequency where heating is maximized. Even so, any reasonable capacitor will survive so long as there's some ventilation -  they're designed that way.

If you want to minimize the heating at a certain frequency you can mix different capacitor types, put multiple capacitors in parallel, etc. It's not difficult to fix.

But ... before you try to 'fix' it you have to find out if it's actually a real problem or not.

It's a complex situation (the inductances around the system are unknowns) so the best way to find out is to build one and see what happens!

[Audioguru]

I fixed most of your scribbled schematic. Two batteries?

[IanBerry]

Thank you for the answers to my question about capacitors and the fixed schematic. I was using two batteries to simulate positive, negative and ground. Also, does anyone have any suggestions on a pair of PNP NPN transistors? I seriously stumped on that. Maybe I should try MOSFETs? But even with a MOSFET pair of N and P channel is hard to find. Should I abandon trying to run it off of rectified wall current and go for something lower like 20 volts? Part of the reason I'm having such a hard time is I have been looking for transistors rated for 400 volts. The reason I've been doing this is because when I tested it on a breadboard instead of it being 9 volts at the peak of the sign wave it was almost 15. So at that same ratio, 170 volts would become 280, and I'm not sure if there is the possibility the voltage could go higher.

What should I use for transistors?

What voltage should I power it at?

[KE5FX]

Give some thought to starting out with a oneTesla kit, maybe.  There's really no way to climb this learning curve via the cut-and-try approach, unless your budget for replacement transistors is near-infinite.