Self-oscillating nano Tesla Coil (14.3 MHz)

[tefatronix]

Hi all, I'm a student from the Czech Republic and this is my first post on this forum, in which I am going to show you my miniature Tesla coil.

I have already posted this in a few Facebook groups before, but in case anybody is interested, here it is.
This is my smallest Tesla coil, the circuit board measures 25x20 mm, the secondary coil has the following parameters: 14.3 MHz fres, 10 mm diameter, 25 mm winding height, 0.1 mm wire diameter.

It is very similar to Slayer exciters and to a self-oscillating coil another member posted here some time ago. The input stage uses extra 1N4148 diodes to decrease the capacitance of the Zener diodes and uses a simple circuit (zener, trimpot, some resistors) to slightly bias the MOSFET gate. The output stage wired in a class E like configuration, however I don't use any capacitor in parallel to the MOSFET's output, as it seems to be in tune even without it. I haven't measured the waveforms, as my scope is quite slow and its input capacitance is not negligible. The MOSFET I use here is an IRF510 - this one was chosen because of its low gate capacitance, low price, easy availability and fairly high speed (I think it would be quite challenging to get an IRFP460 to run at such frequencies  )

I built this coil to have an "improved version" to replace my previous nano Tesla coil, which got damaged and was very inefficient anyway (based on a BD139 NPN BJT in a very similar circuit to this one - it had a different bias circuit and a 150 pF tuning capacitor). I've already had this coil laying here for a few weeks or even months, but I was too lazy to write any documentation, but finally got myself to do it (full documentation is here).

This coil can run off a 12-14 volt power supply without overheating, it can momentarily tolerate higher voltages (I tested it at voltages over 24 V) as long as the input is pulsed, otherwise it will overheat fairly quickly. At voltages under approximately 15 V or so, the discharge has to be started manually with a piece of metal, etc.

[Mads Barnkob]

That is a wonderful little coil!

How long can you run it at 12VDC input with that heat sink staying below burning-my-fingers temperature?

Would tuning capacitor get more critical with lower resonant frequency?, as I imagine the low internal capacitance would then no longer be enough.

Did you try to replace the metal screws/nuts and spacers to a plastic material to see if it improved efficiency, not that I think it is a problem at these extremely small power levels, but it is still very close to the primary/secondary circuit.

[tefatronix]

That is a wonderful little coil!

How long can you run it at 12VDC input with that heat sink staying below burning-my-fingers temperature?

Would tuning capacitor get more critical with lower resonant frequency?, as I imagine the low internal capacitance would then no longer be enough.

Did you try to replace the metal screws/nuts and spacers to a plastic material to see if it improved efficiency, not that I think it is a problem at these extremely small power levels, but it is still very close to the primary/secondary circuit.

Thanks!

I have usually only used it for a few minutes at 12-14 VDC and maybe 30 s - 1 min at higher voltages, the heating isn't that extreme at 12 V and I think it could run at that voltage without being at finger-burning temperatures for much longer than that, but it creates a lot of EMI (the effect of EMI is MUCH more noticeable at this frequency than at (for example) 1 MHz) and it makes only a small discharge at these voltages...

Haven't tested plastic screws or decreasing the operating frequency, I plan to do so sometime later (however I have lots of other stuff to do + this coil needs an interrupter so I can increase the input voltage to 24 V for longer periods of time, I plan to build a battery+boost+interrupter pack), but with the previous BJT-based version it played a visible role, decreasing or increasing it too far would result in much higher heating and/or lower spark output (if you are interested in the schematic of the old version, it's documented here, although it is not very impressive - essentially a modified BJT based Slayer exciter with the same output stage as what's in this TC). Adding a small capacitance here results in no spark length increase or even some decrease.

[mkarliner]

This looks really nice. However...
14.3 Mhz is right in the middle of the 20M amateur radio
band, and you are likely to be causing havoc for up to
hundreds of miles. The 20M is the best long distance
radio band and I can communicate round the world with
just a few watts of power. A Tesla coil operating on this
frequency is really bad news. Sorry.

Mike

[tefatronix]

This looks really nice. However...
14.3 Mhz is right in the middle of the 20M amateur radio
band, and you are likely to be causing havoc for up to
hundreds of miles. The 20M is the best long distance
radio band and I can communicate round the world with
just a few watts of power. A Tesla coil operating on this
frequency is really bad news. Sorry.

Mike

 I will try to read more on that subject, meanwhile I do not think the actual radiated power is very high and I operate the Tesla coil very rarely, I can try to decrease the operating frequency by a few hundred kHz to (or as close as possible to) the 13.56 MHz ISM band, together with an added tuning capacitor, this could improve performance of the coil as well.