How exactly Antenna or Feedback Core works?

[moisesdias]

Hello there,

i'm building a Tesla coil based on Steve Ward's SSTC5 (http://stevehv.4hv.org/SSTC5.htm)
i have searched all the components and how they relate to each other, but after a lot of research i still haven't figured it out how the feedback works, i mean, what exactly is the signal that the antenna (or the feedback core, which is used in others tutorials) send to the pin 1 of the 74hc14? and why this signal makes the coil run at the resonant frequency?
if someone could explain it to me i would apreciate a lot, i want to understand the whole circuit and the coil before start mounting

[oneKone]

Welcome to the forum.

Hopefully this makes sense:

Say you had a Tesla coil already running and you put an oscilloscope probe as an antenna you would see a sine wave, this sine wave is fed into the 74hc14 (by antenna or ct) then is basically converted to a square wave. With this arrangement the 74hc14 will then drive the gate drivers at whatever the frequency the secondary is at.

[moisesdias]

hi oneKone, thank you for answering,
but, could you be more sepecific? unfortunately i can't use an oscilloscope right now,
the sine wave on the antenna is generated by the electric field
on the secondary, it's proportional to the voltage on the secondary and it's
limited between 0 and 5V due to the 1n60 diodes right?

and let's say that the voltage peak on the antenna is 'Va'
this sine wave is limited between Va and -Va or between Va and 0?
i make a drawing if it isn't clear what i said


and you said "With this arrangement the 74hc14 will then drive
 the gate drivers at whatever the frequency the secondary is at"
but why does it occur? what's the relation between the voltage on the antenna and the correct time
to switch the mosfets?

[Mads Barnkob]

Hi moisesdias and welcome to HVF!

The circuit is self starting due to capacitive coupling of energy through the gate drive transformer (I actually have a video of this phenomen here: https://www.youtube.com/watch?v=eRxyJ9-xuRE )

So once that either of the MOSFET drivers are on from start, it "pings" the secondary circuit even if there was no mains supply connected to the bridge, this small amount of energy is enough to radiate energy to light up a CFL tube. The antenna picks up this signal as well, limits it to -5 to -5 V as you saw yourself.

There is no need for correct switching times as this is a SSTC, not a dual resonant, so there is not two circuits that has to swing at the same time in order to get a energy transfer, in a SSTC we just pump as much as we can at the feedback resonant frequency of the secondary system.

[alan sailer]

"and it's limited between 0 and 5V due to the 1n60 diodes right?"

That is close but the diodes are not perfect and have a forward voltage drop.

"and let's say that the voltage peak on the antenna is 'Va'
this sine wave is limited between Va and -Va or between Va and 0?"

The two diodes will limit the voltage between zero and 5 volts (neglecting the
diode forward voltage). The whole point of this circuit is to limit the potentially
very high Va so that you don't burn out the logic circuit.

Do a search on diode clippers and see if you can get some useful information.

"what's the relation between the voltage on the antenna and the correct time
to switch the mosfets?"

This is the point where I might get the details wrong but I think the overall idea is correct.
To get the maximum voltage out of a tesla coil you need to be at (or close to) the resonant
frequency of the coil. The whole point of the feedback is to allow the coil to self-tune itself to the
resonant point. You could eliminate the antenna or the current feedback and use a
variable frequency signal generator. As you tune the generator you can watch
the output of the Tesla coil increase and peak as you reach the resonance of the system.

Cheers.

[oneKone]

Sorry for not replying. Luckily Mads and Alan have pretty much covered it (to my knowledge anyways)

[Uspring]

and you said "With this arrangement the 74hc14 will then drive
 the gate drivers at whatever the frequency the secondary is at"
but why does it occur? what's the relation between the voltage on the antenna and the correct time
to switch the mosfets?

See also https://highvoltageforum.net/index.php?topic=122.msg912#msg912

Now imagine the running frequency does not cause the overall phase shift to be 0, e.g. the feedback signal arrives earlier than that. This implies, that the input frequency is somewhat higher than the output frequency, causing the coil to increase frequency.
The overall phase shift of the coil is strongly dependent on the frequency so the phase will change in such a way, that the feedback signal will arrive later so compensating the too early feedback signal at the previous frequency. A stable 0 phase shift point can thus be found.

This point does not have to be exactly at resonance but it is most likely to be near there, since the change of phase shift with frequency is largest around resonance and therefore the likelihood of the 0 phase shift point to be near there is correspondingly large. The coil might not resonate at all for some polarity conditions of the input, when there is no zero degrees conditition.

Another condition for a stable feedback is, that there is enough secondary output voltage to drive the antenna and the input stage. That usually requires operation near secondary resonance.