High Voltage Forum

Tesla coils => Solid state Tesla coils => Topic started by: nabzim on June 27, 2017, 08:03:57 PM

Title: Help with secondary-feedback?
Post by: nabzim on June 27, 2017, 08:03:57 PM
I made a post last night about my SSTC, but I realized I may as well ask for help with this issue I had. I have been only successful with with antenna feedback, and while it works great, it would be nice to not need a giant antenna sticking up and out of the base, always needing to be re-bent into a good position if it gets shifted while moving around and whatnot.
I attempted to use this current transformer that I wound: https://goo.gl/photos/3Z2faavbYM8r9SvL9 (https://goo.gl/photos/3Z2faavbYM8r9SvL9)
It has 54 turns of 24awg and the core is this one: http://www.ebay.com/itm/Toroid-Core-Magnetics-ZW-42507-TC-Power-Ferrite-1-00-OD-2-PCS-AL-7825/261788162368?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649 (http://www.ebay.com/itm/Toroid-Core-Magnetics-ZW-42507-TC-Power-Ferrite-1-00-OD-2-PCS-AL-7825/261788162368?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649)

So, basically it works but only at low voltage. When I turn it up, it cuts-out past a certain point on my variac. I had been trying to use loneocean's sstc2 as the reference point to set up secondary feedback, so my feedback circuit is set up like his (except I do not have .1nF cap between the UCC input and ground). http://www.loneoceans.com/labs/sstc2/sstc2schematicv10.jpg (http://www.loneoceans.com/labs/sstc2/sstc2schematicv10.jpg)
 I also tried a 1k burden resistor across the transformer's leads, after reading about current transformers, although that seems to be used for measuring current with a multimeter.

So I'm looking for some idea as to what I may be doing wrong or what I should try next... maybe different burden resistors, more turns of wire, or that the .1nF cap is more important than I assumed?
Title: Re: Help with secondary-feedback?
Post by: Mads Barnkob on June 28, 2017, 10:30:55 AM
Leaving the filter capacitor out from the output of the hex Schmidt inverters could be a potential problem, that induced noise at higher powers is enough to knock the circuit out of order.

A current transformer should have a burden resistor, else you do not have a voltage across its output, just a current across the diode, which then maybe works from a voltage drop point of view and can activate the input of the IC, but that should really be made correct with a resistor in my opinion.

Secondary base current is possibly around 50 mA and up to max 1 A, I am really guessing a little bit here, not completely sure how much it is in a small coil, so you have to design the CT and burden resistor to provide enough feedback for the inverter input to switch properly.

It could also be that you saturate the CT core, is there a difference between continues wave mode, audio interruption or normal interruption from what voltage level it stops working?
Title: Re: Help with secondary-feedback?
Post by: nabzim on June 28, 2017, 09:53:38 PM
Leaving the filter capacitor out from the output of the hex Schmidt inverters could be a potential problem, that induced noise at higher powers is enough to knock the circuit out of order.

A current transformer should have a burden resistor, else you do not have a voltage across its output, just a current across the diode, which then maybe works from a voltage drop point of view and can activate the input of the IC, but that should really be made correct with a resistor in my opinion.

Secondary base current is possibly around 50 mA and up to max 1 A, I am really guessing a little bit here, not completely sure how much it is in a small coil, so you have to design the CT and burden resistor to provide enough feedback for the inverter input to switch properly.

It could also be that you saturate the CT core, is there a difference between continues wave mode, audio interruption or normal interruption from what voltage level it stops working?
So, I tried the .1nF filter cap and that worked, pretty much. I can still get it to cut-out like it did before by adjusting to the position of the primary on the secondary. It works flawlessly when the primary is less coupled and starts to cut out with higher coupling. I am assuming then that too much current flowing through the secondary is causing the "cutting-out" to happen somehow.
Also there is this effect (at the proper coupling) where the output is very similar to a "ramped" type of tesla coil, with single, non-branched sparks that are much quieter than normal ones. I took a video of it you can see here: https://goo.gl/photos/ayWtdxZWsgqYksEi6 (https://goo.gl/photos/ayWtdxZWsgqYksEi6)

Also, I tried the resistor again after the filter cap worked, but there was no significant change that I noticed, other than when turning up the variac, I wouldn't get any spark until 25v, but before I would start to see spark output somewhere between 5 to 10 volts.
Title: Re: Help with secondary-feedback?
Post by: Mads Barnkob on July 01, 2017, 08:22:12 AM
If they sparks get quiet like that, it means that you have a much lower peak current in the primary and the waveform is close to a non-distorted sinus wave, so the energy transfer to the secondary circuit is properly better.

Higher and higher input voltages to achieve break out is mostly a phenomenon that I know from the DRSSTCs, because we de-tune the primary circuit to account for the secondary spark loading de-tuning, so in extreme examples we do not get any output before some hundred volts on the DC bus. It could be that for your SSTC you do not have enough gain in the feedback loop for it to work at low voltage.