@matko123, "I am not a beginner..." Sorry but some of your questions indicate that you are more of a beginner than perhaps you are willing to admit. As such it would be irresponsible for me or anyone else on this forum to advise you on the specifics of building something as potentially lethal as a Tesla coil. * To answer your questions within the scope of the above statement. Please note that this schematic: [http://onetesla.com//media/wysiwyg/downloads/tsschem.png](http://onetesla.com//media/wysiwyg/downloads/tsschem.png) appears to be incomplete. "- how can we do "zero crossing switching" when the negative current passes through current transformer? is not the voltage negative, too?..." The current flow through an inductor is proportional to the integral of the voltage applied across it. Another way of saying this is that the voltage across an inductor is proportional to the derivative of the current through it. Therefore if the current through it stops changing, the voltage across it falls to zero. "...how we know the current is zero?" Very much depending on the application, there are several ways to detect the current flowing through an inductor. Fig 28 in: [https://www.st.com/resource/en/datasheet/stcmb1.pdf](https://www.st.com/resource/en/datasheet/stcmb1.pdf) shows the simplest, direct way using a sense resistor but it also shows another way using the fact that the voltage across a capacitor is proportional to the integral of the current through it and therefore if you differentiate the voltage across it you have a voltgage that is proportional to the current through it. Another couple of techniques are to use a current transformer or to use the voltage across a secondary winding with a high impedance load as illustrated in the screenshot below:  "- what is the behavior of 74hc74 flip-flop?" Again, this is pretty fundamental digital electronics. The generic 7474 is a dual D type flip-flop. For more, see: [https://en.wikipedia.org/wiki/Flip-flop_(electronics)](https://en.wikipedia.org/wiki/Flip-flop_(electronics)) [https://www.electronics-tutorials.ws/sequential/seq_4.html](https://www.electronics-tutorials.ws/sequential/seq_4.html) See also: [https://docs.easyeda.com/en/Simulation/Headings/index.html](https://docs.easyeda.com/en/Simulation/Headings/index.html) then play with: DFFEE D type flip flop demo in: [https://easyeda\.com/example/Easy\_logic\_device\_simulation\_in\_V2\_3\_x\_onwards\-RaKIG2oJ5](https://easyeda.com/example/Easy_logic_device_simulation_in_V2_3_x_onwards-RaKIG2oJ5) "- why is the high-side UCC inverted? I had a look at the previous version of oneTesla TC and the MOS-drivers were flipped. So, the phasing of the primary of the GDT does not matter?" This question does not contain enough supporting information to be answered and anyway it should be directed to the designers of the original DRSSTC: [https://www.stevehv.4hv.org/DRSSTC1.htm](https://www.stevehv.4hv.org/DRSSTC1.htm) and the OneTesla products: [http://onetesla.com/](http://onetesla.com/) "- can be the optical transmitter and receiver (IF-D95T) replaced by a optocoupler? Is the normal state of IF-D95T 5V or 0V (normal state = i mean when there is not signal from transmitter)?" This is also question for the designer of the OneTesla products: [http://onetesla.com/](http://onetesla.com/) "- can i use 200:1 current transformer instead of 300:1 (oneTesla generally uses 300:1, but it is not available anywhere)" This is also question for the designer of the OneTesla products: [http://onetesla.com/](http://onetesla.com/) who obviously supply a suitable transformer in their DIY kits. "- please, describe the whole working principle of the circuit. Please, i cannot do it by my self." This question is beyond the scope of the free help available from EasyEDA. There is information on the working principles here: [http://onetesla.com/tutorials](http://onetesla.com/tutorials) [https://www.stevehv.4hv.org/DRSSTC1.htm](https://www.stevehv.4hv.org/DRSSTC1.htm) and elsewhere on the web. For more information, I suggest you contact: [http://onetesla.com/forum/](http://onetesla.com/forum/)

Thank you man! What I have to say is that I already asked oneTesla for help, but it seems like people have so many things to do that they will not respond me anyways (and anymore). I have built some Tesla Coils thow, but i have never tried primary current feedback. That is why i am curious about it, before I start making the circuit. My biggest problem up to now was that I always used very low voltage ( I mean low in comparison to other DRSSTCs ) - 15V max. That limited the spark length to around 2 inches. Now I want to use serious 240V and for that, I want to make the circuit working fine so I will not burn out all IGBTs. Thanks for answers thow, I am ready to make some LOONG SPARKS :)     Thank you