I'm thinking of choosing and creating step-up convertor for spark-gap demo. Currently I'm thinking of a kind of pulse transformer to raise the voltage to 10-30 kV.

For now I have plenty of time so I'm interesting in trying to create the thing myself. I have no problem creating pulse voltage of 300 V at 50-100 kHz (rectifying it and slicing with ne555 controlled transistor). But then I need to make calculations about the transformer itself.

I understand I need to fetch wire and ferrite core from the local shop. But how to choose minimum core section area and minimum amount of turns, depending on frequency and desired output power? Internet is bloating on "transformer calculation" query, but it is mostly about simple ratio of turns. So could you please help me with some links/instructions?

Is the idea viable, by the way? I understand that unless I take some precautions, I can get breakdown of isolation between turns due to high voltage in the secondary coil...

  • 2
    \$\begingroup\$ look at automotive spark coils... get one .... dont try to re-invent the wheel until you understand how it is done. \$\endgroup\$ May 24, 2017 at 13:46
  • \$\begingroup\$ See Tony's comment, buy an auto coil, so much is already done for you. \$\endgroup\$
    – Neil_UK
    May 24, 2017 at 15:14
  • \$\begingroup\$ diyphysics.com/category/instrumentation/… \$\endgroup\$
    – jonk
    May 24, 2017 at 16:13
  • \$\begingroup\$ Thanks for good suggestion! I will definitely try this, but at the end I want to plug it to 300V, not 12V, so the ignition coil probably would not be good. Surely I can step down to 12V first, but surely I prefer to find or design single transformer instead. Perhaps I should look for CRT display high voltage transformer... \$\endgroup\$ May 25, 2017 at 8:56

1 Answer 1


Although you could build your own high voltage transformer, the cost of the materials, will cost you more than buying/getting a scrap TV high voltage transformer. In addition you save your time and effort.

If you still want to build your own, you start with the output power. Lets say you want 20KV at 0.01 amp, p = 200W. Assume 80% efficiency, you then need 250W input. your input current will be I = 250/300 = 0.8A, but for safety, you need 2A. The turns ratio is 67 (20k/300).
So, the input side should handle 300V @ 2A max. and the output should handle 20KV @ 20ma.
The rest of the calculations depend on the iron core that you get. Be sure to get the spec sheet for the one you get. Good Luck!

  • \$\begingroup\$ Thanks for your answer. As I mentioned I understand basic calculations of voltage / current ratios well enough. What I do not know is "the rest of calculations" :) And I'm going to use ferrite rather than iron core and higher frequency... The problem with TV flyback transformers seems to be that they are also designed for low primary voltage. Though I'm studying this. And you are very right about costs, I've already found that the core alone in the local shops is more expensive than the whole transformer :) \$\endgroup\$ May 26, 2017 at 9:26

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