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High voltage transformer construction

Project description and construction

I attempted construction of a high voltage transformer with the intent of using it to drive a tesla coil by base feed method, instead of the classic air cored transformer approach. I wanted this transformer to be as robust as possible, with less voltage gain than typical for TV flyback transformers – hence I opted for a single layer secondary coil encapsulated in polyester resin, and a primary coil located underneath it for maximized coupling. The pictures speak for themselves:

OLYMPUS DIGITAL CAMERA

Secondary parts – pieces of suitably cut and drilled PVC pipe, one of them bearing the secondary coil winding.

OLYMPUS DIGITAL CAMERA

After careful putting-together, a seal has been made with a special kind of non-drying putty

Resin has been poured from both sides, sealing the secondary coil. The primary is shownto the right, wound on it's form

Resin has been poured from both sides, sealing the secondary coil. The primary is shownto the right, wound on it’s form

Finished transformer

Finished transformer

Results

The primary purpose of this transformer was to attempt base-driving a Tesla coil. This means powering the base of the coil directly from a high frequency HV source, in this case the ferrite transformer! The results were, however, disappointing, and I believe that a good part of reason is that the transformer is not up to the task. Recently I’ve spoken to Steve Ward about this, who in meanwhile ran some electrostatic simulations for his base feed experiments, and it seems that a single layer secondary winding actually creates higher parasitic capacitance than a layered one. The high dielectric constant of the resin I used, as well as proximity of the core and primary makes the self-resonant frequency of my transformer fall within Tesla coil range, which is, a bad thing. Moreover, signs of corona could be seen within the transformer in operation, and this can be noticed in the following video as blue glimmer if one watches carefully. I’m not completely sure whether the corona occurs in the air space between primary and secondary, or within the insulating resin itself. In any case there is evidence that Simply sealing the winding in resin without vacuum potting may still leave tiny amounts of air within, greatly increasing susceptibility to corona and breakdown.

Conclusion

This project is open ended, and there is still a lot of research to be done. Designing a high frequency, high voltage transformer was always a demanding task due to parasitic capacitances and leakage inductance which hamper the output once the drive frequency is comparable to self-resonant frequency of the transformer. In industry, this problem is generally solved by having the HV secondary split into sections by HV rectifiers – so called “split diode” transformers: the diodes act to de-Q the parasitic RLC circuits, increasing the overall self-resonant frequency of the transformer up to order of magnitude of one of those sections. Unfortunately, transformers like these are only able to provide DC, and we need AC to drive Tesla coils!This is where an idea sprang to me – what if we use two diode-split transformers in anti-parallel, or better, two diode split windings in anti-parallel, on the same transformer, in order to balance out the volt-seconds in every half-cycle. After consulting with experts, we were left somewhat puzzled whether this idea would work. In any case, a lot more work is to be done!

 

 

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