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Re: Build your own HV transformer

Original poster: Steve Ward <steve.ward@xxxxxxxxx>

Its not particularly simple by any means.  You must acquire the proper
ferrite cores (i found them on ebay).  1500VA and 3kVA cores were
amoung the largest available.

The HV coil is made from a few hundred turns of suitably sized magnet
wire, typically in the 24-30awg range.  Each layer of the winding
should not use more than maybe 50 turns of wire.  The larger ferrites
typically allow for 20-25V/turn of wire, which means that the total
turns needed for HV is not much compared to a 60hz transformer running
~1V/turn.  Between each layer of winding i use several (10-20) layers
of 4-6mil polyethylene film for insulation.  You can never use too
much insulation on a high frequency HV winding!!  Rules of thumb for
determining the insulation thickness go out the window, the HF current
couples its way through everything, heating it, melting it, and
eventually burning right through ruining the winding.  After winding
the coil you need a vacuum chamber to then "pot" the winding in a
suitable insulator, epoxy is my choice at the moment.  The vacuum is
needed to pull the air out of the layers of winding.  You must cycle
the vacuum several times before the epoxy begins to set in order to
work the air bubbles out.  Any air bubbles and the coil will fail much
sooner than you'd like.

You then need a bunch of experience at debugging full-bridge solid
state drivers.  I have had plenty of experience here over the past 2
years.  Its not an in-expensive endeavor when you go through handfulls
of IGBTs and you still haven't quite figured out why they are
exploding when you start that power arc on the transformer output.  I
have successfully built a 15kv (peak) 1.5kVA HV transformer and driver
seen here:


It has NOT been tested as a tesla coil power supply.  One would need
HV, ultrafast rectification on the output to charge a tank capacitor.
Protecting those diodes might be a project in itself.

Anyway, i (with a friend) am still working out the details on a more
powerful unit, but the plan is for 6kva.  We have the cores and
materials, just not the time and energy to keep loading in more IGBTs
as it just blasts them apart at the moment.  Currently researching
other topologies for work at these higher power levels, possibly a
resonant primary drive, but im still not sure if this is the best way
to go (bad things could happen with no load).

Thats about all the information i can offer up at the moment.  Im
still working out the designs.  It might be something to think about
if you are already familiar with building solid state drivers and have
the equipment to do vacuum potting.  Otherwise you might have a little
better luck with winding many many more turns on a big 50/60hz iron
core.  Neither way is gonna give instant success, but you might want
to consider what your really after and where your skills really are.

Hope that helps answer your question.

Steve Ward

On Apr 8, 2005 8:28 PM, Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: Sebastiaan Draaisma <sebas@xxxxxxxxxxxxxxxx>
> Chiang Mai, Saturday, April 9, 2005
> In reply to: About MOTs..
> Posted by:
> Posted on: Saturday, April 9, 2005
> Hello Steve,
> That sounds interesting! Would you like to share your knowledge on how to
> build your own transformer, I have been looking on google but, not much
> information in the first 100 results :)
> I already wanted to experiment with trying to rewind the secondary of a
> MOT, but if you had success building your own in a different way, I rather
> try that approach.
> Best regards,
> Sebastiaan Draaisma
> Tesla> Original poster: Steve Ward
> <<mailto:steve.ward@xxxxxxxxx>steve.ward@xxxxxxxxx>
> Tesla> On Apr 8, 2005 9:39 AM, Tesla list
> <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx> wrote:
> >> Original poster: Jim Lux
> <<mailto:jimlux@xxxxxxxxxxxxx>jimlux@xxxxxxxxxxxxx>
> >> You'll never get that, not because of breakdown, but because the
> primary of
> >> the second transformer will be horribly saturated by putting 2400V on it.
> >> I doubt you could put 150V into a MOT without encountering saturation.
> Tesla> Hah, 150V? try 120V. Most of the MOTs ive played with start
> Tesla> saturating at about 90VAC input. They typically have a power factor
> Tesla> of .12 at full input voltage, though this was with an open circuit on
> Tesla> the HV secondary. I think the PF might have gotten up to .4 or so
> Tesla> with a heavy load... still junk in my opinion. I guess im lucky
> Tesla> enough to know how to use silicon (IGBTs) and ferrite to make my HV
> Tesla> transformers... 6kVA at about 10 lbs ;-) and no, they dont saturate.
> Tesla> Steve Ward