Re: Air-core power transfomers

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rob-at-pythonemproject-dot-com>

Tesla list wrote:
> 
> Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<rob-at-pythonemproject-dot-com>
> 
> Tesla list wrote:
> >
> > Original poster: "Marry Krutsch by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <u236-at-earthlink-dot-net>
> >
> > Hi all.
> >
> >         I've been interested in those solid state MOTs ever since someone
> here
> > mentioned them.  Since discarded microwaves are scarce around here
> > (that, or someone is beating me to them ;-)), I wanted to make one
> > myself to power a DC coil, and to feed smoother power to magnetrons.
> >
> >         Anyway, I've got plenty of wire, but no ferrite for the core.
Can I
> > make an air core "MOT"?  What are the potential problems with this?
> > Should I let the inductive reactance of the primary limit the current,
> > or should external limiting be used?  Does air "saturate" the way iron
> > does?  I'm thinking that operation at 50 kHz or so is good.  And now the
> > stupid question:  Does the primary (drive coil) need to be wrapped
> > around the secondary (driven coil)?  I thought that coupling would be
> > better in this case, but really don't know.  Thanks for any help.
> >
> > Winston
> 
> If you use high voltage FETS in a half bridge, you may be able to drive
> the coil directly from the FETS through a blocking cap.  I haven't tried
> this, but its on my list of things to do.  I don't fool around building
> such power supplies.  I just get a good >300W switcher, and cut the
> board in half.  Then feed my own signal into the FET gate-source
> isolation transformers.  Rob.
> 
> --
> -----------------------------
> The Numeric Python EM Project
> 
> www.pythonemproject-dot-com

As I'm off from work today, I actually tried this.  Two IRF740's in a
half bridge, and two 1uF blocking caps. 1 goes to ground, the other to
the coil.  Unfortunately, my 300khz signal doesn't make it thru the gate
drive transformer.  And I also remembered that you need dead time. 
Otherwise both FET's conduction cycles overlap and tend to blow up the
fets.  The 1uF blocking caps also present a big shock hazard, so you
need serious grounding.  Maybe a more sane approach is just a 1:1
ferrite isolation transformer.  I have to work on the dead time problem
as I have no intention of using a SMPS controller.  Or I may convert the
circuit to a single FET or IGBT.  Rob.


-- 
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www.pythonemproject-dot-com