Re: MOT level shifter

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

Original poster: "Gregory Hunter by way of Terry Fritz <twftesla-at-qwest-dot-net>" <ghunter31014-at-yahoo-dot-com>

Hi Herwig,

--- Tesla list <tesla-at-pupman-dot-com> wrote:
> Original poster: "Herwig Roscher by way of Terry
> Fritz <twftesla-at-qwest-dot-net>" <herwig.roscher-at-gmx.de>
> 
> 
> List, experienced coilers,
> 
> Not-current-limited transformers like MOTs and pigs

MOTs *ARE* current limited. They have shunts. I have
examined dozens of MOTs from both sides of the
Atlantic, and they all had shunts. The shunts are not
as effective as NST shunts, but they do work. I've got
a 1900VAC MOT in my stash right now that only draws
17A from a 120V wall outlet in a dead short-circuit
condition. I use it as a small ballast. My MOT twins
make a fine 240V/20A ballast with their secondaries
shorted out.

> need a 
> resistive/inductive/capacitive ballast. When using
> an inductive one, 
> it's value needs to be considered when designing a
> LTR-type main 
> cap.
> 
> Level shifters IMO are kind of complex capacitive
> ballasts as well.

Yeah. I experimented with 4 level-shifter MOT caps in
series instead of only two. This halved the
capacitance and cut the power down to about 1750VA. It
performed well on the 4" x 24" coil; 3 foot or better
sparks. Capacitive ballasting on the HV side is a
practical route since surplus MOT caps are so cheap
and plentiful.

> Do simulations predict similar resonant effects like
> with inductive

Can't help you there. I'm no engineer.

> ballasts? If so, need this capacitive ballast to be
> considered when 
> designing the primary coil?
>
----------------------------------------------------------------------------
> -------------
> NSTs mainly deliver sinusodial waveforms.
> Calculation of peak 
> current, peak voltage etc for the main cap is
> relatively easy 
> therefore.
> 
> MOTs with level shifters IMO deliver pulsed signals.

Yes, 60Hz (or 50Hz)pulses with a half-cycle dead spot
between each pulse. I couldn't figure out how to
optimize a tank cap for such a weird supply either, so
I gave up and tried 47nF. This was grossly undersize,
and I'm sure the stiff 12kv pulses forced it to fire
in "burst mode", with a cluster of bangs each time a
pulse came out of the level shifter. The resulting
sparks from a 4" x 24" coil were insane, with frantic,
multi-branched streamers up to 60" long when running
unballasted at 240V/20A.

> How should I 
> calculate the relevant design data for the main cap?
>      Assuming 
> V peak to be 12 kV and I load to be 0.5 A?

Dunno. I think it wise to keep the tank cap smallish
to avoid loading down the level-shifter cap. Not very
scientific, but it worked well for me. Finnish
coiler/engineer Marco Denicolai ran a twin MOT/level
shifter on his first coil, and I think he used a .5uF
tank cap with good results. I don't think the reso
cap, LTR cap, or any of the other familiar NST stuff
is even relevant with the MOT/level shifter supply.

> 
> Cheers,
> 
> Herwig
> 

Regards,

Greg
http://hot-streamer-dot-com/greg
 


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