Re: MOT charging mysteries (was: Re: The RAT coil) (fwd)

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Original poster: Tesla List Moderator <mod1-at-poodle.pupman-dot-com>



---------- Forwarded message ----------
Date: Tue, 05 Dec 2000 07:25:15 -0600
From: Bert Hickman <bert.hickman-at-aquila-dot-net>
To: Tesla list <tesla-at-pupman-dot-com>
Subject: Re: MOT charging mysteries (was: Re: The RAT coil)

Bob and all,

I'm still investigating this circuit a bit more... but it does appear to
offer some intrigueing possibilities to eliminate the need for inductive
ballasting! It appears that the characteristic of the transformer is not
very critical as long as it behaves "mostly" like a voltage source.  The
appropriate size of the caps looks to be more a function of the tank cap
size, desired maximum allowable short circuit on the primary side, and
charging current capability of the diodes.  

Marco's circuit charges the 1 uF caps on one half cycle of mains voltage
through the diodes. On the other mains half cycle, the stored capacitor
voltages level is shifted, and added to, the voltage coming out of the
MOT's. If the gap never fired, the voltage on the tank cap (not shown,
but previously 0.05 uF) would go to 4 times the peak voltage of the
MOT's. 

Normally, the gap is adjusted to fire at a voltage between 2*Vmot (peak)
and 4*Vmot (peak). If this condition exists, the MOT's recharge the 1 uF
caps on one mains half-cycle (the "charging" half-cycle), then partially
or completely discharge them over one or more gap firings on the next ac
mains half-cycle (the "discharging half-cycle). Under these conditions,
the gap fires only on the alternate "discharge" half cycles, and not
during alternate "charge" half cycles. Every time the gap fires, the
voltage stored on the 1 uF caps is decreased, reducing the level shifted
offset. By balancing the ratio of capacitances of the energy storage
caps and the tank cap, and setting Vgap properly, multiple firings can
be achieved on "discharging" half-cycles.

I am still looking more thoroughly at the transient behavior of this
interesting circuit... :^)

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "bob golding" <yubba-at-clara-dot-net>
> 
> Bert all,
>         What does one need to need to know about the trandsformer in order to
> calculate the size of cap to use.? I am using a 8200v 250mA radar
> transformer. My inductive ballast works fine, but it would be nice not
> to have drag 75 lbs of iron and copper about. I assume one could use
> more or less any type of cap as long as the voltage stand off is high
> enough as it is in the 50/60 hz part of the circuit and the high current
> part of the circuit is the "other side" of the gap. Have I got this
> right.?
> 
> cheers
>  bob golding
> 
> cc:    (bcc: Marco Denicolai/MARTIS)
> > Subject:  Re: MOT charging mysteries (was: Re: The RAT coil)
> >
> > >The particular MOT and voltage doubler circuit in your system does not
> > >need external ballasting since it uses 1 uF caps in series with the high
> > >voltage MOT output legs (see http://www.saunalahti.fi/dncmrc/tank.html).
> > >When the main gap fires, there's never a true short circuit placed
> > >directly across your MOT's. In effect, your 1 uF caps are acting much
> > >like a "capacitive" current ballast for your MOT supply, limiting the
> > >short circuit current! While inductive ballasting is used in most
> > >systems, capacitive ballasting is another option. In fact, a pig, if
> > >used with this circuit, would not require external inductive ballasting!
> >
> > Hi Bert.
> > You are right. The 1 uF caps must work as a ballast. I didn't see that...
> >
> > But so let's put it the other way: I haven't simulated it yet, but why not to
> > use the doubler
> > scheme instead of 4 MOTs? You get ballasting for free and the same voltage.
> >
> > Surely the charging speed (i.e. voltage) will not be the same, but (at least
> > from Finn's
> > scope pictures) looks like the MOT current protection plays nasty tricks
> and he
> > is loosing
> > "bangs" anyway.
> >
> > Regards