Re: [TCML] Single MOT Coil?

[Atomic <atomicrox@xxxxxxxxx>]

Jason, do you think it'd be possible to limit current to a low enough level
and use a static spark gap?
(as a poor man's NST substitute on a small coil)


On Fri, Aug 16, 2013 at 1:23 PM, Jason Johnson <jasonmsusolar@xxxxxxxxx>wrote:

> I too have had good luck with a single MOT. Utilizing a level shifter
> circuit (as the thing was designed for after all) is a great way to both
> increase voltage and limit current. To lower current, you just lower the
> capacitance in the level shifter. I have used three microwave caps in
> series on a small coil to provide low, medium and high with no variac or
> heavy ballast (capacitors are way lighter than iron cores and copper
> windings). That particular coil uses a hyperbaric style single static gap
> that is made from 3/4" copper pipe fittings, a length of 4" PVC pipe and a
> vacuum cleaner blower (the blower motor is on a dimmer set to about 50%). I
> have also built rotary gaps that work well at these voltages (~5 kV firing
> voltage). Total gap spacing is around 1/16".
>
> In the microwave, there is very little current limiting from the magnetron.
> Most of the impedance that is seen by the plug-in side of the world is
> because of the level shifter capacitor on the high voltage side. The
> magnetron is approximately 50% efficient - meaning that a 750 watt oven
> (typical) will have a 1500 watt transformer/level shifter. To get 1500
> watts at 2000 volts (again typical values) you need about 2667 ohms of
> impedance. At 60 Hz, a 1 uF capacitor is 2653 ohms. With transformer
> saturation and component value tolerances thrown in, I think it is a wash
> and is safe to say that microwave oven transformers are designed to be
> capacitively ballasted on the high side!
>
> To get half the wattage, you can use two caps in series for half the total
> capacitance. You could also use those two caps in parallel to double the
> wattage. Simple jumper arrangements on the cap terminals allow you to
> change the power settings easily, just be sure to do it when the coil is
> off and the caps have had a minute to discharge. All modern microwave
> capacitors seem to have an internal bleeder, but I like to add an extra one
> anyways.
>
> Biggest piece of advice by far that I have learned about level shifters
> driving Tesla coils is this - *use lots and lots of voltage headroom on
> your level shifter diode*! Filtering helps too, but for example I use 30
> 1N4007 diodes in series for my single stage level shifter (i.e. 30 kV at 1
> amp rating for a 5600 volt nominal level shifter peak voltage). That
> finally seemed to be enough to handle the RF nastiness - faster diodes may
> require less safety factor but I haven't tried.
>
> Also, it seems to me that the tiniest MOTs are still capable of delivering
> LOTS of current, so use the smallest one in your collection. With one or
> two microwave caps strapped to the top or side of the little beasty you are
> very hard pressed to find a more compact, lighter 1kW supply (at least one
> based on an iron core). The duty cycle is limited, of course - but you
> would also be hard pressed to run your coil for long enough to bother the
> MOT and even then you can add oil cooling with not much extra mass or
> volume.
>
> Good luck!
>
> - Jason
>
>
> On Thu, Aug 15, 2013 at 7:48 PM, Dan Kline <misterpaslow@xxxxxxxxx> wrote:
>
> > Thanks, Ted.
> > Very interesting information! I have considered building a DC coil in the
> > past so that I could tune in a perfect break rate and get max sparkage
> that
> > way.
> > I may go this route since I can build upward but not outward :)  But
> about
> > the space issues, I think I may just move to larger apartment sometime in
> > the near future.
> > Dan
> >
> >
> >
> > >________________________________
> > > From: Tesla <tesla@xxxxxxxxxxxxxxx>
> > >To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
> > >Sent: Thursday, August 15, 2013 3:46 PM
> > >Subject: Re: [TCML] Single MOT Coil?
> > >
> > >
> > >Team
> > >
> > >I'd qualify some of the points Mark makes.
> > >
> > >MOT's are magnetically shunted and do have some degree of current
> > limiting. The current limiting characteristics and low voltage are very
> > impacting for classic  AC designs but as the DC  dual level shifter with
> > resonant charging designs illustrate it is possible to get 20 kV from a
> > single MOT which is entirely adequate. Current input can be controlled by
> > the size of the primary capacitor and break rate avoiding the current
> > limiting issues. See theory at
> > http://www.richieburnett.co.uk/dcreschg.html (Richie has a wonderful
> site
> > with the best material on DC designs I’ve found)
> > >
> > >I advocate the use of some power factor correction in these designs to
> > lower the input current drawn.
> > >
> > >http://scopeboy.com/tesla/tc2schem.html
> > >http://www.capturedlightning.org/hot-streamer/drilling/dccoil.html
> > >
> > >
> > >Rgds
> > >Ted in NZ
> > >
> > >
> >
> >
> >--------------------------------------------------------------------------------
> > >
> > >-----Original Message-----
> > >From: Mark X2
> > >Sent: Thursday, August 15, 2013 10:19 PM
> > >To: Tesla Coil Mailing List
> > >Subject: Re: [TCML] Single MOT Coil?
> > >
> > >Gary's right, go for a small NST. A MOT isn't current limited and the
> > >voltage is just too low to get a decent working spark gap.
> > >
> > ><SNIP>
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