Re: [TCML] Single MOT Coil?

[Jason Johnson <jasonmsusolar@xxxxxxxxx>]

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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> >
> >
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