Re: [TCML] Single MOT Coil?

[Dan Kline <misterpaslow@xxxxxxxxx>]

Hi Jason,
This is excellent information. Do you have any kind of a schematic you could send a link to?
Thanks,
Dan



>________________________________
> From: Jason Johnson <jasonmsusolar@xxxxxxxxx>
>To: Tesla Coil Mailing List <tesla@xxxxxxxxxx> 
>Sent: Friday, August 16, 2013 12:23 PM
>Subject: Re: [TCML] Single MOT Coil?
> 
>
>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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