Re: [TCML] QCW sparks

[<lightningfor@xxxxxxxxxxxxxxxxxxxxxx>]

Thats amazing steve!
Nice piece of detective work...
You mentioned that the impedance of the spark appears to ramp down, over
the duration of the pulse period...
Do you think this could be part of the reason that CW sparks are shorter
and fatter?

I also aggree with what Aron says.
Makes sense really. Basically the same principle as a DC collector device
like a Van De Graff machine, except the method of charging is different.

Carlos


On Sun, 29 May 2011 17:53:07 -0400, Aron Koscho <kc5uto@xxxxxxxxx> wrote:
> That's interesting Steve, this seems to imply that the output voltage of
a
> coil is basically the voltage at which its top load breaks down,
regardless
> of streamer length. This would then make streamer length a function of
> current and time, variables which you are controlling with the QCW
setup. I
> have a 100kV vacuum capacitor divider laying around, if I can find the
time
> I'll repeat your experiment. Very cool results!
> 
> Aron
> 
> On Sat, May 28, 2011 at 10:36 PM, Steve Ward <steve.ward@xxxxxxxxx>
wrote:
> 
>> Greetings,
>>
>> First, a bit of rambling: I recently obtained a Jennings vacuum
capacitor
>> voltage divider.  While i could not find a manual for it, i did see
>> mention
>> of it on the Jennings site as being rated for 50kV peak and 60hz to
>> 30MHz.
>>  I did my own calibration with a 40khz AC source up to 5kVpk or so, and
>> found the division ratio to be right at 800:1 for this unit, i may look
>> for
>> more ways to verify this later as its important, but for now i at least
>> have
>> relative data to compare.  The Jennings website mentioned ~600:1 for
this
>> unit, so i have some uncertainty.  I'll go with 800:1 since it gives
what
>> should be "worst-case" voltages.
>>
>> So what have i done?  Well ive been working on this "QCW"
>> (quasi-continuous-wave) DRSSTC.  The power supply is unique in that it
>> can
>> ramp the bus voltage to the DRSSTC from ~30V (enough to reliably start
>> oscillation of the tesla coil) to 300V, basically a high power
arbitrary
>> wave-form generator feeding the H-bridge drive of the tesla coil.  I
>> discovered that keeping the Fres of the system suitably high (>
300khz),
>> and
>> providing a linear ramp in supply voltage over 10-15mS could grow
>> extremely
>> long, straight, sparks compared to the size of the coil (best is 65"
>> sparks
>> from a 9.25" length winding of 30awg).  The top voltage was suspected
to
>> be
>> low because of the lack of flash-over between primary and secondary,
and
>> because of the small 2" minor diameter of the toroid.
>>
>>
http://www.flickr.com/photos/kickermagnet/<http://www.flickr.com/photos/kickermagnet/4386617170/in/photostream>
>> 4386617170/in/photostream
>>
>> So finally i made some measurements!  So far what i see is the top
>> voltage
>> ramps quickly to about 45kV, at which point the breakdown process
begins.
>> The secondary base current is about 1.76A pk at this point.  From here
it
>> takes relatively little extra voltage to grow sparks up to 4 feet long,
i
>> measure just 56kV!  The base current is just over 3A at this point,
which
>> means the secondary current is growing faster than the voltage, so the
>> spark
>> impedance is going down over the 12mS pulse.  Waveform can be seen
here:
>>
>> http://www.flickr.com/photos/kickermagnet/5770183734/in/photostream
>>
>> Further pictures of the setup can be browsed from my home page:
>>
>> http://www.flickr.com/photos/kickermagnet/
>>
>> Im not 100% sure what to make of it yet.  The explanation i like to
give
>> is
>> that its a "self-stretching arc".  If you consider the arc length you
can
>> stretch from a 15kV transformer (many feet if there is enough
current...
>> say
>> 2A), then the fact that this tesla coil functions at relatively low
>> voltage,
>> yet still producing long sparks, makes sense.  The spark just takes a
>> loooooong time to get to that length.
>>
>> Another interesting point is that the top voltage is a lot lower than V
=
>> IZ
>> would suggest for a lumped model of my secondary coil.  Basically, the
>> secondary is ~25mH, and with 3Apk current through 25mH at 325khz, id
>> expect
>> a peak voltage of 153kV.  The only way i can account for this large
>> discrepancy in apparent impedance of the coil is that there must be
>> significant capacitance from the secondary to primary, so the base
>> current
>> looks much larger compared to say, the toroid current.  I'd like to
next
>> work on measuring the current between secondary and toroid and see if
it
>> is
>> consistent with my voltage measurement claims.  The fact that the 50kV
>> probe
>> hasnt shown any signs of stress makes me think that the voltage really
>> isnt
>> 150kV, but really is more like the 56kV i claim.
>>
>> The next step will be to measure the ramp voltage and work on a spice
>> model
>> to determine the streamer impedance.
>>
>> Thoughts?
>>
>> Steve
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