Re: [TCML] QCW sparks

[Aron Koscho <kc5uto@xxxxxxxxx>]

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
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla