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Re: [TCML] QCW sparks



Hey---

The low-frequency response of a capacitive voltage divider can be extended to lower frequencies by paralleling it with a resistive one. For instance, you can use the resistor from a 40kV TV probe, which is usually 1000 megohms, across the HV capacitor. Then put a 1/4 watt resistor of that resistance divided by the capacitive ratio (e. g. 1200 ohms = 1E9/800) across the low voltage cap. This will extend the range to DC, and if the ratios are well matched it will have a smooth frequency response.

It is also necessary to use a high impedance scope input to avoid changing the ratio.

Take good care of your Jennings--it's a jewel.

---Carl





On 5/28/11 7:36 PM, Steve Ward 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.


To a first order, then, the maximum voltage on the secondary is in the 70kV range.. which fits with the rest of your data.

Is that actually a "resonant frequency" or the frequency at which you're driving the system (more Fop than Fres) without concern for where the 1/sqrt(LC) happens to be?



http://www.flickr.com/photos/kickermagnet/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:



Is that the actual base current (measured how?)


I don't know that the spark impedance is going down, but certainly, the capacitance of the spark is increasing (and inductance, to a lesser extent), so it's reasonable to think that you can push more current into it.

I think you might think of your source as more a stiff AC source than a traditional LC primary tank that's coupled.





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.

Yes.. you could probably do some sort of energy balance calculation to see how much power you're radiating and conducting away from the hot spark.



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 li

I'd guess more that the spark is almost doubling the capacitance of the secondary system. Capacitance (for "thin" wires) is more dependent on the length than the diameter. You've got a long spark..


One test you could do is to just hook a wire onto your system in the same place as the spark, and run it at low voltage (below breakout) and see what the waveforms look like. Something like a long coathanger, supported with monofilament fishing line.

Or, if you want to get fancy, some thing like a string of 1-10k 1/8W resistors..


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