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Size DOES matter?



Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Hi all,

I was pondering the relative lack of spark length from my DRSSTC. There seemed to be no shortage of energy going into the thing. No parts were heating excessively, so I assumed most of it was going into the discharge. But the sparks were not so impressive. I was getting maybe 33" at any breakrate above 100hz, and John Freau's formula predicts 40" for 5.5 joule bangs at 100Hz.

What's more, they didn't grow that much with breakrate. I well remember Richie Burnett writing about a classic coil that made maybe 14" sparks in single shot mode but grew dramatically to 66" at 200Hz breakrate. (Richie was not born yesterday so I assume he checked that the tank capacitor charging voltage was the same in both cases.)

Mine did 22" with a single bang and still only about 33" at 200Hz with 1kW power input. (by way of comparison my classic coil did 40" with 1kW input at 400Hz, and 17" with single bangs of very roughly the same energy.)

So I got to thinking, maybe size does matter? Maybe the ultimate limit to spark growth is not how much power/energy your oscillator can deliver, but the size of the resonator? It's often been said that three times your secondary length is the maximum spark length you should attempt. Well maybe there is a law of physics that says you just can't get more than that no matter how much power you supply!

So how could that be? Well for a start it's not too hard to see how toroid size could influence spark length. Streamers need a certain field strength to grow. And, the maximum field strength at the surface of any toroid is limited to the breakdown strength of air. So it seems reasonable that a smaller toroid might not "throw" the field (and hence possibly the streamers) out to such a long distance: simply because it has less surface area to accommodate field lines.

The resonator length may also have an effect. If we admit the argument above, then the toroid should be brought almost to its breakdown voltage for most efficient streamer throwing. If you try to drive a large toroid with a small resonator then the resonator will flash over end-to-end before the toroid reaches breakout. Hence (even with a breakout point) the spark length will be limited by flashover rather than power availability.

To prevent flashover of an undersized resonator you need to reduce the output voltage by fitting a breakout point and/or turning the peak power down (this is achieved by loosening the coupling on a classic coil, or turning down the DC link voltage- and increasing burst length to restore bang energy- on a DRSSTC) The reduced output voltage will probably lead to smaller sparks.


Does anyone have any comments as to whether any of this makes sense ;)

Steve Conner