Re: Big x-ray transformers and Jacobs Ladders.

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FIFTYGUY@xxxxxxx 

In a message dated 4/22/07 11:59:05 P.M. Eastern Daylight Time, 
tesla@xxxxxxxxxx writes:
>It's my understanding that the arc length of a JL
>is primarily dependent upon the current
>available, not the voltage.

    It's my experience that the maximum arc length of a JL is 
primarily dependent on the *Volt-Amps* available. At least under the 
typical conditions of continuous sine wave low frequency AC power sources.

> Once the arc is established, the voltage drop across it is only a
>few hundred volts, if that much. Supplying it
>with more voltage doesn't do anything to increase
>the arc length.

    Well, the rising of the arc tends to lengthen it, and it will 
take more voltage to sustain the longer arc.

>Other list members have reported
>that X-Ray transformers have made disappointing
>JLs, because they can supply relatively little current.

        Who reported that? I haven't found anything like that in the archives.

        I've got both a 10kVA 14.4kV pole pig (from Resonance 
Research) and a ~125kVP 300mA XRT from Jack King.

        First a word on my ballast: I made a core out of pieces I 
cut out of an old 25kVA SCR Drive Isolation Transformer. Much cutting 
and cursing later, I had an open "E" core with a pretty big area (I 
can measure if you really care). I wound the middle leg with four 
parallel 12 ga THHN wire (IIRC it was true bifiliar, with 
interspersed reverse-parallel pairs). Everything is nice and cool 
after 5 minutes of running, and with no closing "I" the open core 
limits short-circuit current to 60 amps.

        Now for my pole pig's JL performance, which I believe I 
mentioned here before:

       Arc starts at typical ~1/2" electrode spacing, and will climb 
until the point at which the electrodes are ~18" apart. Doesn't seem 
to matter much whether the electrodes are a steep 90 degree "horn" 
gap or a gentle 6' tall JL with the bottom 2/3 in a 6" diameter Pyrex tube.
        Now the interesting thing: My pig has a switch on the side 
to convert it to 7.2kV operation - half the voltage at twice the 
current. A little tougher to start the arc, and less reliable to keep 
it going, but mostly it looks the same! Same maximum arc span, too.

        My XRT performance:

        I had to build another JL base for the XRT (yes, the pole 
pig runs this JL the same way it ran the first one...). After all, 
the XRT will *start* an arc with the electrodes maybe 12" apart! And 
sometimes the arc just wants to hang out at the bottom, writhing 
along the base, instead of climbing! The solution to that was to 
almost "bridge" the electrodes about halfway up with extra "starter" 
points - two 10" pieces of copper tubing hose, one clamped to each 
electrode. The "starter" pieces each point down and into the JL's 
"V", and have their points ~8" apart. Thus the arc starts at them, 2' 
away from the support base for the JL . Even then, it sometimes 
doesn't want to climb. Max arc span is perhaps 24".

        Now, I think if I could enclose the XRT's JL then it might 
do a bit better, maybe 36". But since the current *in* to the XRT is 
limited to 60 Amps, it can't sustain much current once the arc is 
started, and thus doesn't do as well as you might expect. After all, 
the rated current in is more than twice that. I think it could be 
pushed harder for a few minutes. Nominal output is 89kV AC, at 300mA. 
Obviously it's starting the arc at that high voltage, but it can't 
deliver that voltage at that current to sustain the arc enough to 
make a significant improvement over the Pole Pig's efforts.

        So over-running my Pole Pig by 50% is a little less 
impressive than under-running my X-Ray Transformer at about 40%. I 
suspect running each at their rated output would reveal a big 
difference ion performance.

        I've got a little XRT I've never run that would make a 
useful comparison, were it not for the fact that it runs at 8kHz. 
That also means I'd have to build an 8kHz 110V 100 Amp  power supply 
for it, which would be non-trivial (maybe a car alternator?).

        While I'm writing about it, I've noticed that sometimes the 
JL arc from my XRT "snaps" loudly. I've chalked this up to cable 
capacitance (I'm using two 15 foot lengths of X-Ray cable to plug 
into the wells, stripped ends to the JL). Sure sounds like a safety 
gap firing when it does this! Yes, the rectifiers and filament 
x-formers are removed, and the milliameter connection is shunted.

-Phil LaBudde




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