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Re: Unexplained arcing current
Hi Terry:
I ran a simulation on my system, observing the current through the series
resistors. I did observe the large current spikes each time the gap
closed, close to 2 Amps. This was due to the charge in the bypass caps
being dumped into the series resistors. The magnitude of the spikes were
the same regardless of the bypass cap value, and this makes sense. The
initial magnitude of the spike is just a function of the bypass cap
voltage divided by the resistor value. The duration of the spikes were
related to the cap value, of course.
But this additional power burned in the resistors due to the bypass caps
discharging is not that much, since the pulse widths are so brief. The
RMS current through the resistors was just over 100 mA, pretty much
regardless of reasonable bypass cap values.
On your model, I get an RMS current through RF2 of between 75-90mA with
your 2700pF bypass cap. Changing the cap to 0.27pF reduced the current
only slightly.
I don't think this explains my heating observation.
Regards, Gary Lau
Waltham, MA USA
>Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
>Hi Gary,
>
> While working on fault condition analysis for my new coil, I (MicroSim)
>figured this darn thing out (finally!!).
>
>When the gap is firing, these resistors see BIG spikes. They faithfully
>absorb this energy and save the neon just like they are supposed to.
>However, when you look at the RMS current, these spike contribute a fair
>amount of power. Even though the spikes are very brief, they contribute
>power in proportion to the square of their level. If we run 60mA and see a
>bunch of 3 amp spikes, the instantaneous power in those spikes is 3^2 /
>0.06^2 = 2500 times greater! In MicroSim, look at the current using the
>RMS() function over some time. You will see that the RMS current can be
>much higher than the 60mA we all expect. This is one case where computer
>modeling did solve a problem that could not be found otherwise. This link
>(below) shows the spikes from my old coil configuration. It is from my
>last paper. This may also be related to having filter caps too big since
>they discharge into these resistors when the gap shorts. However, your
>caps are not large at all. There is also considerable noise that these
>resistors eat due to the fact that the gap is spiking at each zero current
>crossing. Without a secondary, this effect is really amplified. Some
>resistors are also much better at dealing with these high frequency spikes
>than others. I like the Ohmites. Some wire wounds cannot take spikes like
>this and will burn up their little wires. Thank goodness it is not burning
>out the neon secondary's little wires!
>
>http://www.peakpeak-dot-com/~terryf/tesla/experiments/modact/Image167.gif
>
> Terry
>
>
>
>At 12:03 AM 5/22/99 -0400, you wrote:
>>I just saw something I can't explain. For as long as I've been coiling,
>>the resistors in my NST protection circuit have been getting hotter than
>>expected. The power defined by I**2 R, I being the NST current, never
>>came close to explaining how hot they got, so I assumed that the
>>additional wattage was due to a reverse transformer action. After the
>>gap quenches, the energy in the secondary could couple back to the
>>primary, although it won't be tuned any longer now that the gap is open...
>>Not a very good theory, I know.
>>
>>Tonight I shorted my gap, essentially shorting the NST, but having the
>>current pass through my RC network. My R's are each 1.6K, 113W, non-
>>inductive. C's are 450pF. Metering the current, I got 73mA (mine is a
>>15KV/60mA NST), and the resistors barely got warm.
>>
>>Next I removed the short across the gap but broke the connection between
>>the tank L and C, so that the gap arcs, but no cap charging/discharging.
>>Now the resistors get hot as a pistol in no time. I tried measuring the
>>current to the gap, but my digital meter just wigged out, I'm probably
>>lucky it still works.
>>
>>So, what's different that causes the R's to see so much more power going
>>to an arcing gap? It's just down to 60 Hz stuff now.
>>
>>Gary Lau
>>Waltham, MA USA