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Re: Questions about current
Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>
Hi Matt, All!
>I have a couple of questions I was wondering if someone could
>answer for me.
>(I am in the process of launching off my first coil and am hoping to
>as much as possible as I go).
>I am fully aware of the basic nature of these questions an appreciate
>1. Is there any current 'lost' to corona?
There can be. In a Van de Graaf (VDG) generator, if it is operated in
air, the topload charges to the point that the air insulation breaks
down and then corona discharge takes place from the topload, removing
further charge as fast as it is added. Thus equilibrium between
charging and discharging is achieved and maintained and the charge on
the topload (and hence the voltage) cannot increase any further.
In a Tesla coil, the charging rate is so very high* that when the air
begins to ionise, corona discharge is totally inadequate to remove the
charge as fast as it is being added, that is why spark breakout has to
occur in order to achieve an equilibrium.
Corona discharge is quite possibly occurring simultaneously with spark
breakout, but the rate of charge removal by this mechanism is pretty
insignificant in comparison with spark breakout. The only time corona
discharge is likely to be a significant loss mechanism in the
secondary circuit is with a very large radius topload which is so
large that spark breakout is suppressed.
Corona can also occur in the primary circuit and from the sides of the
secondary coil. Sharp edges and points will promote this, and in my
micro-magnifier, if I join the coupling coil to the secondary with
very thin wire (the tiny radius corresponds to a "sharp edge") I see a
lot of corona discharge all along the thin wire. Despite the very
visible corona extending at least half an inch from the thin wire,
there are still plenty of sparks coming from the topload under these
conditions, which, given the low power I'm running (15W) suggests that
even highly-visible corona is not a serious power drain.
I think the short answer to your question about corona is yes, it can
consume some current, but it may not necessarily be a serious source
* In comparison with a VDG, that is. A VDG charging rate might be
hundreds of microamps or perhaps a few milliamps at most, whereas with
a Tesla coil the charging current can amount to several tens of amps,
i.e. the Tesla coil charging rate can easily be a thousand or ten
thousand times higher than a VDG with the same size topload.