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Re: Why does top capacitance work?
Tesla List wrote:
>
> Subscriber: sfalco-at-worldnet.att-dot-net Mon Feb 10 21:49:24 1997
> Date: Sat, 08 Feb 1997 10:47:25 -0500
> From: Steve Falco <sfalco-at-worldnet.att-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Why does top capacitance work?
>
> I must be suffering from a misconception about top capacitance - i.e.
> the toroid. People talk about putting a larger toroid on top as a way
> to get a larger spark. The claim is that the larger toroid has to
> charge up to a higher voltage before breaking out.
>
> The problem I have with this, is that the coil will be resonating with
> an A.C. voltage, at 50 kHz or 100 kHz or whatever. So, won't the top
> capacitance charge and discharge at the same rate? I would think the
> average voltage would be zero. (I know I'm wrong here - I've seen the
> sparks!)
>
> So what is going on? Is there some sort of rectification effect here?
> Can anyone tell me what they think a plot of the voltage versus time
> would look like assuming we could hook a strip-chart recorder to our
> tesla coil and record the instantaneous voltage? Or do we get an
> oscillation that builds over time where eventually the swing exceeds
> the breakdown voltage? This seems like the most likely description to
> me.
>
> Thanks for any insights,
> Steve Falco
> sfalco-at-worldnet.att-dot-net
Steve,
It turns out that adding capacitance to the top actually reduces the
maximum output voltage that the coil can generate! However, adding a
toroid-shaped top-load can prevent the secondary from starting to
break-out with corona/streamers until a higher voltage is reached. This
allows more energy to be transferred from the primary to the secondary
before we start to generate streamers. There are other, more complex,
factors at work as well, but this is the major reason. And yes, the
output does oscillate at the operating frequency of the coil, since the
combination of the secondary inductance, and the capacitance of the coil
plus toroid form a series LC circuit which is "excited" by the primary
coil.
Each time the gap fires, this circuit "rings" and builds up voltage
until (hopefully) breakout occurs. The coil output waveform actually
looks like a rising amplitude sinewave that reaches a maximum, then
declines at a rate governed by whether we're generating streamers and if
the primary gap has ceased firing. There are some jpegs on the funet
site (QNCH and UNQNCH.JPG) that show simulations of how these look (with
no breakout). The "average" voltage is, indeed zero.
Safe coilin to ya!
-- Bert H --