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# Re: [TCML] TC Max Spark Length & Output Voltage

```Arc voltages come in a wide range. For a spark gap with rounded
electrodes you need about 30kV per cm for a spark to fire.
In an arc lamp, voltages around a 100V for a 1cm gap are sufficient.
An arc lamp has to be ignited, though, e.g. in an antique carbon lamp,
the electrodes first are touching and then pulled apart.

The difference between these 2 situations results from the way,
free electrons, which are the carriers of conductivity, are created.

In the spark gap, free electrons have to be accelerated by the electric field
against the resistance of air molecules to be able to hit air
molecules fast enough to liberate new electrons from them.
That causes an avalanche of electrons and requires high voltages.

In an arc lamp, electrons are liberated by high temperature
air molecules hitting each other. Thus voltage (and current)
is required only to keep the air hot enough.

Tesla coil arcs are special, because they can end in mid air.
Once the arc breaks out, which requires several tens of kV
by the means of electron acceleration, it produces a plasma
channel. The once created plasma channel has a lot lower
voltage drop such as in an arc lamp.

That channel transports most of the top load voltage to the tip of
the arc, so that it can grow from there. A TC arc is a mixture
of both the arc lamp and spark gap situations, i.e. around 100V/cm near
the breakout point and 30kV/cm at the arc tip.

The voltage-length relationship in a TC arc is complicated. It also
depends on the duration of the burst, since arc growth takes some
time. Also involved is the frequency of the coil. A higher frequency
will cause more charge movement along the arc, increasing power
dissipation along the arc, which implies a better conductivity of it.
So less voltage is then required for a given arc length.

Udo

```
----- Original Message ----- From: "William Fox" <wm9fox@xxxxxxxxx>
```To: <tesla@xxxxxxxxxxxxxxxxxx>
Sent: Wednesday, January 16, 2019 3:08 AM
Subject: [TCML] TC Max Spark Length & Output Voltage

Hello Everyone,

There has been much discussion over the decades about estimating maximum TC spark length & corresponding voltage... To my knowledge,
much investigation has been done by our fellow members, John Freau (Length= 1.7 * SQRT(kVA in) & Greg Leyh V = 200 kV/m)... Thanks,
guys!  But with the advent of so many different types of TC’s beyond the SGTC (e.g. Class E, DRSSTC, QRSSTC, SSTC, VTTC, etc.) these
issues are getting much more complicated.  It would be very interesting, if those who are much more knowledgeable in this area than
I, take a shot at tabulating field data & see if, from that data, some equations of approximation might be derived.  Assuming
discharge is issuing from a breakout point on topload or pointed terminal.  For example, QRSSTC’s maintain the arc channel longer,
time-wise, allowing the discharge to grow to 5 feet or so from a very short, squatty coil, but actual voltage as low as 75 kV (this
from offline discussion with Bert Hickman)... Thanks, Bert!  What do you members have to say about this?

Thanks,

Bill

Sent by Mačak's humble servant.

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