# [TCML] MAX SPARK FOR FIXED VOLTAGE

bartb bartb at classictesla.com
Tue Mar 10 19:24:17 MST 2009

```Hi Dex,

Your question is good, but it is based on the assumption that potential
voltage equals a given spark length. This isn't the case in a Tesla coil
output spark like it is in the potential required for 2 bodies of some
size and geometry to arc to one-another. Over and over again it's been
shown that spark "lengths" in TC service follow a power curve. It
depends on the power delivered to the streamers and time. The actual
voltage at the top load cannot be directly linked to spark length
without the inclusion of the change in current over time. The main
parameters that supply this are the inductances, capacitances, power
supply, and top load limit on voltage.

A better question and answerable question is what is the approximate max
spark length possible for a given power. Then we can easily quote John
Freau's 1.7 x sqrt(input watts) spark length equation which has held
well for more than a decade. There are certainly situations which affect
even this spark length, but it is the most accurate equation for spark
lengths over the spectrum of variations with systems.

I can accurately say that if you used 100kV, 200kV, 300kV and kept power
unchanged, then for a 1000W coil, expect the following ideally.
1) 100kV = 53.7"
2) 200kV = 53.7"
3) 300kV = 53.7"

This is not a trick. It states that the terminal voltage is not
affecting the spark length directly. Maintaining power delivered to the
spark at 1000W, the current is adjusting (10mA, 5mA, 3.3mA). However, if
you were to double the power to 2000W, then (ideally):
1) 100kV = 76"
2) 200kV = 76"
3) 300kV = 76"

Note the spark length increased only by a ratio of 76/53.7 for twice the
power. This is the reality of Tesla Coils. BTW, by "ideally" I'm
referring to the fact that not all coils will achieve the spark lengths
for the power given. Most will be below this length, some at this
length, and a select few a little better (John's system during these
tests were quite efficient).

Another reality not thought about is that you will be hard-pressed to
keep the top terminal below 100kV and above 400kV. The top terminal
limits voltage but can also break out at peak voltages which are far
from average voltages. From a completely academic standpoint, it's fine
to contemplate the top terminal voltage, but for breakout, consider the
peak voltage the top terminal will see (punching out). However, after
the initial breakout, the current delivered to the streamer is what will
dictate to some degree the spark lengths achievable. The power is what
pumps the current into the spark channel (and the more power, the longer
and higher the current is available).

I can probably speak for a lot of coilers when I say that reducing gap
losses (more power delivered in the emf at the primary and thus the
secondary) and more power in the supply itself is what has increased
spark lengths on any given coil. There is of course secondary
inductances better equipped than others at delivering the power to the
top terminal and the terminal C itself determining when breakdown
occurs, but it is "power" that ultimately delivers short or long sparks
(power is the predominant player with spark lengths).

Take care,
Bart

Dex Dexter wrote:
> I have a good question I don't know answer to.
> QUESTION:
> What is aproximately maximum spark lenght possible for a given output voltage Tesla system generates?
>
> Regardless of the efficiency,how long spark can be produced by a Tesla-type impulsive devices with voltage levels:
> 1) 100 kV
> 2) 200 kV
> 3) 300 kV
>
> Can a high power impulse source,like a Tesla transformer,put out a 10 feet long spark with 'only' 100 kV voltage peaks?
> To accomplish that it is free to use any power input level,any size and shape of  a terminal capacity,any firing rate wanted,andthe best high frequency excitation waves mix.DRSSTC,VTTC,etc.
> The only limiting thing is not to exceed 100 000 V terminal potential.
> Supposing a normal atmosphere enviroment is that possible?
> I know that in no circumstance 50 kV can close 5' wide gap unless directly conducting path is formed somehow,or a
> strong external ionization occurs ,like exposure to a flame,extremely powerful lasers,but Tesla transformers are the
> champs of 'spark pumping' and I was wondering what could be done with 100 kV.
>
> Dex
>
>
>
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```