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Re: Spark Gaps & *!*SAFETY*!*
Welcome to the list Tabraze!
Whoa there! The electrode spacing determines purely the *voltage* at which the
spark gap fires. *Nothing else*!. In air at ATP, it should be around 0.028
inches per kilovolt for multiple static gaps using 1/4 inch pipe sections.
Power rating really is irrelevant, except for the fact that if you are running
lots of power then static gaps will tend to get overwhelmed and overheated
very fast.
The frequency (upon which skin depth depends) is *purely* dependent upon the
values of inductance and capacitance in the tesla coil itself (ie the primary
cap and coil and the secondary cap and coil). The frequency of these two
circuits alone is important to the output frequency. Lprimary * Cprimary
should therefore be equal to Lsecondary * Csecondary.
The total gap distance in a static system will only affect "break rate" and
the charging voltage of the primary tank cap. Smaller spacings for the same
power will result in more breaks per second and less cap voltage. For maximum
output you will need to find an optimum, which will be somewhere near the
spacing that the transformer and gap *alone* (test this empirically with a
variac) initiate spark conduction. There is always a compromise between break
rate and power per bang in a static system that will result in the longest
spark.
**** Crucial Safety Note ***
I am a little worried, as I am sure other list members may be also, that you
are considering doing the so-called 'skin effect' tricks at this early stage
before having thoroughly digested all the available information. There is
serious doubt that skin effect is that significant in the human body at the
frequuencies we see in tesla coils over say 2" diameter. There have been a few
deaths associated with these risky pursuits, maybe Terry would be prepared to
repost his known accounts. It may indeed be that the main reason why
attracting the discharge of a small coil doesn't shock is that the
pain-sensing nerves are not sensitive to these frequencies. This does not
necessarily apply to your heart or brain nerve-cells, and it may also be that
the blood vessels form a fairly nice conductor to these areas. Also, be very,
very, aware that either a power arc from your fingers to a nearby earthed
object, or yet worse one to a live mains cable, may transmit far more current
through the body than you can survive. The worst case is perhaps an arc from
secondary to primary while you are doing this, which will connect you diectly
to a source of *high* voltage 50/60Hz line power. You will almost certainly
die in this case. I *don't* think I can imagine myself doing this trick with
even my modest coil running at 2kW input. It's simply too frightening when
I see what the discharge can do to (a) my hearing and (b) nearby inanimate
objects for me to risk. Read the safety FAQ for more details. Don't leave
anything to chance here - as it says,, your first mistake could well be your
last. Seriously.
Main dangers are -
240V 50Hz from the wall (lethal)
??kV 50Hz in your PSU (lethal too)
Stored charge in your primary tank capacitance (possibly lethal, at least very
painful)
Very high power RF in the primary and secondary circuits (even at the
secondary base if your ground is not very good) - nasty burns and internal
heating.
Trip hazards from trailing wires / tools and equipment
Unexpected circuits completed by power arcs.
RFI (your neigbours may suddenly decide to form a lynch mob)
Damage to property, especially starting fires (this could even happen inside a
wall where you can't see it!)
Using or adjusting the coil while drunk or under the influence of (any) drugs.
UV radiation from the Spark Gap
I would say as I have said before, read more, much more, start slowly, have
extreme respect for the power you are using at all times, and double check
everything before you even start building, then quadruple check every day you
test and run the thing. The first light can often be such a frightening and
awe-inspiring experience that it teaches you a lot of respect for your coil,
but if you die first - there's not much we can help you with.
Play safe, the very best of luck, and happy coiling.
Alex Crow
PS I notice you are in the UK - where do you live? I am based in London at the
moment, however my coil is stuck in Birmingham. If you are near either I'd
love to hear from you. There is a growing Tesla community in the UK, and there
may be an Autumn Teslathon in the offing (looks like late October) so keep in
touch. Watch out for Bob Golding, Mike Tucknott, Richie Burnett, Alan Sharp
and myself on the list (which is *the* most valuable resource).
> Original Poster: "Tabraze Malik" <tab-at-malik13.freeserve.co.uk>
>
> Hi it's Tabraze,
>
> Before I make my first coil I want to get an idea of it in my head. Are
> there any tables or formulae that let you calculate how far the 2
> electrodes need to be to make the spark gap for transformers of a given
> power level? In static gaps with multiple electrodes how far does the
> total gap length need to be? does the size of the gap vary between
> transformers of different power ratings?
> I may want to use my coil for special effects when using the skin effect.
> Does making the gap smaller increase the frequency?if so does this make the
> skin effect more reliable?
> Is there any way to find the frequency of a gap if you know the distance
> between electrodes, inductance etc?
> I don't know if this information will effect your response but you may need
> it; I live in the UK, here the line voltage in 230V a.c at 50hz
> If there are any tables or formulae then please could you attach them to
> your reply or tell me where I can find them?
> If there is no such information then I would welcome any suggestions from
> experienced coilers.
> I know this is asking for a lot but I don't want to spend money on parts
> and get it wrong, I'm also very keen on Tesla coils.
>
>
> Thanks
> Tabraze Malik