Newbie with a Car Coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "James Prior by way of Terry Fritz <twftesla-at-qwest-dot-net>" <james.prior-at-metrognome.demon.co.uk>

Hi Tesla group,

I've enjoyed reading and digesting the information on various web 
sites, and the Tesla list over the last couple of weeks since I've 
renewed my interest in Tesla Coils (actually my interest was 
aroused again after a mention on the news about recent anti-gravity 
work, which led to me re-investigating all this amazing stuff on the 
internet).

When I was about 12 years old I built a TC, based upon plans in a 
very old book called "The Science of Kinetic Art", which explained 
how to build the coils and a Leyden Jar (a bottle cap!). Luckily for 
naive me, in hindsight, it never worked very well, otherwise I'm sure 
I would have killed myself!! ;-)

Anyways, I'm 40 now and I've started down the road again, and 
have just built a basic car ignition coil power supply and jacobs 
ladder based upon the  schematics on a couple of web sites in the 
TC webring, with a view to bolting on the Tesla 
primary/secondary/SG and caps, etc.

The type of circuit I'm talking about can be found at
http://hot-streamer-dot-com/greg/cararc.htm
Simple, quick-n-dirty. Mains into a light dimmer, then cap, then 12v 
car coil (although I've built it with a bit more care than just throwing 
together a couple of bits on a workbench that might fall over - not 
that there's anything wrong with the basic approach ;-).

I wonder if you'd mind me asking a couple of questions, and I hope 
this isn't too basic or not quite "on topic" since I'm only at the 
power stage!  So please bear in mind I'm an experimental  
beginner, and my initial outlay will be basic/cheap (but depends on 
peoples comments).

Being based in the UK, I'm on 240V AC, using a 700W dimmer 
switch, a 12v car ignition coil and a 10uF 450v cap. (I've also got a 
3uF 450v cap which I started with and can parallel with the 10uF for 
more umph). It all works quite nicely and produces a good intense 
spark up to about 3 to 4cm. On the Jacobs ladder it starts at 1cm 
going up to about 6cm where it breaks and with still air it goes right 
to the top.

Now, the bit I'm interested in your thoughts on is that the mains 
plug is protected with a 3A fuse, and when I run the Jacobs ladder 
with the 10uF cap it will operate for about 10-20 seconds then 
blows the fuse. Surely this setup isn't drawing 3A x 240v = 720W ?
Or is that quite feasible? I'm not sure what a 12v coil driven at 240V 
through the cap will draw in terms of current, but its got to be in 
excess of 3A to blow the fuse repeatedly. Thats almost like a 1KW 
electric fire isn't it!  I'm just concerned about the possibility of the 
ignition coil blowing up - is that likely? Will my dimmer blow-up 
first?

Also I have linked the case of the car coil, and one or two other 
metal parts (the caps are mounted on) to the earth of the mains 
plug to give some basic safety protection of exposed metal parts.  
At one point when I set the spark gap really wide, the HT of the coil 
was finding these earthed parts instead of the -ve side of the spark 
gap, and these "leak" sparks tracking across the chipboard base 
were about 6 or 7cm long!! (not bad from a car coil ?)
Well, I've insulated these to stop this now. 
Is this behaviour to be expected? i.e. a longer spark to ground, 
than to the 240v negative.

The coil gets slightly warm after about 5-10 minutes, and on the 
web page it says it hardly gets warm even running all day (at 110v), 
but I am running at double the voltage in UK. 

It seems there are differing opinions that this 240v into a car coil 
can run a TC, but that an NST is more effective. Why? Aren't both 
an ignition coil and an NST using 50/60Hz as their frequency?  
Would it be better/safer to build a 12v driver using a 555 timer IC 
and power transistor, thereby isolating the mains supply, and also 
offering fine control over the characteristics of the frequency and 
ratio.  

Where does the HIGH frequency so important to TC's come from? 
Is it (depending on PSU) the 50Hz mains, the 555 or the actual 
spark?

I guess my main concern is understanding whats safe and whats 
not!  Am I being overly concerned about the mains side when I 
should pay more respect for the HT side?
Is my 240v driven car coil going to give too much current when an 
NST will be more refined?  Is it bad news to mix the 240v mains too 
closely with a TC?

Finally - Is that too many questions in one mail? 8-}

Would appreciate your advice.

Jim (being 12 again).
Smell that ozone.
-----------------------------------------------------------
James Prior
Home e-mail: james.prior-at-metrognome.demon.co.uk
Home: 01476 552040
Work: 01476 552041
Mobile: 07050 129516
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