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Re: 70kv system



Original poster: "Peter Lawrence by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>

Johnathan,
          if the top of your coil is a 2" brass door knob (that's what I
first used) then you might have a problem getting sparks to break out
with the 12v ignition coil power supply, take the ball off and just let
the sparks fly off the end of the screw or whatever you have on the end
of your coil, you should get 1" sparks.

Using a 10kv-28ma power supply is the high end of what I use on these "small"
TCs. It will put out ~12" sparks, not something you want children within 10'
of, they will reach out at anything close and an arm can stray pretty far when
you are not watching. (Dont forget to tell them to put their fingers in
their ears, if you have not hear a TC first hand you will be supprised!)

At these power levels you've got to be careful about coupling (having too
small diameter primary winding) or you will get inter-winding sparks along
your secondary. And your capacitor has to be GOOD (built from Terry Fritz'
caps) or it will burn quickly (been there, done that, many times...)

10Kohms verses 100Kohms, this does not make sense. The primary should be
less than 100ohms, the secondary should be maybe 50Kohms.  If you have two
sides of a secondary and one side is 10K to the case/ground and the other side
is 100K to the case/ground something is very wrong...

Connecting a TC to an OBIT with two output terminals means that the TC
secondary bottom must be connected to ground; and the TC primary coil and
cap are connected to the OBIT, neither side of which is grounded anymore.
I hope that makes sense...

12 caps, .001 mfd, 7.5kv. These have to be Mylar, they will last a little
while with the 12v ignition coil supply; and will burn up immediately with
the OBIT, regardless of how you series/parallel them.

2 strings of 6 caps in series = .001 * 2 / 6 = .00033 mfd,  6*7.5kv = 45kv
3 strings of 4 caps in series = .001 * 3 / 4 = .00075 mfd,  4*7.5kv = 30kv
4 strings of 3 caps in series = .001 * 4 / 3 = .00133 mfd,  3*7.5kv = 22.5kv
6 strings of 2 caps in series = .001 * 6 / 2 = .003   mfd,  2*7.5kv = 15kv

which combination you should chose depends on the resonant frequency of
your secondary and the inductance of your primary. Given the large wire
size and few turns of your secondary (4~500 turns) I'ld guess that this
coil will require the .003 mfd combo, and at 15kv that does not leave
much margin for safety. Mylar has the bad habit of conducting surface sparks
at high voltages and frequences, resulting in sparks wrapping around the
edges of the mylar dialectric and shorting out the capacitor, the resulting
spark burns the mylar, which makes it even more prone to arcing etc etc etc,
very quick capacitor death...

Terry sells .056mfd/1600v polypropelene caps for $1.80 each, a bargain. One
string of 12 caps is .0047mfd (probably close enough for your coil) at 19.2kv
which is plenty (polypropelene has excelent high voltage high frequence
characteristics) your total: $21.60.


-Peter.



>Date: Wed, 04 Apr 2001 11:30:44 -0700
>From: Jonathan Peakall <jpeakall-at-mcn-dot-org>
>MIME-Version: 1.0
>To: Peter Lawrence <Peter.Lawrence-at-Sun-dot-com>
>Subject: Re: 70kv system
>Content-Transfer-Encoding: 7bit
>
>Peter,
>
>So is it basically impossible get an ignition coil TC to break out on
>it's own, or will it always need a metal object to pull streamers from?
>I am interested because I teach science to elementary school kids, and
>want to have a coil they can experiment with safely. So a ignition coil
>TC that breaks out would be cool.
>
>What's the stupidity factor on this:
>
>I have a weak oil furnace transformer, rated 10,000 volts at 28 ma. One
>side of the OFT reads 11.6 k ohms, and the other 108.7 k ohms. Anyway to
>figure out what it's output would be? Can I hook it up to the coils I
>previously described, adding a safety gap and changing the primary cap?
>As the OTF is mid-point grounded, I assume that I attach to the case for
>ground connection? All I have handy for primary caps are the 7.5 kv .001
>uf caps I described earlier. I have twelve of them, which, if I
>understand correctly, I could make into a series parallel combo
>resulting in .003 uf 15k. Is this true? How big a risk would it be to
>run them in a combo resulting in .004 uf 7.5kv?
>
>You are the only one so far to really read my post and answer. The only
>way I can say thanks is to promise to help others out, when I know
>enough to give safe advice.
>
>Thanks again,
>
>Jonathan
>
>Peter Lawrence wrote:
>> 
>> Johnathan,
>>           I too built one of those (almost 20 years ago). The spark-coil
>> powered TCs are always going to be disappointing. I got maybe 1" sparks
>> from the top of my TC that way, but the spark-coil that powered it could
>> do 1" sparks so what was the point of the TC in that case?
>> 
>> I soon obtained a (very small) 6kv-20ma Neon Sign Transformer, and got
>> 4~5" sparks right away. Cool.
>> 
>> It is impossible that small changes like #22 verses #24 wire size are going
>> to make any difference.  (I've been experimenting with difference wire
>> sizes and number of turns, everything from #24 all the way down to #36, and
>> its hard to see any difference with less than 4 wire guage sizes appart).
>> Same thing with winding diameter. Same thing with primary wire guage and
>> diameter. I've used everything from 3/16" tubing to #14 (0.064") solid wire,
>> its all the same on these small sized TCs. You get close to some reference
>> design, and then adjust your primary tap point to tune the coil, it always
>> works.
>> 
>> Here are some of my observations on small systems:
>> 
>> get a NST power supply (MOTs have too much current for a static spark gap),
>> 
>> build an MMC cap (contact Terry Fritz for the caps, I use 2 or 3 strings of
>> 12 caps each for my 6kv-20ma, 7.5kv-30ma, and 9kv-30ma NSTs; 12 long strings
>> is overkill for these voltages but hey I've never blown a cap this way!),
>> 
>> the best static gap I've used has tungsten-carbide balls for the electrodes
>> (brass and stainless steel corrode and the corrosion tends to diminish TC
>> performance, requiring frequent cleaning),
>> 
>> get a torroid for the top (a ball top will not suppress the corona on the
>> last few windings of the secondary),
>> 
>> if you are going to use more than 7.5kv input then put a 2" ball about 4"
>> above the center of the torroid as a break out, this way the sparks go UP,
>> rather than OUT; the difference is the outward sparks will tend to hit
>> the primary or the NST or anything else nearby, maybe even you.
>> 
>> built a safety gap, or you will blow your (possibly expensive) NST. This
>> warning comes from experience!!!
>> 
>> put 1-kohm-25-watt resistors between the safety gap and the primary 
capacitor,
>> that way the cap won't put out an explosive discharge across the safety gap
>> when it fires. (dont worry about power loss in the resistor, the NST has an
>> internal dc resistance of 20-50 kohms, so you are only adding 2-5%).
>> 
>> For years I thought I was doing something wrong not getting the 8~10" sparks
>> that the plans said were possible. Not so, the only way to get that long
>> of a spark on a 12" high coil is with 7.5kv input, and 9kv will get 12~14".
>> 
>> -Peter Lawrence.