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RE: site update + filter question



Hi Jan:

A couple of comments on your filter question.

First, since you have the luxury of using an NST where neither side nor
center tap is grounded, you can ground one side, and then you need a filter
on just the other side.  This will immediately cut your losses in half.

Second, your total resistance of 60K Ohms is way too high and this causes it
to eat so much of your available power.  I'd recommend not much more than
1-2K total.  As far as the low pass filters cutoff frequency, one's first
thought is that it should be well below the tank frequency of the coil.
Your coil's frequency is probably at least 500KHz (WAG), so 10KHz is
needlessly low, but there is more to consider.  When the tank is oscillating
at this frequency that we think we want to filter out, remember that the
spark gap is conducting, essentially shorting out this frequency as far as
the filter can see.  When the gap quenches and this "short" is removed,
there's virtually no more energy left in the primary circuit to worry about.

Then why do we even need the filters?  Because as the primary rings and the
gap is conducting, the gap conduction ceases very briefly each time the gap
current passes through zero, and this sets up very brief, very high voltage
oscillations in the primary coil in the tens of megaHertz region.  Terry
Fritz discovered, measured, and documented this in one of the papers on his
web site.  The good news is, megahertz-range transients are very esy to
filter out with modest R-C networks.

Regards, Gary Lau
Waltham, MA USA


		-----Original Message-----
		From:	Tesla List [mailto:tesla-at-pupman-dot-com]
		Sent:	Sunday, April 23, 2000 5:04 PM
		To:	tesla-at-pupman-dot-com
		Subject:	site update + filter question

		Original Poster: "Jan Florian Wagner" <jwagner-at-cc.hut.fi> 

		Hi all,

		in my series "small, crappy, but working", I've got some
pics of my tiny
		twin coil at www.hut.fi/~jwagner/tesla that might be
interesting,
		especially those TV flyback-like arcs. :o)

		Other thing, I'd like to ask about RC filter design. I've a
3kV
		50mA NST for that tiny twin TC. The tank cap is 8.5nF.

		The current filter looks like this

		NST side         tank side
		---/\/\/\---+---/\/\/\----
		            |
		           === 470pF 6kV max (ceramic)
		            |
		---/\/\/\---+---/\/\/\----
		15kOhm power resistors

		It's symmetric because the NST has no center tap and is not
grounded.

		Those 60 kOhm total at 3kVAC (=>50mA) shouldn't be a
problem, as it is
		50mA max from the NST anyway? Or?

		The filter should now be a low-pass below 10kHz. Is that ok,
or should
		the freq be even lower?

		The problem now is that once the filter is connected,
there's a dramatic
		performance drop (about half of streamer length), so there's
probably
		something wrong with that filter design?

		How do you calculate the resistor values? And what wattage
should they be?

		many thanks,
		and happy easter coiling to all :),

			Jan

		--
		*************************************************
		Jan Florian Wagner
		http://www.hut.fi/~jwagner