Re. LC filter adequate? Please, no replies about the NST connections.

>Original Poster: Doug Brunner <dabrunner-at-earthlink-dot-net> 
>OK, everybody, the NSTs have no problems. I've already powered up the
>supply, and it works. What I really need is feedback on the LC
>protection circuit.
>I'm building a .88 kVA coil with a set of neons as a power source (one
>7.5/60, one 7.5/35, one 7.5/18, with the two small ones in parallel, and
>the big one in series with them). Fortunately, the casings have not
>shorted. I would have liked to get a 15/60, but I couldn't find anything
>under $100.  I've currently got a LC protection circuit installed, made
>up of a 8.8 nF cap in parallel with the HV
>input, and a 4 mF inductor in series, after the cap. The coil frequency
>is 317 kHz. Will this circuit be enough, or do I need to install a
>resistor somewhere?

1) Either you or your NST's are doomed if series connected.  You should
   not ignore this.
2) You must have the same circuit in each HV leg, it sounds like you have
   only one inductor.  I assume you meant 4mH inductor, not mF.  If you
   have only one inductor, the leg without it is unprotected.
3) Filters with inductors are bad in that they have their own resonant
   frequency, ringing there instead of at the tank's frequency.  Neither
   frequency is good for the NST's.  If you insist upon using L's, you
   must have 1-5KOhms in series with each to dampen the ringing.
4) To be effective, an L-C filter must be referenced to RF ground.
   There are normally two identical legs to a filter, each with a cap from
   the NST HV terminal to ground, and R or (R and L) in series.  If you do
   not have the circuit referenced to ground, RF will be applied to at
   least one leg of the power supply.
5) 8.8 nF for a filter cap is too high.  While it will make for a good
   filter, you'll be wasting too much energy.  Both the tank cap and the
   filter cap are charged to the power supply voltage, and both are
   discharged when the spark gap fires.  The tank cap's energy goes into
   the secondary and pretty sparks, but the filter cap's energy goes into
   the L and/or R and is dissipated as heat.  If the filter cap is a
   significant fraction of the tank cap's value, the same fraction of the
   power supply's energy is burnt as heat.  The filter cap should not be
   more that about 10% of the tank cap's value (a totally arbitrary number,
   just a figure representing an "acceptable" loss).

Gary Lau
Waltham, MA USA