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
>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
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).
Waltham, MA USA