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Re: Terry filter alternates?

Original poster: "Mark Broker" <mbroker@xxxxxxxxxxxxxxxx>

By my math I get 7.9W, not 29, for the discharging filter caps with the specs below. And since there are two resistors, it's ~4W each. A 30mA NST may have a short circuit current of 80mA, or 6.4W. That's only ~10W.

The 100W resistors in the filter on my 15/90 TC never get much above ambient when things are running well. There is a small fan blowing air across them, which I'm sure helps. When the system is changed and tuning needs to be done (when things aren't "running well"), the resistors can get quite warm in a hurry. So I wouldn't skimp too much on the wattage. I'm using 50W on my 12/60 TC, though I honestly haven't checked their temperature....

Since the original question was on making the filter less expensive, I would suggest making the RC portion of the filter as shown in the design, and omit the MOVs. You can also use 50W resistors, but make sure there's nothing that can get scorched within proximity to the resistors.

Chokes can actually exacerbate the overvoltage on the NST because they ring with the filter capacitors when the spark gap is fired. Terry did an excellent writeup on this on his website.

Cheers

Mark Broker
Chief Engineer, The Geek Group


On Fri, 03 Dec 2004 18:24:32 -0700, Tesla list <tesla@xxxxxxxxxx> wrote:


You are also discharging the filter caps each time the gap shorts.

http://hot-streamer.com/TeslaCoils/MyPapers/modact/Image165.gif

300pF at 21kV 120BPS....

E = 1/2 C V^2 x BPS = 29 watts!!

So we now have 43.4 watts in the resistors.  At 100 watts, they have a 300C
temp rise.  At 43 watts, they are cooking at 155C  Darn toasty!!

Problem is the normal full rated temp for the power resistors is stunning
300C!!  So we massively over rate them to keep the temperatures
sane.  Originally I used much smaller resistors, but everyone whined that
they were getting supper hot.  So I upped the size of the resistors like 4X
and no more complaints ;-))

Cheers,

         Terry