Re: Terry filter alternates?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Mark,

Microsim says that my big coil's filter resistors dissipate 7.64 watts 
(15/60 LTR) and my small coil's resistors 1.76 watts.

So your right.  I had almost gone all week without messing up too :o))

Since the filter was designed for up to 250mA systems, smaller NST systems 
can use smaller resistors but still beware they the get hot.  The resistors 
also see 1/2 of the NST voltage for short times so they have to have 
considerable voltage handling.  Ceramics are great for this.  I would worry 
about other smaller types from the voltage standpoint.

Hope this helps "my" confusion %:-)

Cheers,

        Terry



At 07:54 PM 12/3/2004, you wrote:
> 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