Re: reason for rf chokes?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Larry,

At 05:50 AM 8/11/2002 +0000, you wrote:
>two more basic nst filter questions.
>
>using the rc filter design, the pair of caps are tied to rf ground.
>as well as the safety gaps to rf ground.  does that mean i need another 
>ground rod or can i use the same one im using for my secondary?  i'm asking 
>because in my mind that doesn't sound safe.  As in it would cause excess 
>signal noise... (perhaps i'm wrong, since the bottom of the secondary caries 
>low voltage high current.)  and if I have poor grounding (i have two four 
>foot zinc coated threaded stock 5/8 dia for ground rods,) then the rc filter 
>wouldn't work as well compared to a better rf ground.  or am i just 
>splitting hairs?

"I" just tie all the RF stuff to a single RF ground.  I never worried about
it too much.  However, there are a few differing opinions on all this.

>
>I had previously looked at a rc filter design, but ruled it out.  why? 
>because the two caps tied to the high voltage side, to me, looked like a 
>short.  i figured the caps would just go up in smoke and the hv caps would 
>be junk.
>

As long as the caps are small value and can take the voltage, there is no
problem.  The resistors isolate them from the main gap.

>in sizing the resistors, (just wanted to make sure i get this right)
>12kv/120ma = 1440watts. there for each resistor would need to be a minimum 
>of 720 watts? or would i multiply 12kv x sqrt(2) for true voltage? 
>thus...2036 watts, 1016watts min.

The NST puts out 120mA so the resistors dissipate 0.120^2 x 1000 = 14.4
watts.  

Power = current squared x resistance.

>
>and here i beat the dead horse.
>a choke can be a filter. a cap can be a filter. a resistor cannot filter by 
>itself. an rc filter: the cap has a charge time, the resistor increases the 
>charge time. an lc filter: the cap has charge time, the inductor acts like a 
>frequency relative resistor which affects charge time with regard to 
>frequency. i understand some of the basics, but the portion of the 
>resistance doesn't ring right with me. (bad pun, sorry :)

Putting 1000 ohms into a 1000 watt system sounds odd but the high voltage
goes right through it.  Consider that the output impedance of a 15/120 NST is:

15000 / 0.12 = 125000 ohms  Another 1000 ohms makes little difference.

Cheers,

	Terry


>
>larry.
>
>