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Filter Choke

 * Original msg to: Coco-at-astroman-dot-com
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting coco-at-astroman-dot-com:

> Basically my question is this.. concerning the protection of my
> neon sign transformers, I have heard two diff. ideas on the 
> design of the chokes..one friend of mind recc. making a a pair
> of coils with a winding length the same as my secondary length,
> but finer gauge wire, on a form similar in proportions to my 
> secondary, then running each side of these to ground with a 
> total of four caps...Another told me he just uses no.12 
> household wire, with about 12 or so turns..(but no
> specs. on form size/proportion) 

My first advice would be to forget getting coiling advice from
this friend. 

> I guess I do not fully understand what these are meant to do, I
> was under the impression that that they are to be roughly the 
> oper. freq. in order to block that freq. from going back to the
> power suppply and causing damage,...if so, how accurate must 
> they be..what kind of range is effective?

I can see it is time again to re-repost the basics we have
learned (as a group) about RF choking and HV RF filter boards.

1) Air wound RF chokes are absolutely worthless.

2) RF chokes can can vary widely in inductance and still be       
   effective, but in values between 3-5 mh work very well.

3) RF chokes are most effective when combined with bypass         
   capacitance.

To get this 3-5 mh inductance (ideal) I (we) have found that
large ferrite toroids or rings work best for the choke core.
These donut shaped cores are sometimes plastic coated; if they
are not plastic coated they should be wrapped with several layers
of insulating plastic electrical tape before winding. The
prepared core should then be wound with at least a couple dozen
turns of thin insulated wire. These inductors can be placed in
series for added protection/better breakdown performance.

The purpose of these inductors, or RF chokes, is to present an
impedance to the RF from the Tesla tank circuit looking into the
power supply. The amplitude of any RF signal on the 60 cycle feed
line should be measurably lower on the power supply side of these
choking inductors.

To further reduce the levels of RF "hash" on the HV transformer
winding, a small value of bypassing capacitance is recommended. 
Bypass capacitance has three basic requirements: a) very small
value  b) very high voltage rating  c) and a very high RF
dissipation factor dielectric. Surplus DC rated HV barium
titanate capacitors seem almost made for the job, but homemade
glass caps work equally well. The value of these capacitors
should be well below .001 microFarads. They are placed across the
HV line in two different ways depending on the step up
transformer.

1) If the step-up transformer is center tap grounded (i.e. a      
   neon) the bypass capacitance needs to be divided into two      
   equal values and the center of the bypass capacitance is       
   grounded with the transformer core. In this case the value
   of the bypass cap on each side of the RF ground needs to be
   less than .001 microFarads.

    X1                                           

   ||O--------------------------------nnnnnnnn--TO-TANK-CIRCUIT->
   ||O            |             |        RF1
   ||O         ------- BC1      |                                 
--O||O         -------          |
  O||O            |             *
  O||O--GRND------|-----GRND----* SG                             
  O||O            |             * 
--O||O         -------          |
   ||O         ------- BC2      |                              
   ||O            |             |        RF2                 
   ||O--------------------------------uuuuuuuu--TO-TANK-CIRCUIT->

Where:

        X1 = Is the step up xfmr with grounded center tap
      GRND = Dedicated RF Ground for Tesla work
       BC1 = Bypass Capacitance
       BC2 = Bypass Capacitance
        SG = Safety Gap w/grounded center post
       RF1 = Radio Frequency Choke
       RF2 = Radio Frequency Choke

On larger transformers (potential, pig) where there is no
grounded center tap on the HV winding: a single (undivided) 
bypass capacitance with a value less than .001 microFarads may 
be placed directly across the HV bushings to provide bypassing.

> Also..concerning additional RF from traveling back down the 
> suply line..can anyone reccomend any type of line filters to 
> put before the power supply? 

I use paralleled commercial line filters in this application.

> Concerning coupling of primary and secondary...what sort of 
> distance is a good staring point between the primary and 
> seconary (for a system with 15kv, 120ma, 0.03 uf, secondary 
> 6.0"OD, roughly 260khz)...I know it will take a little 
> experimenting, but I would like to start in the ball park. 

This depends on the physcial shape of the primary. Using an
inverted conical section "saucer" or flat spiral "pancake"
primary I find that around 1-1/2" inches distance is about right. 
This works out to an inner primary turn about 3" larger in
diameter than the secondary.

Richard Quick


... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12