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More thoughts on protection chokes


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From:  Greg Leyh [SMTP:lod-at-pacbell-dot-net]
Sent:  Tuesday, February 24, 1998 5:27 AM
To:  Tesla List
Subject:  Re: More thoughts on protection chokes

Gary Lau wrote:


> VSIN is 7.5KV RMS = 10.6KV 60 Hz, half of 15KV/60mA NST secondary.
> L3/R3 are lumped secondary components, 352 Hernys and 2.75K.
> C3 is my bypass cap, 600 pF.
> L2/R2/C2 is my ferrite core choke, 14mH, 0.5Ohm, 14.8pF
> R1 is my series damping resistor, 500 Ohms.
> SW1 & SW2 are switches representing spark gap - on-at-8.333ms, off-at-8.7ms
> R4 is spark gap resistance, arbitrarilly set at 4 Ohms.
> L1/C1 are primary tank components, 30.1 uH, .01 uF (290 KHz).
> 
>   +------+--R1--+--L2--R2--+------L3--R3-+
>   |      |      |          |             |
>   |     SW1      ----C2----+            VSIN
>   L1     |                 |             |
>   |      |                C3             |
>   |      |                 |             |
>   |      R4                +-------------+--GND
>   |      |                 |             |
>   |      |                C3             |
>   C1     |                 |             |
>   |     SW2      ----C2----+            VSIN
>   |      |      |          |             |
>   +------+--R1--+--L2--R2--+------L3--R3-+
> 
> I've found some unexpected things.  Looking at R2/L3 (xfmr terminal), I
> don't see any evidence of the tank resonance energy, regardless of how I
> include or exclude the C3 bypass caps.  This may be due to my simplistic
> modeling of the gap conducting for > the duration of the tank ringdown.
> When the gap is conducting, there is essentially no voltage across it,
> and when the gap opens up, there's no longer have a tank circuit at the
> frequency of the ringing secondary for it to couple into.  So I'm having
> trouble remembering what we're trying to protect against!


That's because you have a properly designed primary
tank circuit!  Your circuit values and configuration
as shown here seem valid to me.

IMHO, most of the reported NST failures posted on this 
List are due to:
A) using marginal or "previously damaged" NST's
B) wiring errors
C) placing the pri cap across the NST, rather than
   the spark gap
D) excessive reactive power levels in the NST

Also, if a designer relies solely on the parasitic
capacitance of the NST to provide C3, then the value
for C3 may vary wildly, depending on the geometry of the
winding, and the potting compound.  This will drastically
alter the performance of the filtering network.

Your addition of an external C3 improves upon this, but 
the task of de-Qing the filter still remains.  My guess is
that the required de-Qing resistances will dissipate 
quite a bit of heat.


[snip]
> What I do see in the PSpice simulations however is my choke and bypass
> cap resonating and ringing down as soon as the gap conducts.  The choke
> ringdown current here peaks at tens of AMPS and is due to the bypass caps
> discharging through the chokes, gaps, and damping R's.

Yes, indeed!
For all of you 'empiricists' out there, this result of
Gary's simulation is quite REAL, and is not at all obvious 
from looking at the schematic.  It was this same choke 
ringdown current that _melted_ a 4" x 20" RFC in my old
coil!  PSPICE revealed that the choke current was not the
expected 5ADC, but rather 20A RMS AC worth of ringdown!
Changing from an L-C to an L-Diode arrangement fixed this.

A picture of the L-Diode filter can be seen at:
ftp://ftp.funet.fi/pub/sci/electrical/tesla/pictures/gl/gl_diode.jpg
 

-GL