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Re: Ferrite chokes & saturation
From: Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: bert.hickman-at-aquila-dot-com
Sent: Thursday, November 20, 1997 9:11 AM
To: Tesla List
Subject: Re: Ferrite chokes & saturation
Tesla List wrote:
>
> From: Gary Lau 19-Nov-1997 1728[SMTP:lau-at-hdecad.ENET.dec-dot-com]
> Sent: Wednesday, November 19, 1997 3:39 PM
> To: tesla-at-pupman-dot-com
> Cc: lau-at-hdecad.ENET.dec-dot-com
> Subject: Ferrite chokes & saturation
>
> Many of us employ ferrite core chokes in our xfmr protection networks.
> While the effectiveness (HF attenuation factor) of an L-C low pass filter
> can be fairly easily calculated, I haven't seen any quantitative
> discussion on whether core saturation is occuring. If it is, that pretty
> much wipes out the benefit of an inductor.
>
> I've looked over the Amidon web site for info on saturating ferrite core
> chokes, but the graphs, beyond being illegable, I just didn't know how to
> apply them.
>
> Does anyone actually know how to determine, either through specs or
> measurement, if a ferrite core choke is likely to saturate in a Tesla xfmr
> protection network? Is it the 60 Hz current or the HF current that is
> significant, or both? How would either of these be calculated?
Of the two, it's the mains 50/60 Hz current that would be most likely to
cause saturation. Ideally, if the choke is "choking", the HF current is
much smaller. A rough estimate for the mains current can be made by
using the sum of the faceplate RMS output current ratings of your neons
and multiplying this by 1.41 to find the peak current. A factor of 2-3
times this can then be applied if you have sized the tank cap to match
the transformer limiting inductance (50/60 Hz resonant case).
>
> If it is the HF current that causes saturation, could one then assume
> that if a primary strike occurs, saturation current may then be reached
> and choke effectiveness lost, at the time it is most needed?
>
The protection chokes are not intended to stand off the voltages you'll
see if you get a primary strike. The odds are quite good that your
protection chokes will flash over under this condition - This is where
your safety gaps come into play.
> And on the topic of xfmr protection networks, assuming safety gaps are
> employed at the xfmr outputs, are series chokes and/or R's integral to
> the safety gap's effectiveness? I'm trying to rationalize why many on
> this list use chokes but no bypass caps, or R's but no chokes. Could
> these series components be viewed as ballast devices, significant only
> if and when the safety gap fires?
>
> Gary Lau
> Waltham, MA
Many coilers have tried various configurations in an attempt to lengthen
the life of their neons, and there's also a degree of "folklore" on the
merits of various configurations. Ideally, you want bypass caps shunting
each HV leg of the neon to ground, and series resistors to provide
adequate damping in addition to single-layer HV chokes. When the main
gap fires, stray (parasitic) capacitances and wiring inductances on the
HV transformer side of the gap will be shock excited and will "ring"
with VHF oscillations.
The combination of the RF chokes, series Resistors, and bypass caps are
there to safely trap these on the gap side of the choke, and shunt any
residual VHF making it through the chokes around the transformer
secondaries via the bypass caps. The series R's should then rapidly
dissipate the any lower frequency shock-induced ringing between the
bypass caps and the protective chokes.
Safe chokin' to you!
-- Bert --