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RE: [TCML] pole pig protection
These comments are in response to Gary Lau, who writes on this thread.
Gary is a very nice person. The following opinions are mine, and for the
record I agree completely with everything Gary has said. Someone please let
me know if I have missed the point entirely, and if that is the case then I
am sorry.
Terry filter protection for a pole pig is pointless. A pole pig is designed
to be struck by lightning over and over again during its lifetime. Have you
ever been hit by lightning? As equipment goes, a pig is typically used to
avoid transformer limitations. You can do just about whatever you want with
a pole pig, and it will still be there (not hindered in any way) after the
explosion.
Miles Waldron
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Lau, Gary
Sent: Tuesday, September 09, 2008 6:25 PM
To: 'Tesla Coil Mailing List'
Subject: RE: [TCML] pole pig protection
Opinions vary on this topic. Here is mine.
First off, in contrast to NST's and MOT's, pole pigs are extremely rugged
and well designed. They are designed to be undamaged by lightening strikes.
While it is not unheard-of, damage to pole pigs is a very rare thing.
As to protecting the pig from RF - I assume that your TC circuit has the
main gap in parallel with the pig HV secondary. Please consider that the
ONLY time there is RF oscillating in the TC primary or secondary, the spark
gap is conducting. If the spark gap is conducting, it is essentially
shorting out the pig's HV secondary, so there's not a lot of RF to protect
against.
Now, it's not entirely true that the gap is conducting continuously during
each bang. Each time the current through the gap passes through zero at the
resonant frequency, the gap arc extinguishes briefly for a few(?)
nanoseconds. During this brief time the primary rings with its
self-capacitance, in the tens of MHz region. The initial magnitude of this
oscillation is Vbang, and since it is in series with the tank cap, also
initially at Vbang, the combination of the two in series (what the HV
winding sees) is 2Vbang. This, IMHO, is the thing that needs to be
filtered, at least for NST's and less sturdy transformers. And this is
easily and effectively attenuated by the R-C filter in the Terry filter.
Simply adding a choke in series with the pig's HV terminal does not a filter
make. The pig's secondary may be viewed as a many thousand HENRY inductor.
Adding a few mH in series with it accomplishes nothing. Low pass filters
are constructed of typically two or more elements - L-C, R-C. L-C-R, etc.
Just adding a choke inductor does not achieve any filtering. You might as
well string together cloves of garlic or St Christopher medallions.
I apologize for being so wordy and blunt, but the use of chokes to "protect"
transformers is a practice that was once done, just because it "seemed" at
first glance like a good idea, and persists despite strong logic not to. It
has since been shown through rigorous circuit analysis, simulation, and
measurement, to be ineffective at best and counterproductive when done with
bypass caps.
Regards, Gary Lau
MA, USA
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Jim Calvin
> Sent: Tuesday, September 09, 2008 3:07 PM
> To: 'Tesla Coil Mailing List'
> Subject: [TCML] pole pig protection
>
>
> I have a 25KVA pole pig I am using for my large tesla coil (the coil
> only uses about 5-6KVA). I am using a hand wound RF choke to protect
> the RF from the coil from getting back into the pig. It is wound with
> 16 gauge wire onto a 8 inch sonatube. I measured the inductance with
> RLC meter to be 10mH. My pole pig has two HV leads with one grounded
> and the other connected through this choke to my coil. I have not
> used the coil in awhile (been a busy summer). I was going to start
> using it again but I wanted to make sure this is sufficient to protect
> my piggy! Any advice would be helful.
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