RE: [TCML] pole pig protection

["Lau, Gary" <Gary.Lau@xxxxxx>]

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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